Multi-strategy object caching service Version: 4.x

Lead Maintainer: Ben Acker

catbox is a multi-strategy key-value object store. It comes with extensions supporting a memory cache, Redis, MongoDB, Memcached, Riak and Amazon S3. catbox provides two interfaces: a low-level Client and a high-level Policy.

Installation

In order to reduce module dependencies, catbox does not includes the external caching strategies. To use other strategies, each service must be manually installed via npm or package dependencies manually. The available strategies are:

• Memory
• Redis
• MongoDB
• Memcached
• Riak
• Amazon S3

Client

The Client object provides a low-level cache abstraction. The object is constructed using new Client(engine, options) where:

• engine - is an object or a prototype function implementing the cache strategy:
  • function - a prototype function with the signature function(options). catbox will call new func(options).
  • object - a pre instantiated client implementation object. Does not support passing options.
• options - the strategy configuration object. Each strategy defines its own configuration options with the following common options:
  • partition - the partition name used to isolate the cached results across multiple clients. The partition name is used as the MongoDB database name, the Riak bucket, or as a key prefix in Redis and Memcached. To share the cache across multiple clients, use the same partition name.

Note that any implementation of client strategies must return deep copies of the stored data as the API assumes that the object returned from a get() is owned by the called and can be safely modified without affecting the cache copy.

API

The Client object provides the following methods:

• start(callback) - creates a connection to the cache server. Must be called before any other method is available. The callback signature is function(err).
• stop() - terminates the connection to the cache server.
• get(key, callback) - retrieve an item from the cache engine if found where:
  • key - a cache key object (see below).
  • callback - a function with the signature function(err, cached). If the item is not found, both err and cached are null. If found, the cached object contains the following: - item - the value stored in the cache using set(). - stored - the timestamp when the item was stored in the cache (in milliseconds). - ttl - the remaining time-to-live (not the original value used when storing the object).
• set(key, value, ttl, callback) - store an item in the cache for a specified length of time, where:
  • key - a cache key object (see below).
  • value - the string or object value to be stored.
  • ttl - a time-to-live value in milliseconds after which the item is automatically removed from the cache (or is marked invalid).
  • callback - a function with the signature function(err).
• drop(key, callback) - remove an item from cache where:
  • key - a cache key object (see below).
  • callback - a function with the signature function(err).

Any method with a key argument takes an object with the following required properties:

• segment - a caching segment name string. Enables using a single cache server for storing different sets of items with overlapping ids.
• id - a unique item identifier string (per segment). Can be an empty string.

Policy

The Policy object provides a convenient cache interface by setting a global policy which is automatically applied to every storage action. The object is constructed using new Policy(options, [cache, segment]) where:

• options - is an object with the following keys:
  • expiresIn - relative expiration expressed in the number of milliseconds since the item was saved in the cache. Cannot be used together with expiresAt.
  • expiresAt - time of day expressed in 24h notation using the 'HH:MM' format, at which point all cache records for the route expire. Uses local time. Cannot be used together with expiresIn.
  • generateFunc - a function used to generate a new cache item if one is not found in the cache when calling get(). The method's signature is function(id, next) where: - id - the id string or object provided to the get() method. - next - the method called when the new item is returned with the signature function(err, value, ttl) where: - err - an error condition. - value - the new value generated. - ttl - the cache ttl value in milliseconds. Set to 0 to skip storing in the cache. Defaults to the cache global policy.
  • staleIn - number of milliseconds to mark an item stored in cache as stale and attempt to regenerate it when generateFunc is provided. Must be less than expiresIn. Alternatively function that returns staleIn value in miliseconds. The function signature is function(stored, ttl) where:
    • stored - the timestamp when the item was stored in the cache (in milliseconds).
    • ttl - the remaining time-to-live (not the original value used when storing the object).
  • staleTimeout - number of milliseconds to wait before checking if an item is stale.
  • generateTimeout - number of milliseconds to wait before returning a timeout error when the generateFunc function takes too long to return a value. When the value is eventually returned, it is stored in the cache for future requests.
  • dropOnError - if true, an error or timeout in the generateFunc causes the stale value to be evicted from the cache. Defaults to true.
• cache - a Client instance (which has already been started).
• segment - required when cache is provided. The segment name used to isolate cached items within the cache partition.

API

The Policy object provides the following methods:

• get(id, callback) - retrieve an item from the cache. If the item is not found and the generateFunc method was provided, a new value is generated, stored in the cache, and returned. Multiple concurrent requests are queued and processed once. The method arguments are:
  • id - the unique item identifier (within the policy segment). Can be a string or an object with the required 'id' key.
  • callback - the return function. The function signature is function(err, value, cached, report) where:
    • err - any errors encountered.
    • value - the fetched or generated value.
    • cached - null if a valid item was not found in the cache, or an object with the following keys:
      • item - the cached value.
      • stored - the timestamp when the item was stored in the cache.
      • ttl - the cache ttl value for the record.
      • isStale - true if the item is stale.
    • report - an object with logging information about the generation operation containing the following keys (as relevant):
      • msec - the cache lookup time in milliseconds.
      • stored - the timestamp when the item was stored in the cache.
      • isStale - true if the item is stale.
      • ttl - the cache ttl value for the record.
      • error - lookup error.
• set(id, value, ttl, callback) - store an item in the cache where:
  • id - the unique item identifier (within the policy segment).
  • value - the string or object value to be stored.
  • ttl - a time-to-live override value in milliseconds after which the item is automatically removed from the cache (or is marked invalid). This should be set to 0 in order to use the caching rules configured when creating the Policy object.
  • callback - a function with the signature function(err).
• drop(id, callback) - remove the item from cache where:
  • id - the unique item identifier (within the policy segment).
  • callback - a function with the signature function(err).
• ttl(created) - given a created timestamp in milliseconds, returns the time-to-live left based on the configured rules.
• `rules(options) - changes the policy rules after construction (note that items already stored will not be affected) where:
  • options - the same options as the Policy constructor.