9 ms·

Events: Fat or Thin?

92 points by senzilla 4y ago

mabbo 4y ago
As always, it depends. Yay engineering and trade-offs. Hey just remember: both is always an option if you're consumers disagree. Thin stream from the consumers who don't trust the fat data, fat stream for the event log and other consumers that prefer it.
ulam2 4y ago
Why not have an option for both? Both have their use cases.
doctor_eval 4y ago
> Thin Events add coupling That’s not my experience. In fact I’d say fat events add coupling because they create an invisible (from the emitter) dependency on the event body, which becomes ossified. So I’d say the opposite: thin events reduce coupling. Sure, the receiver might call an API and that creates coupling with the API. But receivers are also free to call or not call any other API they want. What if they don’t care about the body of the object? So I’m on team thin. Every time I’ve been tempted by the other team, I’ve regretted it. It’s also in my experience a lot more difficult to version events than it is to version APIs, so reducing their surface area also solves other problems.
- SideburnsOfDoom 4y ago
  > thin events reduce coupling. Sure, the receiver might call an API and that creates coupling with the API. You make a statement in the first sentence, and in the next sentence produce evidence ... that the statement is wrong. And, YMMV. It is my experience that thin events add coupling. If service B receives an event, and wants to process it ASAP (i.e. near real time) and so calls back over http to Service A for the details, then a) there is additional latency for a http call. And time variance - Even if the average latency of a http request round-trip is fine, the P99 might be bad. b) You're asking for occasional "eventual consistency" trouble when A's state lags or has moved on ahead of the event c) Worst of all: When service A is down or unreachable, Service B is unable to do work: Service B uptime must be <= Service A uptime. You have coupled their reliability, and if Service B is identified as mission-critical, then you have the choice of either making Service A equally critical, or decoupling them e.g. with "fat events". I don't believe that it's accurate to say "receivers are also free to call or not call..." it's not choosing a flavor of ice-cream, you do the calls that the work at hand _needs_. If you find that you never need to call back to service A then yes, "thin events" would suit your case better. That has not been my experience. It's fair that event data format versioning is a lot of work with fat events - nothing is without downside. But in your case, do you have "dependency on the event body" ? All of it? If a thin event is all that you need, then you depend on a couple of ids in the event body, and not the rest. Json reading is very forgiving of added / removed fields, you can ignore the parts of a fat event that you don't care about.
  - FeepingCreature 4y ago
    > b) You're asking for occasional "eventual consistency" trouble when A's state lags or has moved on ahead of the event To be noted that this is the default if B is recovering after an outage. Personally, I consider events to be insane. "We create an immutable database so that the state of the system is always recoverable." Okay, cool, very functional programming of you. "But then to actually work with the event from the immutable database, you have to query a stateful service." ??? What? And even fat events only go so far to get you out of that. So with a stream of n events, you don't have n states that the application can be in, but n times the product of all possible states of every other service that you query. How does this help?!
    - PaulRobinson 4y ago
      The bit you seem to be missing is the events are the source of truth, not the databases. Lose your database? Roll up all the events. Got a lot of them? Take snapshots and then roll up from the last trusted snapshot. In true event sourced systems, the databases and stateful systems are artefacts that can be thrown away and rebuilt. The event log is the actual “true” database. Once you design around that, your objections melt away. And if you think this is some faddish trend, this is how finance has worked since the invention of book keeping and how your databases under your stateful services are working under the hood.
      - FeepingCreature 4y ago
        This only works if your events are in a single globally ordered stream or all your code is eventually consistent over every stream it consumes. Specifically, you cannot do the "query a service for the aggregate state" thing this article espouses for thin events, ever.
        insanitybit 4y ago
        You can achieve strong eventual consistency with this system.
  - 4y ago
- avereveard 4y ago
  assume the data format changes, it would change in the called api as well. as long as the fat event sends data that it's in the same format that the api would return, you'd have the same level of coupling. I think fat vs thin is more about how much other services the event have to travel, because thin event would multiply reads by a fair factor, with the tradeoff being the performance hit for the queue system to store and ship large events
  - doctor_eval 4y ago
    With an API you can publish a new endpoint (/v1, /v2 etc). It’s normally reasonably easy to maintain an old API even while you add features to the new API, and the runtime penalty is minimal because clients would be expected to call just one version of the API for any given event. (You can also see who’s calling the old API and ask them to change) But this is not true for events. If you change the body such that you now need to maintain two versions of an event, then you have to publish both events simultaneously, which means double the server side effort, storage etc for each event version. It’s pretty inefficient, and painful. You can work out who subscribes to the old event but there is still a big efficiency hit. You might be right about many reads per event in a simplistic way; if you have a lot of clients then it could be expensive if you don’t have a server side cache. But there would typically be a lot of temporality in such a system so it seems like an easy problem to solve for most use cases; you don’t have to cache for long, but caches are of course tricky if your use case is not very simple. That said, if there is already a HTTP connection open then the additional latency and bandwidth hit cause by this events are going to be minimal in most cases, and probably drowned out entirely if you need to push multiple versions. As I said in another thread, I should have said that thin is my default. There are cases when fat makes more sense, but normally I’d start with thin and see if I need to flesh it out. Whenever I’ve started fat I’ve ended up reverting.
    - manfre 4y ago
      Supporting multiple versions of an event schema is a solved problem. Apache Avro with a published schema hash in a message header is one solution. https://avro.apache.org/ https://avro.apache.org/
- paphillips 4y ago
  I also disagree with the article - thin events don't always result in more coupling, and I'll add that thin events can remove temporal or state coupling as illustrated below. However, the caveat is: as with many things I think choosing one team or the other has nuance and depends on the specific scenario. An example: I'm using thin events in a master data application integration scenario to send a 'sync this record' type of command message into a queue. The message body does not have the record details, only the basic information to uniquely identify the record. It also doesn't identify the type of change except for a flag to identify deletes. The 'sync' message is generalized to work for all entities and systems, so routing, logging, and other functions preceding the mapping and target operation have no coupling to any system or entity and can expect a fixed message format that will probably never change. Thus versioning isn't a concern. Choosing team 'thin event' does result in an extra read of the target system, but that is a feature for this scenario and what I want to enforce. I can't assume a target system is in any particular state, and the operation to be performed will be determined from the target system at whatever point in time a message is processed, which could be more than once. If the message ended up in a dead letter queue, it can be reprocessed later without issue. If one production system's data is cloned down to a lower environment, the integrations continue to work even if the source and target environment data is mismatched. No state is stored or depended upon from either system and the design is idempotent (ignoring a target system's business rules that may constrain valid operations over time). In contrast, other scenarios may benefit from or require a fat event. I've never used event sourcing, but as others mention, if current state can be built from all previous events 'rolled forward' or 'replayed', then each event must be a stand-alone immutable record with all information - thin events cannot be used. Or, if a scenario requires high performance we might need to use a fat event to eliminate the extra read, and then compensate for the other consequences that arise.
- drewcoo 4y ago
  Thank you. Came here to say that. When I've seen this fat event pattern it's been because different services' responsibilities were not fully separated. And that's tight coupling. Fat events imply tight coupling. The "thin" pattern described in the article goes like this: 1) service FOO gets an event 2) FOO then has to query BAR (and maybe BAZ and QUUX) to determine the overall state of everything to determine what to do next And #2 means all of that is kind of "thin" is tightly coupled, too. I've also personally seen thin events that are not the article's thin strawman. I sometimes wonder if people understand coupling or design.
eurasiantiger 4y ago
Delta events sounds an awful lot like Change Data Capture.
- thom 4y ago
  It seems rare that every operation in a business is atomic across a single aggregate, so I’ve always been wary of events that are pure CRUD, whatever you choose to call them.
ivan_gammel 4y ago
I‘d pick neither and just let the system in possession of data send with the event only the part of data it owns (i.e. something in between fat and thin). Saves API call back, the body doesn’t have to be fully deserialized, so no format coupling, the rest can be fetched from other services on demand (coherent state though is not guaranteed, but that’s usually not critical with well designed bounded contexts).
- SideburnsOfDoom 4y ago
  > just let the system in possession of data send with the event only the part of data it owns (i.e. something in between fat and thin). Is that really different from a fat event?
  - ivan_gammel 4y ago
    Yes. The owned part of saga can be as small as an acknowledgment of something that happened. Basically, you do not create the pattern of fat event in your architecture and then stick to it, instead you send both fat, thin and in between depending on context.
deleted 4y ago
[deleted]
aerffrea 4y ago
Thin events have the benefit of easy retry/resend logic. Depending on your message queue solution you might need to sometimes resend events. If the event is 'user account changed', receiving it a few times too many causes only performance issues, but not correctness problems. Sometimes this is the better tradeoff. It is easier to send an event 'user account changed' than to analyze in detail what exactly changed, which also allows you to decouple the event logic from everything else. Of course not every system benefits from such solutions, but sometimes simplicity wins.
sithlord 4y ago
I'm a fan of fat events, and let the receiver decide if they want to trust the event or not, or go ahead and make a call to the service to get the data. for example: if one receiver wants to know if you have read a book, then there is no reason to make a call to the service. but if a service wants to know the last book you read, and doesn't trust the events to be in order, then it would make sense to just call the service.
- SideburnsOfDoom 4y ago
  > if a service wants to know the last book you read, and doesn't trust the events to be in order, then it would make sense to just call the service. It would make more sense to me if the events had an increasing sequence number, version number or accurate timestamp, so that if I record that "'sithlord' last read 'The Godfather' at event '123456'" I can record that, and ignore any event related to "sithlord last read" with event < 123456. This is not a new problem, there are existing solutions to it.
nfgivivu 4y ago
I'd settle for developers just making sure events were events that have happened and not action commands they want to happen. Nothing is worse than an event driven system polluted with action commands.
- mrkeen 4y ago
  I agree in principle, but I also to need action commands when crossing boundaries in and out of the event system (e.g. to the user, or to an external provider.) Of course I still phrase them in the past tense: PurchaseRequested { userId = ... } PaymentRequestSentToStripe { userId = ... } It really helps with debugging, especially if the user gets into some failed state (and the events tell you that they accidentally requested twice)
  - nfgivivu 4y ago
    Of course you need command actions. All I'm asking is that you stop mixing them up with events. Whether you're mixing or not is hard to tell if you're fudging the wording. Whats the point of the payment request sent to stripe event? Does it trigger a stripe API request or is it emitted after an API request to stripe is made?
majke 4y ago
I'll bite. Neither. Both. Depending on system. When the "state" is large, or changes often, obviously you can't send full state every time - that would be too much for end-nodes to process on every event. Both cpu - deserialization, and bandwidth. Delta is the answer. Delta though is hard, since there always is an inherent race between getting the first full snapshot, and subscribing to updates. On the other hand doing delta is hard. Therefore, for simple small updated not-often things, fat events carrying all state might be okay. There is a linear tradeoff on the "data delivery" component: - worse latency saves cpu and bandwidth (think: batching updates) - better latency burns more cpu and bandwidth Finally, the receiver system always requires some domain specific API. In some cases passing delta to application is fine, in some cases passing a full object is better. For example, sometimes you can save a re-draw by just updating some value, in other cases the receiver will need to redraw everything so changing the full object is totally fine. I would like to see a pub/sub messaging system that solves these issues. That you can "publish" and object, select latency goal, and "subscribe" to the event on the receiver and allow the system to choose the correct delivery method. For example, the system might choose pull vs push, or appropriate delta algorithm. As a programmer, I really just want to get access to the "synchronized" object on multiple systems.
- SideburnsOfDoom 4y ago
  This omits the issues with "thin events" - it may be fine most of the time, but as it usually involves a "get more details" call over http or of some other kind, it has more moving parts, is therefor more prone to failures and slowdowns due to the extra coupling. This can kick in when load goes up or some other issue affects reliability, and cause cascading failure.
- knome 4y ago
  >Delta though is hard, since there always is an inherent race between getting the first full snapshot, and subscribing to updates. Since the deltas include a version identifier for what they should be applied on top of, then you should always be able to safely start by requesting the deltas, then ask for the object. Buffer the deltas till your full copy is received, then discard deltas for previous versions until the stream applies to yours, applying them thereafter to keep it up to date.
andsbf 4y ago
When will people settle on the fact that there is no silver bullet? It's always a matter of trade offs according to the system needs.
- SideburnsOfDoom 4y ago
  Because the very real downsides of thin events are not simple or obvious. It may be fine most of the time, but as it usually involves a "get more details" call over http or of some other kind, it has more moving parts, is therefor more prone to failures and slowdowns due to the extra coupling. This can kick in when load goes up or some other issue affects reliability, and cause cascading failure.
  - jayd16 4y ago
    But it's a lot less prone to data races and other upsides so it really is contextual to your needs. I would also push back that it has more moving parts. You'll often need to pull information and getting pushed information as well can duplicate code on both the service side and client. In practice thin events are easier to get right, despite the extra API requests. I also think the cases where there's some kind of outage but it's an outage such that the information in the event is enough is fairly rare. I would guess it's more rare than outages that also disable event triggering anyhow.
    - SideburnsOfDoom 4y ago
      > But it's a lot less prone to data races What's the reasoning for that statement? My other comments have detailed cases of data races, caused by using thin events, solved by using fat events, so I'm going to push back on that: In my experience, the idea that thin events are "less prone to data races" has not been true at all, and that data race is inherent in the model of "receive an event, go back and get more data over http about it". How would you qualify this as "a lot" - how much is that lot? Citations very much needed. > I would also push back that it has more moving parts. You'll often need to pull information "Often" is not always. You seem to be saying that when you don't get the signature benefit of fat events (not having to pull information) ... there's no benefit. Yes, that's a tautology, and also an encouragement to further refine the design until you do get the benefit. > I also think the cases where there's some kind of outage but it's an outage such that the information in the event is enough Are you speaking from experience here? Doesn't seem like it. I refer you to point c) here https://news.ycombinator.com/item?id=33392655 https://news.ycombinator.com/item?id=33392655 It helps reliability to shorten the list of services that your service depends upon being 100% up and responsive.
acjohnson55 4y ago
For me, this depends on the semantics of the system. Is the sender commanding the receiver to carry out the rest of the process, or is the sender broadcasting information to a dynamic set of interested parties? In other words, are you building a pipeline or a pub-sub? If the former, there is inherently tight coupling between sender and receiver, and the sender should send all necessary context to simplify the system design. If the latter, then we talking about a decoupled system, where the sender cannot make assumptions about what info the receiver does or doesn't need to take further action. A thin event is called for to keep the contract simple. One of my frustrations with the event-driven trend is that people don't always seem to think through what they're designing. It's easy to end up with a much more complex system than a transactional architecture. Generally, I favor modeling as much of my system as possible as pipelines, and use pub-subs sparingly, as places where you have fan out to parallel pipelines. Raw events are like GOTOs. They are extremely powerful, but also very difficult to reason about.
didip 4y ago
I have a different take, why not just send RPC functions with all of their parameters? Then the payload is guaranteed to be small but still able to handle complex operations.
- mrkeen 4y ago
  If you send to multiple receivers, some of the messages may not make it through. Then you're in the position of deciding whether 'the message was sent' or not.
kgeist 4y ago
Fat events once overloaded our message broker with OOM under high load and the broker's default behavior was to block all publishers until the queue was emptied (to release memory) - downtime as a result. Another issue was that under high load, if the event queue was too large, handlers would end up processing very stale data resulting in all kinds of broken behavior (from the point of view of the user). Thin events resulted in DDoS of our service a few times because handlers would call our APIs too frequently to retrieve object state (which was partially mitigated by having separate machines serve incoming traffic and process events). (A trick we used which worked for both fat and thin events was to add versioning to objects to avoid unnecessary processing). We also used delta events as well but they had same issues as thin events because handlers usually have to retrieve full object state anyway to have meaningful processing (not always, depends on business logic and the architecture). There are so many ways to shoot yourself in the foot with all three approaches and I still hesitate a lot when choosing what kind of events to use for the next project.

92 points by senzilla 4y ago

As always, it depends. Yay engineering and trade-offs. Hey just remember: both is always an option if you're consumers disagree. Thin stream from the consumers who don't trust the fat data, fat stream for the event log and other consumers that prefer it.

ulam2 4y ago

Why not have an option for both? Both have their use cases.

doctor_eval 4y ago

> Thin Events add coupling That’s not my experience. In fact I’d say fat events add coupling because they create an invisible (from the emitter) dependency on the event body, which becomes ossified. So I’d say the opposite: thin events reduce coupling. Sure, the receiver might call an API and that creates coupling with the API. But receivers are also free to call or not call any other API they want. What if they don’t care about the body of the object? So I’m on team thin. Every time I’ve been tempted by the other team, I’ve regretted it. It’s also in my experience a lot more difficult to version events than it is to version APIs, so reducing their surface area also solves other problems.

SideburnsOfDoom 4y ago
> thin events reduce coupling. Sure, the receiver might call an API and that creates coupling with the API. You make a statement in the first sentence, and in the next sentence produce evidence ... that the statement is wrong. And, YMMV. It is my experience that thin events add coupling. If service B receives an event, and wants to process it ASAP (i.e. near real time) and so calls back over http to Service A for the details, then a) there is additional latency for a http call. And time variance - Even if the average latency of a http request round-trip is fine, the P99 might be bad. b) You're asking for occasional "eventual consistency" trouble when A's state lags or has moved on ahead of the event c) Worst of all: When service A is down or unreachable, Service B is unable to do work: Service B uptime must be <= Service A uptime. You have coupled their reliability, and if Service B is identified as mission-critical, then you have the choice of either making Service A equally critical, or decoupling them e.g. with "fat events". I don't believe that it's accurate to say "receivers are also free to call or not call..." it's not choosing a flavor of ice-cream, you do the calls that the work at hand _needs_. If you find that you never need to call back to service A then yes, "thin events" would suit your case better. That has not been my experience. It's fair that event data format versioning is a lot of work with fat events - nothing is without downside. But in your case, do you have "dependency on the event body" ? All of it? If a thin event is all that you need, then you depend on a couple of ids in the event body, and not the rest. Json reading is very forgiving of added / removed fields, you can ignore the parts of a fat event that you don't care about.
- FeepingCreature 4y ago
  > b) You're asking for occasional "eventual consistency" trouble when A's state lags or has moved on ahead of the event To be noted that this is the default if B is recovering after an outage. Personally, I consider events to be insane. "We create an immutable database so that the state of the system is always recoverable." Okay, cool, very functional programming of you. "But then to actually work with the event from the immutable database, you have to query a stateful service." ??? What? And even fat events only go so far to get you out of that. So with a stream of n events, you don't have n states that the application can be in, but n times the product of all possible states of every other service that you query. How does this help?!
  - PaulRobinson 4y ago
    The bit you seem to be missing is the events are the source of truth, not the databases. Lose your database? Roll up all the events. Got a lot of them? Take snapshots and then roll up from the last trusted snapshot. In true event sourced systems, the databases and stateful systems are artefacts that can be thrown away and rebuilt. The event log is the actual “true” database. Once you design around that, your objections melt away. And if you think this is some faddish trend, this is how finance has worked since the invention of book keeping and how your databases under your stateful services are working under the hood.
    - FeepingCreature 4y ago
      This only works if your events are in a single globally ordered stream or all your code is eventually consistent over every stream it consumes. Specifically, you cannot do the "query a service for the aggregate state" thing this article espouses for thin events, ever.
      - insanitybit 4y ago
        You can achieve strong eventual consistency with this system.
- 4y ago
avereveard 4y ago
assume the data format changes, it would change in the called api as well. as long as the fat event sends data that it's in the same format that the api would return, you'd have the same level of coupling. I think fat vs thin is more about how much other services the event have to travel, because thin event would multiply reads by a fair factor, with the tradeoff being the performance hit for the queue system to store and ship large events
- doctor_eval 4y ago
  With an API you can publish a new endpoint (/v1, /v2 etc). It’s normally reasonably easy to maintain an old API even while you add features to the new API, and the runtime penalty is minimal because clients would be expected to call just one version of the API for any given event. (You can also see who’s calling the old API and ask them to change) But this is not true for events. If you change the body such that you now need to maintain two versions of an event, then you have to publish both events simultaneously, which means double the server side effort, storage etc for each event version. It’s pretty inefficient, and painful. You can work out who subscribes to the old event but there is still a big efficiency hit. You might be right about many reads per event in a simplistic way; if you have a lot of clients then it could be expensive if you don’t have a server side cache. But there would typically be a lot of temporality in such a system so it seems like an easy problem to solve for most use cases; you don’t have to cache for long, but caches are of course tricky if your use case is not very simple. That said, if there is already a HTTP connection open then the additional latency and bandwidth hit cause by this events are going to be minimal in most cases, and probably drowned out entirely if you need to push multiple versions. As I said in another thread, I should have said that thin is my default. There are cases when fat makes more sense, but normally I’d start with thin and see if I need to flesh it out. Whenever I’ve started fat I’ve ended up reverting.
  - manfre 4y ago
    Supporting multiple versions of an event schema is a solved problem. Apache Avro with a published schema hash in a message header is one solution. https://avro.apache.org/ https://avro.apache.org/
paphillips 4y ago
I also disagree with the article - thin events don't always result in more coupling, and I'll add that thin events can remove temporal or state coupling as illustrated below. However, the caveat is: as with many things I think choosing one team or the other has nuance and depends on the specific scenario. An example: I'm using thin events in a master data application integration scenario to send a 'sync this record' type of command message into a queue. The message body does not have the record details, only the basic information to uniquely identify the record. It also doesn't identify the type of change except for a flag to identify deletes. The 'sync' message is generalized to work for all entities and systems, so routing, logging, and other functions preceding the mapping and target operation have no coupling to any system or entity and can expect a fixed message format that will probably never change. Thus versioning isn't a concern. Choosing team 'thin event' does result in an extra read of the target system, but that is a feature for this scenario and what I want to enforce. I can't assume a target system is in any particular state, and the operation to be performed will be determined from the target system at whatever point in time a message is processed, which could be more than once. If the message ended up in a dead letter queue, it can be reprocessed later without issue. If one production system's data is cloned down to a lower environment, the integrations continue to work even if the source and target environment data is mismatched. No state is stored or depended upon from either system and the design is idempotent (ignoring a target system's business rules that may constrain valid operations over time). In contrast, other scenarios may benefit from or require a fat event. I've never used event sourcing, but as others mention, if current state can be built from all previous events 'rolled forward' or 'replayed', then each event must be a stand-alone immutable record with all information - thin events cannot be used. Or, if a scenario requires high performance we might need to use a fat event to eliminate the extra read, and then compensate for the other consequences that arise.
drewcoo 4y ago
Thank you. Came here to say that. When I've seen this fat event pattern it's been because different services' responsibilities were not fully separated. And that's tight coupling. Fat events imply tight coupling. The "thin" pattern described in the article goes like this: 1) service FOO gets an event 2) FOO then has to query BAR (and maybe BAZ and QUUX) to determine the overall state of everything to determine what to do next And #2 means all of that is kind of "thin" is tightly coupled, too. I've also personally seen thin events that are not the article's thin strawman. I sometimes wonder if people understand coupling or design.

eurasiantiger 4y ago

Delta events sounds an awful lot like Change Data Capture.

thom 4y ago
It seems rare that every operation in a business is atomic across a single aggregate, so I’ve always been wary of events that are pure CRUD, whatever you choose to call them.

ivan_gammel 4y ago

I‘d pick neither and just let the system in possession of data send with the event only the part of data it owns (i.e. something in between fat and thin). Saves API call back, the body doesn’t have to be fully deserialized, so no format coupling, the rest can be fetched from other services on demand (coherent state though is not guaranteed, but that’s usually not critical with well designed bounded contexts).

SideburnsOfDoom 4y ago
> just let the system in possession of data send with the event only the part of data it owns (i.e. something in between fat and thin). Is that really different from a fat event?
- ivan_gammel 4y ago
  Yes. The owned part of saga can be as small as an acknowledgment of something that happened. Basically, you do not create the pattern of fat event in your architecture and then stick to it, instead you send both fat, thin and in between depending on context.

deleted 4y ago

[deleted]

aerffrea 4y ago

Thin events have the benefit of easy retry/resend logic. Depending on your message queue solution you might need to sometimes resend events. If the event is 'user account changed', receiving it a few times too many causes only performance issues, but not correctness problems. Sometimes this is the better tradeoff. It is easier to send an event 'user account changed' than to analyze in detail what exactly changed, which also allows you to decouple the event logic from everything else. Of course not every system benefits from such solutions, but sometimes simplicity wins.

sithlord 4y ago

I'm a fan of fat events, and let the receiver decide if they want to trust the event or not, or go ahead and make a call to the service to get the data. for example: if one receiver wants to know if you have read a book, then there is no reason to make a call to the service. but if a service wants to know the last book you read, and doesn't trust the events to be in order, then it would make sense to just call the service.

SideburnsOfDoom 4y ago
> if a service wants to know the last book you read, and doesn't trust the events to be in order, then it would make sense to just call the service. It would make more sense to me if the events had an increasing sequence number, version number or accurate timestamp, so that if I record that "'sithlord' last read 'The Godfather' at event '123456'" I can record that, and ignore any event related to "sithlord last read" with event < 123456. This is not a new problem, there are existing solutions to it.

nfgivivu 4y ago

I'd settle for developers just making sure events were events that have happened and not action commands they want to happen. Nothing is worse than an event driven system polluted with action commands.

mrkeen 4y ago
I agree in principle, but I also to need action commands when crossing boundaries in and out of the event system (e.g. to the user, or to an external provider.) Of course I still phrase them in the past tense: PurchaseRequested { userId = ... } PaymentRequestSentToStripe { userId = ... } It really helps with debugging, especially if the user gets into some failed state (and the events tell you that they accidentally requested twice)
- nfgivivu 4y ago
  Of course you need command actions. All I'm asking is that you stop mixing them up with events. Whether you're mixing or not is hard to tell if you're fudging the wording. Whats the point of the payment request sent to stripe event? Does it trigger a stripe API request or is it emitted after an API request to stripe is made?

majke 4y ago

I'll bite. Neither. Both. Depending on system. When the "state" is large, or changes often, obviously you can't send full state every time - that would be too much for end-nodes to process on every event. Both cpu - deserialization, and bandwidth. Delta is the answer. Delta though is hard, since there always is an inherent race between getting the first full snapshot, and subscribing to updates. On the other hand doing delta is hard. Therefore, for simple small updated not-often things, fat events carrying all state might be okay. There is a linear tradeoff on the "data delivery" component: - worse latency saves cpu and bandwidth (think: batching updates) - better latency burns more cpu and bandwidth Finally, the receiver system always requires some domain specific API. In some cases passing delta to application is fine, in some cases passing a full object is better. For example, sometimes you can save a re-draw by just updating some value, in other cases the receiver will need to redraw everything so changing the full object is totally fine. I would like to see a pub/sub messaging system that solves these issues. That you can "publish" and object, select latency goal, and "subscribe" to the event on the receiver and allow the system to choose the correct delivery method. For example, the system might choose pull vs push, or appropriate delta algorithm. As a programmer, I really just want to get access to the "synchronized" object on multiple systems.

SideburnsOfDoom 4y ago
This omits the issues with "thin events" - it may be fine most of the time, but as it usually involves a "get more details" call over http or of some other kind, it has more moving parts, is therefor more prone to failures and slowdowns due to the extra coupling. This can kick in when load goes up or some other issue affects reliability, and cause cascading failure.
knome 4y ago
>Delta though is hard, since there always is an inherent race between getting the first full snapshot, and subscribing to updates. Since the deltas include a version identifier for what they should be applied on top of, then you should always be able to safely start by requesting the deltas, then ask for the object. Buffer the deltas till your full copy is received, then discard deltas for previous versions until the stream applies to yours, applying them thereafter to keep it up to date.

andsbf 4y ago

When will people settle on the fact that there is no silver bullet? It's always a matter of trade offs according to the system needs.

SideburnsOfDoom 4y ago
Because the very real downsides of thin events are not simple or obvious. It may be fine most of the time, but as it usually involves a "get more details" call over http or of some other kind, it has more moving parts, is therefor more prone to failures and slowdowns due to the extra coupling. This can kick in when load goes up or some other issue affects reliability, and cause cascading failure.
- jayd16 4y ago
  But it's a lot less prone to data races and other upsides so it really is contextual to your needs. I would also push back that it has more moving parts. You'll often need to pull information and getting pushed information as well can duplicate code on both the service side and client. In practice thin events are easier to get right, despite the extra API requests. I also think the cases where there's some kind of outage but it's an outage such that the information in the event is enough is fairly rare. I would guess it's more rare than outages that also disable event triggering anyhow.
  - SideburnsOfDoom 4y ago
    > But it's a lot less prone to data races What's the reasoning for that statement? My other comments have detailed cases of data races, caused by using thin events, solved by using fat events, so I'm going to push back on that: In my experience, the idea that thin events are "less prone to data races" has not been true at all, and that data race is inherent in the model of "receive an event, go back and get more data over http about it". How would you qualify this as "a lot" - how much is that lot? Citations very much needed. > I would also push back that it has more moving parts. You'll often need to pull information "Often" is not always. You seem to be saying that when you don't get the signature benefit of fat events (not having to pull information) ... there's no benefit. Yes, that's a tautology, and also an encouragement to further refine the design until you do get the benefit. > I also think the cases where there's some kind of outage but it's an outage such that the information in the event is enough Are you speaking from experience here? Doesn't seem like it. I refer you to point c) here https://news.ycombinator.com/item?id=33392655 https://news.ycombinator.com/item?id=33392655 It helps reliability to shorten the list of services that your service depends upon being 100% up and responsive.

acjohnson55 4y ago

For me, this depends on the semantics of the system. Is the sender commanding the receiver to carry out the rest of the process, or is the sender broadcasting information to a dynamic set of interested parties? In other words, are you building a pipeline or a pub-sub? If the former, there is inherently tight coupling between sender and receiver, and the sender should send all necessary context to simplify the system design. If the latter, then we talking about a decoupled system, where the sender cannot make assumptions about what info the receiver does or doesn't need to take further action. A thin event is called for to keep the contract simple. One of my frustrations with the event-driven trend is that people don't always seem to think through what they're designing. It's easy to end up with a much more complex system than a transactional architecture. Generally, I favor modeling as much of my system as possible as pipelines, and use pub-subs sparingly, as places where you have fan out to parallel pipelines. Raw events are like GOTOs. They are extremely powerful, but also very difficult to reason about.

didip 4y ago

I have a different take, why not just send RPC functions with all of their parameters? Then the payload is guaranteed to be small but still able to handle complex operations.

mrkeen 4y ago
If you send to multiple receivers, some of the messages may not make it through. Then you're in the position of deciding whether 'the message was sent' or not.

kgeist 4y ago

Fat events once overloaded our message broker with OOM under high load and the broker's default behavior was to block all publishers until the queue was emptied (to release memory) - downtime as a result. Another issue was that under high load, if the event queue was too large, handlers would end up processing very stale data resulting in all kinds of broken behavior (from the point of view of the user). Thin events resulted in DDoS of our service a few times because handlers would call our APIs too frequently to retrieve object state (which was partially mitigated by having separate machines serve incoming traffic and process events). (A trick we used which worked for both fat and thin events was to add versioning to objects to avoid unnecessary processing). We also used delta events as well but they had same issues as thin events because handlers usually have to retrieve full object state anyway to have meaningful processing (not always, depends on business logic and the architecture). There are so many ways to shoot yourself in the foot with all three approaches and I still hesitate a lot when choosing what kind of events to use for the next project.