17.20.2 InnoDB memcached Architecture

The InnoDB memcached plugin implements memcached as a MySQL plugin daemon that accesses the InnoDB storage engine directly, bypassing the MySQL SQL layer.

The following diagram illustrates how an application accesses data through the daemon_memcached plugin, compared with SQL.

Figure 17.4 MySQL Server with Integrated memcached Server

Shows an application accessing data in the InnoDB storage engine using both SQL and the memcached protocol. Using SQL, the application accesses data through the MySQL Server and Handler API. Using the memcached protocol, the application bypasses the MySQL Server, accessing data through the memcached plugin and InnoDB API. The memcached plugin is comprised of the innodb_memcache interface and optional local cache.

Features of the daemon_memcached plugin:

memcached as a daemon plugin of mysqld. Both mysqld and memcached run in the same process space, with very low latency access to data.
Direct access to InnoDB tables, bypassing the SQL parser, the optimizer, and even the Handler API layer.
Standard memcached protocols, including the text-based protocol and the binary protocol. The daemon_memcached plugin passes all 55 compatibility tests of the memcapable command.
Multi-column support. You can map multiple columns into the “value” part of the key-value store, with column values delimited by a user-specified separator character.
By default, the memcached protocol is used to read and write data directly to InnoDB, letting MySQL manage in-memory caching using the InnoDB buffer pool. The default settings represent a combination of high reliability and the fewest surprises for database applications. For example, default settings avoid uncommitted data on the database side, or stale data returned for memcached get requests.
Advanced users can configure the system as a traditional memcached server, with all data cached only in the memcached engine (memory caching), or use a combination of the “memcached engine” (memory caching) and the InnoDB memcached engine (InnoDB as back-end persistent storage).

Control over how often data is passed back and forth between InnoDB and memcached operations through the innodb_api_bk_commit_interval, daemon_memcached_r_batch_size, and daemon_memcached_w_batch_size configuration options. Batch size options default to a value of 1 for maximum reliability.
The ability to specify memcached options through the daemon_memcached_option configuration parameter. For example, you can change the port that memcached listens on, reduce the maximum number of simultaneous connections, change the maximum memory size for a key-value pair, or enable debugging messages for the error log.
The innodb_api_trx_level configuration option controls the transaction isolation level on queries processed by memcached. Although memcached has no concept of transactions, you can use this option to control how soon memcached sees changes caused by SQL statements issued on the table used by the daemon_memcached plugin. By default, innodb_api_trx_level is set to READ UNCOMMITTED.
The innodb_api_enable_mdl option can be used to lock the table at the MySQL level, so that the mapped table cannot be dropped or altered by DDL through the SQL interface. Without the lock, the table can be dropped from the MySQL layer, but kept in InnoDB storage until memcached or some other user stops using it. “MDL” stands for “metadata locking”.