class Hoodoo::TransientStore::MemcachedRedisMirror

Hoodoo::TransientStore plugin supporting storage into both Memcached and Redis simultaneously.

The implementation uses Hoodoo::TransientStore::Memcached and Hoodoo::TransientStore::Redis to talk to the two storage engines.

When looking up data with get, the requested item must be found in both storage engines. If it is found in only one, the other one is deleted to keep maximum pool space available in both and nil will be returned for the lookup.

Note unusual requirements for the connection URI data provided to the initialize call.

The mirroring storage engine plug-in is useful if migrating from one of these engines to another without invalidating data present in the one from which you are migrating away. Change to using the mirrored storage engine for as long as the maximum item expiry period in the old engine, then once you know all old engine items must have been expired, cut over to just the new engine.

Attributes

get_keys_from[RW]

Command an instance to allow get to return values which are only found in one or the other storage engine, or in both.

Permitted values are:

:memcached: Only Memcached needs to have a value for a key
:redis: Only Redis needs to have a value for a key
both: Both engines must have the key

This is useful in migration scenarios where moving from Memcached to Redis or vice versa. If wishing to be able to still read old data only in Memcached, set :memcached; else :redis. That way, data only in the old engine but not yet in the new is still considered valid and read back. For true mirroring which requires both stores to have the value, use :both.

The default is :both.

Public Class Methods

new( storage_host_uri:, namespace: ) click to toggle source

See Hoodoo::TransientStore::Base::new for details.

Do not instantiate this class directly. Use Hoodoo::TransientStore::new.

The storage_host_uri parameter is necessarily unusual here. It must be either _a Hash_ with Symbol keys :memcached and :redis, or a serialised JSON string representing the same information. These values define the actual storage engine host URI for the respective engines. For example, to connect to locally running engines configured on their default ports, pass this Hash in storage_host_uri:

{
  :memcached => 'localhost:11211',
  :redis     => 'redis://localhost:6379'
}

…or:

"{
  \"memcached\": \"localhost:11211\",
  \"redis\":     \"redis://localhost:6379\"
}"

See Hoodoo::TransientStore::Memcached::new and Hoodoo::TransientStore::Redis::new for details of connection URI requirements for those engines.

The value of the namespace parameter applies equally to both engines.

Calls superclass method Hoodoo::TransientStore::Base::new

# File lib/hoodoo/transient_store/transient_store/memcached_redis_mirror.rb, line 88
def initialize( storage_host_uri:, namespace: )
  super # Pass all arguments through -> *not* 'super()'

  storage_host_uri = deserialize_and_symbolize( storage_host_uri ) if storage_host_uri.is_a?( String )

  unless storage_host_uri.is_a?( Hash ) &&
         storage_host_uri.has_key?( :memcached ) &&
         storage_host_uri.has_key?( :redis )
    raise 'Hoodoo::TransientStore::MemcachedRedisMirror: Bad storage host URI data passed to constructor'
  end

  @get_keys_from   = :both
  @memcached_store = Hoodoo::TransientStore::Memcached.new( storage_host_uri: storage_host_uri[ :memcached ], namespace: namespace )
  @redis_store     =     Hoodoo::TransientStore::Redis.new( storage_host_uri: storage_host_uri[ :redis     ], namespace: namespace )
end

Public Instance Methods

close() click to toggle source

See Hoodoo::TransientStore::Base#close for details.

# File lib/hoodoo/transient_store/transient_store/memcached_redis_mirror.rb, line 178
def close
  @memcached_store.close() rescue nil # Rescue so that Redis "close()" is still attempted.
      @redis_store.close()
end

delete( key: ) click to toggle source

See Hoodoo::TransientStore::Base#delete for details.

# File lib/hoodoo/transient_store/transient_store/memcached_redis_mirror.rb, line 152
def delete( key: )
  exception = nil

  begin
    @memcached_store.delete( key: key )
  rescue => e
    exception = e
  end

  # But allow Redis delete to still be attempted...

  begin
    @redis_store.delete( key: key )
  rescue => e
    exception ||= e
  end

  if exception.nil?
    return true
  else
    raise exception
  end
end

get( key: ) click to toggle source

See Hoodoo::TransientStore::Base#get for details.

The requested item must be found in both Memcached and Redis. If it is found in only one, the other one is deleted to keep maximum pool space available in and nil will be returned.

If get_keys_from is configured for :both and the data for some reason has ended up differing in the two stores - most likely because something modified just one of them (perhaps there is outdated code kicking around which is writing to just one) - then the Memcached copy will “win” and the Redis value will be ignored.

# File lib/hoodoo/transient_store/transient_store/memcached_redis_mirror.rb, line 125
def get( key: )
  case @get_keys_from
    when :both
      memcached_result = @memcached_store.get( key: key )
      redis_result     =     @redis_store.get( key: key )

      if @get_keys_from == :both && ( memcached_result.nil? || redis_result.nil? )
        delete( key: key )
        return nil
      else
        return memcached_result
      end

    when :memcached
      return @memcached_store.get( key: key )

    when :redis
      return @redis_store.get( key: key )

    else
      raise "Hoodoo::TransientStore::Base\#get: Invalid prior value given in \#get_keys_from= of '#{ @get_keys_from.inspect }' - only ':both', ':memcached' or ':redis' are allowed"

  end
end

set( key:, payload:, maximum_lifespan: ) click to toggle source

See Hoodoo::TransientStore::Base#set for details.

# File lib/hoodoo/transient_store/transient_store/memcached_redis_mirror.rb, line 106
def set( key:, payload:, maximum_lifespan: )
  memcached_result = @memcached_store.set( key: key, payload: payload, maximum_lifespan: maximum_lifespan )
  redis_result     =     @redis_store.set( key: key, payload: payload, maximum_lifespan: maximum_lifespan )

  return memcached_result && redis_result
end