Context cache

Overview

Requests are often served differently depending on their "context". The context of a request is composed of multiple dimensions. Here are some examples of common dimensions:

• environment { production, staging, regression, development, etc. }
• lang { en-US, en-GB, fr-FR, fr-CA, etc. }
• device { desktop, tablet, phone, etc. }
• partner { foo, bar, baz, etc. }
• experiment { A, B, C, etc. }

Oftentimes, meta-data necessary to handle a request has to be computed based on its context. That computing can be expensive, so the result is usually cached. Unfortunately, as you can see, the number of contexts can be extremely high since it is the combination of the values each dimension can take. This results in a very large cache containing a large number of objects. This in turns slows down garbage collection (GC). At Yahoo!, we've seen instances where GC ends up representing 70% of the average time needed to serve a request! Additionally, in some cases, only a small number of contexts may really be needed to serve a large percentage of traffic. For example, at Yahoo! Search, a node may cache meta-data for 1,000+ contexts, but we noticed that the 100 most requested contexts serve over 98% of our traffic.

This utility module solves this specific issue by caching data only for the most requested contexts. This will result in low latency for most of your traffic and low memory consumption, which is a requirement for efficient GC.

Usage

Install the context-cache npm package:

npm install context-cache

You can then create a ContextCache instance in your application code:

var ContextCache = require('./context-cache');
var cc = ContextCache.create();

You can pass configuration settings to create:

var cc = ContextCache.create({
    maxCacheSize: 200,
    cacheHitThreshold: 10,
    storeObjectsSerialized: true,
    hotcacheTTL: 300,
    isolationMode: 'clone'
});

You can then use that instance to store/retrieve in/from the cache:

var ctx = JSON.stringify(context);
cc.set(ctx, data);
var data = cc.get(ctx);

Finally, you can obtain information about the cache itself:

console.log(cc.getInfo());