OCI8 Connection Handling and Connection Pooling

Connection Functions

The OCI8 extension provides three different functions for connecting to Oracle. The standard connection function is oci_connect(). This creates a connection to an Oracle database and returns a resource used by subsequent database calls.

Connecting to an Oracle server is a reasonably expensive operation in terms of the time that it takes to complete. The oci_pconnect() function uses a persistent cache of connections that can be re-used across different script requests. This means that the connection overhead will typically only occur once per PHP process (or Apache child).

If the application connects to Oracle using a different set of credentials for each web user, the persistent cache employed by oci_pconnect() will become less useful as the number of concurrent users increases, to the point where it may start to adversely affect the overall performance of the Oracle server due to maintaining too many idle connections. If the application is structured in this way, it is recommended to either tune the application using the oci8.max_persistent and oci8.persistent_timeout configuration settings (these will give control over the persistent connection cache size and lifetime), use Oracle Database Resident Connection Pooling (in Oracle Database 11g or later), or use oci_connect() instead.

Both oci_connect() and oci_pconnect() employ a connection cache; if multiple calls to oci_connect() use the same parameters in a given script, the second and subsequent calls return the existing connection handle. The cache used by oci_connect() is cleaned up at the end of the script run, or when the connection handle is explicitly closed. The function oci_pconnect() has similar behavior, although its cache is maintained separately and survives between HTTP requests.

This caching feature means the two handles are not transactionally isolated (they are in fact the same connection handle, so there is no isolation of any kind). If the application needs two separate, transactionally isolated connections, then use oci_new_connect().

The oci_pconnect() cache is cleared and any database connections closed when the PHP process terminates, so effective use of persistent connections requires that PHP be an Apache module or used with FCGI, or similar. Persistent connections will not have any benefits over oci_connect() when PHP is used with CGI or via the command-line.

The function oci_new_connect() always creates a new connection to the Oracle server, regardless of what other connections might already exist. High traffic web applications should avoid using oci_new_connect(), especially in the busiest sections of the application.

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DRCP Connection Pooling

PHP from 5.3 (PECL OCI8 1.3) supports Oracle Database Resident Connection Pooling (DRCP). DRCP allows more efficient use of database machine memory and provides high scalability. No, or minimal, application changes are needed to use DRCP.

DRCP is suited for applications that connect using few database schemas and hold database connections open for a short period of time. Other applications should use Oracle's default Dedicated database server processes, or use Shared servers.

DRCP benefits all three connection functions, but gives the highest scalability when connections are created with oci_pconnect().

For DRCP to be available in OCI8, Oracle client libraries used by PHP and the version of the Oracle Database must both be 11g or greater.

Documentation on DRCP is found in several Oracle manuals. For example, see » Configuring Database Resident Connection Pooling in the Oracle documentation for usage information. A » DRCP white paper contains background information on DRCP.

To use DRCP, build PHP with the OCI8 1.3 (or later) extension and Oracle 11g (or later) libraries and then follow these steps:

• As a privileged database administrator, use a program like SQL*Plus to start the connection pool in the database:

      SQL> execute dbms_connection_pool.start_pool;
  

• Optionally use dbms_connection_pool.alter_param() to configure DRCP settings. The current pool settings can be queried from the DBA_CPOOL_INFO view.

• Update the connection strings used. For PHP applications that currently connect using a Network Connect Name like MYDB:

      $c = oci_pconnect("myuser", "mypassword", "MYDB");
  

  modify the tnsnames.ora file and add a (SERVER=POOLED) clause, for example:

      MYDB = (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp) (HOST=myhost.dom.com)
             (PORT=1521))(CONNECT_DATA=(SERVICE_NAME=sales)
             (SERVER=POOLED)))
  

  Alternatively, modify the Easy Connect syntax in PHP and add :POOLED after the service name:

      $c = oci_pconnect("myuser", "mypassword", "myhost.dom.com:1521/sales:POOLED");
  

• Edit php.ini and choose a connection class name. This name indicates a logical division of the connection pool and can be used to isolate pooling for separate applications. Any PHP applications with the same user name and connection class value will be able to share connections in the pool, giving greater scalability.

      oci8.connection_class = "MY_APPLICATION_NAME"
  

• Run the application, connecting to the 11g (or later) database.

Hinweis:

Applications using Oracle 10g that require the performance of persistent connections can reduce the amount of database server memory needed by using Oracle Shared servers (previously known as Multi Threaded Servers). Refer to Oracle documentation for information.

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DRCP Recommendations and Known Limitations

Changing a password over DRCP connections will fail with the error ORA-56609: Usage not supported with DRCP. This is a documented restriction of Oracle Database 11g.

From OCI8 1.3, persistent connections can be closed by the user, allowing greater control over connection resource usage. Persistent connections will now also be closed automatically when there is no PHP variable referencing them, such as at the end of scope of a PHP user function. This will rollback any uncommitted transaction. These changes to persistent connections make them behave similarly to non-persistent connections, simplifying the interface, allowing for greater application consistency and predictability. Use oci8.old_oci_close_semantics set to On to retain the historical behavior.

If the Oracle Database is version 11.1.0.6, then the Oracle database patch for Oracle bug 6474441 must be applied to use DRCP. Without this patch, errors such as ORA-01000: maximum open cursors exceeded, ORA-01001 invalid cursor or ORA-01002 fetch out of sequence may occur. This bug was fixed in Oracle 11.1.0.7 onwards.

If the Oracle 11.1.0.6 database patch cannot be applied, one of the following three workarounds can be used instead:

• Connect using Oracle Dedicated or Shared servers instead of DRCP.
• Set PHP's oci8.statement_cache_size to 0.
• Set an event in the database initialization parameter file: event="56699 trace name context forever, level 128".

Oracle Database 11.1.0.7 and the Oracle Database 11.1.0.6 patch for Oracle bug 6474441 allow PHP applications with DRCP connection to use a database LOGON trigger to set session properties at the time of session creation. Examples of such settings are the NLS language and the date format.

If the Oracle 11.1.0.6 database patch cannot be applied, one of the following workarounds can be used instead of using LOGON triggers:

• After logon, explicitly set the session properties using PHP application code.
• Connect using Oracle Dedicated or Shared servers instead of DRCP.

The automatic re-establishment of PHP persistent connections after an Apache or FCGI process respawns means LOGON triggers in PHP are only recommended for setting session attributes and not for per-application user connection requests. This is even more so with DRCP due to the automatic pool sizing and with the way LOGON triggers fire with DRCP authentication.