Question

I need a way to detect changes of my database because I’m implementing a caching system on a client’s site. (caching the whole page, not just the queries)

At the top of the every page (i have a “master page” so just one place in code) I query the database to see if its size is changed like so.

I sum the size and get a “unique” number. Compare it to the previous one, and if the values are the same, deliver the cached page, if not, run the “normal” page and then cache that.

So I think in theory this could work, because I have just 1 database query, and based on that deliver the site (cached or uncached depending on the result) to the user.

Could this work, and if not – why?

EDIT:

what about cheking for the last update timestamp? LINK

Answer 1

Who is updating the database? Suggest that the process causing the change should be the one invalidating your cache. When a user/admin causes a change to occur to the table, you could:

• if the admin and the public-facing site share the same application space (i.e. they ARE the same app), then simply update the cache at the time the admin makes a change.
• set a status bit somewhere (session variable perhaps), and periodically check to reload your cached data.

Suggest size is not a good indicator of ‘change occurred’. Forget rows – what if a bool/bit value changes? That’d satisfy the condition to invalidate the cache, but the ‘size’ would always be the same. You could improve this with a last-change-datetime, but you’re still having the same basic problem – querying the database to check for a change flag. Kind of defeats the purpose of a cache.

Consider another approach – reload your cache every n minutes. Whether there’s been a change or not, just reload it. n will be a value that fits in your business requirements. You could have stale cache, and be serving old/stale data for at most n minutes. This wouldn’t be my suggestions, but something to consider.