14 ms·

How Discord Stores Billions of Messages Using Cassandra

438 points by jhgg 10y ago

no_protocol 10y ago
Publicly naming a group of users who caused a problem with their system makes me uncomfortable enough to avoid trying their service. How do I know the next blog post won't be calling me out for doing something wrong? Also, apparently you can't search past messages...yet.
- Vishnevskiy 10y ago
  We make friends with owners of large public Discords and they had no problem with being included. The owner of that Discord even enjoyed reading the blog post. You can also search past messages and the blog post ends with saying the follow-up will be about search. Search is not rolled out to everyone but will next week. https://support.discordapp.com/hc/en-us/articles/115000468588-Using-Search https://support.discordapp.com/hc/en-us/articles/11500046858...
  - no_protocol 10y ago
    Neat. That seems like a major addition. Can I download the full transcript of a channel for safekeeping in case things are deleted in the future?
    - Vishnevskiy 10y ago
      We don't have an archive downloader, but you can use our API to scan over the history and save it. https://discordapp.com/developers/docs/resources/channel https://discordapp.com/developers/docs/resources/channel
      - no_protocol 10y ago
        Thanks for sharing. I guess I'm not quite in the target market. It seems like a very interesting evolution beyond prior closed chat systems like AIM, ICQ, etc.
- owaislone 10y ago
  They didn't do anything wrong. They just used a feature Discord provided. It was Discord who failed to anticipate a possible problem with the feature. It's not like they are blaming or publicly shaming someone.
jakebasile 10y ago
I use Discord a fair amount, and something that annoys me about it is that everyone has their own server. I realize this is a key part of the product, but the way I tend to use it is split into two modes: - I hang out on a primary server with a few friends. We use it when we play games together. - I get invited to someone else's server when I join up with them in a game. The former use case is fine but the latter annoys me. I end up having N extra servers on my Discord client that I'll likely never use again. I get pings from their silly bot channels (seemingly even if I turn notifications off for that server/channel), and I show up in their member lists until I remove myself. I wish there was a way to accept an invite as "temporary", so that it automatically goes away when I leave or shut down Discord. Maybe keep a history somewhere if I want to go back (and the invite is still valid). Aside from that, it's a great product and really cleaned up the gamer-focused voice chat landscape. It confuses me that people will still use things like TeamSpeak or (god help you) Ventrilo when you can get a server on Discord for free with far better features. Now that I posted this, I realize this has little to do with TFA. Sorry. edit: formatting, apology
- Vishnevskiy 10y ago
  We have plans to make using temporary sessions in games a much better experience so look out for that in the future.
  - Lutin 10y ago
    There's also a "Grant temporary membership" option when creating the invite that will automatically kick users when they disconnect unless a role has been assigned to them, but having that as an option when accepted would be cool.
- acron0 10y ago
  It's possible to leave servers. I was also irritated by the exact behaviour you described, until I figured out how to leave servers. You can also mute all notifications from a server.
- markcerqueira 10y ago
  "seemingly even if I turn notifications off for that server/channel" This kills me the most. When I turn off notifications for a server, I do not want to see the red dot on the app icon in my Dock.
joaodlf 10y ago
Not surprised to see other companies facing issues with Cassandra and tombstones. Don't get me wrong, I understand the need for tombstones in a distributed system like Cassandra... It doesn't make it any less of a pain though :).
- jjirsa 10y ago
  The tombstone problem described is due to misuse - probably from improper use of prepared statements. Looks like they worked around it well.
  - dwenzek 10y ago
    > The tombstone problem described is due to misuse. My concerns with Cassandra are precisely here: this is easy to misuse it. There are a lot of constraints on the schema (more specifically on the design of partition & clustering keys). Each choice leads to many restrictions on what can be requested/updated/removed; and to different issues with tombstones and GC. The Discord's story is exactly what I experimented: a series of surprises, and even really bad surprises in production. In both cases, the story ended with an efficient system, but with by far more engineering work and rework than initially planned.
    - krinchan 10y ago
      This article is extremely similar to almost every other "OMG! We used Cassandra and it was nothing like a SQL database!" article by Netflix, Spotify, and so many more. The fact that every single one contains the same 6 or 7 self-inflicted issues is pretty funny to me. I mean, I thought we lived in the age of the Google Dev. Cassandra does require you to know more of it's internals than most other data stores. Unfortunately, the move to CQL and very SQL-like names for things that are nothing at ALL like their SQL counterparts is not helping. Also, our own personal death by tombstone: A developer who didn't even know those existed checked in some logic that would write null into a column every time a thing succeeded. After that passed QA and went into production, all hell broke loose with queries timing out everywhere. SUCH FUN.
flyingramen 10y ago
It is fascinating that more and more people are using Cassandra. DataStax believes they have fixed problems with prior guarantees claims that were exposed by Jepsen. But there has been no official Jepsen testing since. On the topic of looking at Scylla next, I wonder why did the team not just start out with it to begin with. Also, are they people with experience running both. How is the performance? And what is the state of reliability?
- Vishnevskiy 10y ago
  The problems that Jepsen found were centered around the "transactions" feature that Cassandra added. We don't use these and don't need them since we don't need 100% consistency and prefer availability (for example we read at quorum to trigger read repair, but downgrade to single node reads if we need to). Also ScyllaDB is a new product and it would be crazy to start off with it. We plan to run a long-term double write experiment before we are comfortable with using it as a primary data store.
  - smharris65 10y ago
    The Jepsen tests were not completely centered around transactions. It also had to do with data loss when replicas go down and pure "last-write-wins" approach. For those wanting more info around this the original post is here: https://aphyr.com/posts/294-jepsen-cassandra https://aphyr.com/posts/294-jepsen-cassandra
    - Vishnevskiy 10y ago
      Last-write-wins is a semantic we are okay with for this data, and dealing with one of our conflicts is outlined the article.
      - smharris65 10y ago
        I enjoyed your article and I do appreciate your transparency!
- 10y ago
mahyarm 10y ago
Discord missed an opportunity a year or two ago to become something like slack for large companies. Hipchat's perf is horrible and slack couldn't scale to +20k users a year ago. Managing a mattermost instance requires staff and is more outage prone. It's really too bad that they didn't take advantage of it, since they were actually scalable compared to their competitors and had good voice chat. Slack has started becoming more scalable recently, so I don't know how much the opportunity is still there.
- Vishnevskiy 10y ago
  We are larger than Slack
  - ceejayoz 10y ago
    In what metrics? Number of users? Number of paying users? Valuation? Headcount?
    - nrjdhsbsid 10y ago
      Hype
    - sciurus 10y ago
      Seconding this question. Can you share the equivalent discord numbers for the slack numbers at the link below? http://expandedramblings.com/index.php/slack-statistics/ http://expandedramblings.com/index.php/slack-statistics/
      - Vishnevskiy 10y ago
        We currently don't share exact metrics for all those stats, but we have shared a few press releases and blog posts which you can easily extrapolate from. :)
    - bpicolo 10y ago
      I'm pretty sure he's suggesting number of users. Looks like roughly 11 vs 4 million? Definitely not paying users / valuation currently. Slack is pretty massive there.
lightedman 10y ago
You're storing messages, how are you guaranteeing safety of those messages when it looks like one can seemingly just blast through your API calls to find messages when one isn't even on that server?
- kbhn 10y ago
  A few seconds of research would have revealed that their API requires an authentication token[1] [1]https://discordapp.com/developers/docs/reference https://discordapp.com/developers/docs/reference
  - lightedman 10y ago
    A few seconds of pen-testing tells me their token revocation is pretty shoddy. I'm now spying on my fiance's Discord chat. Bravo to those that downvoted me without even bothering to use their brains (let alone test something.)
    - Vishnevskiy 10y ago
      The normal token system revokes on password change, if you want to revoke and have extra security we offer MFA login which has unique tokens per login. If security is of importance to you then use MFA.
      - lightedman 10y ago
        Why are you not revoking tokens after session end across the board? Token re-use is one of the faster-rising security breach factors now days.
    - Sacho 10y ago
      Your first claim seems to be wrong - why wouldn't people downvote it?
      - lightedman 10y ago
alfg 10y ago
Love Discord. Most of my friends and I have switched over from using Mumble and it's been great. I run a small Mumble host [1] and I've always thought of the idea of wrapping the Mumble client and server APIs to function like Discord/Slack as an open source alternative. Mumble is great and all, but the UI/UX appeal of Discord is so much better. Keep up the great work! Also, is this is the same Stanislav of Guildwork? Ha, I remember when Guildwork was being formed back in the FFXI days. [1] https://guildbit.com https://guildbit.com
- Vishnevskiy 10y ago
  I am :) glad people remember me from FFXI haha Add me on Discord if you ever wanna chat, Stanislav#7943
jolux 10y ago
Discord is great but I have intermittent performance issues with it that make it almost unusable in comparison to Slack which never has any noticeable latency.
- traskjd 10y ago
  This may or may not be your issue. But I found that the Windows Defender system would start going nuts on files related to Discord. This would cause all sorts of problems (though most of the files involved seemed to be more related to their auto-update system, which shouldn't have impacted the running process... or so I would have thought!).
  - jolux 10y ago
    No this was on Mac...
- terda12 10y ago
  This is pretty good to hear honestly. Competition is always good for the consumer. This rivalry between Discord and Slack will only make things better for everyone.
- Vishnevskiy 10y ago
  This should not be the case, we strive for Discord be lightning fast at all times. Please email me stanislav@discordapp.com I would love more details.
  - jolux 10y ago
    I can't really give you "more details," I was trying to use Discord with some people and every couple minutes or so the chat would freeze up and then flood with messages. It wasn't just me, it was everyone in the room's Discord doing it, and it really doesn't seem like a client-side bug.
sparrish 10y ago
If you're deleting often, I recommend running a full compact (after your repair) to free up space and rid yourself of those tombstones once and for all. Repairs without compacts make those SSTables grow and grow. It's amazing how much space a compact clears up.
- coredog64 10y ago
  I had to delete a shitload of data from Cassandra recently and it required dropping gc_grace_seconds to a very low value in order for the tombstoned records to be dropped during compaction (this was mentioned in the article)
- jjirsa 10y ago
  Full compactions really shouldn't be needed - if the tombstones are problematic you may be able to data model around it.
ve55 10y ago
Discord seems to me like it has a very polished user experience, and it's no surprise that users are trashing programs like Skype in favor of Discord when it is better in every area. Discord seems to take security seriously, as they should, but I'm curious about their stance on privacy and openness. For example, I wonder if they would consider: - Allowing end-to-end encryption to be used between users for private communications - Allowing users to connect to Discord servers using IRC or other clients (or, at least having an API that easily allows this)[1] - Allow users to have better control over their own data, such as providing local/downloadable logs so that they can search or otherwise use logs themselves Discord is definitely succeeding within the gaming market, but I'm curious what other markets they would like to take a stab at. [1] I'm aware Discord has an API, but if I understand it correctly, normal users cannot easily use Discord from anything other the official Discord apps, as this API is specifically for Discord 'bots'. I see there's a discord-irc bridge, but not much more than that. I may be incorrect on this.
- b1naryth1ef 10y ago
  - E2E/OTR encryption is something some of us are interested in, but due to the nature of our platform probably isn't going to happen anytime soon (we'd want to do it right, which requires time and effort). - Some libraries support connecting through user accounts, and there are various third-party tools for "linking" chat rooms, incl. some client plugins for irssi and such. We don't officially support it, but it's definitely possible. - Search is currently live on our alpha-testing client, and should be rolling out globally soon. It's also possible to save or log channels through the API fairly easily.
  - ve55 10y ago
    thanks for the informative response. I will look into how difficult it is to connect to a server using a user account from an IRC client, as that would make the experience much nicer for users like me. I'm curious about the logging API permissions - it seems kind of weird that I could potentially join someone's Discord server and then download logs of their conversations for the past year instantly after joining, but I suppose this is already possible by viewing history in the client? EDIT: looking at the API on https://discordpy.readthedocs.io/en/latest/api.html https://discordpy.readthedocs.io/en/latest/api.html, it seems you need permission for the channel logs, but that can't prevent someone from writing code to collect them manually, regardless of permissions?
niftich 10y ago
These kinds of write-ups offer valuable insight into a popular project's requirements and decision-making, and are some of the most instructive resources one can find: these show not only the kinds of challenges one has to face at scale, but also how architectural choices are made. It's far more valuable to understand why Discord uses Cassandra than to merely be aware they do. Out of curiosity, did you consider HBase and Riak? Did you entertain going fully hosted with Bigtable? If so, what criteria resulted in Cassandra winning out?
- Vishnevskiy 10y ago
  Let me take a stab at that. Riak is not a good model since its more a blob store and we wanted to simply range scan through messages rather than sharding blobs (Cassandra is REALLY good at this). HBase would have been fine for this model, but the open source version of HBase has much lower adoption than Cassandra so that was a big factor. We also don't care about consistency and HBase is a CP database, we prefer AP for this use case. As far as using GCP's BigTable (HBase compat), we made this decision before we moved to GCP, but we are also not fans of using platform lock-in. While BigTable has the same API as HBase we would hate to go to an less widely adopted version where we have a hard time getting community support if we decided to leave GCP. Hope that helps.
  - runeks 10y ago
    > As far as using GCP's BigTable (HBase compat), we made this decision before we moved to GCP, but we are also not fans of using platform lock-in. Did you consider GCP Datastore as well? It has strong consistency for a single "entity group", but eventual consistency for queries on multiple entity groups. So by storing data only relevant to a single user in an entity group, you can have strongly consistent, atomic transactions on that group (albeit limited to 1 tx/s), and at the same time do global queries on all user data with eventual consistency.
    - Vishnevskiy 10y ago
      The pricing model does not fit our needs, and that is even more locked in than the BigTable variant.
maktouch 10y ago
It's really interesting to see that you're using Cassandra for this. IIRC, Cassandra was created by Facebook for their messaging, and realized that eventual consistency was a bad model for chat, so they moved to HBase instead. (source: http://highscalability.com/blog/2010/11/16/facebooks-new-real-time-messaging-system-hbase-to-store-135.html http://highscalability.com/blog/2010/11/16/facebooks-new-rea...) The tombstone issue was really interesting ! Thanks for sharing.
- Vishnevskiy 10y ago
  They did make the original and the core model is the same, but Cassandra in 2017 is quite different from what they open sourced in features, usability and stability. I guess everyone makes their own tradeoffs though. This has been working wonderfully for us.
- jjirsa 10y ago
  You can have strong consistency in Cassandra - ING gave a talk at Cassandra summit 2015 on their multi-DC Strongly consistent use cases Cassandra let's you choose - per query - how many replicas must ack the query. Strong consistency is just a query parameter away.
  - nvarsj 10y ago
    Need to be careful about the wording, "strong consistency". I dislike that datastax uses that in their documentation, because it's misleading and really confuses people. There is no commit protocol in place - Cassandra is still an AP db under the hood, so even having multiple replicas acknowledge doesn't mean the data is consistent. For that you need paxos or something similar. This becomes very obvious if you are doing updates from multiple sources to the same key.
    - jjirsa 10y ago
      You do realize that Cassandra implements paxos and has a CAS system right?
smaili 10y ago
Does anyone know what protocol/transport Discord uses? XMPP, web sockets, JSON, etc?
- b1naryth1ef 10y ago
  Our realtime messaging is done over Websockets using either JSON (for web/non-native) or ETF (http://erlang.org/doc/apps/erts/erl_ext_dist.html http://erlang.org/doc/apps/erts/erl_ext_dist.html). Almost all user-based actions are sent over our HTTP API using JSON.
  - lobster_johnson 10y ago
    Since Cassandra is eventually consistent, how do clients get a consistent sequence of messages? Do you actually use Cassandra range queries to poll for new messages, or do clients use some kind of queue to get notified? Say messages A, B, C are created in that sequence. But isn't it then possible that a client asking for new messages only gets A and C, and B only shows up a few milliseconds later, which would be missed unless the client actually asked for "everything since before A"? Or is that not possible?
    - jhgg 10y ago
      We have a real time distribution system written in Erlang/Elixir that keeps the clients in sync. We do not poll for messages. That wouldn't work at our scale :)
cookiecaper 10y ago
I'm one of the people who nagged you on the redis post, and particularly expressed skepticism that such a transition would've been necessary. I haven't read this yet, but I just want to say thanks for actually following up to that thread and posting it. Looking forward to it! --------- EDIT: Just read the post, and while it provides a good perspective on Discord's rationale to introduce Cassandra in the first place and does a great job pointing out some unexpected pitfalls, it doesn't specifically respond to replacing Redis with Cassandra due to clustering difficulty, per the prior thread. [0] Redis is only specifically called out as something they "didn't want to use", which I guess is probably the most honest answer. The bucket logic applied to Cassandra seems like it could've been applied to redis + a traditional permanent storage backend nearly as easily. The biggest downside here would be crossing the boundary for cold data, but that's a pretty common thing that we know lots of ways to address, right? And Cassandra effectively has to do the same thing anyway, it just abstracts it away. Again, I'm left wondering what specific value Cassandra brings to the table that couldn't have been brought by applying equal-or-lesser effort to the system they already had. I also found it amusing that they're already contemplating the need to transition to a datastore that runs on a non-garbage-collected platform. [0] https://news.ycombinator.com/item?id=13368754 https://news.ycombinator.com/item?id=13368754
- Vishnevskiy 10y ago
  This post was about using Cassandra for message storage. You are basically advocating for plugging 2 systems together, which out of the box don't provide elasticity. Or we could just use Cassandra. It is a simpler solution and Cassandra is not new tech. Aside from caching empty bucket information we have nothing sitting in front of Cassandra. It works great and the effort was minimal. The Redis comment by jhgg was referring to our service which tracks read states for each user. We might write about that later, but it's not as interesting. The most interesting about that experience was reusing memory in Go to avoid thrashing the GC. We care about seamless elasticity for our services which Redis doesn't provide out of the box except with Redis Cluster which does not seem to be wildly adopted and forces routing to clients.
  - cookiecaper 10y ago
jjirsa 10y ago
Wildly biased Cassandra person, but I find this very well written and explained, and I'm especially happy that when you bumped into problems like wide partition and tombstone memory pressure, you didn't just throw up your hands, but you worked around it. The wide partition memory problem should be fixed in 4.0, for what it's worth.
treenyc 10y ago
I'm curious why would people use a closed source software, when you can use something like https://riot.im https://riot.im Please let me know. I may be missing something.
- eikenberry 10y ago
  Games vs general use. By targeting gamers Discord is able to customize the experience to make it better for games.
  - Zaheer 10y ago
    Kind of like Twitch vs Youtube even
    - eikenberry 10y ago
      Exactly.
- romanovcode 10y ago
  Marketing.
  - Crespyl 10y ago
    The incredibly slick ease of use/user onboarding doesn't hurt either.
- roastbeef 10y ago
  why do people always push matrix on ycombinator when other companies launch their own chat products?
  - treenyc 10y ago
    when there is an equally good x in open source, why would anyone use closed sourced alternative? Is it not a valid question? If it is the marketing, service of support then let me know.
simooooo 10y ago
What's an upsert?
- Vishnevskiy 10y ago
  Update if exists, otherwise insert.
beck5 10y ago
Serious question, how do you backup a casandra database of that size. Do you even back it up or just rely on the sharing to prevent dataloss?
- Vishnevskiy 10y ago
  Cassandra has a snapshat command that creates a directory by symlinking files that hold data (this is safe cause Cassandra files are immutable). Then you just upload them to your backup storage. This is obviously for recovery scenarios that are catastrophic. Normally though since the data is replicated on 3 nodes, you can technically loose a node completely and rebuild it from the other nodes.
- manigandham 10y ago
  Cassandra natively supports multi-cluster replication - so you can run an entirely separate cluster that also has a copy of the entire dataset (which itself has configurable replication within a cluster) which can be used as an online fully-active backup. We run 3 geo-distributed clusters with no offline backups because of this.
pilif 10y ago
> While Cassandra has schemas not unlike a relational database, they are cheap to alter and do not impose any temporary performance impact in most relational databases, the schema is cheap to alter and does not impose a temporary performance impact. In-fact, all of their requirements (aside of linear scalability) could also be met with a relational database. Doing so would gain you much more flexible access to querying for various reports and it would reduce the engineering effort required for retrieval of data as they add more features (relational databases are really good at being queried for arbitrary constraints). I think people tend to dismiss relational databases a bit too quickly these days.
- threeseed 10y ago
  a) I'm not aware of any relational database that can alter a schema in real time on a hot table with billions of records. b) You were quite okay to just dismiss scalability there except that's the most important requirement for a company such as this. People don't just choose Cassandra lightly given how significant its tradeoffs are. c) Most companies are offloading analytics/reporting workloads into Hadoop/Spark and then exporting the results back to their EDW. This allows for far more functionality and keeps your primary data store free from adhoc internal workloads. d) Nobody dismisses relational databases quickly. In almost all cases they are the first choice because they are so well understood. The issue is that most of them do have issues with linear scalability and the cost to support them quite prohibitive e.g. Teradata, Oracle.
  - macobo 10y ago
    Regarding a), a sharded postgresql (i.e. with citus data) can easily accomodate that workflow with just a tiny bit of extra overhead. Re c) this strongly depends on the usecase, I've seen companies use a) to avoid split-brain problems and having to manage two data pipelines to great success at similar scales. You might find https://www.youtube.com/watch?v=NVl9_6J1G60 https://www.youtube.com/watch?v=NVl9_6J1G60 interesting.
  - ianamartin 10y ago
    I sort of agree with threeseed and and the GP comment, so upvotes to both of you. 1) Altering schema is vague. It was used vaguely in the article (although, given the clarity of the article, I suspect the authors knew exactly what they meant). Some alterations on relational database tables are fine, even when hot and have billions of rows. Others are not. Add a new column: fine. Index the new column: fine. Create a new index on a column with billions of rows: definitely not fine. But the index plan described in the article was very specific about what they wanted. It doesn't sound like they had to add any new indices. 2) Mostly agree here. Linear scalability is a big deal here, and it's fucking hard to do well for most RDBMS systems. I slightly disagree, however, because the article explicitly states that the requirements are willing to trade C for A in CAP theorem. This is important. The hardest parts of linear scaling in RDBMS are enforcing C. Think transactional systems that absolutely must be consistent. Like your bank account. This isn't that, and the blog post clearly states it. Takes a lot of pressure off the relational database when it comes to scaling. 3) Strongly disagree. Most companies don't have the resources or manpower to do that. It takes a lot of time and a lot of effort. Hell, most companies don't even have an EDW. Let alone a pipeline from the OLTP server to Spark/Hadoop to the non-existent EDW. 4) We seem to run in different circles. Almost everyone I know dismisses relational databases without question. Mongo is the way to go. And I get called out as the resident old fart/luddite who insists on using postgres. Speaking of which, if the first things you think of with relational DBs are Teradata and Oracle, we are definitely operating in different contexts. If your opinion is that relational databases are generally well understood by--and therefore often the first choice for--developers . . . I want to know where you work. Because that's not a different context from where I am. That's a different universe. The reality is that storing and retrieving data is a hard problem, and there's no set answer that works for everyone in every situation. If you're building a new product from scratch, you should go with what your team knows, provided that the team knows enough to not put yourself in the situation where you're just losing data in a partition scenario (well-made point in the original article. Mongo is fine on one node. Scale it out, and you might as well write your data to /dev/null) Almost any datastore will serve the needs of a new product until it needs to scale horizontally. Relational, NoSQL, Object store, whatever. When it comes to scaling linearly, you have to take factors into account. 1) Which part of CAP theorem are you willing to sacrifice? You always have to let go of one. 1a) If you want a CP system, you have no choice but to deal with scaling problems of relational databases. You must have transactional guarantees for this to work. 1b) If you need an AP system, you have choices, but the choices lean in favor of systems like cassandra. It's just easier than seting up multi-node postgres and doing sharding. It's also worth pointing out that people very often dismiss vertical scaling too soon. Take a look at Joel Spolsky's articles about infrastructure at StackOverflow. You can do quite a lot with the available firepower of modern technology by just buying bigger and better hardware. I'm not suggesting that going bigger would have been the right choice for Discord. But sometimes it can be the right choice. If there's something I fundamentally disagree with about the article, it's this: trying to do everything in a single data store. I think--much like what you suggested above--that it's better to have separate systems for reading and writing. Since the use case is definitively AP, I can't see a reason not to have a transactional system in an RDBMS and a streaming pipeline to a cassandra cluster for reading. Use the right tools for the right job, is basically my point.
marknadal 10y ago
Wow! This is an incredible article. I do research and development for systems like this at GUN, and this article nails a lot of important pieces. Particularly there ability to jump to an old message quickly. We built a prototype of a similar system that handled 100M+ messages a day for about $10, 2 minute screen cast here: https://www.youtube.com/watch?v=x_WqBuEA7s8 https://www.youtube.com/watch?v=x_WqBuEA7s8 . However, this was without FTS or Mentions tagging, so I want to explore some thoughts here: 1. The bucketing approach is what we did as well, it is quite effective. However, warning to outsiders, this only effective for append-only data (like chat apps, twitter, etc.) and not good for data that gets a lot of recurring updates. 2. The more indices you add, the more expensive it gets. If you are getting a 100M+ messages a day, and you then want to update the FTS index and mentions index (user messages' index, hashtag index, etc.) you'll be doing significantly more writes. And you'll notice that those writes are updates to an index - this is the gotcha and will increase your cost. 3. Our system by default backs up / replicates to S3, which is something they mention they want to perhaps do in the future. This has huge perks to it, including price reductions, fault tolerance, and less DevOps - which is something they (and you) should value! There backend team is amazingly small. These guys and gals seem exceptionally talented and making smart decisions. I'm looking forward to the future post on FTS!
Globz 10y ago
I love Discord and use it on a daily basis, one of our main concern with my gaming group is the voice latency compared to TS, Mumble or Ventrilo but this is mainly due to the inability to host your own server. One of the big missing feature we would like to have in Discord is the ability to assign special permission to our groups leader so they can communicate over voice chat to other other group leaders in other channels (global voice chat). When we play PVP MMO's and have 40+ users all in the same channel calling shots its impossible to coordinate properly. What we normally do is split the group in 4 so 10 players in 4 different channels and each group leaders are calling shots independently BUT can also communicate via voice chat to other group leaders. Basically there's a global voice chat for group leaders that no one else can hear but them. Other than that Discord is amazing!
mastax 10y ago
For a bit more information about the tombstone issue from the perspective of the person who caused it: https://www.reddit.com/r/programming/comments/5oynbu/_/dcnxy5s https://www.reddit.com/r/programming/comments/5oynbu/_/dcnxy...
glidek 10y ago
> Having a large partition also means the data in it cannot be distributed around the cluster. Why can't a large partition be distributed around the cluster?
- manigandham 10y ago
  Cassandra uses consistent hashing. A partition is a segment of data identified by the partition key to determine which node in the consistent hash ring owns that data. You cant break down partitions any further because it's just a name for the smallest cohesive set of data owned by a hash key, so instead it's advisable to use more partitions with data modeling rather than making them huge.

438 points by jhgg 10y ago

Publicly naming a group of users who caused a problem with their system makes me uncomfortable enough to avoid trying their service. How do I know the next blog post won't be calling me out for doing something wrong? Also, apparently you can't search past messages...yet.

Vishnevskiy 10y ago
We make friends with owners of large public Discords and they had no problem with being included. The owner of that Discord even enjoyed reading the blog post. You can also search past messages and the blog post ends with saying the follow-up will be about search. Search is not rolled out to everyone but will next week. https://support.discordapp.com/hc/en-us/articles/115000468588-Using-Search https://support.discordapp.com/hc/en-us/articles/11500046858...
- no_protocol 10y ago
  Neat. That seems like a major addition. Can I download the full transcript of a channel for safekeeping in case things are deleted in the future?
  - Vishnevskiy 10y ago
    We don't have an archive downloader, but you can use our API to scan over the history and save it. https://discordapp.com/developers/docs/resources/channel https://discordapp.com/developers/docs/resources/channel
    - no_protocol 10y ago
      Thanks for sharing. I guess I'm not quite in the target market. It seems like a very interesting evolution beyond prior closed chat systems like AIM, ICQ, etc.
owaislone 10y ago
They didn't do anything wrong. They just used a feature Discord provided. It was Discord who failed to anticipate a possible problem with the feature. It's not like they are blaming or publicly shaming someone.

jakebasile 10y ago

I use Discord a fair amount, and something that annoys me about it is that everyone has their own server. I realize this is a key part of the product, but the way I tend to use it is split into two modes: - I hang out on a primary server with a few friends. We use it when we play games together. - I get invited to someone else's server when I join up with them in a game. The former use case is fine but the latter annoys me. I end up having N extra servers on my Discord client that I'll likely never use again. I get pings from their silly bot channels (seemingly even if I turn notifications off for that server/channel), and I show up in their member lists until I remove myself. I wish there was a way to accept an invite as "temporary", so that it automatically goes away when I leave or shut down Discord. Maybe keep a history somewhere if I want to go back (and the invite is still valid). Aside from that, it's a great product and really cleaned up the gamer-focused voice chat landscape. It confuses me that people will still use things like TeamSpeak or (god help you) Ventrilo when you can get a server on Discord for free with far better features. Now that I posted this, I realize this has little to do with TFA. Sorry. edit: formatting, apology

Vishnevskiy 10y ago
We have plans to make using temporary sessions in games a much better experience so look out for that in the future.
- Lutin 10y ago
  There's also a "Grant temporary membership" option when creating the invite that will automatically kick users when they disconnect unless a role has been assigned to them, but having that as an option when accepted would be cool.
acron0 10y ago
It's possible to leave servers. I was also irritated by the exact behaviour you described, until I figured out how to leave servers. You can also mute all notifications from a server.
markcerqueira 10y ago
"seemingly even if I turn notifications off for that server/channel" This kills me the most. When I turn off notifications for a server, I do not want to see the red dot on the app icon in my Dock.

joaodlf 10y ago

Not surprised to see other companies facing issues with Cassandra and tombstones. Don't get me wrong, I understand the need for tombstones in a distributed system like Cassandra... It doesn't make it any less of a pain though :).

jjirsa 10y ago
The tombstone problem described is due to misuse - probably from improper use of prepared statements. Looks like they worked around it well.
- dwenzek 10y ago
  > The tombstone problem described is due to misuse. My concerns with Cassandra are precisely here: this is easy to misuse it. There are a lot of constraints on the schema (more specifically on the design of partition & clustering keys). Each choice leads to many restrictions on what can be requested/updated/removed; and to different issues with tombstones and GC. The Discord's story is exactly what I experimented: a series of surprises, and even really bad surprises in production. In both cases, the story ended with an efficient system, but with by far more engineering work and rework than initially planned.
  - krinchan 10y ago
    This article is extremely similar to almost every other "OMG! We used Cassandra and it was nothing like a SQL database!" article by Netflix, Spotify, and so many more. The fact that every single one contains the same 6 or 7 self-inflicted issues is pretty funny to me. I mean, I thought we lived in the age of the Google Dev. Cassandra does require you to know more of it's internals than most other data stores. Unfortunately, the move to CQL and very SQL-like names for things that are nothing at ALL like their SQL counterparts is not helping. Also, our own personal death by tombstone: A developer who didn't even know those existed checked in some logic that would write null into a column every time a thing succeeded. After that passed QA and went into production, all hell broke loose with queries timing out everywhere. SUCH FUN.

flyingramen 10y ago

It is fascinating that more and more people are using Cassandra. DataStax believes they have fixed problems with prior guarantees claims that were exposed by Jepsen. But there has been no official Jepsen testing since. On the topic of looking at Scylla next, I wonder why did the team not just start out with it to begin with. Also, are they people with experience running both. How is the performance? And what is the state of reliability?

Vishnevskiy 10y ago
The problems that Jepsen found were centered around the "transactions" feature that Cassandra added. We don't use these and don't need them since we don't need 100% consistency and prefer availability (for example we read at quorum to trigger read repair, but downgrade to single node reads if we need to). Also ScyllaDB is a new product and it would be crazy to start off with it. We plan to run a long-term double write experiment before we are comfortable with using it as a primary data store.
- smharris65 10y ago
  The Jepsen tests were not completely centered around transactions. It also had to do with data loss when replicas go down and pure "last-write-wins" approach. For those wanting more info around this the original post is here: https://aphyr.com/posts/294-jepsen-cassandra https://aphyr.com/posts/294-jepsen-cassandra
  - Vishnevskiy 10y ago
    Last-write-wins is a semantic we are okay with for this data, and dealing with one of our conflicts is outlined the article.
    - smharris65 10y ago
      I enjoyed your article and I do appreciate your transparency!
10y ago

mahyarm 10y ago

Discord missed an opportunity a year or two ago to become something like slack for large companies. Hipchat's perf is horrible and slack couldn't scale to +20k users a year ago. Managing a mattermost instance requires staff and is more outage prone. It's really too bad that they didn't take advantage of it, since they were actually scalable compared to their competitors and had good voice chat. Slack has started becoming more scalable recently, so I don't know how much the opportunity is still there.

Vishnevskiy 10y ago
We are larger than Slack
- ceejayoz 10y ago
  In what metrics? Number of users? Number of paying users? Valuation? Headcount?
  - nrjdhsbsid 10y ago
    Hype
  - sciurus 10y ago
    Seconding this question. Can you share the equivalent discord numbers for the slack numbers at the link below? http://expandedramblings.com/index.php/slack-statistics/ http://expandedramblings.com/index.php/slack-statistics/
    - Vishnevskiy 10y ago
      We currently don't share exact metrics for all those stats, but we have shared a few press releases and blog posts which you can easily extrapolate from. :)
  - bpicolo 10y ago
    I'm pretty sure he's suggesting number of users. Looks like roughly 11 vs 4 million? Definitely not paying users / valuation currently. Slack is pretty massive there.

lightedman 10y ago

You're storing messages, how are you guaranteeing safety of those messages when it looks like one can seemingly just blast through your API calls to find messages when one isn't even on that server?

kbhn 10y ago
A few seconds of research would have revealed that their API requires an authentication token[1] [1]https://discordapp.com/developers/docs/reference https://discordapp.com/developers/docs/reference
- lightedman 10y ago
  A few seconds of pen-testing tells me their token revocation is pretty shoddy. I'm now spying on my fiance's Discord chat. Bravo to those that downvoted me without even bothering to use their brains (let alone test something.)
  - Vishnevskiy 10y ago
    The normal token system revokes on password change, if you want to revoke and have extra security we offer MFA login which has unique tokens per login. If security is of importance to you then use MFA.
    - lightedman 10y ago
      Why are you not revoking tokens after session end across the board? Token re-use is one of the faster-rising security breach factors now days.
  - Sacho 10y ago
    Your first claim seems to be wrong - why wouldn't people downvote it?
    - lightedman 10y ago

alfg 10y ago

Love Discord. Most of my friends and I have switched over from using Mumble and it's been great. I run a small Mumble host [1] and I've always thought of the idea of wrapping the Mumble client and server APIs to function like Discord/Slack as an open source alternative. Mumble is great and all, but the UI/UX appeal of Discord is so much better. Keep up the great work! Also, is this is the same Stanislav of Guildwork? Ha, I remember when Guildwork was being formed back in the FFXI days. [1] https://guildbit.com https://guildbit.com

Vishnevskiy 10y ago
I am :) glad people remember me from FFXI haha Add me on Discord if you ever wanna chat, Stanislav#7943

jolux 10y ago

Discord is great but I have intermittent performance issues with it that make it almost unusable in comparison to Slack which never has any noticeable latency.

traskjd 10y ago
This may or may not be your issue. But I found that the Windows Defender system would start going nuts on files related to Discord. This would cause all sorts of problems (though most of the files involved seemed to be more related to their auto-update system, which shouldn't have impacted the running process... or so I would have thought!).
- jolux 10y ago
  No this was on Mac...
terda12 10y ago
This is pretty good to hear honestly. Competition is always good for the consumer. This rivalry between Discord and Slack will only make things better for everyone.
Vishnevskiy 10y ago
This should not be the case, we strive for Discord be lightning fast at all times. Please email me stanislav@discordapp.com I would love more details.
- jolux 10y ago
  I can't really give you "more details," I was trying to use Discord with some people and every couple minutes or so the chat would freeze up and then flood with messages. It wasn't just me, it was everyone in the room's Discord doing it, and it really doesn't seem like a client-side bug.

sparrish 10y ago

If you're deleting often, I recommend running a full compact (after your repair) to free up space and rid yourself of those tombstones once and for all. Repairs without compacts make those SSTables grow and grow. It's amazing how much space a compact clears up.

coredog64 10y ago
I had to delete a shitload of data from Cassandra recently and it required dropping gc_grace_seconds to a very low value in order for the tombstoned records to be dropped during compaction (this was mentioned in the article)
jjirsa 10y ago
Full compactions really shouldn't be needed - if the tombstones are problematic you may be able to data model around it.

ve55 10y ago

Discord seems to me like it has a very polished user experience, and it's no surprise that users are trashing programs like Skype in favor of Discord when it is better in every area. Discord seems to take security seriously, as they should, but I'm curious about their stance on privacy and openness. For example, I wonder if they would consider: - Allowing end-to-end encryption to be used between users for private communications - Allowing users to connect to Discord servers using IRC or other clients (or, at least having an API that easily allows this)[1] - Allow users to have better control over their own data, such as providing local/downloadable logs so that they can search or otherwise use logs themselves Discord is definitely succeeding within the gaming market, but I'm curious what other markets they would like to take a stab at. [1] I'm aware Discord has an API, but if I understand it correctly, normal users cannot easily use Discord from anything other the official Discord apps, as this API is specifically for Discord 'bots'. I see there's a discord-irc bridge, but not much more than that. I may be incorrect on this.

b1naryth1ef 10y ago
- E2E/OTR encryption is something some of us are interested in, but due to the nature of our platform probably isn't going to happen anytime soon (we'd want to do it right, which requires time and effort). - Some libraries support connecting through user accounts, and there are various third-party tools for "linking" chat rooms, incl. some client plugins for irssi and such. We don't officially support it, but it's definitely possible. - Search is currently live on our alpha-testing client, and should be rolling out globally soon. It's also possible to save or log channels through the API fairly easily.
- ve55 10y ago
  thanks for the informative response. I will look into how difficult it is to connect to a server using a user account from an IRC client, as that would make the experience much nicer for users like me. I'm curious about the logging API permissions - it seems kind of weird that I could potentially join someone's Discord server and then download logs of their conversations for the past year instantly after joining, but I suppose this is already possible by viewing history in the client? EDIT: looking at the API on https://discordpy.readthedocs.io/en/latest/api.html https://discordpy.readthedocs.io/en/latest/api.html, it seems you need permission for the channel logs, but that can't prevent someone from writing code to collect them manually, regardless of permissions?

niftich 10y ago

These kinds of write-ups offer valuable insight into a popular project's requirements and decision-making, and are some of the most instructive resources one can find: these show not only the kinds of challenges one has to face at scale, but also how architectural choices are made. It's far more valuable to understand why Discord uses Cassandra than to merely be aware they do. Out of curiosity, did you consider HBase and Riak? Did you entertain going fully hosted with Bigtable? If so, what criteria resulted in Cassandra winning out?

Vishnevskiy 10y ago
Let me take a stab at that. Riak is not a good model since its more a blob store and we wanted to simply range scan through messages rather than sharding blobs (Cassandra is REALLY good at this). HBase would have been fine for this model, but the open source version of HBase has much lower adoption than Cassandra so that was a big factor. We also don't care about consistency and HBase is a CP database, we prefer AP for this use case. As far as using GCP's BigTable (HBase compat), we made this decision before we moved to GCP, but we are also not fans of using platform lock-in. While BigTable has the same API as HBase we would hate to go to an less widely adopted version where we have a hard time getting community support if we decided to leave GCP. Hope that helps.
- runeks 10y ago
  > As far as using GCP's BigTable (HBase compat), we made this decision before we moved to GCP, but we are also not fans of using platform lock-in. Did you consider GCP Datastore as well? It has strong consistency for a single "entity group", but eventual consistency for queries on multiple entity groups. So by storing data only relevant to a single user in an entity group, you can have strongly consistent, atomic transactions on that group (albeit limited to 1 tx/s), and at the same time do global queries on all user data with eventual consistency.
  - Vishnevskiy 10y ago
    The pricing model does not fit our needs, and that is even more locked in than the BigTable variant.

maktouch 10y ago

It's really interesting to see that you're using Cassandra for this. IIRC, Cassandra was created by Facebook for their messaging, and realized that eventual consistency was a bad model for chat, so they moved to HBase instead. (source: http://highscalability.com/blog/2010/11/16/facebooks-new-real-time-messaging-system-hbase-to-store-135.html http://highscalability.com/blog/2010/11/16/facebooks-new-rea...) The tombstone issue was really interesting ! Thanks for sharing.

Vishnevskiy 10y ago
They did make the original and the core model is the same, but Cassandra in 2017 is quite different from what they open sourced in features, usability and stability. I guess everyone makes their own tradeoffs though. This has been working wonderfully for us.
jjirsa 10y ago
You can have strong consistency in Cassandra - ING gave a talk at Cassandra summit 2015 on their multi-DC Strongly consistent use cases Cassandra let's you choose - per query - how many replicas must ack the query. Strong consistency is just a query parameter away.
- nvarsj 10y ago
  Need to be careful about the wording, "strong consistency". I dislike that datastax uses that in their documentation, because it's misleading and really confuses people. There is no commit protocol in place - Cassandra is still an AP db under the hood, so even having multiple replicas acknowledge doesn't mean the data is consistent. For that you need paxos or something similar. This becomes very obvious if you are doing updates from multiple sources to the same key.
  - jjirsa 10y ago
    You do realize that Cassandra implements paxos and has a CAS system right?

smaili 10y ago

Does anyone know what protocol/transport Discord uses? XMPP, web sockets, JSON, etc?

b1naryth1ef 10y ago
Our realtime messaging is done over Websockets using either JSON (for web/non-native) or ETF (http://erlang.org/doc/apps/erts/erl_ext_dist.html http://erlang.org/doc/apps/erts/erl_ext_dist.html). Almost all user-based actions are sent over our HTTP API using JSON.
- lobster_johnson 10y ago
  Since Cassandra is eventually consistent, how do clients get a consistent sequence of messages? Do you actually use Cassandra range queries to poll for new messages, or do clients use some kind of queue to get notified? Say messages A, B, C are created in that sequence. But isn't it then possible that a client asking for new messages only gets A and C, and B only shows up a few milliseconds later, which would be missed unless the client actually asked for "everything since before A"? Or is that not possible?
  - jhgg 10y ago
    We have a real time distribution system written in Erlang/Elixir that keeps the clients in sync. We do not poll for messages. That wouldn't work at our scale :)

cookiecaper 10y ago

I'm one of the people who nagged you on the redis post, and particularly expressed skepticism that such a transition would've been necessary. I haven't read this yet, but I just want to say thanks for actually following up to that thread and posting it. Looking forward to it! --------- EDIT: Just read the post, and while it provides a good perspective on Discord's rationale to introduce Cassandra in the first place and does a great job pointing out some unexpected pitfalls, it doesn't specifically respond to replacing Redis with Cassandra due to clustering difficulty, per the prior thread. [0] Redis is only specifically called out as something they "didn't want to use", which I guess is probably the most honest answer. The bucket logic applied to Cassandra seems like it could've been applied to redis + a traditional permanent storage backend nearly as easily. The biggest downside here would be crossing the boundary for cold data, but that's a pretty common thing that we know lots of ways to address, right? And Cassandra effectively has to do the same thing anyway, it just abstracts it away. Again, I'm left wondering what specific value Cassandra brings to the table that couldn't have been brought by applying equal-or-lesser effort to the system they already had. I also found it amusing that they're already contemplating the need to transition to a datastore that runs on a non-garbage-collected platform. [0] https://news.ycombinator.com/item?id=13368754 https://news.ycombinator.com/item?id=13368754

Vishnevskiy 10y ago
This post was about using Cassandra for message storage. You are basically advocating for plugging 2 systems together, which out of the box don't provide elasticity. Or we could just use Cassandra. It is a simpler solution and Cassandra is not new tech. Aside from caching empty bucket information we have nothing sitting in front of Cassandra. It works great and the effort was minimal. The Redis comment by jhgg was referring to our service which tracks read states for each user. We might write about that later, but it's not as interesting. The most interesting about that experience was reusing memory in Go to avoid thrashing the GC. We care about seamless elasticity for our services which Redis doesn't provide out of the box except with Redis Cluster which does not seem to be wildly adopted and forces routing to clients.
- cookiecaper 10y ago

jjirsa 10y ago

Wildly biased Cassandra person, but I find this very well written and explained, and I'm especially happy that when you bumped into problems like wide partition and tombstone memory pressure, you didn't just throw up your hands, but you worked around it. The wide partition memory problem should be fixed in 4.0, for what it's worth.

treenyc 10y ago

I'm curious why would people use a closed source software, when you can use something like https://riot.im https://riot.im Please let me know. I may be missing something.

eikenberry 10y ago
Games vs general use. By targeting gamers Discord is able to customize the experience to make it better for games.
- Zaheer 10y ago
  Kind of like Twitch vs Youtube even
  - eikenberry 10y ago
    Exactly.
romanovcode 10y ago
Marketing.
- Crespyl 10y ago
  The incredibly slick ease of use/user onboarding doesn't hurt either.
roastbeef 10y ago
why do people always push matrix on ycombinator when other companies launch their own chat products?
- treenyc 10y ago
  when there is an equally good x in open source, why would anyone use closed sourced alternative? Is it not a valid question? If it is the marketing, service of support then let me know.

simooooo 10y ago

What's an upsert?

Vishnevskiy 10y ago
Update if exists, otherwise insert.

beck5 10y ago

Serious question, how do you backup a casandra database of that size. Do you even back it up or just rely on the sharing to prevent dataloss?

Vishnevskiy 10y ago
Cassandra has a snapshat command that creates a directory by symlinking files that hold data (this is safe cause Cassandra files are immutable). Then you just upload them to your backup storage. This is obviously for recovery scenarios that are catastrophic. Normally though since the data is replicated on 3 nodes, you can technically loose a node completely and rebuild it from the other nodes.
manigandham 10y ago
Cassandra natively supports multi-cluster replication - so you can run an entirely separate cluster that also has a copy of the entire dataset (which itself has configurable replication within a cluster) which can be used as an online fully-active backup. We run 3 geo-distributed clusters with no offline backups because of this.

pilif 10y ago

> While Cassandra has schemas not unlike a relational database, they are cheap to alter and do not impose any temporary performance impact in most relational databases, the schema is cheap to alter and does not impose a temporary performance impact. In-fact, all of their requirements (aside of linear scalability) could also be met with a relational database. Doing so would gain you much more flexible access to querying for various reports and it would reduce the engineering effort required for retrieval of data as they add more features (relational databases are really good at being queried for arbitrary constraints). I think people tend to dismiss relational databases a bit too quickly these days.

threeseed 10y ago
a) I'm not aware of any relational database that can alter a schema in real time on a hot table with billions of records. b) You were quite okay to just dismiss scalability there except that's the most important requirement for a company such as this. People don't just choose Cassandra lightly given how significant its tradeoffs are. c) Most companies are offloading analytics/reporting workloads into Hadoop/Spark and then exporting the results back to their EDW. This allows for far more functionality and keeps your primary data store free from adhoc internal workloads. d) Nobody dismisses relational databases quickly. In almost all cases they are the first choice because they are so well understood. The issue is that most of them do have issues with linear scalability and the cost to support them quite prohibitive e.g. Teradata, Oracle.
- macobo 10y ago
  Regarding a), a sharded postgresql (i.e. with citus data) can easily accomodate that workflow with just a tiny bit of extra overhead. Re c) this strongly depends on the usecase, I've seen companies use a) to avoid split-brain problems and having to manage two data pipelines to great success at similar scales. You might find https://www.youtube.com/watch?v=NVl9_6J1G60 https://www.youtube.com/watch?v=NVl9_6J1G60 interesting.
- ianamartin 10y ago
  I sort of agree with threeseed and and the GP comment, so upvotes to both of you. 1) Altering schema is vague. It was used vaguely in the article (although, given the clarity of the article, I suspect the authors knew exactly what they meant). Some alterations on relational database tables are fine, even when hot and have billions of rows. Others are not. Add a new column: fine. Index the new column: fine. Create a new index on a column with billions of rows: definitely not fine. But the index plan described in the article was very specific about what they wanted. It doesn't sound like they had to add any new indices. 2) Mostly agree here. Linear scalability is a big deal here, and it's fucking hard to do well for most RDBMS systems. I slightly disagree, however, because the article explicitly states that the requirements are willing to trade C for A in CAP theorem. This is important. The hardest parts of linear scaling in RDBMS are enforcing C. Think transactional systems that absolutely must be consistent. Like your bank account. This isn't that, and the blog post clearly states it. Takes a lot of pressure off the relational database when it comes to scaling. 3) Strongly disagree. Most companies don't have the resources or manpower to do that. It takes a lot of time and a lot of effort. Hell, most companies don't even have an EDW. Let alone a pipeline from the OLTP server to Spark/Hadoop to the non-existent EDW. 4) We seem to run in different circles. Almost everyone I know dismisses relational databases without question. Mongo is the way to go. And I get called out as the resident old fart/luddite who insists on using postgres. Speaking of which, if the first things you think of with relational DBs are Teradata and Oracle, we are definitely operating in different contexts. If your opinion is that relational databases are generally well understood by--and therefore often the first choice for--developers . . . I want to know where you work. Because that's not a different context from where I am. That's a different universe. The reality is that storing and retrieving data is a hard problem, and there's no set answer that works for everyone in every situation. If you're building a new product from scratch, you should go with what your team knows, provided that the team knows enough to not put yourself in the situation where you're just losing data in a partition scenario (well-made point in the original article. Mongo is fine on one node. Scale it out, and you might as well write your data to /dev/null) Almost any datastore will serve the needs of a new product until it needs to scale horizontally. Relational, NoSQL, Object store, whatever. When it comes to scaling linearly, you have to take factors into account. 1) Which part of CAP theorem are you willing to sacrifice? You always have to let go of one. 1a) If you want a CP system, you have no choice but to deal with scaling problems of relational databases. You must have transactional guarantees for this to work. 1b) If you need an AP system, you have choices, but the choices lean in favor of systems like cassandra. It's just easier than seting up multi-node postgres and doing sharding. It's also worth pointing out that people very often dismiss vertical scaling too soon. Take a look at Joel Spolsky's articles about infrastructure at StackOverflow. You can do quite a lot with the available firepower of modern technology by just buying bigger and better hardware. I'm not suggesting that going bigger would have been the right choice for Discord. But sometimes it can be the right choice. If there's something I fundamentally disagree with about the article, it's this: trying to do everything in a single data store. I think--much like what you suggested above--that it's better to have separate systems for reading and writing. Since the use case is definitively AP, I can't see a reason not to have a transactional system in an RDBMS and a streaming pipeline to a cassandra cluster for reading. Use the right tools for the right job, is basically my point.

marknadal 10y ago

Wow! This is an incredible article. I do research and development for systems like this at GUN, and this article nails a lot of important pieces. Particularly there ability to jump to an old message quickly. We built a prototype of a similar system that handled 100M+ messages a day for about $10, 2 minute screen cast here: https://www.youtube.com/watch?v=x_WqBuEA7s8 https://www.youtube.com/watch?v=x_WqBuEA7s8 . However, this was without FTS or Mentions tagging, so I want to explore some thoughts here: 1. The bucketing approach is what we did as well, it is quite effective. However, warning to outsiders, this only effective for append-only data (like chat apps, twitter, etc.) and not good for data that gets a lot of recurring updates. 2. The more indices you add, the more expensive it gets. If you are getting a 100M+ messages a day, and you then want to update the FTS index and mentions index (user messages' index, hashtag index, etc.) you'll be doing significantly more writes. And you'll notice that those writes are updates to an index - this is the gotcha and will increase your cost. 3. Our system by default backs up / replicates to S3, which is something they mention they want to perhaps do in the future. This has huge perks to it, including price reductions, fault tolerance, and less DevOps - which is something they (and you) should value! There backend team is amazingly small. These guys and gals seem exceptionally talented and making smart decisions. I'm looking forward to the future post on FTS!

Globz 10y ago

I love Discord and use it on a daily basis, one of our main concern with my gaming group is the voice latency compared to TS, Mumble or Ventrilo but this is mainly due to the inability to host your own server. One of the big missing feature we would like to have in Discord is the ability to assign special permission to our groups leader so they can communicate over voice chat to other other group leaders in other channels (global voice chat). When we play PVP MMO's and have 40+ users all in the same channel calling shots its impossible to coordinate properly. What we normally do is split the group in 4 so 10 players in 4 different channels and each group leaders are calling shots independently BUT can also communicate via voice chat to other group leaders. Basically there's a global voice chat for group leaders that no one else can hear but them. Other than that Discord is amazing!

mastax 10y ago

For a bit more information about the tombstone issue from the perspective of the person who caused it: https://www.reddit.com/r/programming/comments/5oynbu/_/dcnxy5s https://www.reddit.com/r/programming/comments/5oynbu/_/dcnxy...

glidek 10y ago

> Having a large partition also means the data in it cannot be distributed around the cluster. Why can't a large partition be distributed around the cluster?

manigandham 10y ago
Cassandra uses consistent hashing. A partition is a segment of data identified by the partition key to determine which node in the consistent hash ring owns that data. You cant break down partitions any further because it's just a name for the smallest cohesive set of data owned by a hash key, so instead it's advisable to use more partitions with data modeling rather than making them huge.