Distribucache

Datastore-independent automatically-repopulating cache. This cache does everything in its power to shield the caller from the delays of the downstream services. It has a unique feature, where the cache will populate itself on a certain interval, and will stop doing so when the values that were being refreshed have not been used.

There are multiple available datastores, including:

• Redis
• Memory

The cache can be used in various ways, ranging from the simplest get / set, to complex scenarios with watermarks for staleness and final expiration.

Usage

Basic

There are many different ways to use the cache. Features are added to the cache, based on the configuration that you use. Below is an example of the simplest cache:

var distribucache = require('distribucache'),
  // create a Redis store (to keep track of the Redis connections)
  // generally performed once in the lifetime of the app
  redisStore = require('distribucache-redis-store'),
  cacheClient = distribucache.createClient(redisStore({
    host: 'localhost',
    port: 6379
  })),
  // create a new cache
  // performed every time a new cache configuration is needed
  cache = cacheClient.create('nsp');

cache.get('k1', function (err, value) {
  if (err) throw err;
  console.log('got value:', value);
});

cache.set('k1', 'v1', function (err) {
  if (err) throw err;
  console.log('set value');
});

cache.del('k1', function (err) {
  if (err) throw err;
  console.log('deleted k1');
});

Promises: if a callback is not provided as the last argument, a Promise will be returned from the get, set and del methods.

Note: the value from the get will be null if the value is not in the cache.

Configuration

The cache can be configured in two places: (a) when creating a cache-client, and (b) when creating a cache. As you expect, the configuration in the cache overrides the configuration of the cache-client:

var cacheClient = distribucache.createClient(store, {
  expiresIn: '2 sec'   // setting globally
});

// overrides the globally set `expiresIn`
var articleCache = cacheClient.create('articles', {
  expiresIn: '1 min'
});

// uses the global `expiresIn`
var pageCache = cacheClient.create('pages');

Populating

A common pattern is to call the get first, and if the item is not in the cache, call set. For this common pattern, you can provide a populate function when creating the cache. On a get, if the cache is empty your populate function will be called to populate the cache, and then the flow will continue to the get callback. This ensures that the get always returns a value, either from the cache or from the downstream service.

var cache = cacheClient.create('nsp', {
  populate: function (key, cb) {
    setTimeout(function () {
      cb(null, 42);
    }, 100);
  }
});

cache.get('k1', function (err, value) {
  console.log(value); // 42
});

Promises: the populate function may return a Promise if you choose.

Expiration / Staleness

When an expiresIn is set, a get request will return null after the time expires. After this, the value will be dropped from the datastore. When the populate function is set, instead of returning null the populate method will be called.

The expiresIn may be set in milliseconds or in the human-readable format.

var cache = cacheClient.create('nsp', {
  expiresIn: 2000  // 2 seconds
});

A staleIn can also be set. It acts as a low-water-mark. When a value is stale it is still returned as is to the caller. Two additional things happen: (a) the stale event is called (with key as the argument) and (b) the populate is called in the background if it is provided; allowing the next get call to get a fresh value, without incurring the delay of accessing a downstream service.

var cache = cacheClient.create('nsp', {
  staleIn: 1000  // 1 second
});

Timer-based Background Population

The more complex, yet most powerful feature of the cache is its ability to update its keys on a specific interval. To do this set the populateIn config. You must also set a pausePopulateIn to make sure the cache is not re-populated forever needlessly.

The cache will use the pausePopulateIn to check whether the key has been used during that interval. The cache does this by tracking the access time of keys. For example, if you want the cache to stop populating when the key hasn't been used for a minute, set pausePopulateIn to 1000 * 60 ms.

var cache = cacheClient.create('nsp', {
  populateIn: 1000  // 1 second
  pausePopulateIn: 1000 * 60  // 1 minute
});

Note: this feature will work even with disruptions to the service, as the burden of determining which keys need to be re-populated is on the store (e.g., in the Redis store this is done using a combination of keyspace events and expiring keys).

API

Distribucache

• createClient(store, config)

Possible config values below.

{String} [config.namespace]
{Boolean} [config.optimizeForSmallValues] defaults to false
{Boolean} [config.optimizeForBuffers] defaults to false
{String} [config.expiresIn] in ms
{String} [config.staleIn] in ms
{Function} [config.populate]
{Number} [config.populateIn] in ms, defaults to 30sec
{Number} [config.populateInAttempts] defaults to 5
{Number} [config.pausePopulateIn] in ms, defaults to 30sec
{Number} [config.timeoutPopulateIn] in ms
{Number} [config.leaseExpiresIn] in ms
{Number} [config.accessedAtThrottle] in ms, defaults to 1000

Notes:

• The values above are allowed for the config and are also available to the CacheClient#create
• See the Optimizations docs for values that begin with optimizeFor

CacheClient

• create(namespace, config)
  • namespace is a String that will identify the particular cache. It is good practice to add a version to the namespace in order to make sure that when you change the interface, you will not get older cached objects (with a possibly different signature). For example: create('articles:v1').
  • config is an Object. See the global config above for all of the possible values.

More

• Events
• Optimizations
• Human-readable Time Intervals
• Debugging

License

MIT