Where to store a system flag?

This may seem a silly question, but having been pondering it for a few days, i've only come up with one answer, so I thought i'd throw it open and see what other people think.

I have a PHP script which runs as a CRON job, and checks that our CDN is up and running. If it is, it needs to set a flag to show 'true', if not to show 'false'. The main PHP webpages then need to check this flag everytime a user visits the site, and set the CDN URL variable appropriately.

The query is where do we store this flag? We could put it in a database, but then the database would get 'hit' every page load. Also, as we're using Memcache it could result in a delay to it being updated (although there are ways around that, obviously).

My next thought was to save it into a simple text file on the s开发者_高级运维erver. But a) we have load-balanced servers, and although we could run the same CRON job on each server, that seems inefficient, and b) opening and reading a text file could, i thought, slow the page load down, as its involving disk activity everytime - that isn't good when the system is delivering 3 million web pages a week!

The ideal would be PHP system variable that retains its value between pages - but of course, that doesn't exist! Its not the kind of information that should be stored as a cookie on the users machine either.

Does any one have any thoughts? All comments welcome!

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If you are already using Memcache, then why do you need the CronJob at all?

Put the check for the CDN into your regular application code. Cache the result in Memcache with a lifetime of what your Cron Interval is right now. If the cache is stale, do the full checking on the CDN, otherwise return the cached result.

If you want to keep the CRON job and are concerned about Disk I/O, consider using a RAM disk or Semaphores or Shared Memory to store the results. But it seems kinda pointless when you are already using memcache.

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Do you already use a database connection?

• If so, use that; you can fit it into your existing replication mechanism pretty easily, I'm sure.

• If not, then whatever you do here is going to add overhead, be it reading a text file on disk or reading from a database where you weren't before.