What is a multi-tier cache?

Question

I've recently come across the phrase "multi-tier cache" relating to multi-tiered architectures, but without a meaningful explanation of what such a cache would be (or how it would be used).

Relevant online searches for that phrase don't really turn up anything either. My interpretation would be a cache servicing all tiers of some n-tier web app. Perhaps a distributed cache with one cache node on each tier.

Has SO ever come across this term before? Am I right? Way off?

Answer 1

After playing around with EhCache for a few weeks it is still not perfectly clear what they mean by the term "multi-tier" cache. I will follow up with what I interpret to be the implied meaning; if at any time down the road someone comes along and knows otherwise, please feel free to answer and I'll remove this one.

A multi-tier cache appears to be a replicated and/or distributed cache that lives on 1+ tiers in an n-tier architecture. It allows components on multiple tiers to gain access to the same cache(s). In EhCache, using a replicated or distributed cache architecture in conjunction with simply referring to the same cache servers from multiple tiers achieves this.

Answer 2

I know this is old, but thought I'd toss in my two cents here since I've written several multi-tier caches, or at least several iterations of one.

Consider this; Every application will have different layers, and at each layer a different form of information can be cached. Each cache item will generally expire for one of two reasons, either a period of time has expired, or a dependency has been updated.

For this explanation, lets imagine that we have three layers:

1. Templates (object definitions)
2. Objects (complete object cache)
3. Blocks (partial objects / block cache)

Each layer depends on it's parent, and we would define those using some form of dependency assignment. So Blocks depend on Objects which depend on Templates. If an Object is changed, any dependencies in Block would be expunged and refreshed; if a Template is changed, any Object dependencies would be expunged, in turn expunging any Blocks, and all would be refreshed.

There are several benefits, long expiry times are a big one because dependencies will ensure that downstream resources are updated whenever parents are updated, so you won't get stale cached resources. Block caches alone are a big help because, short of whole page caching (which requires AJAX or Edge Side Includes to avoid caching dynamic content), blocks will be the closest elements to an end users browser / interface and can save boatloads of pre-processing cycles.

The complication in a multi-tier cache like this though is that it generally can't rely on a purely DB based foreign key expunging, that is unless each tier is 1:1 in relation to its parent (ie. Block will only rely on a single object, which relies on a single template). You'll have to programmatically address the expunging of dependent resources. You can either do this via stored procedures in the DB, or in your application layer if you want to dynamically work with expunging rules.

Hope that helps someone :)

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Edit: I should add, any one of these tiers can be clustered, sharded, or otherwise in a scaled environment, so this model works in both small and large environments.