catbox

Multi-strategy object caching service Version: 10.x

Lead Maintainer: Gil Pedersen

catbox is a multi-strategy key-value object store. It comes with extensions supporting a memory cache, Redis, MongoDB, Memcached, 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 include 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
• Amazon S3
• Local Disk

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:

• await start() - creates a connection to the cache server. Must be called before any other method is available. Any errors are thrown.
• stop() - terminates the connection to the cache server.
• await get(key) - retrieve an item from the cache engine if found where:
  • key - a cache key object (see below).
  • return value:
    • null is the item is not found.
    • throws an error if the request failed.
    • otherwise, an object with the following properties:
      • 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).
• await set(key, value, ttl) - 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).
  • any errors are thrown.
• await drop(key) - remove an item from cache where:
  • key - a cache key object (see below).
  • any errors are thrown.
• isReady() - returns true if cache engine determines itself as ready, false if it is not ready.
• validateSegmentName(segment) - returns null if the segment name is valid (see below), otherwise should return an instance of Error with an appropriate message.

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 optional keys (unless noted otherwise):
  • 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 async function(id, flags) where:
    • id - the id string or object provided to the get() method.
    • flags - an object used to pass back additional flags:
      • 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 milliseconds. 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 returning a stale value while generateFunc is generating a fresh value.
  • 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. Required if generateFunc is present. Set to false to disable timeouts which may cause all get() requests to get stuck forever.
  • dropOnError - if true, an error or timeout in the generateFunc causes the stale value to be evicted from the cache. Defaults to true.
  • generateOnReadError - if false, an upstream cache read error will stop the get() method from calling the generate function and will instead pass back the cache error. Defaults to true.
  • generateIgnoreWriteError - if false, an upstream cache write error will be passed back with the generated value when calling the get() method. Defaults to true.
  • pendingGenerateTimeout - number of milliseconds while generateFunc call is in progress for a given id, before a subsequent generateFunc call is allowed. Defaults to 0, no blocking of concurrent generateFunc calls beyond staleTimeout.
  • getDecoratedValue - if true, the return value of policy.get() calls is an object with { value, cached, report }. Defaults to false which returns a plain value.
• 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:

• await get(id) - 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.
  • return value:
    • the requested item if found, otherwise null.
    • any errors are thrown.
    • if getDecoratedValue is true, returns an object with the following properties:
      • 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.
• await set(id, value, ttl) - 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.
  • any errors are thrown.
• await drop(id) - remove the item from cache where:
  • id - the unique item identifier (within the policy segment).
  • any errors are thrown.
• 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.
• isReady() - returns true if cache engine determines itself as ready, false if it is not ready or if there is no cache engine set.
• stats - an object with cache statistics where:
  • sets - number of cache writes.
  • gets - number of cache get() requests.
  • hits - number of cache get() requests in which the requested id was found in the cache (can be stale).
  • stales - number of cache reads with stale requests (only counts the first request in a queued get() operation).
  • generates - number of calls to the generate function.
  • errors - cache operations errors.