Best way to handle concurrency issues

Question

i have a LAPP (linux, apache, postgresql and php) environment, but the question is pretty the same both on Postgres or Mysql.

I have an cms app i developed, that handle

Answer 1

Donnie's answer (polling) is probably your best option - simple and works. It'll cover almost every case (its unlikely a simple PK lookup would hurt performance, even on a very popular site).

For completeness, and if you wanted to avoid polling, you can use a push-model. There's various ways described in the Wikipedia article. If you can maintain a write-through cache (everytime you update the record, you update the cache), then you can almost completely eliminate the database load.

Don't use a timestamp "last_updated" column, though. Edits within the same second aren't unheard of. You could get away with it if you add extra information (server that did the update, remote address, port, etc) to ensure that, if two requests came in at the same second, to the same server, you could detect the difference. If you need that precision, though, you might as well use a unique revision field (it doesn't necessarily have to be an incrementing integer, just unique within that record's lifespan).

Someone mentioned persistent connections - this would reduce the setup cost of the polling queries (every connection consumes resources on the database and host machine, naturally). You would keep a single connection (or as few as possible) open all the time (or as long as possible) and use that (in combination with caching and memoization, if desired).

Finally, there are SQL statements that allow you to add a condition on UPDATE or INSERT. My SQl is really rusting, but I think its something like UPDATE ... WHERE .... To match this level of protection, you would have to do your own row locking prior to sending the update (and all the error handling and cleanup that might entail). Its unlikely you'd need this; I'm just mentioning it for completness.

Edit:

Your solution sounds fine (cache timestamps, proxy polling requests to a another server). The only change I'd make is to update the cached timestamps on every save. This will keep the cache fresher. I'd also check the timestamp directly from the db when saving to prevent a save sneaking in due to stale cache data.

If you use APC for caching, then a second HTTP server doesn't make sense - you'd have to run it on the same machine (APC uses shared memory). The same physical machine would be doing the work, but with the additional overhead of a second HTTP server. If you want to off load the polling requests to a second server (lighttpd, in your case), then it would be better to setup lightttpd in front of Apache on a second physical machine and use a shared caching server (memcache) so that the lighttpd server can read the cached timestamps, and Apache can update the cached timestamps. The rationale for putting lighttpd in front of Apache is, if most requests are polling requests, to avoid the heavier-weight Apache process usage.

You probably don't need a second server at all, really. Apache should be able to handle the additional requests. If it can't, then I'd revisit your configuration (specifically the directives that control how many worker processes you run and how many requests they are allowed to handle before being killed).