Use a cached version of a php page unless the database has changed

Question

I've looked at similar questions about caching in PHP and I'm still stumped as to how to check whether the database has changed without making a new call to the database, which would defeat the point of caching.

I understand technically how to implement caching in PHP -- using ETag and Last Modified headers, output buffering, storing static files, etc. What is tripping me up is how to determine when to serve up a new version of a page instead of a cached version. If the database content has changed, I want to show the new version and not the cached version.

For example, let's say I have a page that displays details about a product. Generally, once the product info is stored in the database, it won't change much. But occasionally there might be an edit to the product description or a price change. If the product has a new price, I don't want to show the user the old price by using a cached version of the page. For that reason, updating the cached content every hour doesn't seem sufficient. Not to mention that that's too often for the content that doesn't change, the real problem is that it won't update the content fast enough when there is a change.

So should I store something (e.g., an ETag value or a static html file) every time the product database is updated through a form in the Admin area of the application? What am I missing here?

[Note: Not interested in using a caching library here. I'd like to learn how to do it in straight PHP for now.]

Answer 1

Caching is a pretty complex topic, because you can cache all sort of data in various places. Usually you implement caching to relieve bottlenecks in your server structure.

In your setup you can cache data at three different locations:

1) Clientside, between client and server You would use this method to save bandwidth and shorten loading times for the user. You can achieve this by setting cache related fields into the http header (Cache-Control, Expires, ETag and so on).

If you use Cache-Control or Expires, the decision wether to load an updated version from the server or not purely depends on the client. So even if there is a new version available, the user won't see it. On the plus side you are saving lots of cpu cycles on the server, because your php script won't be executed.

If you use ETag, you can inform the client on each request, if the version of the requested content has changed. But your php script will be executed on each request, even if the ETag is unchanged.

2) Serverside, between client and server This kind of caching primarily reduces high cpu load on your server. It won't affect the amount of traffic generated between client and server.

You can use a client proxy like Varnish to store rendered responses on the server side. The good thing is, that you have full control over the cache. If an updated version of a requested content is available, you can simply purge the old version from the cache, so that a new version is generated from your php script and stored in your cache.

Every response that is cacheable will only be generated one time and then be served from cache to the clients.

3) In your application If you are heavily using your database, you should consider using a fast key value store like memcached to cache query results. Of course you have to adjust your database classes for this (first ask memcached, if memcached doesn't have the result ask the database and store the result into memcached), but the performance gain will be quite impressive, because memcached is really fast.

Sometimes it even makes sense to store data solely in memcached, if the data doesn't has to be persisted permanently (php sessions for example).

Answer 2

I had also faced the same problem long back (I dont know if you will find my way to be correct).

Why I needed the cache :- What my site use to do was, it use to update the database by running the script on cron.php file and index.php use to show the listing from database (this use to take ages to load )

my solution :-

every time a new list was created or updated I unlinked the cache file then on index.php page I checked if cache file exists load cache or else load content from database also at the same time write this data to the cache file so next time when user requests for index.php file