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15.19.8 InnoDB memcached Plugin Internals

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InnoDB API for the InnoDB memcached Plugin

The InnoDB memcached engine accesses InnoDB through InnoDB APIs, most of which are directly adopted from embedded InnoDB. InnoDB API functions are passed to the InnoDB memcached engine as callback functions. InnoDB API functions access the InnoDB tables directly, and are mostly DML operations with the exception of TRUNCATE TABLE.

memcached commands are implemented through the InnoDB memcached API. The following table outlines how memcached commands are mapped to DML or DDL operations.

Table 15.28 memcached Commands and Associated DML or DDL Operations

memcached Command DML or DDL Operations
get a read/fetch command
set a search followed by an INSERT or UPDATE (depending on whether or not a key exists)
add a search followed by an INSERT or UPDATE
replace a search followed by an UPDATE
append a search followed by an UPDATE (appends data to the result before UPDATE)
prepend a search followed by an UPDATE (prepends data to the result before UPDATE)
incr a search followed by an UPDATE
decr a search followed by an UPDATE
delete a search followed by a DELETE
flush_all TRUNCATE TABLE (DDL)

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InnoDB memcached Plugin Configuration Tables

This section describes configuration tables used by the daemon_memcached plugin. The cache_policies table, config_options table, and containers table are created by the innodb_memcached_config.sql configuration script in the innodb_memcache database.

  1. mysql> USE innodb_memcache;
  2. Database changed
  3. mysql> SHOW TABLES;
  4. +---------------------------+
  5. | Tables_in_innodb_memcache |
  6. +---------------------------+
  7. | cache_policies            |
  8. | config_options            |
  9. | containers                |
  10. +---------------------------+

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cache_policies Table

The cache_policies table defines a cache policy for the InnoDB memcached installation. You can specify individual policies for get, set, delete, and flush operations, within a single cache policy. The default setting for all operations is innodb_only.

  • innodb_only: Use InnoDB as the data store.

  • cache_only: Use the memcached engine as the data store.

  • caching: Use both InnoDB and the memcached engine as data stores. In this case, if memcached cannot find a key in memory, it searches for the value in an InnoDB table.

  • disable: Disable caching.

Table 15.29 cache_policies Columns

Column Description
policy_name Name of the cache policy. The default cache policy name is cache_policy.
get_policy The cache policy for get operations. Valid values are innodb_only, cache_only, caching, or disabled. The default setting is innodb_only.
set_policy The cache policy for set operations. Valid values are innodb_only, cache_only, caching, or disabled. The default setting is innodb_only.
delete_policy The cache policy for delete operations. Valid values are innodb_only, cache_only, caching, or disabled. The default setting is innodb_only.
flush_policy The cache policy for flush operations. Valid values are innodb_only, cache_only, caching, or disabled. The default setting is innodb_only.

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config_options Table

The config_options table stores memcached-related settings that can be changed at runtime using SQL. Supported configuration options are separator and table_map_delimiter.

Table 15.30 config_options Columns

Column Description
Name Name of the memcached-related configuration option. The following configuration options are supported by the config_options table:

  • separator: Used to separate values of a long string into separate values when there are multiple value_columns defined. By default, the separator is a | character. For example, if you define col1, col2 as value columns, and you define | as the separator, you can issue the following memcached command to insert values into col1 and col2, respectively: 

    set keyx 10 0 19
valuecolx|valuecoly

    valuecol1x is stored in col1 and valuecoly is stored in col2.

  • table_map_delimiter: The character separating the schema name and the table name when you use the @@ notation in a key name to access a key in a specific table. For example, @@t1.some_key and @@t2.some_key have the same key value, but are stored in different tables.

Value The value assigned to the memcached-related configuration option.

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containers Table

The containers table is the most important of the three configuration tables. Each InnoDB table that is used to store memcached values must have an entry in the containers table. The entry provides a mapping between InnoDB table columns and container table columns, which is required for memcached to work with InnoDB tables.

The containers table contains a default entry for the test.demo_test table, which is created by the innodb_memcached_config.sql configuration script. To use the daemon_memcached plugin with your own InnoDB table, you must create an entry in the containers table.

Table 15.31 containers Columns

Column Description
name The name given to the container. If an InnoDB table is not requested by name using @@ notation, the daemon_memcached plugin uses the InnoDB table with a containers.name value of default. If there is no such entry, the first entry in the containers table, ordered alphabetically by name (ascending), determines the default InnoDB table.
db_schema The name of the database where the InnoDB table resides. This is a required value.
db_table The name of the InnoDB table that stores memcached values. This is a required value.
key_columns The column in the InnoDB table that contains lookup key values for memcached operations. This is a required value.
value_columns The InnoDB table columns (one or more) that store memcached data. Multiple columns can be specified using the separator character specified in the innodb_memcached.config_options table. By default, the separator is a pipe character (“|”). To specify multiple columns, separate them with the defined separator character. For example: col1|col2|col3. This is a required value.
flags The InnoDB table columns that are used as flags (a user-defined numeric value that is stored and retrieved along with the main value) for memcached. A flag value can be used as a column specifier for some operations (such as incr, prepend) if a memcached value is mapped to multiple columns, so that an operation is performed on a specified column. For example, if you have mapped a value_columns to three InnoDB table columns, and only want the increment operation performed on one columns, use the flags column to specify the column. If you do not use the flags column, set a value of 0 to indicate that it is unused.
cas_column The InnoDB table column that stores compare-and-swap (cas) values. The cas_column value is related to the way memcached hashes requests to different servers and caches data in memory. Because the InnoDB memcached plugin is tightly integrated with a single memcached daemon, and the in-memory caching mechanism is handled by MySQL and the InnoDB buffer pool, this column is rarely needed. If you do not use this column, set a value of 0 to indicate that it is unused.
expire_time_column The InnoDB table column that stores expiration values. The expire_time_column value is related to the way memcached hashes requests to different servers and caches data in memory. Because the InnoDB memcached plugin is tightly integrated with a single memcached daemon, and the in-memory caching mechanism is handled by MySQL and the InnoDB buffer pool, this column is rarely needed. If you do not use this column, set a value of 0 to indicate that the column is unused. The maximum expire time is defined as INT_MAX32 or 2147483647 seconds (approximately 68 years).
unique_idx_name_on_key The name of the index on the key column. It must be a unique index. It can be the primary key or a secondary index. Preferably, use the primary key of the InnoDB table. Using the primary key avoids a lookup that is performed when using a secondary index. You cannot make a covering index for memcached lookups; InnoDB returns an error if you try to define a composite secondary index over both the key and value columns.

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containers Table Column Constraints

  • You must supply a value for db_schema, db_name, key_columns, value_columns and unique_idx_name_on_key. Specify 0 for flags, cas_column, and expire_time_column if they are unused. Failing to do so could cause your setup to fail.

  • key_columns: The maximum limit for a memcached key is 250 characters, which is enforced by memcached. The mapped key must be a non-Null CHAR or VARCHAR type.

  • value_columns: Must be mapped to a CHAR, VARCHAR, or BLOB column. There is no length restriction and the value can be NULL.

  • cas_column: The cas value is a 64 bit integer. It must be mapped to a BIGINT of at least 8 bytes. If you do not use this column, set a value of 0 to indicate that it is unused.

  • expiration_time_column: Must mapped to an INTEGER of at least 4 bytes. Expiration time is defined as a 32-bit integer for Unix time (the number of seconds since January 1, 1970, as a 32-bit value), or the number of seconds starting from the current time. For the latter, the number of seconds may not exceed 60*60*24*30 (the number of seconds in 30 days). If the number sent by a client is larger, the server considers it to be a real Unix time value rather than an offset from the current time. If you do not use this column, set a value of 0 to indicate that it is unused.

  • flags: Must be mapped to an INTEGER of at least 32-bits and can be NULL. If you do not use this column, set a value of 0 to indicate that it is unused.

A pre-check is performed at plugin load time to enforce column constraints. If mismatches are found, the plugin is not loaded.

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Multiple Value Column Mapping

  • During plugin initialization, when InnoDB memcached is configured with information defined in the containers table, each mapped column defined in containers.value_columns is verified against the mapped InnoDB table. If multiple InnoDB table columns are mapped, there is a check to ensure that each column exists and is the right type.

  • At run-time, for memcached insert operations, if there are more delimited values than the number of mapped columns, only the number of mapped values are taken. For example, if there are six mapped columns, and seven delimited values are provided, only the first six delimited values are taken. The seventh delimited value is ignored.

  • If there are fewer delimited values than mapped columns, unfilled columns are set to NULL. If an unfilled column cannot be set to NULL, insert operations fail.

  • If a table has more columns than mapped values, the extra columns do not affect results.

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The demo_test Example Table

The innodb_memcached_config.sql configuration script creates a demo_test table in the test database, which can be used to verify InnoDB memcached plugin installation immediately after setup.

The innodb_memcached_config.sql configuration script also creates an entry for the demo_test table in the innodb_memcache.containers table. 

mysql> SELECT * FROM innodb_memcache.containers\G
*************************** 1. row ***************************
                  name: aaa
             db_schema: test
              db_table: demo_test
           key_columns: c1
         value_columns: c2
                 flags: c3
            cas_column: c4
    expire_time_column: c5
unique_idx_name_on_key: PRIMARY

mysql> SELECT * FROM test.demo_test;
+----+------------------+------+------+------+
| c1 | c2               | c3   | c4   | c5   |
+----+------------------+------+------+------+
| AA | HELLO, HELLO     |    8 |    0 |    0 |
+----+------------------+------+------+------+

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Document created the 26/06/2006, last modified the 26/10/2018
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