Web Caching

Using a Hierarchy

I covered the advantages and disadvantages of hierarchical caching in Chapter 7. Though a hierarchy can improve your cache’s performance, it can also create some new problems. In most cases, the performance improvements are marginal, affecting 5–10% of requests. The nature and extent of any problems depend mostly on who is operating the other caches. If all caches in the hierarchy are managed by a single organization, the potential for problems is significantly diminished. Interorganizational hierarchies, on the other hand, are more susceptible to issues that outweigh any performance gains.

Performance improvements come in the form of additional cache hits and lower average response times. The percentage of requests that are hits in neighbor caches depends on the size of the hierarchy. Cache hits become more common as the hierarchy membership grows. Hierarchies and meshes also offer better scaling characteristics, which can be critical for organizations with large networks. In other words, an intraorganizational hierarchy may not perform significantly better but may be easier to manage.

If you feel that a cache hierarchy will benefit your service, be sure to find out which intercache protocols the candidate products support. Most products support ICP, which is good if the other caches in a hierarchy are made by different companies. Don’t forget to consider the effects of the different intercache protocols. ICP and HTCP add delays to cache misses. Cache Digests ...