DOCSP-7543 support prewarming mongos routing table and connection pool

source/reference/parameters.txt

@@ -2379,7 +2379,7 @@ Sharding Parameters
 
    Default: 1
 
-   |mongos-only|
+   |both|
 
    Minimum number of outbound connections each TaskExecutor connection
    pool can open to any given :binary:`~bin.mongod` instance.
@@ -2390,6 +2390,19 @@ Sharding Parameters
    connections until :parameter:`ShardingTaskExecutorPoolHostTimeoutMS`
    milliseconds pass without any application using that pool.
 
+   For a :binary:`~bin.mongos` using the
+   :parameter:`warmMinConnectionsInShardingTaskExecutorPoolOnStartup`
+   parameter, the ``ShardingTaskExecutorPoolMinSize`` parameter also
+   controls how many connections to each shard host are established on
+   startup of the :binary:`~bin.mongos` instance before it begins
+   accepting incoming client connections.
+
+   .. note::
+
+      In MongoDB 4.4, the
+      :parameter:`warmMinConnectionsInShardingTaskExecutorPoolOnStartup`
+      parameter is enabled by default for the :binary:`~bin.mongos`.
+
    You can only set this parameter during start-up and cannot change
    this setting using the :dbcommand:`setParameter` database command.
 
@@ -2401,7 +2414,8 @@ Sharding Parameters
    pools, where ``n`` is the number of cores. See
    :parameter:`taskExecutorPoolSize`.
 
-   .. seealso:: :parameter:`ShardingTaskExecutorPoolMaxSize`
+   .. seealso:: - :parameter:`ShardingTaskExecutorPoolMaxSize`
+                - :parameter:`warmMinConnectionsInShardingTaskExecutorPoolOnStartup`
 
 .. parameter:: ShardingTaskExecutorPoolRefreshRequirementMS
 
@@ -2574,6 +2588,99 @@ Sharding Parameters
 
       - :parameter:`ShardingTaskExecutorPoolMinSize`
 
+.. parameter:: loadRoutingTableOnStartup
+
+   .. versionadded:: 4.4
+
+   Type: boolean
+
+   *Default*: true
+
+   |mongos-only|
+
+   Configures a :binary:`~bin.mongos` instance to preload the routing
+   table for a sharded cluster on startup. With this setting
+   enabled, the :binary:`~bin.mongos` caches the cluster-wide routing
+   table for each sharded collection as part of its startup procedure,
+   before it begins accepting client connections.
+
+   Without this setting enabled, the :binary:`~bin.mongos` only loads
+   a routing table as needed for incoming client connections, and only
+   loads the specific routing table for the namespace of a given
+   request.
+
+   A :binary:`~bin.mongos` instance with the
+   :parameter:`loadRoutingTableOnStartup` parameter enabled may
+   experience longer startup times, but will result in faster servicing
+   of initial client connections once started.
+
+   :parameter:`loadRoutingTableOnStartup` is enabled by default.
+
+   You can only set this parameter on startup, using either the
+   :setting:`setParameter` configuration file setting or the
+   :option:`--setParameter <mongos --setParameter>` command line option.
+
+.. parameter:: warmMinConnectionsInShardingTaskExecutorPoolOnStartup
+
+   .. versionadded:: 4.4
+
+   Type: boolean
+
+   *Default*: true
+
+   |mongos-only|
+
+   Configures a :binary:`~bin.mongos` instance to prewarm its connection
+   pool on startup. With this parameter enabled, the
+   :binary:`~bin.mongos` attempts to establish
+   :parameter:`ShardingTaskExecutorPoolMinSize` network
+   connections to each shard server as part of its startup procedure,
+   before it begins accepting client connections.
+
+   A timeout for this behavior can be configured with the
+   :parameter:`warmMinConnectionsInShardingTaskExecutorPoolOnStartupWaitMS`
+   parameter. If this timeout is reached, the :binary:`~bin.mongos` will
+   begin accepting client connections regardless of the size of its
+   connection pool.
+
+   A :binary:`~bin.mongos` instance with this parameter enabled may
+   experience longer startup times, but will result in faster servicing
+   of initial client connections once started.
+
+   :parameter:`warmMinConnectionsInShardingTaskExecutorPoolOnStartup` is
+   enabled by default.
+
+   You can only set this parameter on startup, using either the
+   :setting:`setParameter` configuration file setting or the
+   :option:`--setParameter <mongos --setParameter>` command line option.
+
+   .. seealso:: - :parameter:`warmMinConnectionsInShardingTaskExecutorPoolOnStartupWaitMS`
+                - :parameter:`ShardingTaskExecutorPoolMinSize`
+
+.. parameter:: warmMinConnectionsInShardingTaskExecutorPoolOnStartupWaitMS
+
+   .. versionadded:: 4.4
+
+   Type: integer
+
+   *Default*: 2000 (i.e. 2 seconds)
+
+   |mongos-only|
+
+   Sets the timeout threshold in milliseconds for a
+   :binary:`~bin.mongos` to wait for :parameter:`ShardingTaskExecutorPoolMinSize`
+   connections to be established per shard host when using the
+   :parameter:`warmMinConnectionsInShardingTaskExecutorPoolOnStartup`
+   parameter. If this timeout is reached, the :binary:`~bin.mongos` will
+   begin accepting client connections regardless of the size of its
+   connection pool.
+
+   You can only set this parameter on startup, using either the
+   :setting:`setParameter` configuration file setting or the
+   :option:`--setParameter <mongos --setParameter>` command line option.
+
+   .. seealso:: - :parameter:`warmMinConnectionsInShardingTaskExecutorPoolOnStartup`
+                - :parameter:`ShardingTaskExecutorPoolMinSize`
 
 .. parameter:: migrateCloneInsertionBatchDelayMS
 

source/release-notes/4.4.txt

@@ -390,6 +390,51 @@ need to be moved) as of the time the command is run or need to be
 moved. With the command, users can verify that initial chunk creation
 and migration has finished.
 
+Improved ``mongos`` Startup Procedure
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Starting with MongoDB 4.4, :binary:`~bin.mongos` adds the following new
+default startup behavior:
+
+- :binary:`~bin.mongos` will now preload a sharded cluster's routing
+  table on startup, rather than doing so on-demand for the first
+  incoming client connection.
+
+- :binary:`~bin.mongos` will now prewarm its connection pool to shard
+  hosts on startup, rather than doing so on-demand for incoming
+  client connections.
+
+This behavior results in faster servicing of initial client
+connections after a :binary:`~bin.mongos` instance is started or
+restarted. In particular, this allows sites that employ multiple
+:binary:`~bin.mongos` instances to restart them as necessary, or add new
+ones, without initial client requests to those instances needing to wait
+on connection establishment.
+
+Both routing table preloading and connection pool prewarming are enabled
+by default.
+
+MongoDB 4.4 adds the following parameters for controlling this behavior:
+
+- :parameter:`loadRoutingTableOnStartup` 
+
+  - *Default*: ``true`` (Enabled)
+  - Enables or disables support for preloading the routing table on
+    startup for the :binary:`~bin.mongos`.
+
+- :parameter:`warmMinConnectionsInShardingTaskExecutorPoolOnStartup`
+
+  - *Default*: ``true`` (Enabled) 
+  - Enables or disables support for prewarming the connection pool on
+    startup for the :binary:`~bin.mongos`.
+
+- :parameter:`warmMinConnectionsInShardingTaskExecutorPoolOnStartupWaitMS`
+
+  - *Default*: ``2000`` (2 seconds)
+  - Sets the timeout in milliseconds before client connections
+    to the :binary:`~bin.mongos` are allowed regardless of established
+    connection pool size.
+
 Platform Support
 ----------------