Facebook Chat Architecture (Erlang)
docs.google.com
docs.google.com
Erlang's definitely got a speed and concurrency advantage. More people should take the time to learn about it, as it fits a lot of use cases.
I wonder what their process is.. Just coping over new .beams and loading them is fine for functional changes only, but you need a system to run code-upgrade hooks if you want to change the state being passed around.
Releases and upgrades do make it simpler to synchronize what needs to be taken down in what order, restarted, left alone, etc. But simple updates are still doable without that.
sys:suspend(Pid), {module, my_module} = code:load_file(my_module), sys:change_code(Pid, my_module, undefined, [], 10000), sys:resume(Pid);
But you still need a way to do that for running processes, something that release_handler does for you with real OTP releases by finding which pids are running for each module, by looking at supervisors and so on.
Would this be a gigabit or gigabyte? Usually I would just assume gigabit but in the case of facebook i dont feel so hasty...
Like... how many sessions are created per day, persisted from day to day, how many messages sent per chat session, how long windows stay open, how many restores, etc.
Info on how to connect to it here: http://www.facebook.com/sitetour/chat.php
It's not rocket science, it's mainly plumbing - moving data around.
Some people are often too quick to try and emulate people/products by picking out some magical quality, using it, and expecting identical results.
It's just like women who latch onto fashion tips/diet tips from celebs - eg "Oooo Angelina Jolie used the X diet. If I use that, I can be like her".
Similarly, some techies think "Ooo facebook used Erlang. If I use Erlang, I won't have any problems scaling".
It's certainly more in how you approach things, and how you architect things than which particular tool you choose to use. There is no magic solution.
FWIW, I run Mibbit which handles a good number of users and messages per second on a few servers. Not facebook numbers yet, but not small either. I think we do a few billion messages a month.
All those evented frameworks are single-threaded. All my servers have at least 16 cores and in few years they'll have 256 cores and more.
Good luck handling 256 instances of node.js per server.
Good luck connecting all your nodes into distributed cluster.
The only real competition for Erlang in this case is Jetty/Netty with Java or Scala.
Erlang/OTP is just a tool. You still need to work very hard to build a system like Facebook Chat!
Facebook chat doesn't use CPU. Why would it need 16 cores? It's more than likely IO bound. It's just moving boring data around.
If you're using serious CPU power to write a chat backend, you're doing something really badly or using a crappy tool.
Your "But I need more CPU cores!!!" argument is moot.
I'm not following your logic here. If something doesn't need 16 CPUs, why is splitting it over 16 cores a good idea? You're going to be adding a lot of inefficiency, complexity etc for no gain.
Most of mine are dual or possibly 4 core.
One core for OS stuff, other processes etc, one core to do networking IO for me.
As I say, networking IO shouldn't be using CPU. If you're doing long or blocking jobs, pass it over to some other thread. (That's a far better way to utilize multiple cores than splitting the whole networking IO over multiple cores).
What you are suggesting here is exactly what Erlang provides for you. Each connection is a light-weight process inside the VM. The VM then schedules these Erlang processes against native threads. The difference is Erlang has perfected and proven that it can do this reliably over 20 years of development. You would be implementing this by hand, and guaranteed to screw up because its a very hard problem and you are not the smartest person in the world.
Hybrid solutions with Evented I/O and Multi-threading end up being the solution, Erlang is a hybrid out of the box.
Erlang "doing it for me" isn't going to be anywhere near as efficient as me doing it myself.
I'm probably not the smartest person in the world, but I'm certainly smart enough to write a comet/web server that's fast enough.
I guess I'm not going to convince anyone here shrug. If you believe Erlang is a magic bullet great for you.
Furthermore, I don't know where everyone gets off saying erlang is so fast either: I'm not sure where things are now, but last time I checked, their scheduling algorithms were leaving quite a lot on the table, standing to benefit quite substantially from even the most basic operations and logistics knowledge.
I run beaconpush.com which is a cloud-based service for browser push messages. We're based on Netty and Java, which works out well for us. We've looked at Erlang and wrote some initial prototypes in it but Java became our language of choice. Mainly because we knew it and had some experiences with Netty, a NIO library for Java. Netty did also outperform Erlang and mochiweb (both vanilla configured).
Anyway, the problem tend to be with long-poll that users re-establish their connection back to the servers each time they receive a message or navigate to another page (or reload the page). This end up taking a big toll on the system. We're to some extent limited to how fast we can perform accept() on our sockets.
If you use long-polling with multi-part you can get away with sending more messages on a single established connection (it becomes request/response/response/response or similar). That can reduce the system load and the use of WebSockets can eliminate the use of reconnections altogether (disregarding any page reloads).
Facebook's use of AJAX navigation (i.e not reloading the entire page when user clicks on links etc) also reduces this load. This due to not having to re-establish a connection each time a user reloads a page.
So yes, we're actually CPU bound by the accept() behavior (at least to how the JVM does accept()). But would our connection be more permanent in nature then no, we would of course be more I/O bound if not completely.
I do around 1k req/s per cpu and never see load avg above 0.5
>> "re-establish their connection back to the servers each time they receive a message or navigate to another page (or reload the page). This end up taking a big toll on the system. We're to some extent limited to how fast we can perform accept() on our sockets."
I'd expect what you're seeing is the overhead of your system creating new objects initializing requests etc rather than anything low level.
(Mibbit uses keep alive long polling XHR / websocket, using custom app server in Java +NIO)
But even if that would be solved or if you used something more low-level like, say nginx, you'd still be limited of how fast you can accept().
Try using ApacheBench and hit one server with keep-alive on and off respectively. Even nginx, which I would say is a performant web server, show quite some discrepancies between having to reconnect each time and having a permanent connection.
I file this under the penalty of having to set up and tear down a socket each time. Something that only can be avoided by doing less reconnecting.
I could of course be wrong but these are the findings from doing performance testing and discussing it on the Netty mailing list. But I'd gladly accept suggestions on how to improve this situation for our service.
As I say, I'd be interested in numbers to back that up. It's certainly not a limitation I've noticed.
If you're not using the right tools, for the right job, you're incompetent.
http://erlang.2086793.n4.nabble.com/node-js-compared-to-erla...