In Web caching, it becomes important to ensure that the cached object being retrieved by the client on a cache hit, is not a stale object. The cached object is said to be stale if the object cached at the proxy server is different from that at the originating server i.e. after the object had been cached by the proxy server the originating server in some way had modified that object. Here, the problem is how to keep the cached pages up to date.
The techniques used in conventional systems may not be adequate for Caching on the Web. For example, invalidation callbacks cannot work because a page could be cached in millions of caches, and it's not practical that the server knows about all these caches to notify them when the page changed. The mechanisms used for maintaining Web cache coherence are based mainly on validation checks. In short, the idea is to have a timestamp associated to each cached object, and then, periodically or on demand, to check with the primary server whether the object's timestamp in the cache is the same with the object's timestamp in the primary server.If the timestamp at the originating server is of higher value than that of the object at the Proxy cache we require to refrain the client from accessing the stale object.
Following are some of the techniques with which we can ensure cache coherence for the Web.