lrucache - A simple LRU-cache based on a hash and a priority queue.

Setup

gem install lrucache

Example

require 'lrucache'
cache = LRUCache.new(:max_size => 3, :default => 42)
cache[1] = 'a'
cache[2] = 'b'
cache[3] = 'c'
puts cache[2] # b
puts cache[1] # a
puts cache[3] # c
puts cache[:does_not_exist] # 42, has no effect on LRU
cache[4] = 'd' # Out of space! Throws out the least-recently used (2 => 'b')
puts cache.keys # [1,3,4]

TTL (time-to-live)

cache = LRUCache.new(:ttl => 1.hour)
cache.store("banana", "yellow")
cache.store("monkey", "banana", Time.now + 3.days)
# or ...
cache.store("monkey", "banana", 3.days)

# Three minutes later ...
cache.fetch("banana") # "yellow"
cache.fetch("monkey") # "banana"

# Three hours later ...
cache.fetch("banana") # nil
cache.fetch("monkey") # "banana"

# Three days later ...
cache.fetch("banana") # nil
cache.fetch("monkey") # nil

SOFT_TTL

Allows you to have two TTLs when calling fetch with a block. After the soft-ttl expires, the block is called to refresh the value. If the block completes normally, the value is replaced and expirations reset. If the block raises a fatal (non-RuntimeError) exception, it bubbles up. But, if the block raises a RuntimeError, the cached value is kept and used a little longer, and the soft-ttl is postponed retry_delay into the future. If the block is not called successfully before the normal TTL expires, then the cached value expires and the block is called for a new value, but exceptions are not handled.

cache = LRUCache.new(:ttl => 1.hour,
                     :soft_ttl => 30.minutes,
                     :retry_delay => 1.minute)
cache.fetch("banana") { "yellow" } # "yellow"
cache.fetch("banana") { "george" } # "yellow"

# 30 minutes later ...
cache.fetch("banana") { raise "ruckus" } # "yellow"
cache.fetch("banana") { "george" } # "yellow"

# 1 more minute later ...
cache.fetch("banana") { "george" } # "george"
cache.fetch("banana") { "barney" } # "george"