Copyright 1997-2012 the PHP Documentation Group.

The plugin is implemented as a PHP extension. See also the installation instructions to install the PECL/mysqlnd_ms extension.

Compile or configure the PHP MySQL extension (API) (mysqli, PDO_MYSQL, mysql) that you plan to use with support for the mysqlnd library. PECL/mysqlnd_ms is a plugin for the mysqlnd library. To use the plugin with any of the PHP MySQL extensions, the extension has to use the mysqlnd library.

Then, load the extension into PHP and activate the plugin in the PHP configuration file using the PHP configuration directive named mysqlnd_ms.enable.

mysqlnd_ms.enable=1
mysqlnd_ms.config_file=/path/to/mysqlnd_ms_plugin.ini

    

The plugin uses its own configuration file. Use the PHP configuration directive mysqlnd_ms.config_file to set the full file path to the plugin-specific configuration file. This file must be readable by PHP (e.g., the web server user). Please note, the configuration directive mysqlnd_ms.config_file superseeds mysqlnd_ms.ini_file since 1.4.0. It is a common pitfall to use the old, no longer available configuration directive.

Create a plugin-specific configuration file. Save the file to the path set by the PHP configuration directive mysqlnd_ms.config_file.

The plugins configuration file is JSON based. It is divided into one or more sections. Each section has a name, for example, myapp. Every section makes its own set of configuration settings.

A section must, at a minimum, list the MySQL replication master server, and set a list of slaves. The plugin supports using only one master server per section. Multi-master MySQL replication setups are not yet fully supported. Use the configuration setting master to set the hostname, and the port or socket of the MySQL master server. MySQL slave servers are configured using the slave keyword.

{
    "myapp": {
        "master": {
            "master_0": {
                "host": "localhost"
            }
        },
        "slave": [

        ]
    }
}

    

Configuring a MySQL slave server list is required, although it may contain an empty list. It is recommended to always configure at least one slave server.

Server lists can use anonymous or non-anonymous syntax. Non-anonymous lists include alias names for the servers, such as master_0 for the master in the above example. The quickstart uses the more verbose non-anonymous syntax.

{
    "myapp": {
        "master": {
            "master_0": {
                "host": "localhost",
                "socket": "\/tmp\/mysql.sock"
            }
        },
        "slave": {
            "slave_0": {
                "host": "192.168.2.27",
                "port": "3306"
            }
        }
    }
}

    

If there are at least two servers in total, the plugin can start to load balance and switch connections. Switching connections is not always transparent and can cause issues in certain cases. The reference sections about connection pooling and switching, transaction handling, fail over load balancing and read-write splitting all provide more details. And potential pitfalls are described later in this guide.

It is the responsibility of the application to handle potential issues caused by connection switches, by configuring a master with at least one slave server, which allows switching to work therefore related problems can be found.

The MySQL master and MySQL slave servers, which you configure, do not need to be part of MySQL replication setup. For testing purpose you can use single MySQL server and make it known to the plugin as a master and slave server as shown below. This could help you to detect many potential issues with connection switches. However, such a setup will not be prone to the issues caused by replication lag.

{
    "myapp": {
        "master": {
            "master_0": {
                "host": "localhost",
                "socket": "\/tmp\/mysql.sock"
            }
        },
        "slave": {
            "slave_0": {
                "host": "127.0.0.1",
                "port": "3306"
            }
        }
    }
}

    

The plugin attempts to notify you of invalid configurations. Since 1.5.0 it will throw a warning during PHP startup if the configuration file cannot be read, is empty or parsing the JSON failed. Depending on your PHP settings those errors may appear in some log files only. Further validation is done when a connection is to be established and the configuration file is searched for valid sections. Setting mysqlnd_ms.force_config_usage may help debugging a faulty setup. Please, see also configuration file debugging notes.

Copyright 1997-2012 the PHP Documentation Group.

The plugin can be used with any PHP MySQL extension (mysqli, mysql, and PDO_MYSQL) that is compiled to use the mysqlnd library. PECL/mysqlnd_ms plugs into the mysqlnd library. It does not change the API or behavior of those extensions.

Whenever a connection to MySQL is being opened, the plugin compares the host parameter value of the connect call, with the section names from the plugin specific configuration file. If, for example, the plugin specific configuration file has a section myapp then the section should be referenced by opening a MySQL connection to the host myapp

{
    "myapp": {
        "master": {
            "master_0": {
                "host": "localhost",
                "socket": "\/tmp\/mysql.sock"
            }
        },
        "slave": {
            "slave_0": {
                "host": "192.168.2.27",
                "port": "3306"
            }
        }
    }
}

    

<?php
/* Load balanced following "myapp" section rules from the plugins config file */
$mysqli = new mysqli("myapp", "username", "password", "database");
$pdo = new PDO('mysql:host=myapp;dbname=database', 'username', 'password');
$mysql = mysql_connect("myapp", "username", "password");
?>

    

The connection examples above will be load balanced. The plugin will send read-only statements to the MySQL slave server with the IP 192.168.2.27 and will listen on port 3306 for the MySQL client connection. All other statements will be directed to the MySQL master server running on the host localhost. If on Unix like operating systems, the master on localhost will be accepting MySQL client connections on the Unix domain socket /tmp/mysql.sock, while TCP/IP is the default port on Windows. The plugin will use the user name username and the password password to connect to any of the MySQL servers listed in the section myapp of the plugins configuration file. Upon connect, the plugin will select database as the current schemata.

The username, password and schema name are taken from the connect API calls and used for all servers. In other words: you must use the same username and password for every MySQL server listed in a plugin configuration file section. The is not a general limitation. As of PECL/mysqlnd_ms 1.1.0, it is possible to set the username and password for any server in the plugins configuration file, to be used instead of the credentials passed to the API call.

The plugin does not change the API for running statements. Read-write splitting works out of the box. The following example assumes that there is no significant replication lag between the master and the slave.

<?php
/* Load balanced following "myapp" section rules from the plugins config file */
$mysqli = new mysqli("myapp", "username", "password", "database");
if (mysqli_connect_errno())
  /* Of course, your error handling is nicer... */
  die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));

/* Statements will be run on the master */
if (!$mysqli->query("DROP TABLE IF EXISTS test")) {
 printf("[%d] %s\n", $mysqli->errno, $mysqli->error);
}
if (!$mysqli->query("CREATE TABLE test(id INT)")) {
 printf("[%d] %s\n", $mysqli->errno, $mysqli->error);
}
if (!$mysqli->query("INSERT INTO test(id) VALUES (1)")) {
 printf("[%d] %s\n", $mysqli->errno, $mysqli->error);
}

/* read-only: statement will be run on a slave */
if (!($res = $mysqli->query("SELECT id FROM test")) {
 printf("[%d] %s\n", $mysqli->errno, $mysqli->error);
} else {
 $row = $res->fetch_assoc();
 $res->close();
 printf("Slave returns id = '%s'\n", $row['id'];
}
$mysqli->close();
?>

    

Slave returns id = '1'

    

Copyright 1997-2012 the PHP Documentation Group.

The plugin changes the semantics of a PHP MySQL connection handle. A new connection handle represents a connection pool, instead of a single MySQL client-server network connection. The connection pool consists of a master connection, and optionally any number of slave connections.

Every connection from the connection pool has its own state. For example, SQL user variables, temporary tables and transactions are part of the state. For a complete list of items that belong to the state of a connection, see the connection pooling and switching concepts documentation. If the plugin decides to switch connections for load balancing, the application could be given a connection which has a different state. Applications must be made aware of this.

{
    "myapp": {
        "master": {
            "master_0": {
                "host": "localhost",
                "socket": "\/tmp\/mysql.sock"
            }
        },
        "slave": {
            "slave_0": {
                "host": "192.168.2.27",
                "port": "3306"
            }
        }
    }
}

    

<?php
$mysqli = new mysqli("myapp", "username", "password", "database");
if (!$mysqli)
  /* Of course, your error handling is nicer... */
  die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));

/* Connection 1, connection bound SQL user variable, no SELECT thus run on master */
if (!$mysqli->query("SET @myrole='master'")) {
 printf("[%d] %s\n", $mysqli->errno, $mysqli->error);
}

/* Connection 2, run on slave because SELECT */
if (!($res = $mysqli->query("SELECT @myrole AS _role"))) {
 printf("[%d] %s\n", $mysqli->errno, $mysqli->error);
} else {
 $row = $res->fetch_assoc();
 $res->close();
 printf("@myrole = '%s'\n", $row['_role']);
}
$mysqli->close();
?>

    

@myrole = ''

    

The example opens a load balanced connection and executes two statements. The first statement SET @myrole='master' does not begin with the string SELECT. Therefore the plugin does not recognize it as a read-only query which shall be run on a slave. The plugin runs the statement on the connection to the master. The statement sets a SQL user variable which is bound to the master connection. The state of the master connection has been changed.

The next statement is SELECT @myrole AS _role. The plugin does recognize it as a read-only query and sends it to the slave. The statement is run on a connection to the slave. This second connection does not have any SQL user variables bound to it. It has a different state than the first connection to the master. The requested SQL user variable is not set. The example script prints @myrole = ''.

It is the responsibility of the application developer to take care of the connection state. The plugin does not monitor all connection state changing activities. Monitoring all possible cases would be a very CPU intensive task, if it could be done at all.

The pitfalls can easily be worked around using SQL hints.

Copyright 1997-2012 the PHP Documentation Group.

SQL hints can force a query to choose a specific server from the connection pool. It gives the plugin a hint to use a designated server, which can solve issues caused by connection switches and connection state.

SQL hints are standard compliant SQL comments. Because SQL comments are supposed to be ignored by SQL processing systems, they do not interfere with other programs such as the MySQL Server, the MySQL Proxy, or a firewall.

Three SQL hints are supported by the plugin: The MYSQLND_MS_MASTER_SWITCH hint makes the plugin run a statement on the master, MYSQLND_MS_SLAVE_SWITCH enforces the use of the slave, and MYSQLND_MS_LAST_USED_SWITCH will run a statement on the same server that was used for the previous statement.

The plugin scans the beginning of a statement for the existence of an SQL hint. SQL hints are only recognized if they appear at the beginning of the statement.

{
    "myapp": {
        "master": {
            "master_0": {
                "host": "localhost",
                "socket": "\/tmp\/mysql.sock"
            }
        },
        "slave": {
            "slave_0": {
                "host": "192.168.2.27",
                "port": "3306"
            }
        }
    }
}

    

<?php
$mysqli = new mysqli("myapp", "username", "password", "database");
if (mysqli_connect_errno())
  /* Of course, your error handling is nicer... */
  die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));

/* Connection 1, connection bound SQL user variable, no SELECT thus run on master */
if (!$mysqli->query("SET @myrole='master'")) {
 printf("[%d] %s\n", $mysqli->errno, $mysqli->error);
}

/* Connection 1, run on master because of SQL hint */
if (!($res = $mysqli->query(sprintf("/*%s*/SELECT @myrole AS _role", MYSQLND_MS_LAST_USED_SWITCH)))) {
 printf("[%d] %s\n", $mysqli->errno, $mysqli->error);
} else {
 $row = $res->fetch_assoc();
 $res->close();
 printf("@myrole = '%s'\n", $row['_role']);
}
$mysqli->close();
?>

    

@myrole = 'master'

    

In the above example, using MYSQLND_MS_LAST_USED_SWITCH prevents session switching from the master to a slave when running the SELECT statement.

SQL hints can also be used to run SELECT statements on the MySQL master server. This may be desired if the MySQL slave servers are typically behind the master, but you need current data from the cluster.

In version 1.2.0 the concept of a service level has been introduced to address cases when current data is required. Using a service level requires less attention and removes the need of using SQL hints for this use case. Please, find more information below in the service level and consistency section.

<?php
$mysqli = new mysqli("myapp", "username", "password", "database");
if (!$mysqli)
  /* Of course, your error handling is nicer... */
  die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));

/* Force use of master, master has always fresh and current data */
if (!$mysqli->query(sprintf("/*%s*/SELECT critical_data FROM important_table", MYSQLND_MS_MASTER_SWITCH))) {
 printf("[%d] %s\n", $mysqli->errno, $mysqli->error);
}
?>

    

A use case may include the creation of tables on a slave. If an SQL hint is not given, then the plugin will send CREATE and INSERT statements to the master. Use the SQL hint MYSQLND_MS_SLAVE_SWITCH if you want to run any such statement on a slave, for example, to build temporary reporting tables.

<?php
$mysqli = new mysqli("myapp", "username", "password", "database");
if (!$mysqli)
  /* Of course, your error handling is nicer... */
  die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));

/* Force use of slave */
if (!$mysqli->query(sprintf("/*%s*/CREATE TABLE slave_reporting(id INT)", MYSQLND_MS_SLAVE_SWITCH))) {
 printf("[%d] %s\n", $mysqli->errno, $mysqli->error);
}
/* Continue using this particular slave connection */
if (!$mysqli->query(sprintf("/*%s*/INSERT INTO slave_reporting(id) VALUES (1), (2), (3)", MYSQLND_MS_LAST_USED_SWITCH))) {
 printf("[%d] %s\n", $mysqli->errno, $mysqli->error);
}
/* Don't use MYSQLND_MS_SLAVE_SWITCH which would allow switching to another slave! */
if ($res = $mysqli->query(sprintf("/*%s*/SELECT COUNT(*) AS _num FROM slave_reporting", MYSQLND_MS_LAST_USED_SWITCH))) {
  $row = $res->fetch_assoc();
  $res->close();
  printf("There are %d rows in the table 'slave_reporting'", $row['_num']);
} else {
  printf("[%d] %s\n", $mysqli->errno, $mysqli->error);
}
$mysqli->close();
?>

    

The SQL hint MYSQLND_MS_LAST_USED forbids switching a connection, and forces use of the previously used connection.

Copyright 1997-2012 the PHP Documentation Group.

The current version of the plugin is not transaction safe by default, because it is not aware of running transactions in all cases. SQL transactions are units of work to be run on a single server. The plugin does not always know when the unit of work starts and when it ends. Therefore, the plugin may decide to switch connections in the middle of a transaction.

No kind of MySQL load balancer can detect transaction boundaries without any kind of hint from the application.

You can either use SQL hints to work around this limitation. Alternatively, you can activate transaction API call monitoring. In the latter case you must use API calls only to control transactions, see below.

[myapp]
{
    "myapp": {
        "master": {
            "master_0": {
                "host": "localhost",
                "socket": "\/tmp\/mysql.sock"
            }
        },
        "slave": {
            "slave_0": {
                "host": "192.168.2.27",
                "port": "3306"
            }
        }
    }
}

    

<?php
$mysqli = new mysqli("myapp", "username", "password", "database");
if (!$mysqli)
  /* Of course, your error handling is nicer... */
  die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));

/* Not a SELECT, will use master */
if (!$mysqli->query("START TRANSACTION")) {
 /* Please use better error handling in your code */
 die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
}

/* Prevent connection switch! */
if (!$mysqli->query(sprintf("/*%s*/INSERT INTO test(id) VALUES (1)", MYSQLND_MS_LAST_USED_SWITCH)))) {
 /* Please do proper ROLLBACK in your code, don't just die */
 die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
}
if ($res = $mysqli->query(sprintf("/*%s*/SELECT COUNT(*) AS _num FROM test", MYSQLND_MS_LAST_USED_SWITCH)))) {
  $row = $res->fetch_assoc();
  $res->close();
  if ($row['_num'] > 1000) {
   if (!$mysqli->query(sprintf("/*%s*/INSERT INTO events(task) VALUES ('cleanup')", MYSQLND_MS_LAST_USED_SWITCH)))) {
     die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
   }
  }
} else {
 die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
}
if (!$mysqli->query(sprintf("/*%s*/UPDATE log SET last_update = NOW()", MYSQLND_MS_LAST_USED_SWITCH)))) {
 die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
}
if (!$mysqli->query(sprintf("/*%s*/COMMIT", MYSQLND_MS_LAST_USED_SWITCH)))) {
 die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
}

$mysqli->close();
?>

    

Starting with PHP 5.4.0, the mysqlnd library allows the plugin to monitor the status of the autocommit mode, if the mode is set by API calls instead of using SQL statements such as SET AUTOCOMMIT=0. This makes it possible for the plugin to become transaction aware. In this case, you do not need to use SQL hints.

If using PHP 5.4.0 or newer, API calls that enable autocommit mode, and when setting the plugin configuration option trx_stickiness=master, the plugin can automatically disable load balancing and connection switches for SQL transactions. In this configuration, the plugin stops load balancing if autocommit is disabled and directs all statements to the master. This prevents connection switches in the middle of a transaction. Once autocommit is re-enabled, the plugin starts to load balance statements again.

API based transaction boundary detection has been improved with PHP 5.5.0 and PECL/mysqlnd_ms 1.5.0 to cover not only calls to mysqli_autocommit but also mysqli_begin, mysqli_commit and mysqli_rollback.

{
    "myapp": {
        "master": {
            "master_0": {
                "host": "localhost",
                "socket": "\/tmp\/mysql.sock"
            }
        },
        "slave": {
            "slave_0": {
                "host": "127.0.0.1",
                "port": "3306"
            }
        },
        "trx_stickiness": "master"
    }
}

    

<?php
$mysqli = new mysqli("myapp", "username", "password", "database");
if (!$mysqli)
  /* Of course, your error handling is nicer... */
  die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));

/* Disable autocommit, plugin will run all statements on the master */
$mysqli->autocommit(FALSE);

if (!$mysqli->query("INSERT INTO test(id) VALUES (1)")) {
 /* Please do proper ROLLBACK in your code, don't just die */
 die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
}
if ($res = $mysqli->query("SELECT COUNT(*) AS _num FROM test")) {
  $row = $res->fetch_assoc();
  $res->close();
  if ($row['_num'] > 1000) {
   if (!$mysqli->query("INSERT INTO events(task) VALUES ('cleanup')")) {
     die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
   }
  }
} else {
 die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
}
if (!$mysqli->query("UPDATE log SET last_update = NOW()")) {
 die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
}
if (!$mysqli->commit()) {
 die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
}

/* Plugin assumes that the transaction has ended and starts load balancing again */
$mysqli->autocommit(TRUE);
$mysqli->close();
?>

    

Please note the restrictions outlined in the transaction handling concepts section.

Copyright 1997-2012 the PHP Documentation Group.

Different types of MySQL cluster solutions offer different service and data consistency levels to their users. An asynchronous MySQL replication cluster offers eventual consistency by default. A read executed on an asynchronous slave may return current, stale or no data at all, depending on whether the slave has replayed all changesets from the master or not.

Applications using an MySQL replication cluster need to be designed to work correctly with eventual consistent data. In some cases, however, stale data is not acceptable. In those cases only certain slaves or even only master accesses are allowed to achieve the required quality of service from the cluster.

As of PECL mysqlnd_ms 1.2.0 the plugin is capable of selecting MySQL replication nodes automatically that deliver session consistency or strong consistency. Session consistency means that one client can read its writes. Other clients may or may not see the clients' write. Strong consistency means that all clients will see all writes from the client.

{
    "myapp": {
        "master": {
            "master_0": {
                "host": "localhost",
                "socket": "\/tmp\/mysql.sock"
            }
        },
        "slave": {
            "slave_0": {
                "host": "127.0.0.1",
                "port": "3306"
            }
        }
    }
}

    

<?php
$mysqli = new mysqli("myapp", "username", "password", "database");
if (!$mysqli)
  /* Of course, your error handling is nicer... */
  die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));

/* read-write splitting: master used */
if (!$mysqli->query("INSERT INTO orders(order_id, item) VALUES (1, 'christmas tree, 1.8m')")) {
   /* Please use better error handling in your code */
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
}

/* Request session consistency: read your writes */
if (!mysqlnd_ms_set_qos($mysqli, MYSQLND_MS_QOS_CONSISTENCY_SESSION))
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));

/* Plugin picks a node which has the changes, here: master */
if (!$res = $mysqli->query("SELECT item FROM orders WHERE order_id = 1"))
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));

var_dump($res->fetch_assoc());

/* Back to eventual consistency: stale data allowed */
if (!mysqlnd_ms_set_qos($mysqli, MYSQLND_MS_QOS_CONSISTENCY_EVENTUAL))
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));

/* Plugin picks any slave, stale data is allowed */
if (!$res = $mysqli->query("SELECT item, price FROM specials"))
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
?>

    

Service levels can be set in the plugins configuration file and at runtime using mysqlnd_ms_set_qos. In the example the function is used to enforce session consistency (read your writes) for all future statements until further notice. The SELECT statement on the orders table is run on the master to ensure the previous write can be seen by the client. Read-write splitting logic has been adapted to fulfill the service level.

After the application has read its changes from the orders table it returns to the default service level, which is eventual consistency. Eventual consistency puts no restrictions on choosing a node for statement execution. Thus, the SELECT statement on the specials table is executed on a slave.

The new functionality supersedes the use of SQL hints and the master_on_write configuration option. In many cases mysqlnd_ms_set_qos is easier to use, more powerful improves portability.

{
    "myapp": {
        "master": {
            "master_0": {
                "host": "localhost",
                "socket": "\/tmp\/mysql.sock"
            }
        },
        "slave": {
            "slave_0": {
                "host": "127.0.0.1",
                "port": "3306"
            }
        },
        "failover" : "master"
    }
}

    

<?php
$mysqli = new mysqli("myapp", "username", "password", "database");
if (!$mysqli)
  /* Of course, your error handling is nicer... */
  die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));

/* Read from slaves lagging no more than four seconds */
$ret = mysqlnd_ms_set_qos($mysqli,
         MYSQLND_MS_QOS_CONSISTENCY_EVENTUAL,
         MYSQLND_MS_QOS_OPTION_AGE, 4);

if (!$ret)
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));

/* Plugin picks any slave, which may or may not have the changes */
if (!$res = $mysqli->query("SELECT item, price FROM daytrade"))
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));


/* Back to default: use of all slaves and masters permitted */
if (!mysqlnd_ms_set_qos($mysqli, MYSQLND_MS_QOS_CONSISTENCY_EVENTUAL))
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
?>

    

The eventual consistency service level can be used with an optional parameter to set a maximum slave lag for choosing slaves. If set, the plugin checks SHOW SLAVE STATUS for all configured slaves. In case of the example, only slaves for which Slave_IO_Running=Yes, Slave_SQL_Running=Yes and Seconds_Behind_Master <= 4 is true are considered for executing the statement SELECT item, price FROM daytrade.

Checking SHOW SLAVE STATUS is done transparently from an applications perspective. Errors, if any, are reported as warnings. No error will be set on the connection handle. Even if all SHOW SLAVE STATUS SQL statements executed by the plugin fail, the execution of the users statement is not stopped, given that master fail over is enabled. Thus, no application changes are required.

To prevent mysqlnd_ms from emitting a warning if no slaves can be found that lag no more than the defined number of seconds behind the master, it is necessary to enable master fail over in the plugins configuration file. If no slaves can be found and fail over is turned on, the plugin picks a master for executing the statement.

If no slave can be found and fail over is turned off, the plugin emits a warning, it does not execute the statement and it sets an error on the connection.

{
    "myapp": {
        "master": {
            "master_0": {
                "host": "localhost",
                "socket": "\/tmp\/mysql.sock"
            }
        },
        "slave": {
            "slave_0": {
                "host": "127.0.0.1",
                "port": "3306"
            }
        }
    }
}

    

<?php
$mysqli = new mysqli("myapp", "username", "password", "database");
if (!$mysqli)
  /* Of course, your error handling is nicer... */
  die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));

/* Read from slaves lagging no more than four seconds */
$ret = mysqlnd_ms_set_qos($mysqli,
         MYSQLND_MS_QOS_CONSISTENCY_EVENTUAL,
         MYSQLND_MS_QOS_OPTION_AGE, 4);

if (!$ret)
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));

/* Plugin picks any slave, which may or may not have the changes */
if (!$res = $mysqli->query("SELECT item, price FROM daytrade"))
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));


/* Back to default: use of all slaves and masters permitted */
if (!mysqlnd_ms_set_qos($mysqli, MYSQLND_MS_QOS_CONSISTENCY_EVENTUAL))
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
?>

    

PHP Warning:  mysqli::query(): (mysqlnd_ms) Couldn't find the appropriate slave connection. 0 slaves to choose from. Something is wrong in %s on line %d
PHP Warning:  mysqli::query(): (mysqlnd_ms) No connection selected by the last filter in %s on line %d
[2000] (mysqlnd_ms) No connection selected by the last filter

    

Copyright 1997-2012 the PHP Documentation Group.

PECL/mysqlnd_ms can either use its own global transaction ID emulation or the global transaction ID feature built-in to MySQL 5.6.5-m8 or later. From a developer perspective the client-side and server-side approach offer the same features with regards to service levels provided by PECL/mysqlnd_ms. Their differences are discussed in the concepts section.

The quickstart first demonstrates the use of the client-side global transaction ID emulation built-in to PECL/mysqlnd_ms before its show how to use the server-side counterpart. The order ensures that the underlying idea is discussed first.

Idea and client-side emulation

In its most basic form a global transaction ID (GTID) is a counter in a table on the master. The counter is incremented whenever a transaction is committed on the master. Slaves replicate the table. The counter serves two purposes. In case of a master failure, it helps the database administrator to identify the most recent slave for promoting it to the new master. The most recent slave is the one with the highest counter value. Applications can use the global transaction ID to search for slaves which have replicated a certain write (identified by a global transaction ID) already.

PECL/mysqlnd_ms can inject SQL for every committed transaction to increment a GTID counter. The so created GTID is accessible by the application to identify an applications write operation. This enables the plugin to deliver session consistency (read your writes) service level by not only querying masters but also slaves which have replicated the change already. Read load is taken away from the master.

Client-side global transaction ID emulation has some limitations. Please, read the concepts section carefully to fully understand the principles and ideas behind it, before using in production environments. The background knowledge is not required to continue with the quickstart.

First, create a counter table on your master server and insert a record into it. The plugin does not assist creating the table. Database administrators must make sure it exists. Depending on the error reporting mode, the plugin will silently ignore the lack of the table or bail out.

CREATE TABLE `trx` (
  `trx_id` int(11) DEFAULT NULL,
  `last_update` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
) ENGINE=InnoDB DEFAULT CHARSET=latin1
INSERT INTO `trx`(`trx_id`) VALUES (1);

    

In the plugins configuration file set the SQL to update the global transaction ID table using on_commit from the global_transaction_id_injection section. Make sure the table name used for the UPDATE statement is fully qualified. In the example, test.trx is used to refer to table trx in the schema (database) test. Use the table that was created in the previous step. It is important to set the fully qualified table name because the connection on which the injection is done may use a different default database. Make sure the user that opens the connection is allowed to execute the UPDATE.

Enable reporting of errors that may occur when mysqlnd_ms does global transaction ID injection.

{
    "myapp": {
        "master": {
            "master_0": {
                "host": "localhost",
                "socket": "\/tmp\/mysql.sock"
            }
        },
        "slave": {
            "slave_0": {
                "host": "127.0.0.1",
                "port": "3306"
            }
        },
        "global_transaction_id_injection":{
            "on_commit":"UPDATE test.trx SET trx_id = trx_id + 1",
            "report_error":true
        }
    }
}

    

<?php
$mysqli = new mysqli("myapp", "username", "password", "database");
if (!$mysqli)
  /* Of course, your error handling is nicer... */
  die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));

/* auto commit mode, transaction on master, GTID must be incremented */
if (!$mysqli->query("DROP TABLE IF EXISTS test"))
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));

/* auto commit mode, transaction on master, GTID must be incremented */
if (!$mysqli->query("CREATE TABLE test(id INT)"))
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));

/* auto commit mode, transaction on master, GTID must be incremented */
if (!$mysqli->query("INSERT INTO test(id) VALUES (1)"))
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));

/* auto commit mode, read on slave, no increment */
if (!($res = $mysqli->query("SELECT id FROM test")))
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));

var_dump($res->fetch_assoc());
?>

    

array(1) {
  ["id"]=>
  string(1) "1"
}

    

The example runs three statements in auto commit mode on the master, causing three transactions on the master. For every such statement, the plugin will inject the configured UPDATE transparently before executing the users SQL statement. When the script ends the global transaction ID counter on the master has been incremented by three.

The fourth SQL statement executed in the example, a SELECT, does not trigger an increment. Only transactions (writes) executed on a master shall increment the GTID counter.

{
    "myapp": {
        "master": {
            "master_0": {
                "host": "localhost",
                "socket": "\/tmp\/mysql.sock"
            }
        },
        "slave": {
            "slave_0": {
                "host": "127.0.0.1",
                "port": "3306"
            }
        },
        "global_transaction_id_injection":{
            "on_commit":"UPDATE test.trx SET trx_id = trx_id + 1",
            "fetch_last_gtid" : "SELECT MAX(trx_id) FROM test.trx",
            "report_error":true
        }
    }
}

    

<?php
$mysqli = new mysqli("myapp", "username", "password", "database");
if (!$mysqli)
  /* Of course, your error handling is nicer... */
  die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));

/* auto commit mode, transaction on master, GTID must be incremented */
if (!$mysqli->query("DROP TABLE IF EXISTS test"))
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));

printf("GTID after transaction %s\n", mysqlnd_ms_get_last_gtid($mysqli));

/* auto commit mode, transaction on master, GTID must be incremented */
if (!$mysqli->query("CREATE TABLE test(id INT)"))
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));

printf("GTID after transaction %s\n", mysqlnd_ms_get_last_gtid($mysqli));
?>

    

GTID after transaction 7
GTID after transaction 8

    

Applications can ask PECL mysqlnd_ms for a global transaction ID which belongs to the last write operation performed by the application. The function mysqlnd_ms_get_last_gtid returns the GTID obtained when executing the SQL statement from the fetch_last_gtid entry of the global_transaction_id_injection section from the plugins configuration file. The function may be called after the GTID has been incremented.

Applications are adviced not to run the SQL statement themselves as this bares the risk of accidently causing an implicit GTID increment. Also, if the function is used, it is easy to migrate an application from one SQL statement for fetching a transaction ID to another, for example, if any MySQL server ever features built-in global transaction ID support.

The quickstart shows a SQL statement which will return a GTID equal or greater to that created for the previous statement. It is exactly the GTID created for the previous statement if no other clients have incremented the GTID in the time span between the statement execution and the SELECT to fetch the GTID. Otherwise, it is greater.

{
    "myapp": {
        "master": {
            "master_0": {
                "host": "localhost",
                "socket": "\/tmp\/mysql.sock"
            }
        },
        "slave": {
            "slave_0": {
                "host": "127.0.0.1",
                "port": "3306"
            }
        },
        "global_transaction_id_injection":{
            "on_commit":"UPDATE test.trx SET trx_id = trx_id + 1",
            "fetch_last_gtid" : "SELECT MAX(trx_id) FROM test.trx",
            "check_for_gtid" : "SELECT trx_id FROM test.trx WHERE trx_id >= #GTID",
            "report_error":true
        }
    }
}

    

<?php
$mysqli = new mysqli("myapp", "username", "password", "database");
if (!$mysqli)
  /* Of course, your error handling is nicer... */
  die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));

/* auto commit mode, transaction on master, GTID must be incremented */
if (!$mysqli->query("DROP TABLE IF EXISTS test") ||
    !$mysqli->query("CREATE TABLE test(id INT)") ||
    !$mysqli->query("INSERT INTO test(id) VALUES (1)"))
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));

/* GTID as an identifier for the last write */
$gtid = mysqlnd_ms_get_last_gtid($mysqli);

/* Session consistency (read your writes): try to read from slaves not only master */
if (false == mysqlnd_ms_set_qos($mysqli, MYSQLND_MS_QOS_CONSISTENCY_SESSION, MYSQLND_MS_QOS_OPTION_GTID, $gtid)) {
    die(sprintf("[006] [%d] %s\n", $mysqli->errno, $mysqli->error));
}

/* Either run on master or a slave which has replicated the INSERT */
if (!($res = $mysqli->query("SELECT id FROM test"))) {
    die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
}

var_dump($res->fetch_assoc());
?>

    

A GTID returned from mysqlnd_ms_get_last_gtid can be used as an option for the session consistency service level. Session consistency delivers read your writes. Session consistency can be requested by calling mysqlnd_ms_set_qos. In the example, the plugin will execute the SELECT statement either on the master or on a slave which has replicated the previous INSERT already.

PECL mysqlnd_ms will transparently check every configured slave if it has replicated the INSERT by checking the slaves GTID table. The check is done running the SQL set with the check_for_gtid option from the global_transaction_id_injection section of the plugins configuration file. Please note, that this is a slow and expensive procedure. Applications should try to use it sparsely and only if read load on the master becomes to high otherwise.

Use of the server-side global transaction ID feature

Starting with MySQL 5.6.5-m8 the MySQL Replication system features server-side global transaction IDs. Transaction identifiers are automatically generated and maintained by the server. Users do not need to take care of maintaining them. There is no need to setup any tables in advance, or for setting on_commit. A client-side emulation is no longer needed.

Clients can continue to use global transaction identifier to achieve session consistency when reading from MySQL Replication slaves. The algorithm works as described above. Different SQL statements must be configured for fetch_last_gtid and check_for_gtid. The statements are given below. Please note, MySQL 5.6.5-m8 is a development version. Details of the server implementation may change in the future and require adoption of the SQL statements shown.

Using the following configuration any of the above described functionality can be used together with the server-side global transaction ID feature. mysqlnd_ms_get_last_gtid and mysqlnd_ms_set_qos continue to work as described above. The only difference is that the server does not use a simple sequence number but a string containing of a server identifier and a sequence number. Thus, users cannot easily derive an order from GTIDs returned by mysqlnd_ms_get_last_gtid.

{
    "myapp": {
        "master": {
            "master_0": {
                "host": "localhost",
                "socket": "\/tmp\/mysql.sock"
            }
        },
        "slave": {
            "slave_0": {
                "host": "127.0.0.1",
                "port": "3306"
            }
        },
        "global_transaction_id_injection":{
            "fetch_last_gtid" : "SELECT @@GLOBAL.GTID_DONE AS trx_id FROM DUAL",
            "check_for_gtid" : "SELECT GTID_SUBSET('#GTID', @@GLOBAL.GTID_DONE) AS trx_id FROM DUAL",
            "report_error":true
        }
    }
}

    

Copyright 1997-2012 the PHP Documentation Group.

Databases clusters can deliver different levels of consistency. As of PECL/mysqlnd_ms 1.2.0 it is possible to advice the plugin to consider only cluster nodes that can deliver the consistency level requested. For example, if using asynchronous MySQL Replication with its cluster-wide eventual consistency, it is possible to request session consistency (read your writes) at any time using mysqlnd_ms_set_quos. Please, see also the service level and consistency introduction.

/* Request session consistency: read your writes */
if (!mysqlnd_ms_set_qos($mysqli, MYSQLND_MS_QOS_CONSISTENCY_SESSION))
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
     

Assuming PECL/mysqlnd has been explicitly told to deliver no consistency level higher than eventual consistency, it is possible to replace a database node read access with a client-side cache using time-to-live (TTL) as its invalidation strategy. Both the database node and the cache may or may not serve current data as this is what eventual consistency defines.

Replacing a database node read access with a local cache access can improve overall performance and lower the database load. If the cache entry is every reused by other clients than the one creating the cache entry, a database access is saved and thus database load is lowered. Furthermore, system performance can become better if computation and delivery of a database query is slower than a local cache access.

{
    "myapp": {
        "master": {
            "master_0": {
                "host": "localhost",
                "socket": "\/tmp\/mysql.sock"
            }
        },
        "slave": {
            "slave_0": {
                "host": "127.0.0.1",
                "port": "3306"
            }
        },
    }
}

    

<?php
$mysqli = new mysqli("myapp", "username", "password", "database");
if (!$mysqli)
  /* Of course, your error handling is nicer... */
  die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));

if (!$mysqli->query("DROP TABLE IF EXISTS test") ||
    !$mysqli->query("CREATE TABLE test(id INT)") ||
    !$mysqli->query("INSERT INTO test(id) VALUES (1)"))
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));

/* Explicitly allow eventual consistency and caching (TTL <= 60 seconds) */
if (false == mysqlnd_ms_set_qos($mysqli, MYSQLND_MS_QOS_CONSISTENCY_EVENTUAL, MYSQLND_MS_QOS_OPTION_CACHE, 60)) {
    die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
}

/* To make this example work, we must wait for a slave to catch up. Brute force style. */
$attempts = 0;
do {
  /* check if slave has the table */
  if ($res = $mysqli->query("SELECT id FROM test")) {
    break;
  } else if ($mysqli->errno) {
    die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
  }
  /* wait for slave to catch up */
  usleep(200000);
} while ($attempts++ < 10);

/* Query has been run on a slave, result is in the cache */
assert($res);
var_dump($res->fetch_assoc());

/* Served from cache */
$res = $mysqli->query("SELECT id FROM test");
?>

    

The example shows how to use the cache feature. First, you have to set the quality of service to eventual consistency and explicitly allow for caching. This is done by calling mysqlnd_ms_set_qos. Then, the result set of every read-only statement is cached for upto that many seconds as allowed with mysqlnd_ms_set_qos.

The actual TTL is lower or equal to the value set with mysqlnd_ms_set_qos. The value passed to the function sets the maximum age (seconds) of the data delivered. To calculate the actual TTL value the replication lag on a slave is checked and subtracted from the given value. If, for example, the maximum age is set to 60 seconds and the slave reports a lag of 10 seconds the resulting TTL is 50 seconds. The TTL is calculated individually for every cached query.

<?php
$mysqli = new mysqli("myapp", "username", "password", "database");
if (!$mysqli)
  /* Of course, your error handling is nicer... */
  die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));

if (!$mysqli->query("DROP TABLE IF EXISTS test") ||
    !$mysqli->query("CREATE TABLE test(id INT)") ||
    !$mysqli->query("INSERT INTO test(id) VALUES (1)"))
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));

/* Explicitly allow eventual consistency and caching (TTL <= 60 seconds) */
if (false == mysqlnd_ms_set_qos($mysqli, MYSQLND_MS_QOS_CONSISTENCY_EVENTUAL, MYSQLND_MS_QOS_OPTION_CACHE, 60)) {
    die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
}

/* To make this example work, we must wait for a slave to catch up. Brute force style. */
$attempts = 0;
do {
  /* check if slave has the table */
  if ($res = $mysqli->query("SELECT id FROM test")) {
    break;
  } else if ($mysqli->errno) {
    die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
  }
  /* wait for slave to catch up */
  usleep(200000);
} while ($attempts++ < 10);

assert($res);

/* Query has been run on a slave, result is in the cache */
var_dump($res->fetch_assoc());

/* Served from cache */
if (!($res = $mysqli->query("SELECT id FROM test")))
 die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
var_dump($res->fetch_assoc());

/* Update on master */
if (!$mysqli->query("UPDATE test SET id = 2"))
 die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));

/* Read your writes */
if (false == mysqlnd_ms_set_qos($mysqli, MYSQLND_MS_QOS_CONSISTENCY_SESSION)) {
    die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
}

/* Fetch latest data */
if (!($res = $mysqli->query("SELECT id FROM test")))
 die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
var_dump($res->fetch_assoc());
?>

    

The quality of service can be changed at any time to avoid further cache usage. If needed, you can switch to read your writes (session consistency). In that case, the cache will not be used and fresh data is read.

Copyright 1997-2012 the PHP Documentation Group.

By default, the plugin does not attempt to fail over if connecting to a host fails. This prevents pitfalls related to connection state. It is recommended to manually handle connection errors in a way similar to a failed transaction. You should catch the error, rebuild the connection state and rerun your query as shown below.

If connection state is no issue to you, you can alternatively enable automatic and silent failover. Depending on the configuration, the automatic and silent failover will either attempt to fail over to the master before issuing and error or, try to connect to other slaves, given the query allowes for it, before attempting to connect to a master. Because automatic failover is not fool-proof, it is not discussed in the quickstart. Instead, details are given in the concepts section below.

  {
    "myapp": {
        "master": {
            "master_0": {
                "host": "localhost",
                "socket": "\/tmp\/mysql.sock"
            }
        },
        "slave": {
            "slave_0": {
                "host": "simulate_slave_failure",
                "port": "0"
            },
            "slave_1": {
                "host": "127.0.0.1",
                "port": 3311
            }
        },
       "filters": { "roundrobin": [] }
    }
 }

    

<?php
$mysqli = new mysqli("myapp", "username", "password", "database");
if (!$mysqli)
  /* Of course, your error handling is nicer... */
  die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));

$sql = "SELECT 1 FROM DUAL";

/* error handling as it should be done regardless of the plugin */
if (!($res = $link->query($sql))) {
  /* plugin specific: check for connection error */
  switch ($link->errno) {
    case 2002:
    case 2003:
    case 2005:
      printf("Connection error - trying next slave!\n");
      /* load balancer will pick next slave */
      $res = $link->query($sql);
      break;
    default:
      /* no connection error, failover is unlikely to help */
      die(sprintf("SQL error: [%d] %s", $link->errno, $link->error));
      break;
  }
}
if ($res) {
  var_dump($res->fetch_assoc());
}
?>

    

Copyright 1997-2012 the PHP Documentation Group.

Database clustering is done for various reasons. Clusters can improve availability, fault tolerance, and increase performance by applying a divide and conquer approach as work is distributed over many machines. Clustering is sometimes combined with partitioning and sharding to further break up a large complex task into smaller, more manageable units.

The mysqlnd_ms plugin aims to support a wide variety of MySQL database clusters. Some flavors of MySQL database clusters have built-in methods for partitioning and sharding, which could be transparent to use. The plugin supports the two most common approaches: MySQL Replication table filtering, and Sharding (application based partitioning).

MySQL Replication supports partitioning as filters that allow you to create slaves that replicate all or specific databases of the master, or tables. It is then in the responsibility of the application to choose a slave according to the filter rules. You can either use the mysqlnd_ms node_groups filter to manually support this, or use the experimental table filter.

Manual partitioning or sharding is supported through the node grouping filter, and SQL hints as of 1.5.0. The node_groups filter lets you assign a symbolic name to a group of master and slave servers. In the example, the master master_0 and slave_0 form a group with the name Partition_A. It is entirely up to you to decide what makes up a group. For example, you may use node groups for sharding, and use the group names to address shards like Shard_A_Range_0_100.

 {
  "myapp": {
       "master": {
            "master_0": {
                "host": "localhost",
                "socket": "\/tmp\/mysql.sock"
            }
        },
        "slave": {
            "slave_0": {
                "host": "simulate_slave_failure",
                "port": "0"
            },
            "slave_1": {
                "host": "127.0.0.1",
                "port": 3311
            }
        },
        "filters": {
            "node_groups": {
                "Partition_A" : {
                    "master": ["master_0"],
                    "slave": ["slave_0"]
                }
            },
           "roundrobin": []
        }
    }
}

    

<?php
function select($mysqli, $msg, $hint = '') {
  /* Note: weak test, two connections to two servers may have the same thread id */
  $sql = sprintf("SELECT CONNECTION_ID() AS _thread, '%s' AS _hint FROM DUAL", $msg);
  if ($hint) {
    $sql = $hint . $sql;
  }
  if (!($res = $mysqli->query($sql))) {
    printf("[%d] %s", $mysqli->errno, $mysqli->error);
    return false;
  }
  $row =  $res->fetch_assoc();
  printf("%d - %s - %s\n", $row['_thread'], $row['_hint'], $sql);
  return true;
}

$mysqli = new mysqli("myapp", "user", "password", "database");
if (!$mysqli)
  /* Of course, your error handling is nicer... */
  die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));

/* All slaves allowed */
select($mysqli, "slave_0");
select($mysqli, "slave_1");

/* only servers of node group "Partition_A" allowed */
select($mysqli, "slave_1", "/*Partition_A*/");
select($mysqli, "slave_1", "/*Partition_A*/");
?>

    

6804 - slave_0 - SELECT CONNECTION_ID() AS _thread, 'slave1' AS _hint FROM DUAL
2442 - slave_1 - SELECT CONNECTION_ID() AS _thread, 'slave2' AS _hint FROM DUAL
6804 - slave_0 - /*Partition_A*/SELECT CONNECTION_ID() AS _thread, 'slave1' AS _hint FROM DUAL
6804 - slave_0 - /*Partition_A*/SELECT CONNECTION_ID() AS _thread, 'slave1' AS _hint FROM DUAL

    

By default, the plugin will use all configured master and slave servers for query execution. But if a query begins with a SQL hint like /*node_group*/, the plugin will only consider the servers listed in the node_group for query execution. Thus, SELECT queries prefixed with /*Partition_A*/ will only be executed on slave_0.