SERVER-58205: Update documentation to include load balancer support

(cherry picked from commit b696e034fe97e7699dd45ac2595422e1d510ba2c)

2 files changed, 39 insertions, 1 deletions

diff --git a/docs/index.md b/docs/index.md
index fd7807cd849..cb678583e0e 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -5,12 +5,22 @@ This is just some internal documentation.
 
 For the full MongoDB docs, please see [mongodb.org](http://www.mongodb.org/)
 
+* [Batons](baton.md)
+* [Build System](build_system.md)
+* [Build System Reference](build_system_reference.md)
 * [Building MongoDB](building.md)
 * [Command Dispatch](command_dispatch.md)
+* [Contextual Singletons](contexts.md)
 * [Egress Networking](egress_networking.md)
 * [Exception Architecture](exception_architecture.md)
 * [Fail Points](fail_points.md)
+* [Futures and Promises](futures_and_promises.md)
+* [Load Balancer Support](load_balancer_support.md)
 * [Memory Management](memory_management.md)
 * [Parsing Stack Traces](parsing_stack_traces.md)
+* [Primary-only Services](primary_only_service.md)
+* [Security Architecture Guide](security_guide.md)
+* [Server Parameters](server-parameters.md)
 * [String Manipulation](string_manipulation.md)
-* [MongoDB Voluntary Product Accessibility Template® (VPAT™)](vpat.md)
+* [Thread Pools](thread_pools.md)
+* [MongoDB Voluntary Product Accessibility Template® (VPAT™)](vpat.md)
\ No newline at end of file
diff --git a/docs/load_balancer_support.md b/docs/load_balancer_support.md
new file mode 100644
index 00000000000..cfb9a6b65df
--- /dev/null
+++ b/docs/load_balancer_support.md
@@ -0,0 +1,28 @@
+# Load balancer support
+
+`mongos` has built-in support for connections made via L4 load balancers. However, placing `mongos`
+endpoints behind load balancers requires proper configuration of the load balancers, `mongos`, and
+any drivers or shells used to connect to the database. Three conditions must be fulfilled for
+`mongos` to be used behind a load balancer:
+* `mongos` must be configured with the [MongoDB Server Parameter](https://docs.mongodb.com/manual/reference/parameters/) `loadBalancerPort` whose value can be specified at program start in any of the ways mentioned in the server parameter documentation.
+This option causes `mongos` to open a second port that expects _only_ load balanced connections. All connections made from load
+balancers _must_ be made over this port, and no regular connections may be made over this port.
+* The L4 load balancer _must_ be configured to emit a [proxy protocol][proxy-protocol-url] header
+at the [start of its connection stream](https://github.com/mongodb/mongo/commit/3a18d295d22b377cc7bc4c97bd3b6884d065bb85). `mongos` [supports](https://github.com/mongodb/mongo/commit/786482da93c3e5e58b1c690cb060f00c60864f69) both version 1 and version 2 of the proxy 
+protocol standard.
+* The connection string used to establish the `mongos` connection must set the `loadBalanced` option,
+e.g., when connecting to a local `mongos` instance, if the `loadBalancerPort` server parameter was set to 20100, the
+connection string must be of the form `"mongodb://localhost:20100/?loadBalanced=true"`.
+
+`mongos` will emit appropiate error messages on connection attempts if these requirements are not
+met.
+
+There are some subtle behavioral differences that these configuration options enable, chief of
+which is how `mongos` deals with open cursors on client disconnection. Over a normal connection,
+`mongos` will keep open cursors alive for a short while after client disconnection in case the
+client reconnects and continues to request more from the given cursor. Since client reconnections
+aren't expected behind a load balancer (as the load balancer will likely redirect a given client
+to a different `mongos` instance upon reconnection), we eagerly [close cursors](https://github.com/mongodb/mongo/commit/b429d5dda98bbe18ab0851ffd1729d3b57fc8a4e) on load balanced
+client disconnects. We also [abort any in-progress transactions](https://github.com/mongodb/mongo/commit/74628ed4e314dfe0fd69d3fbae1411981a869f6b) that were initiated by the load balanced client.
+
+[proxy-protocol-url]: https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt