Persistent Database Connections

What are Persistent Connections?

Persistent connections are links that do not close when the execution of your script ends. When a persistent connection is requested, PHP checks if there's already an identical persistent connection (that remained open from earlier) - and if it exists, it uses it. If it does not exist, it creates the link. An 'identical' connection is a connection that was opened to the same host, with the same username and the same password (where applicable).

There's no method of requesting a specific connection, or guaranteeing whether you get an existing connection or a brand new one (if all existing connections are in use, or the request is being served by a different worker, which has a separate pool of connections).

This means that you cannot use PHP's persistent connections to, for example:

assign a specific database session to a specific web user
create a large transaction across multiple requests
initiate a query on one request and collect the results on another

Persistent connections do not give you any functionality that wasn't possible with non-persistent connections.

Web Requests

There are two ways in which your web server can utilize PHP to generate web pages:

The first method is to use PHP as a CGI "wrapper". When run this way, an instance of the PHP interpreter is created and destroyed for every page request (for a PHP page) to your web server. Because it is destroyed after every request, any resources that it acquires (such as a link to an SQL database server) are closed when it is destroyed. In this case, you do not gain anything from trying to use persistent connections - they simply don't persist.

The second, and most popular, method is to run PHP-FPM, or PHP as a module in a multiprocess web server, which currently only includes Apache. These setups typically have one process (the parent) which coordinates a set of processes (its children) who actually do the work of serving up web pages. When a request comes in from a client, it is handed off to one of the children that is not already serving another client. This means that when the same client makes a second request to the server, it may be served by a different child process than the first time. When opening a persistent connection, every following page requesting SQL services can reuse the same established connection to the SQL server.

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You can check which method your web requests use by checking the value of "Server API" in the output of phpinfo() or the value of PHP_SAPI, run from a web request.

If the Server API is "Apache 2 Handler" or "FPM/FastCGI", then persistent connections will be used across requests served by the same worker. For any other value, persistent connections will not persist after each request.

Command-line Processes

As command-line PHP uses a new process for each script, persistent connections are not shared between command-line scripts, so there is no value in using them in transient scripts such as crons or commands. However, they may be useful if, for example, you're writing a long-running application server that serves many requests or tasks and each may need their own database connection.

Why Use Them?

Persistent connections are good if the overhead to create a link to your SQL server is high. Whether or not this overhead is really high depends on many factors. Like, what kind of database it is, whether or not it sits on the same computer on which your web server sits, how loaded the machine the SQL server sits on is and so forth. The bottom line is that if that connection overhead is high, persistent connections help you considerably. They cause the child process to simply connect only once for its entire lifespan, instead of every time it processes a page that requires connecting to the SQL server. This means that for every child that opened a persistent connection will have its own open persistent connection to the server. For example, if you had 20 different child processes that ran a script that made a persistent connection to your SQL server, you'd have 20 different connections to the SQL server, one from each child.

Potential Drawbacks: Connection Limits

Note, however, that this can have some drawbacks if you are using a database with connection limits that are exceeded by persistent child connections. If your database has a limit of 16 simultaneous connections, and in the course of a busy server session, 17 child threads attempt to connect, one will not be able to. If there are bugs in your scripts which do not allow the connections to shut down (such as infinite loops), the database with only 16 connections may be rapidly swamped.

Persistent connections will usually increase the number of connections open at any given time because idle workers will still hold the connections for the previous requests they served. If a large number of workers is spun up to handle an influx of requests, the connections they opened will remain until the worker is killed or the database server closes the connection.

Ensure that the maximum number of connections allowed by the database server is greater than the maximum number of web request workers (plus any other usage such as crons or administrative connections).

Check your database documentation for information on handling abandoned or idle connections (timeouts). Long timeouts may significantly increase the number of persistent connections open at any one time.

Potential Drawbacks: Maintaining Connection State

Some database extensions perform automatic cleanup when the connection is reused; others leave this task at the discretion of the application developer. Depending on the chosen database extension and the application design, manual cleanup may be needed before the script exits. Changes that may leave connections in an unexpected state include:

Selected / default database
Table locks
Uncommitted transactions
Temporary tables
Connection specific settings or features such as profiling

Table locks and transactions that are not cleaned up or closed may cause other queries to be blocked indefinitely and/or cause subsequent reuse of the connection to cause unexpected changes.

Having the wrong database selected will cause subsequent reuse of the connection to be unable to execute queries as expected (or execute them on the wrong database if schemas are similar enough).

If temporary tables are not cleaned up, subsequent requests will not be able to recreate the same table.

You can implement cleanup using class destructors or register_shutdown_function(). You may also want to consider dedicated connection pooling proxies that include this as part of their functionality.

Final Words

Given their behavior and potential drawbacks described above, you should not use persistent connections without careful consideration. They should not be used without implementing additional changes to your application and careful configuration of your database server and web server and/or PHP-FPM.

Consider alternative solutions such as investigating and fixing the causes of connection creation overheads (for example, disabling reverse DNS lookups on the database server), or dedicated connection pooling proxies.

For high volume web APIs, consider using alternative runtimes or long-running application servers.