=================================
Connection Monitoring and Pooling
=================================

:Title: Connection Monitoring and Pooling
:Author: Dan Aprahamian
:Advisory Group: Jeff Yemin, Matt Broadstone
:Approvers: Bernie Hackett, Dan Pasette, Jeff Yemin, Matt Broadstone, Sam Rossi, Scott L'Hommedieu
:Status: Accepted
:Type: Standards
:Minimum Server Version: N/A
:Last Modified: June 11, 2019
:Version: 1.1.0

.. contents::

Abstract
========

Drivers currently support a variety of options that allow users to configure connection pooling behavior. Users are confused by drivers supporting different subsets of these options. Additionally, drivers implement their connection pools differently, making it difficult to design cross-driver pool functionality. By unifying and codifying pooling options and behavior across all drivers, we will increase user comprehension and code base maintainability.

META
====

The keywords “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in `RFC 2119 <https://www.ietf.org/rfc/rfc2119.txt>`_.

Definitions
===========

Endpoint
~~~~~~~~

For convenience, an Endpoint refers to either a **mongod** or **mongos** instance.

Thread
~~~~~~

For convenience, a Thread refers to:

-  A shared-address-space process (a.k.a. a thread) in multi-threaded drivers
-  An Execution Frame / Continuation in asynchronous drivers
-  A goroutine in Go

Behavioral Description
======================

Which Drivers this applies to
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This specification is solely concerned with drivers that implement a connection pool. A driver SHOULD implement a connection pool, but is not required to.

Connection Pool Options
~~~~~~~~~~~~~~~~~~~~~~~

All drivers that implement a connection pool MUST implement and conform to the same MongoClient options. There can be slight deviation in naming to make the options idiomatic to the driver language.

Connection Pool Behaviors
~~~~~~~~~~~~~~~~~~~~~~~~~

All driver connection pools MUST provide an API that allows the driver to check out a connection, check in a connection back to the pool, and clear all connections in the pool. This API is for internal use only, and SHOULD NOT be documented as a public API.

Connection Pool Monitoring
~~~~~~~~~~~~~~~~~~~~~~~~~~

All drivers that implement a connection pool MUST provide an API that allows users to subscribe to events emitted from the pool.

Detailed Design
===============

.. _connection-pool-options-1:

Connection Pool Options
~~~~~~~~~~~~~~~~~~~~~~~

Drivers that implement a Connection Pool MUST support the following ConnectionPoolOptions:

.. code:: typescript

    interface ConnectionPoolOptions {
      /**
       *  The maximum number of connections that may be associated
       *  with a pool at a given time. This includes in use and
       *  available connections.
       *  If specified, MUST be an integer >= 0.
       *  A value of 0 means there is no limit.
       *  Defaults to 100.
       */
      maxPoolSize?: number;

      /**
       *  The minimum number of connections that MUST exist at any moment
       *  in a single connection pool.
       *  If specified, MUST be an integer >= 0. If maxPoolSize is > 0
       *  then minPoolSize must be <= maxPoolSize
       *  Defaults to 0.
       */
      minPoolSize?: number;

      /**
       *  The maximum amount of time a connection should remain idle
       *  in the connection pool before being marked idle.
       *  If specified, MUST be a number >= 0.
       *  A value of 0 means there is no limit.
       *  Defaults to 0.
       */
      maxIdleTimeMS?: number;
    }

Additionally, Drivers that implement a Connection Pool MUST support the following ConnectionPoolOptions UNLESS that driver meets ALL of the following conditions:

-  The driver/language currently has an idiomatic timeout mechanism implemented
-  The timeout mechanism conforms to `the aggressive requirement of timing out a thread in the WaitQueue <#w1dcrm950sbn>`__

.. code:: typescript

    interface ConnectionPoolOptions {
      /**
       *  The maximum amount of time a thread can wait for a connection
       *  to become available.
       *  If specified, MUST be a number >= 0.
       *  A value of 0 means there is no limit.
       *  Defaults to 0.
       */
      waitQueueTimeoutMS?: number;
    }

These options MUST be specified at the MongoClient level, and SHOULD be named in a manner idiomatic to the driver's language. All connection pools created by a MongoClient MUST use the same ConnectionPoolOptions.

When parsing a mongodb connection string, a user MUST be able to specify these options using the default names specified above.

Deprecated Options
------------------

The following ConnectionPoolOptions are considered deprecated. They MUST NOT be implemented if they do not already exist in a driver, and they SHOULD be deprecated and removed from drivers that implement them as early as possible:

.. code:: typescript

    interface ConnectionPoolOptions {
      /**
       *  The maximum number of threads that can simultaneously wait
       *  for a connection to become available.
       */
      waitQueueSize?: number;

      /**
       *  An alternative way of setting waitQueueSize, it specifies
       *  the maximum number of threads that can wait per connection.
       *  waitQueueSize === waitQueueMultiple \* maxPoolSize
       */
      waitQueueMultiple?: number
    }

Connection Pool Members
~~~~~~~~~~~~~~~~~~~~~~~

Connection
----------

A driver-defined wrapper around a single TCP/IP connection to an Endpoint. A Connection has the following properties:

-  **Single Endpoint:** A Connection MUST be associated with a single Endpoint. A Connection MUST NOT be associated with multiple Endpoints.
-  **Single Lifetime:** A Connection MUST NOT be used after it is closed.
-  **Single Owner:** A Connection MUST belong to exactly one Pool, and MUST NOT be shared across multiple pools
-  **Single Track:** A Connection MUST limit itself to one request / response at a time. A Connection MUST NOT multiplex/pipeline requests to an Endpoint.
-  **Monotonically Increasing ID:** A Connection MUST have an ID number associated with it. Connection IDs within a Pool MUST be assigned in order of creation, starting at 1 and increasing by 1 for each new Connection.
-  **Valid Connection:** A connection MUST NOT be checked out of the pool until it has successfully and fully completed a MongoDB Handshake and Authentication as specified in the `Handshake <https://github.com/mongodb/specifications/blob/master/source/mongodb-handshake/handshake.rst>`__, `OP_COMPRESSED <https://github.com/mongodb/specifications/blob/master/source/compression/OP_COMPRESSED.rst>`__, and `Authentication <https://github.com/mongodb/specifications/blob/master/source/auth/auth.rst>`__ specifications.
-  **Perishable**: it is possible for a connection to become **Perished**. A connection is considered perished if any of the following are true:

   -  **Stale:** The connection's generation does not match the generation of the parent pool
   -  **Idle:** The connection is currently available and readyToUse, and has been for longer than **maxIdleTimeMS**.
   -  **Errored:** The connection has experienced an error that indicates the connection is no longer recommended for use. Examples include, but are not limited to:

      -  Network Error
      -  Network Timeout
      -  Endpoint closing the connection
      -  Driver-Side Timeout
      -  Wire-Protocol Error

.. code:: typescript

    interface Connection {
      /**
       *  An id number associated with the connection
       */
      id: number;

      /**
       *  The address of the pool that owns this connection
       */
      address: string;

      /**
       *  An integer representing the “generation” of the pool
       *  when this connection was created
       */
      generation: number;
    }

WaitQueue
---------

A concept that represents pending requests for connections. When a thread requests a Connection from a Pool, the thread enters the Pool's WaitQueue. A thread stays in the WaitQueue until it either receives a Connection or times out. A WaitQueue has the following traits:

-  **Thread-Safe**: When multiple threads attempt to enter or exit a WaitQueue, they do so in a thread-safe manner.
-  **Ordered/Fair**: When connections are made available, they are issued out to threads in the order that the threads entered the WaitQueue.
-  **Timeout aggressively:** If **waitQueueTimeoutMS** is set, members of a WaitQueue MUST timeout if they are enqueued for longer than waitQueueTimeoutMS. Members of a WaitQueue MUST timeout aggressively, and MUST leave the WaitQueue immediately upon timeout.

The implementation details of a WaitQueue are left to the driver.
Example implementations include:

-  A fair Semaphore
-  A Queue of callbacks

Connection Pool
---------------

A driver-defined entity that encapsulates all non-monitoring connections associated with a single Endpoint. The pool has the following properties:

-  **Thread Safe:** All Pool behaviors MUST be thread safe.
-  **Not Fork-Safe:** A Pool is explicitly not fork-safe. If a Pool detects that is it being used by a forked process, it MUST immediately clear itself and update it's pid
-  **Single Owner:** A Pool MUST be associated with exactly one Endpoint, and MUST NOT be shared between Endpoints.
-  **Emit Events:** A Pool MUST emit pool events when dictated by this spec (see `Connection Pool Monitoring <https://docs.google.com/document/d/1AT1vCSBXB-0MQUqkusIukMngSvlyNU93qe9tBxbcfiY/edit?disco=AAAACWk9Jo4&ts=5bec82a4#heading=h.75j6jnmktzyn>`__). Users MUST be able to subscribe to emitted events in a manner idiomatic to their language and driver.
-  **Closeable:** A Pool MUST be able to be manually closed. When a Pool is closed, the following behaviors change:

   -  Checking in a Connection to the Pool automatically closes the connection
   -  Attempting to check out a Connection from the Pool results in an Error

-  **Capped:** a pool is capped if **maxPoolSize** is set to a non-zero value. If a pool is capped, then it's total number of connections (including available and in use) MUST NOT exceed **maxPoolSize**

.. code:: typescript

    interface ConnectionPool {
      /**
       *  The Queue of threads waiting for a connection to be available
       */
      waitQueue: WaitQueue;

      /**
       *  A generation number representing the SDAM generation of the pool
       */
      generation: number;

      /**
       *  An integer expressing how many total connections
       *  (active + in use) the pool currently has
       */
      totalConnectionCount: number;

      /**
       *  An integer expressing how many connections are currently
       *  available in the pool.
       */
      availableConnectionCount: number;

      /**
       *  Returns a connection for use
       */
      checkOut(): Connection;

      /**
       *  Check in a connection back to the connection pool
       */
      checkIn(connection: Connection): void;

      /**
       *  Mark all current connections as stale.
       */
      clear(): void;

      /**
       *  Closes the pool, preventing the pool from creating and returning new Connections
       */
      close(): void;
    }

.. _connection-pool-behaviors-1:

Connection Pool Behaviors
~~~~~~~~~~~~~~~~~~~~~~~~~

Creating a Connection Pool
--------------------------

Pool creation is mostly an implementation detail specific to the driver language. If minPoolSize is set, the pool MUST immediately create enough connections such that totalConnections >= minPoolSize. These connections MUST be created in a non-blocking manner. Drivers SHOULD additionally connect these connections if it is possible to do so in a non-blocking manner, either via the use of a Background Thread or via asynchronous I/O.

.. code::

    set generation to 0
    emit PoolCreatedEvent
    if minPoolSize is set:
      while totalConnectionCount < minPoolSize:
        create connection
        # If this can be done without blocking, ensure that connection
        # is ready to use


Closing a Connection Pool
-------------------------

When a pool is closed, it MUST first close all available connections in that pool. This results in the following behavior changes:

-  In use connections MUST be closed when they are checked in to the closed pool.
-  Attempting to check out a connection MUST result in an error.

.. code::

    mark pool as CLOSED
    for connection in availableConnections:
      close connection
    emit PoolClosedEvent

Creating a Connection (Internal Implementation)
-----------------------------------------------

When creating a connection, the initial Connection is in a “set up” state. This only creates a “virtual” connection, and performs no I/O. The Connection MUST NOT be made available until it has completed its connection as described `here <#connecting-a-connection-internal-implementation>`__.

.. code::

    connection = new Connection()
    mark connection as available
    emit ConnectionCreatedEvent
    return connection

Connecting a Connection (Internal Implementation)
-------------------------------------------------

Before a connection can be returned outside of the pool, it must be “set up”. This process involves performing the initial handshake, handling OP_COMPRESSED, and performing authentication.

.. code::

    try:
      connect connection via TCP / TLS
      perform connection handshake
      handle OP_COMPRESSED
      perform connection authentication
      mark connection as readyToUse
      emit ConnectionReadyEvent
      return connection
    except error:
      close connection
      throw error # Propagate error in manner idiomatic to language.


Closing a Connection (Internal Implementation)
----------------------------------------------

When a Connection is closed, it MUST first be marked as closed, removing it from being counted as available, in use, or setup. Once the connection is marked as closed, the Connection can perform whatever teardown is necessary. The Driver SHOULD perform the teardown in a non-blocking manner.

.. code::

    mark connection as closed
    emit ConnectionClosedEvent

    # The following can happen at a later time
    connection.close()

Checking Out a Connection
-------------------------

A Pool MUST have a method of allowing the driver to check out a Connection. Checking out a Connection involves entering the WaitQueue, and waiting for a Connection to become available. If the thread times out in the WaitQueue, an error is thrown.

If, in the process of iterating available connections in the pool by the checkOut method, a perished connection is encountered, such a connection MUST be closed and the iteration of available connections MUST continue until either a non-perished available connection is found or the list of available connections is exhausted. If no connections are available and the total number of connections is less than maxPoolSize, the pool MUST create and return a new Connection.

If the pool is closed, any attempt to check out a connection MUST throw an Error, and any items in the waitQueue MUST be removed from the waitQueue and throw an Error.

If minPoolSize is set, the Connection Pool must have at least minPoolSize total connections. If the pool does not implement a background thread as specified here, the checkOut method is responsible for ensuring this requirement.

A Connection MUST NOT be checked out until it is readyToUse. In addition, the Pool MUST NOT block other threads from checking out connections while waiting for a connection to be readyToUse.


.. code::

    connection = Null
    emit ConnectionCheckOutStartedEvent
    try:
      enter WaitQueue
      wait until at top of wait queue
      # Note that in a lock-based implementation of the wait queue would
      # only allow one thread in the following block at a time
      while connection is Null:
        if a connection is available:
          while connection is Null and a connection is available:
            connection = next available connection
            if connection is perished:
              close connection
              connection = Null
        else if totalConnectionCount < maxPoolSize:
          connection = create connection
        # If there is no background thread, the pool MUST ensure that
        # at least minPoolSize connections are in the pool.
        # This SHOULD be done in a non-blocking manner
        while totalConnectionCount < minPoolSize:
          create connection
    except pool is closed:
      emit ConnectionCheckOutFailedEvent(reason="poolClosed")
      throw PoolClosedError
    except timeout:
      emit ConnectionCheckOutFailedEvent(reason="timeout")
      throw WaitQueueTimeoutError
    finally:
      # This must be done in all drivers
      leave wait queue

    # If the connection has not been connected yet, the connection
    # (TCP, TLS, handshake, compression, and auth) must be performed
    # before the connection is returned. This MUST NOT block other threads
    # from acquiring connections.
    if connection is not readyToUse:
      try:
        set up connection
      except set up connection error:
        emit ConnectionCheckOutFailedEvent(reason="error")
        throw

    mark connection as in use
    emit ConnectionCheckedOutEvent
    return connection

Checking In a Connection
------------------------

A Pool MUST have a method of allowing the driver to check in a Connection. The driver MUST NOT be allowed to check in a Connection to a Pool that did not create that Connection, and MUST throw an Error if this is attempted.

When the Connection is checked in, it is closed if any of the following are true:

-  The connection is perished.
-  The pool has been closed.

Otherwise, the connection is marked as available.

.. code::

    emit ConnectionCheckedInEvent
    if connection is perished OR pool is closed:
      close connection
    else:
      mark connection as available

Clearing a Connection Pool
--------------------------

A Pool MUST have a method of clearing all Connections when instructed. Rather than iterating through every Connection, this method should simply increment the generation of the Pool, implicitly marking all current connections as stale. The checkOut and checkIn algorithms will handle clearing out stale connections. If a user is subscribed to Connection Monitoring events, a PoolClearedEvent MUST be emitted after incrementing the generation.

Forking
-------

A Connection is explicitly not fork-safe. The proper behavior in the case of a fork is to ResetAfterFork by:

-  clear all Connection Pools in the child process
-  closing all Connections in the child-process.

Drivers that support forking MUST document that connections to an Endpoint are not fork-safe, and document the proper way to ResetAfterFork in the driver.

Drivers MAY aggressively ResetAfterFork if the driver detects it has been forked.

Optional Behaviors
------------------

The following features of a Connection Pool SHOULD be implemented if they make sense in the driver and driver's language.

Background Thread
^^^^^^^^^^^^^^^^^

A Pool SHOULD have a background Thread that is responsible for
monitoring the state of all available connections. This background
thread SHOULD

-  Create and connect connections to ensure that the pool always satisfies **minPoolSize**
-  Remove and close perished available connections.

withConnection
^^^^^^^^^^^^^^

A Pool SHOULD implement a scoped resource management mechanism idiomatic to their language to prevent Connections from not being checked in. Examples include `Python's "with" statement <https://docs.python.org/3/whatsnew/2.6.html#pep-343-the-with-statement>`__ and `C#'s "using" statement <https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/using-statement>`__. If implemented, drivers SHOULD use this method as the default method of checking out and checking in Connections.

.. _connection-pool-monitoring-1:

Connection Pool Monitoring
~~~~~~~~~~~~~~~~~~~~~~~~~~

All drivers that implement a connection pool MUST provide an API that allows users to subscribe to events emitted from the pool. If a user subscribes to Connection Monitoring events, these events MUST be emitted when specified in “Connection Pool Behaviors”. Events SHOULD be created and subscribed to in a manner idiomatic to their language and driver.

Events
------


.. code:: typescript

    /**
     *  Emitted when a Connection Pool is created
     */
    interface PoolCreatedEvent {
      /**
       *  The ServerAddress of the Endpoint the pool is attempting to connect to.
       */
      address: string;

      /**
       *  Any non-default pool options that were set on this Connection Pool.
       */
      options: {...}
    }

    /**
     *  Emitted when a Connection Pool is cleared
     */
    interface PoolClearedEvent {
      /**
       *  The ServerAddress of the Endpoint the pool is attempting to connect to.
       */
      address: string;
    }

    /**
     *  Emitted when a Connection Pool is closed
     */
    interface PoolClosedEvent {
      /**
       *  The ServerAddress of the Endpoint the pool is attempting to connect to.
       */
      address: string;
    }

    /**
     *  Emitted when a Connection Pool creates a Connection object.
     *  NOTE: This does not mean that the connection is ready for use.
     */
    interface ConnectionCreatedEvent {
      /**
       *  The ServerAddress of the Endpoint the pool is attempting to connect to.
       */
      address: string;

      /**
       *  The ID of the Connection
       */
      connectionId: number;
    }

    /**
     *  Emitted when a Connection has finished its setup, and is now ready to use
     */
    interface ConnectionReadyEvent {
      /**
       *  The ServerAddress of the Endpoint the pool is attempting to connect to.
       */
      address: string;

      /**
       *  The ID of the Connection
       */
      connectionId: number;
    }

    /**
     *  Emitted when a Connection Pool closes a Connection
     */
    interface ConnectionClosedEvent {
      /**
       *  The ServerAddress of the Endpoint the pool is attempting to connect to.
       */
      address: string;

      /**
       *  The ID of the Connection
       */
      connectionId: number;

      /**
       * A reason explaining why this connection was closed.
       * Can be implemented as a string or enum.
       * Current valid values are:
       *   - "stale":           The pool was cleared, making the connection no longer valid
       *   - "idle":            The connection became stale by being available for too long
       *   - "error":           The connection experienced an error, making it no longer valid
       *   - "poolClosed":      The pool was closed, making the connection no longer valid
       */
      reason: string|Enum;
    }

    /**
     *  Emitted when the driver starts attempting to check out a connection
     */
    interface ConnectionCheckOutStartedEvent {
      /**
       * The ServerAddress of the Endpoint the pool is attempting
       * to connect to.
       */
      address: string;
    }

    /**
     *  Emitted when the driver's attempt to check out a connection fails
     */
    interface ConnectionCheckOutFailedEvent {
      /**
       *  The ServerAddress of the Endpoint the pool is attempting to connect to.
       */
      address: string;

      /**
       *  A reason explaining why connection check out failed.
       *  Can be implemented as a string or enum.
       *  Current valid values are:
       *   - "poolClosed":      The pool was previously closed, and cannot provide new connections
       *   - "timeout":         The connection check out attempt exceeded the specified timeout
       *   - "connectionError": The connection check out attempt experienced an error while setting up a new connection
       */
      reason: string|Enum;
    }

    /**
     *  Emitted when the driver successfully checks out a Connection
     */
    interface ConnectionCheckedOutEvent {
      /**
       *  The ServerAddress of the Endpoint the pool is attempting to connect to.
       */
      address: string;

      /**
       *  The ID of the Connection
       */
      connectionId: number;
    }

    /**
     *  Emitted when the driver checks in a Connection back to the Connection Pool
     */
    interface ConnectionCheckedInEvent {
      /**
       * The ServerAddress of the Endpoint the pool is attempting to connect to.
       */
      address: string;

      /**
       *  The ID of the Connection
       */
      connectionId: number;
    }

Connection Pool Errors
~~~~~~~~~~~~~~~~~~~~~~

A connection pool throws errors in specific circumstances. These Errors
MUST be emitted by the pool. Errors SHOULD be created and dispatched in
a manner idiomatic to the Driver and Language.

.. code:: typescript

    /**
     *  Thrown when the driver attempts to check out a
     *  Connection from a closed Connection Pool
     */
    interface PoolClosedError {
      message: 'Attempted to check out a connection from closed connection pool';
      address: <pool address>;
    }

    /**
     *  Thrown when a driver times out when attempting to check out
     *  a Connection from a Pool
     */
    interface WaitQueueTimeoutError {
      message: 'Timed out while checking out a connection from connection pool';
      address: <pool address>;
    }

Test Plan
=========

See `tests/README.rst <tests/README.rst>`_

Design Rationale
================

Why do we set minPoolSize across all members of a replicaSet, when most traffic will be against a Primary?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Currently, we are attempting to codify our current pooling behavior with minimal changes, and minPoolSize is currently uniform across all members of a replicaSet. This has the benefit of offsetting connection swarming during a Primary Step-Down, which will be further addressed in our `Advanced Pooling Behaviors <#advanced-pooling-behaviors>`__.

Why do we have separate ConnectionCreated and ConnectionReady events, but only one ConnectionClosed event?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

ConnectionCreated and ConnectionReady each involve different state changes in the pool.

-  ConnectionCreated adds a new “setting-up” connection, meaning the totalConnectionCount increases by one
-  ConnectionReady establishes that the connection is ready for use, meaning the availableConnectionCount increases by one

ConnectionClosed indicates that the connection is no longer a member of the pool, decrementing totalConnectionCount and potentially availableConnectionCount. After this point, the connection is no longer a part of the pool. Further hypothetical events would not indicate a change to the state of the pool, so they are not specified here.

Why are waitQueueSize and waitQueueMultiple deprecated?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

These options are not implemented across many drivers. Additionally, they have the effect of prioritizing older requests over newer requests, which is not necessarily the behavior that users want. They can also can result in cases where queue access oscillates back and forth instead of restricting access until the size drops. We may eventually pursue an alternative solutions in `Advanced Pooling Behaviors <#advanced-pooling-behaviors>`__.

Why is waitQueueTimeoutMS optional for some drivers?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We are anticipating eventually introducing a single client-side timeout mechanism, making us hesitant to introduce another granular timeout control. Therefore, if a driver/language already has an idiomatic way to implement their timeouts, they should leverage that mechanism over implementing waitQueueTimeoutMS.


Backwards Compatibility
=======================

As mentioned in `Deprecated Options <#deprecated-options>`__, some drivers currently implement the options ``waitQueueSize`` and/or ``waitQueueMultiple``. These options will need to be deprecated and phased out of the drivers that have implemented them.


Reference Implementations
=========================

- JAVA (JAVA-3079)
- RUBY (RUBY-1560)

Future Development
==================

SDAM
~~~~

This specification does not dictate how SDAM Monitoring connections are managed. SDAM specifies that “A monitor SHOULD NOT use the client's regular connection pool”. Some possible solutions for this include:

-  Having each Endpoint representation in the driver create and manage a separate dedicated Connection for monitoring purposes
-  Having each Endpoint representation in the driver maintain a separate pool of maxPoolSize 1 for monitoring purposes.
-  Having each Pool maintain a dedicated connection for monitoring purposes, with an API to expose that connection.

Advanced Pooling Behaviors
~~~~~~~~~~~~~~~~~~~~~~~~~~

This spec does not address any advanced pooling behaviors like predictive pooling, aggressive connection creation, or handling high request volume. Future work may address this.

Add support for OP_MSG exhaustAllowed
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Exhaust Cursors may require changes to how we close connections in the future, specifically to add a way to close and remove from its pool a connection which has unread exhaust messages.


Change log
==========

:2019-06-06: Add "connectionError" as a valid reason for ConnectionCheckOutFailedEvent
