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 = 
"0.0.1"

Class Attribute Summary collapse

Class Method Summary collapse

Class Attribute Details

.redis ⇒ Object 



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# File 'lib/atomic_redis_cache.rb', line 55

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

Class Method Details

.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