Wednesday, October 19, 2011

Long Polling with Suneido

It started with our summer student deciding to build an "instant messenger" in Suneido. I'm not sure what prompted this, but it seemed like an interesting challenge so I let him run with it.

The trick is the "instant" part. His initial approach was to store the messages in a database table, and poll the table for changes. To make it "instant" he wanted to poll very frequently, at least once a second, and preferably more. We have about 50 users, if they are all polling every second, that's an extra 50 queries per second. I wasn't too keen on this.

I suggested we set up a separate HttpServer for the messenger, and keep the "outstanding" messages in memory, so polling didn't have to hit the database. (New messages were still written to a history table in the database, but that's not as frequent as the polling.) This took the load off the main database server, and made the requests faster.

I explained that a better way to make it "instant" was to use Comet style long polling where the server simply doesn't respond to the request until something is available. This means a lot more connections to the server since every client always has an outstanding request, which can be an issue depending on the server. But we were using jSuneido for the messenger HTTP server, and 50 connections wasn't a problem.

But the client side is also an issue. If we're going to be waiting a potentially long time for a response, the client can't be waiting "blocked", we need to make the request asynchronously. Suneido doesn't have a lot of support for aynchronous programming, but it does have a Thread function that allows you to execute code in a separate "fiber" - not a real thread, more like a coroutine. (On jSuneido "Thread" uses a real thread.)

We've never used Thread much because it has always seemed a little flaky. But it is basically sound because it's what the Suneido database server uses to handle multiple users - and that is solid. And it's also how SocketServer is implemented, and that also works well (e.g. in HttpServer)

One issue is that you want to avoid updating the GUI from a thread. That is easy to do by using Delayed with a 0 delay to process the response. Delayed works via the Windows message queue, which is processed by the main fiber. The code running inside the thread looks like:

     if "" isnt response = Http.Get(uri)
          Delayed(0, ResponseProcessor(response))

(plus catching exceptions etc.)

However, we haven't eliminated polling, we've just moved it to the messenger HTTP server, since it must poll for new messages. (Since Suneido doesn't have any way to "wait" for something.) The code looks something like:

     if Suneido.MessagesAvailable
          return ...
return ""

400 times 100ms is 40 seconds, less than the default timeout of 60 seconds.  Checking once every 100 ms (1/10 of a second) is a light load on the server, most of the time it's sleeping. (Note: this wouldn't work well on a cSuneido HTTP server because Sleep would block all the other fibers.)

This has ended up working remarkably well - the messenger is "instant" with little load on the server or client.

No comments: