# redis lru cache [![Build Status](https://secure.travis-ci.org/facundoolano/redis-lru.png)](http://travis-ci.org/facundoolano/redis-lru)

A least recently used (LRU) cache backed by Redis, allowing data to be shared by
multiple Node.JS processes. API inspired by [node-lru-cache](https://github.com/isaacs/node-lru-cache).

```js
var redis = require('redis').createClient(port, host, opts);
var lru = require('redis-lru');

var personCache = lru(redis, 5); // up to 5 items

personCache.set('john', {name: 'John Doe', age: 27})
  .then(() => personCache.set('jane', {name: 'Jane Doe', age: 30}))
  .then(() => personCache.get('john'))
  .then(console.log) // prints {name: 'John Doe', age: 27}
  .then(() => personCache.reset()) //clear the cache

var bandCache = lru(redis, {max: 2, namespace: 'bands', maxAge: 15000}); // use a different namespace and set expiration

bandCache.set('beatles', 'john, paul, george, ringo')
  .then(() => bandCache.set('zeppelin', 'jimmy, robert, john, bonzo'))
  .then(() => bandCache.get('beatles')) // now beatles are the most recently accessed
  .then(console.log) // 'john, paul, george, ringo'
  .then(() => bandCache.set('floyd', 'david, roger, syd, richard, nick')) // cache full, remove least recently accessed
  .then(() => bandCache.get('zeppelin'))
  .then(console.log) // null, was evicted from cache
```

Works both with [node_redis](https://github.com/NodeRedis/node_redis) and [ioredis](https://github.com/luin/ioredis) clients.

## Installation

```
npm install redis-lru
```

## Options

* `max`: Maximum amount of items the cache can hold. This option is required; if no
other option is needed, it can be passed directly as the second parameter when creating
the cache.
* `namespace`: Prefix appended to all keys saved in Redis, to avoid clashes with other applications
and to allow multiple instances of the cache.
* `maxAge`: Maximum amount of milliseconds the key will be kept in the cache; after that getting/peeking will
resolve to `null`. Note that the value will be removed from Redis after `maxAge`, but the key will
be kept in the cache until next time it's accessed (i.e. it will be included in `count`, `keys`, etc., although not in `has`.).
* `score`: function to customize the score used to order the elements in the cache. Defaults to `() => new Date().getTime()`
* `increment`: if `true`, on each access the result of the `score` function is added to the previous one,
rather than replacing it.

## API

All methods return a Promise.

* `set(key, value, maxAge)`: set value for the given key, marking it as the most recently accessed one.
Keys should be strings, values will be JSON.stringified. The optional `maxAge` overrides for this specific key
the global expiration of the cache.
* `get(key)`: resolve to the value stored in the cache for the given key or `null` if not present.
If present, the key will be marked as the most recently accessed one.
* `getOrSet(key, fn, maxAge)`: resolve to the value stored in the cache for the given key. If not present,
execute `fn`, save the result in the cache and return it. `fn` should be a no args function that
returns a value or a promise. If `maxAge` is passed, it will be used only if the key is not already in the cache.
* `peek(key)`: resolve to the value stored in the cache for the given key, without changing its
last accessed time.
* `del(key)`: removes the item from the cache.
* `reset()`: empties the cache.
* `has(key)`: resolves to true if the given key is present in the cache.
* `keys()`: resolves to an array of keys in the cache, sorted from most to least recently accessed.
* `values()`: resolves to an array of values in the cache, sorted from most to least recently accessed.
* `count()`: resolves to the number of items currently in the cache.

### Using as a LFU cache

By using a custom `score` function and the `increment` option, one can turn the cache
into a least frequently used (LFU), where the items that have been accessed more times
(rather than most recently) are preserved:

```js
var redis = require('redis').createClient(port, host, opts);
var lru = require('redis-lru');

var bandLfu = lru(redis, {max: 2, score: () => 1, increment: true});

bandLfu.set('beatles', 'john, paul, george, ringo')
  .then(() => bandLfu.get('beatles')) // accessed twice
  .then(() => bandLfu.set('zeppelin', 'jimmy, robert, john, bonzo'))
  .then(() => bandLfu.set('floyd', 'david, roger, syd, richard, nick')) // cache full, remove least frequently accessed
  .then(() => bandLfu.get('zeppelin'))
  .then(console.log) // null, was evicted from cache
```

## Implementation

Each item in the cache is stored as a regular key/value in Redis. Additionally,
a [ZSET](https://redis.io/topics/data-types#sorted-sets) is used to keep an
index of the keys sorted by last-accessed timestamp.

Requires Redis 3.0.2 or greater, since it uses the
[XX option of ZADD](https://redis.io/commands/zadd#zadd-options-redis-302-or-greater).