• Connecting
  • Connection Pooling (version 1.2.0-1.2.12

  • Connection Pooling (version 1.2.0-1.2.12 *only*)
  • Connection Pooling (version 1.2.0-1.2.12

    Connection Pooling (version 1.2.0-1.2.12


    This section is no longer relevant as of the 1.3.0
    release of the driver and only serves as a historical information
    on how the pooling used to work.

    The latest versions of the driver have no concept
    of pooling anymore and will maintain only one connection per
    process, for each connection type (ReplicaSet/standalone/mongos),
    for each credentials combination.

    Creating connections is one of the most heavyweight
    things that the driver does. It can take hundreds of milliseconds
    to set up a connection correctly, even on a fast network. Thus, the
    driver tries to minimize the number of new connections created by
    reusing connections from a pool.

    When a user creates a new instance of MongoClient, all
    necessary connections will be taken from their pools (replica sets
    may require multiple connections, one for each member of the set).
    When the MongoClient instance goes out of scope, the
    connections will be returned to the pool. When the PHP process
    exits, all connections in the pools will be closed.

    “Why do I have so many open connections?”

    Connection pools can generate a large number of
    connections. This is expected and, using a little arithmetic, you
    can figure out how many connections will be created. There are
    three factors in determining the total number of connections:

    • connections_per_pool

      Each connection pool will create, by default, an
      unlimited number of connections. One might assume that this is a
      problem: if it can create an unlimited number of connections,
      couldn’t it create thousands and the server would run out of file
      descriptors? In practice, this is unlikely, as unused connections
      are returned to the pool to be used later, so future connections
      will use the same connection instead of creating a new one. Unless
      you create thousands of connections at once without letting any go
      out of scope, the number of connections open should stay at a
      reasonable number.

      You can see how many connections you have in a pool
      using the MongoPool::info() function. Add up the
      “in use” and “in pool” fields for a given server. That is the total
      number of connections for that pool.

    • pools_per_process

      Each MongoDB server address you’re connecting to
      gets its own connection pool. For example, if your local hostname
      is “”, connecting to “”,
      “localhost:27017”, and “/tmp/mongodb-27017.sock” will create three
      connection pools. You can see how many connection pools you have
      open using MongoPool::info().

    • processes

      Each PHP process has a separate set of pools.
      PHP-FPM and Apache generally create between 6 and a couple of dozen
      PHP worker children. Check your settings to see what the max number
      of PHP processes is that can be spawned.

      If you are using PHP-FPM, estimating the number of
      connections can be tricky because it will spawn more PHP-FPM
      workers under heavy load. To be on the safe side, look at the
      max_children parameter or add up spare_servers +
      start_servers (choose whichever number is higher). That
      will indicate how many PHP processes (i.e. sets of pools) you
      should plan for.

    The three variables above can be multiplied
    together to give the max number of connections expected:
    connections_per_pool * pools_per_process *
    processes. Note that connections_per_pool can be
    different for different pools, so connections_per_pool
    should be the max.

    For example, suppose we’re getting 30 connections
    per pool, 10 pools per PHP process, and 128 PHP processes. Then we
    can expect 38400 connections from this machine. Thus, we should set
    this machine’s file descriptor limit to be high enough to handle
    all of these connections or it may run out of file descriptors.

    See MongoPool for more information on connection