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MyPDOMS

MyPDOMS is intended to be a drop-in replacement for most common tasks performed with mysqlnd_ms.

Requires PHP 5.6 or above.

While this is based on PDO, it only supports MySQL.

Table of Contents

• Configuring
• Establishing a Connection
• Differences from PDO
• Query Routing
• SQL Hints
• Slave Selection
• Prepared Statements

Configuring

Before you instantiate an instance of MyPDOMS, you need to configure it. Configuration is accomplished using the static setConfig method, which expects a single parameter of type array. The structure of the expected associative array is:

• [config name] - The name of a configuration. Configurations are collections of servers.
  • master - Database configuration for your master server
    • host - The host where this database is running (required)
    • port - The port on which this database is running (optional; defaults to 3306)
    • username - Database username (optional; defers to constructor argument if missing)
    • password - Database password (optional; defers to constructor argument if missing)
  • slaves - Contains your slave server database configurations
    • [slave name] - Database configuration for a slave server (can be anything except master)
      • host - The host where this database is running (required)
      • port - The port on which this database is running (optional; defaults to 3306)
      • username - Database username (optional; defers to constructor argument if missing)
      • password - Database password (optional; defers to constructor argument if missing)

So, for example, you might want to do this:

You don't need to supply any slave configuration. If you don't configure any slaves, then all queries will go to the master.

Establishing a Connection

Slave connections are lazily-established, but a master connection is established when you construct a new MyPDOMS instance. The constructor is identical to the standard PDO constructor but with these caveats:

• The host in the DSN should be the name of one of your configurations (in the above example, my_site_1 or my_site_2)
• The port in the DSN is ignored if supplied
• If you supply database credentials ($username and $passwd) in both the constructor and in your config (setConfig), the constructor wins
  • It's recommended you supply your credentials in setConfig, to prevent any possible credential leakage e.g. in stack traces
• Any connection $options you supply will be used for establishing connections to the master and to all slave connections

Here's an example:

Differences from PDO

MyPDOMS is a subclass of PDO so PDO's documentation also applies to MyPDOMS with these core differences:

• The differences noted above in the Establishing a Connection section
• The lastUsedHost property contains the name of the last host that was sent a query (e.g. master or slave_1)
• These methods will always be sent to the master connection:
  • beginTransaction
  • commit
  • rollBack
  • inTransaction
  • lastInsertId
  • quote
    • Although quote does not result in any network I/O, it's always called on the master connection
• These methods will be sent to the connection named by lastUsedHost:
  • errorCode
  • errorInfo
• getAvailableDrivers will always return ['mysql']
• Calling setAttribute will result in this sequence of events:
  • The attribute and value you passed in will be stored internally in the MyPDOMS object
  • The attribute will be set on the master connection
  • The attribute will be set on any established slave connections
  • When a new slave connection is established, all previously-set attributes will be immediately set on it
  • Returns true iff all connections returned true when setAttribute was called on them
• Calling getAttribute will return the value from the internal cache, not from a PDO connection object
• prepare, query, and exec will route to a connection based on the criteria noted in Query Routing
• You can set the property alwaysUseMaster to true to always use the master connection, despite wherever the query would ordinarily be routed

Query Routing

Queries will be routed to either the master or to a slave depending on this sequence of checks:

1. Leading comments in the SQL will be checked for SQL Hints
   1. If HINT_MASTER is found then the query will be sent to the master
   2. If HINT_SLAVE is found then the query will be sent to a slave
   3. If HINT_LAST_USED is found then the query will be sent to the last used connection
2. If the first SQL-word is one of INSERT, UPDATE, DELETE, REPLACE, LOAD, ALTER, CREATE, DROP, RENAME, or TRUNCATE then the query will be sent to the master
3. If the first SQL-word is SELECT and the query ends in FOR UPDATE then the query will be sent to the master
4. If none of the above match, then the query will be sent to a slave

Note: The routing logic does not check to see if a transaction is open, because all queries that can result in updates or locks are already automatically routed to the master.

SQL Hints

SQL hints can be used to override the default query routing logic. These SQL hints are available, and should be prepended to queries in a comment:

• MyPDOMS::HINT_MASTER - Send this query to the master
• MyPDOMS::HINT_SLAVE - Send this query to a slave
• MyPDOMS::HINT_LAST_USED - Send this query to the server last used
  • This may be the master, if the master was last used
  • If the last used server was a slave, then the query will be sent to that slave

Example:

If you want to replace mysqlnd_ms and not go back and update all your code, you can use this snippet:

Slave Selection

When a query is due to be routed to a slave, a slave is selected per-query. That is, slaves are not selected per-request but are selected every time a query is executed. Presently, the only supported selection mechanism is unweighted random, in which every query will be sent to a random slave, with all slaves having an equal probability of being chosen.

Example:

Different selection algorithms are expected to be added in later releases, but if you wish to define your own selection algorithm, you may extend MyPDOMS and override the getSlave method.

Prepared Statements

Both emulated and non-emulated prepared statements are fully supported, as they are assigned a connection at prepare-time. That is, after a PDOStatement is returned from prepare(), the statement will always use the same database each time it is executed.

Note: lastUsedHost is updated when a statement is prepared, but not when it is executed. This means that the following is possible: