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[GSoC] MDEV-4674 Port InnoDB memcached interface to MariaDB - Final report


Hi all,

as the pencils down date has passed, I am posting a report on what I have during the whole project.

Repository of my project is available here:


Some stats:

33 commits before the midterm
60 commits after the midterm
93 commits total

What have been done in details:


1. Code of InnoDB Memcached plugin has been imported from MySQL into the MariaDB tree.

2. There were some differences between MariaDB and MySQL, which blocked the plugin from out-of-the-box compilation and correct operation. They have been resolved.

3. Test suite for the plugin has been imported from MySQL. Compatibility changes were introduced into these test cases which required them. The test suite was modified to assign memcached port number dynamically from mtr worker's pool, what allows to run the same test in parallel for many combinations.

The plugin fully passes the test suite, both for 'innodb_plugin' and 'xtradb' combinations.

4. Some enhancements and bug fixes have been introduced into the plugin:

- detection of memcached daemon initialization has been fixed -- this eliminated unnecessary wait (15 seconds) on plugin shutdown and memory leak (in cases when plugin failed to initialize properly before)

- memcached arithmetic commands (incr/decr) handling has been improved -- now numeric columns values are handled in a consistent and predictable way

- incorrect handling of unsigned not null integers columns has been fixed


5. Memcached daemon code has been updated to the latest version from upstream memcached/engine-pu branch.

6. Plugin architecture has been changed to "one table per socket", as originally proposed in MDEV-4674 bug report. This allows usage of well known security solutions (firewalls, unix file permissions) to restrict access to different tables for different users.

Now user can specify which tables should be accessible on which sockets. Memcached daemon will listen on all these sockets and execute commands on appropriate table, depending on which socket client connected.

Supported protocols are TCP, UDP and UNIX stream sockets. Sockets are configured in 'daemon_memcached/containers' table:

INSERT INTO containers VALUES ("tcp://*:11211", "db0", "table0",
                               "key_col", "val_col", "c3", "c4", "c5",
                               "PRIMARY", "|");

INSERT INTO containers VALUES ("udp://localhost:11222", "db0", "table1",
                               "key_col", "val_col", "c3", "c4", "c5",
                               "PRIMARY", "|");

INSERT INTO containers VALUES ("unix:/tmp/mc.sock", "db1", "table0",
                               "key_col", "val_col", "c3", "c4", "c5",
                               "PRIMARY", "|");

7. Plugin has been decoupled from InnoDB.

Now plugin is split into two parts:

  - generic 'daemon_memcached.so'
  - InnoDB specific 'daemon_memcached_engine_ib.so'

The first library is a MariaDB plugin, which should be installed with command:

  install plugin daemon_memcached soname "daemon_memcached.so";

This library is generic. It provides Memcached protocol handling and it loads the second library, which is API-specific engine for accessing the database. Which engine library should be loaded can be specified in 'daemon_memcached_engine_lib_name' MariaDB server option.

Now, the only existing engine library is daemon_memcached_engine_ib.so, which utilizes InnoDB API to access the data. It can operate on InnoDB and XtraDB tables.

8. Some enhancements and bug fixes have been introduced into the plugin:

- CMake options and #cmakedefines associated with the plugin have been cleaned up

- embedded default Memcached engine has been removed

- duplicated code in Memcached source has been removed

- fixed MySQL-originating bug with flush_all of non default table -- command flush_all always flushed the default table, not the current one

- removed support for in-connection table switching -- after introduction of one table/port architecture it was redundant

- plugin shutdown time has been improved -- from constant time of 2 seconds to average time of 0.5 second

- logging of plugin has been refactored -- now plugin send log messages to MariaDB log instead of stderr

- fixed Memcached-originating bug with get_thread_stats() -- bug resulted in out of bound array accesses and writes, what leaded to SEGFAULT

- fixed MySQL-originating crash during 'flush_all' memcached command bug -- crash happened when table was heavily loaded with DML operations and because of that ib_cb_cursor_set_lock() failed to obtain a lock on the table, returning DB_LOCK_WAIT_TIMEOUT error

- fixed MySQL-originating multiple memcached_sync_count for the same transaction bug -- the memcached_sync_count was incremented multiple times for the same transaction and after the end of transaction, it was not reset to zero, what blocked TRUNCATE command on any InnoDB table, on which Memcached had operated during program lifetime, even after uninstalling the plugin

- support for numeric values has been finished -- now BIGINT overflows in the same way as smaller types, test to check over/under-flowing of all integer types has been added

For details of all changes please see commit messages associated with them: https://github.com/piotrjurkiewicz/mariadb-server/commits/10.1-memcached


Moreover, I planned to implement a new interface inside the Memcached plugin, which will use Handler API for database access. This would make possible to use the plugin to access databases based on storage engines other than InnoDB/XtraDB.

However, during testing I discovered many serious bugs, either in Memcached or InnoDB API engine, which originated from upstream Memcached or MySQL and resulted in crashes or deadlocks. I decided that first I should fix them before implementing new functionalities. I lost a lot of time on investigating these bugs and I was unable to finish the implementation of Handler API engine.

Nevertheless, I think the whole plugin is ready to be merged into MariaDB. All user-facing changes (like one port/table architecture) have been completed. Eventual later introduction of Handler API engine will not be associated with any user-facing compatibility changes. It will just lift the requirement that all databases accessed via the Memcached plugin must be InnoDB/XtraDB.

Now I am going to write a separate email to coordinate the merge of my code into MariaDB repository.