DOCS-10739 - Adding listenBacklog parameter

source/includes/options-mongod.yaml

@@ -120,6 +120,55 @@ post: |
      ``--bind_ip`` and ``--bind_ip_all`` are mutually exclusive. That
      is, you can specify one or the other, but not both.
 
+---
+program: mongod
+name: listenBacklog
+default: Target system ``SOMAXCONN`` constant
+args: <number>
+directive: option
+description: |
+   .. versionadded:: 3.6
+
+   The maximum number of connections that can exist in the listen
+   queue.
+
+   .. warning ::
+
+      Consult your local system's documentation to understand the
+      limitations and configuration requirements before using this
+      parameter.
+
+   .. important::
+
+      To prevent undefined behavior, specify a value for this
+      parameter between ``1`` and the local system ``SOMAXCONN``
+      constant.
+
+   The default value for the ``listenBacklog`` parameter is set at
+   compile time to the target system ``SOMAXCONN`` constant.
+   ``SOMAXCONN`` is the maximum valid value that is documented for
+   the *backlog* parameter to the *listen* system call.
+
+   Some systems may interpret ``SOMAXCONN`` symbolically, and others
+   numerically. The actual *listen backlog* applied in practice may
+   differ from any numeric interpretation of the ``SOMAXCONN`` constant
+   or argument to ``--listenBacklog``, and may also be constrained by
+   system settings like ``net.core.somaxconn`` on Linux.
+
+   Passing a value for the ``listenBacklog`` parameter that exceeds the
+   ``SOMAXCONN`` constant for the local system is, by the letter of the
+   standards, undefined behavior. Higher values may be silently integer
+   truncated, may be ignored, may cause unexpected resource
+   consumption, or have other adverse consequences.
+
+   On systems with workloads that exhibit connection spikes, for which
+   it is empirically known that the local system can honor higher
+   values for the *backlog* parameter than the ``SOMAXCONN`` constant,
+   setting the ``listenBacklog`` parameter to a higher value may reduce
+   operation latency as observed by the client by reducing the number
+   of connections which are forced into a backoff state.
+
+optional: true
 ---
 program: mongod
 name: maxConns
@@ -130,8 +179,8 @@ description: |
   accept. This setting has no effect if it is higher than your operating
   system's configured maximum connection tracking threshold.
 
-  Do not assign too low of a value to this option, or you will encounter errors
-  during normal application operation.
+  Do not assign too low of a value to this option, or you will
+  encounter errors during normal application operation.
 post: |
   .. include:: /includes/note-max-conns-max.rst
 optional: true

source/reference/program/mongod.txt

@@ -57,6 +57,8 @@ Core Options
 
 .. include:: /includes/option/option-mongod-ipv6.rst
 
+.. include:: /includes/option/option-mongod-listenBacklog.rst
+
 .. include:: /includes/option/option-mongod-maxConns.rst
 
 .. include:: /includes/option/option-mongod-logpath.rst