The PHP replication and load balancing mysqlnd plugin as been released as an alpha through PECL. Alpha is for those who want to try out the 1.0 feature set as early as possible. Alpha is for those who do not fear debugging, if need be. Notes on troubleshooting.

Testing load balancing and read/write split

There are three ways to verify that load balancing and read/write split works as it should:

• within PHP script: checking thread/connection ids
• within PHP script: monitoring statistics
• externally: mysqlnd debug log file

Thread/connection ids to distinguish connections

PECL/mysqlnd_ms changes the semantics of a PHP MySQL connection handle. If using the plugin, a connection handle returned by any of the three PHP MySQL extensions ( …

Replication is as old as life on earth. No life without replication. MySQL replication is as old as MySQL, almost. No MySQL without replication. The PECL/mysqlnd_ms 1.0.0 alpha release is brand new. My dream, no PHP without the mysqlnd library replication and load balancing plugin, which works with all the PHP MySQL extensions (mysql, mysqli, PDO_MySQL)!

PECL/mysqlnd_ms (download alpha release) is a transparent plugin for the mysqlnd library. It …

Replication is as old as life on earth. No life without replication. MySQL replication is as old as MySQL, almost. No MySQL without replication. The PECL/mysqlnd_ms 1.0.0 alpha release is brand new. My dream, no PHP without the mysqlnd library replication and load balancing plugin, which works with all the PHP MySQL extensions (mysql, mysqli, PDO_MySQL)!

PECL/mysqlnd_ms (download alpha release) is a transparent plugin for the mysqlnd library. It …

PECL/mysqlnd_ms is now transaction aware, sometimes. A new experimental configuration option trx_stickiness=master disables load balancing if autocommit mode is turned off via API. This makes the mysqlnd library plugin transaction safe. Unfortunately it requires PHP 5.3.99 and, it can be fooled by using SQL to control autocommit. It requires some discipline to use.

Past and today: SQL hints for transactions

By default the PHP replication and load balancing plugin (mysqlnd_ms) is not transaction safe, because it is not transaction aware. The plugin does not know when a transaction starts and when it ends. Thus, it may decide to load balance statements and switch connections in the middle of a transaction. So far, SQL hints have been the only way to prevent …

PECL/mysqlnd_ms is now transaction aware, sometimes. A new experimental configuration option trx_stickiness=master disables load balancing if autocommit mode is turned off via API. This makes the mysqlnd library plugin transaction safe. Unfortunately it requires PHP 5.3.99 and, it can be fooled by using SQL to control autocommit. It requires some discipline to use.

Past and today: SQL hints for transactions

By default the PHP replication and load balancing plugin (mysqlnd_ms) is not transaction safe, because it is not transaction aware. The plugin does not know when a transaction starts and when it ends. Thus, it may decide to load balance statements and switch connections in the middle of a transaction. So far, SQL hints have been the only way to prevent …

Hours ago, I have committed the first documentation draft for PECL/mysqlnd_ms. The mirrors should show it on Saturday. PECL/mysqlnd_ms is a replication and load balancing plugin for the mysqlnd library. Its latest feature, master_on_write, helps to work around the issue of replication lag. If master_on_write=1, the plugin stops load balancing statements after the first non read-only statement has been run by a PHP script. Instead of running read-only statements on the slaves, the plugin runs all statements on the master. This is done to ensure that the client will always read the latest data, even the data just written and not yet replicated to the slaves. Thank you, …

Hours ago, I have committed the first documentation draft for PECL/mysqlnd_ms. The mirrors should show it on Saturday. PECL/mysqlnd_ms is a replication and load balancing plugin for the mysqlnd library. Its latest feature, master_on_write, helps to work around the issue of replication lag. If master_on_write=1, the plugin stops load balancing statements after the first non read-only statement has been run by a PHP script. Instead of running read-only statements on the slaves, the plugin runs all statements on the master. This is done to ensure that the client will always read the latest data, even the data just written and not yet replicated to the slaves. Thank you, …

Introducing the best load balancing policy for PECL/mysqlnd_ms: "random_once". A (PHP) web request is short-lived. A web server can handle multiple requests per second. There are many web servers running an application. For each web request choose a random slave to use for the rest of the request. Don’t do load balancing, don’t switch connections during a web request . Result: even load distribution, less connection overhead and less connections overall, cache loves you, less hassle switching stateful slave connections…

Load balancing policies featured by PECL/mysqlnd_ms

As of today, PECL/mysqlnd_ms, our PHP mysqlnd toy plugin for replication load balancing, features four built-in load balancing policies:

1. random: for each read query, choose a random slave
2. round-robin: for each read query, choose the next slaves from the slave …

Introducing the best load balancing policy for PECL/mysqlnd_ms: "random_once". A (PHP) web request is short-lived. A web server can handle multiple requests per second. There are many web servers running an application. For each web request choose a random slave to use for the rest of the request. Don’t do load balancing, don’t switch connections during a web request . Result: even load distribution, less connection overhead and less connections overall, cache loves you, less hassle switching stateful slave connections…

Load balancing policies featured by PECL/mysqlnd_ms

As of today, PECL/mysqlnd_ms, our PHP mysqlnd toy plugin for replication load balancing, features four built-in load balancing policies:

1. random: for each read query, choose a random slave
2. round-robin: for each read query, choose the next slaves from the slave …

Write-read sequences can be tricky for users of MySQL replication, if what has been written needs to be available for reading immediately. Slaves could be behind. You have to run the sequence on the master. If MySQL semisynchronous replication would tell the client which slave is in sync, a client library could select that slave for load balancing.

The more we look into load balancing for PECL/mysqlnd_ms, the PHP mysqlnd master-slave replication load balancing proof-of-concept plugin, the more restrictions and issues come up. An obvious restriction is that connections have a state.

No solution: binlog monitoring

But not …