4 ms·

yes, redis is single threaded. The author's stated position is that if you want to use more cores, you run more instances of redis on the target machine (but th

by zemo 7y ago

yes, redis is single threaded. The author's stated position is that if you want to use more cores, you run more instances of redis on the target machine (but then you have a scheme to map keys to running redis instances as if it were multiple servers). Keeping it single threaded is that it makes reasoning about the data guarantees simple. You're guaranteed that no two redis commands are in flight at the same time. serialize an object to an opaque run of bytes, writing it to redis with SET, get it back with GET, is like ... the majority use case of redis. It's basically just a big hash table with a nice networking layer, that's all it is. It'll do a few hundred thousand operations per second if all you're doing is getting and setting opaque values. You don't have to use the other features, and their existence doesn't cost you anything, but you can have, for example, a hash table of strings, or a list of strings, or a skiplist of strings, or a set of strings, and those things all come in handy. For example, if you have a set, you can just add elements to the set, and if you want to know if the set has something, you just send the value and it gives you a bool back; you don't have to read the entire set to check it for one value. yeah, you're describing a very naive implementation of modulo sharding. That's a pretty old technique, although you wouldn't generally put the shard index into the key. If you do that, you're making it impossible to move the value, because you'd have to change the key. `foo1` and `foo2` are different keys; you have to be able to move the value without changing the key. So generally you wouldn't put the offset in the key itself, you would take a hash of the key and modulo the hash to find the index of the node containing that key. But anyway partitioning the data that way is easy. Nobody is contending that writing it that way once is easy. The problem occurs when you want to add or remove servers. Re-partitioning a live dataset is much harder; adding and removing a node means having to reassign and move a bunch of keys. Unless you build some sort of query-routing proxy, you're also requiring the clients to be aware of the cluster topology, and you have to coordinate cluster changes with the clients. TCP buffer as command queue is hilariously idiosyncratic, I've never given it much though. Dunno what the failure modes are there. TCP flow control should dictate that the sender stop sending data, but I don't know what that means practically; it might cause the sender to block until the queue clears.

graycat 7y ago
Many thanks. Your points on the topology of the architecture of the servers is well taken. My idea for taking a session state server off line has been just (1) to send it a message to decline more new pairs and (2) to wait until all its key-value pairs have timed out and then just shut it down. But there's a flaw here: An actual point in server farm system management uses a variable part of the Web site logic, that is, the time out intervals. That's not so good. But other ways of session state server shutdown would about have to be able to move key-value pairs from the server to be shutdown to another key value server. Maybe easier would be just to replace each session state server with two essentially identical. Then if don't have a proxy (itself a single point of failure?) have each client send everything to both of the session state servers. In a serious data base environment, there would have to be commit logic, etc. Hmm. Otherwise, when want to shutdown a session state server, then just shutdown one of a pair. To bring new pairs on-line, then just send to all the clients a table of all the session state servers they could use. Lock the table, change it, and unlock it? Or just have the table accessed via a pointer and in an atomic operation change the pointer to the new table? Then to delete the old table have to wait until it is no longer in use? Hmm ... concurrency issues which I'm trying to avoid! Hmm, put the new table on some server and then send all the clients a message to go to the server and get their copy of the new table -- single threaded solution! Moving key-value pairs from one key-value server to another sounds to me like Cadillac, filtered air, humidity controlled air conditioning in an open Jeep! Wow. My startup isn't for stock trading, banking, or health care! If a key-value store fails, then on some Web servers there will be some TCP/IP session time outs and some Web site users will get some messages with apologies and some cartoon of a mess up. Hashing could be faster than AVL trees. There is a tweak to hashing in Ronald Fagin, Jurg Nievergelt, Nicholas Pippenger, H. Raymond Strong, 'Extendible hashing—a fast access method for dynamic files', "ACM Transactions on Database Systems", ISSN 0362-5915, Volume 4, Issue 3, September 1979, Pages: 315 - 344. that has a graceful way around collisions. We used that in some work at IBM's Watson lab for their Resource Object Data Manager (RODM) shipped with NetView. For one of the collection classes I'm using, for detecting time outs, the key is a time-date stamp, and I want a fast way to read the pairs in ascending order on time-date stamp, right, starting with the oldest time-date stamp. So, mere hashing would not offer that. Ah, I fell in love with AVL trees when I first read about them in Knuth, The Art .... I'm just hoping that the .Net class is based on AVL trees or something equivalent. Somewhere between the user's fingers and the most distant parts of the server farm there have to be some flow controls of some kind. So, if the TCP/IP input queue to a session state server fills, then likely, right, the Web server making the request will block, that is, wait, maybe time out. So, that would be one source of flow control. Maybe I should think about implementing more. There's an old approach to flow control: Just don't respond to the user so fast. Instead, make the user wait a little. That approach, in effect, slows the rate of data input to the whole Web site. Maybe I should think, at least for the future, say, version 2.0, about actually designed means of flow control, rate limiting, etc. What do people do about that, say, at high end sites?

by zemo 7y ago

graycat 7y ago

Many thanks. Your points on the topology of the architecture of the servers is well taken. My idea for taking a session state server off line has been just (1) to send it a message to decline more new pairs and (2) to wait until all its key-value pairs have timed out and then just shut it down. But there's a flaw here: An actual point in server farm system management uses a variable part of the Web site logic, that is, the time out intervals. That's not so good. But other ways of session state server shutdown would about have to be able to move key-value pairs from the server to be shutdown to another key value server. Maybe easier would be just to replace each session state server with two essentially identical. Then if don't have a proxy (itself a single point of failure?) have each client send everything to both of the session state servers. In a serious data base environment, there would have to be commit logic, etc. Hmm. Otherwise, when want to shutdown a session state server, then just shutdown one of a pair. To bring new pairs on-line, then just send to all the clients a table of all the session state servers they could use. Lock the table, change it, and unlock it? Or just have the table accessed via a pointer and in an atomic operation change the pointer to the new table? Then to delete the old table have to wait until it is no longer in use? Hmm ... concurrency issues which I'm trying to avoid! Hmm, put the new table on some server and then send all the clients a message to go to the server and get their copy of the new table -- single threaded solution! Moving key-value pairs from one key-value server to another sounds to me like Cadillac, filtered air, humidity controlled air conditioning in an open Jeep! Wow. My startup isn't for stock trading, banking, or health care! If a key-value store fails, then on some Web servers there will be some TCP/IP session time outs and some Web site users will get some messages with apologies and some cartoon of a mess up. Hashing could be faster than AVL trees. There is a tweak to hashing in Ronald Fagin, Jurg Nievergelt, Nicholas Pippenger, H. Raymond Strong, 'Extendible hashing—a fast access method for dynamic files', "ACM Transactions on Database Systems", ISSN 0362-5915, Volume 4, Issue 3, September 1979, Pages: 315 - 344. that has a graceful way around collisions. We used that in some work at IBM's Watson lab for their Resource Object Data Manager (RODM) shipped with NetView. For one of the collection classes I'm using, for detecting time outs, the key is a time-date stamp, and I want a fast way to read the pairs in ascending order on time-date stamp, right, starting with the oldest time-date stamp. So, mere hashing would not offer that. Ah, I fell in love with AVL trees when I first read about them in Knuth, The Art .... I'm just hoping that the .Net class is based on AVL trees or something equivalent. Somewhere between the user's fingers and the most distant parts of the server farm there have to be some flow controls of some kind. So, if the TCP/IP input queue to a session state server fills, then likely, right, the Web server making the request will block, that is, wait, maybe time out. So, that would be one source of flow control. Maybe I should think about implementing more. There's an old approach to flow control: Just don't respond to the user so fast. Instead, make the user wait a little. That approach, in effect, slows the rate of data input to the whole Web site. Maybe I should think, at least for the future, say, version 2.0, about actually designed means of flow control, rate limiting, etc. What do people do about that, say, at high end sites?