Module: AtomicRedisCache

Defined in:
lib/atomic_redis_cache.rb,
lib/atomic_redis_cache/version.rb

Constant Summary collapse

DEFAULT_EXPIRATION =

86400 seconds in a day

60*60*24
DEFAULT_RACE_TTL =

seconds to acquire new value

30
MAX_RETRIES =

recalc attempts before expiring cache

3
VERSION = 
'0.2.1'

Class Attribute Summary collapse

Class Method Summary collapse

Class Attribute Details

.redis ⇒ Object

Raises:

  • (ArgumentError) 


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# File 'lib/atomic_redis_cache.rb', line 90

def redis
  raise ArgumentError.new('AtomicRedisCache.redis must be set') unless @redis
  @redis.respond_to?(:call) ? @redis.call : @redis
end

Class Method Details

.delete(key) ⇒ Object

Delete the cache entry completely, including timer



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# File 'lib/atomic_redis_cache.rb', line 81

def delete(key)
  redis.del(key, timer(key)) == 2
end

.fetch(key, opts = {}, &blk) ⇒ Object

Fetch from cache with fallback, just like ActiveSupport::Cache. The main differences are in the edge cases around expiration.

  • when cache expires, we avoid dogpile/thundering herd from multiple processes recalculating at once

  • when calculation takes too long (i.e., due to network traffic) we return the previously cached value for several attempts

Options: :expires_in - expiry in seconds; defaults to a day :race_condition_ttl - time to lock value for recalculation :max_retries - # of times to retry cache refresh before expiring



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# File 'lib/atomic_redis_cache.rb', line 21

def fetch(key, opts={}, &blk)
  expires_in = opts[:expires_in] || DEFAULT_EXPIRATION
  race_ttl   = opts[:race_condition_ttl] || DEFAULT_RACE_TTL
  retries    = opts[:max_retries] || MAX_RETRIES

  now        = Time.now.to_i
  ttl        = expires_in + retries * race_ttl
  t_key      = timer(key)

  if val = redis.get(key)              # cache hit
    if redis.get(t_key).to_i < now     # expired entry or dne
      redis.set t_key, now + race_ttl  # block other callers for recalc duration
      begin
        Timeout.timeout(race_ttl) do   # if recalc exceeds race_ttl, abort
          val = Marshal.dump(blk.call) # determine new value
          redis.multi do               # atomically cache + mark as valid
            redis.setex key, ttl, val
            redis.set t_key, now + expires_in
          end
        end
      rescue Timeout::Error => e       # eval timed out, use cached val
      end
    end
  else                                 # cache miss
    val = Marshal.dump(blk.call)       # determine new value
    redis.multi do                     # atomically cache + mark as valid
      redis.setex key, ttl, val
      redis.set t_key, now + expires_in
    end
  end

  Marshal.load(val)
end

.read(key) ⇒ Object

Fetch from the cache atomically; return nil if empty or expired



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# File 'lib/atomic_redis_cache.rb', line 56

def read(key)
  val, exp = redis.mget key, timer(key)
  Marshal.load(val) unless exp.to_i < Time.now.to_i
end

.timer(key) ⇒ Object 



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# File 'lib/atomic_redis_cache.rb', line 85

def timer(key)
  "timer:#{key}"
end

.write(key, val, opts = {}) ⇒ Object

Write to the cache unconditionally, returns success as boolean Accepts the same options and uses the same defaults as .fetch() Note that write() ignores locks, so it can be called multiple times; prefer .fetch() unless absolutely necessary.



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# File 'lib/atomic_redis_cache.rb', line 65

def write(key, val, opts={})
  expires_in = opts[:expires_in] || DEFAULT_EXPIRATION
  race_ttl   = opts[:race_condition_ttl] || DEFAULT_RACE_TTL
  retries    = opts[:max_retries] || MAX_RETRIES
  ttl        = expires_in + retries * race_ttl
  expiry     = Time.now.to_i + expires_in

  response = redis.multi do
    redis.setex key, ttl, Marshal.dump(val)
    redis.set timer(key), expiry
  end

  response.all? { |ret| ret == 'OK' }
end