Docs: Add Nginx loadbalancing example with sticky mxid for workers (#15411)

* Docs: Add Nginx loadbalancing example with sticky mxid for workers

Add example nginx configuration snippet that

* does load balancing for workers
* respects mxid part of the token
  * from both url parameter and auth header
  * and handles since parameter

Thanks to @olmari for pushing me to write this and testing the configs

Signed-off-by: Tatu Wikman <tatu.wikman@gmail.com>

* Add changelog entry

Signed-off-by: Tatu Wikman <tatu.wikman@gmail.com>

* Update codeblock formatter

Co-authored-by: Dirk Klimpel <5740567+dklimpel@users.noreply.github.com>

* Remove indirectly related nginx-config

Signed-off-by: Sami Olmari <sami@olmari.fi>

* Proper definition of action how to target username for worker

Signed-off-by: Sami Olmari <sami@olmari.fi>

* Change "nginx" to general "reverse proxy" as it's concept now.

Signed-off-by: Sami Olmari <sami@olmari.fi>

* Wording in better English

Co-authored-by: Tatu Wikman <tatu.wikman@gmail.com>

* rename changelog entry to have correct extension

---------

Signed-off-by: Tatu Wikman <tatu.wikman@gmail.com>
Signed-off-by: Sami Olmari <sami@olmari.fi>
Co-authored-by: Dirk Klimpel <5740567+dklimpel@users.noreply.github.com>
Co-authored-by: Sami Olmari <sami@olmari.fi>
Co-authored-by: Sami Olmari <sami+github@olmari.fi>

changelog.d/15411.doc

@@ -0,0 +1 @@
+Docs: Add Nginx loadbalancing example with sticky mxid for workers.

docs/workers.md

@@ -325,8 +325,7 @@ load balancing can be done in different ways.
 
 For `/sync` and `/initialSync` requests it will be more efficient if all
 requests from a particular user are routed to a single instance. This can
-be done e.g. in nginx via IP `hash $http_x_forwarded_for;` or via
+be done in reverse proxy by extracting username part from the users access token.
-`hash $http_authorization consistent;` which contains the users access token.
 
 Admins may additionally wish to separate out `/sync`
 requests that have a `since` query parameter from those that don't (and
@@ -335,6 +334,69 @@ when a user logs in on a new device and can be *very* resource intensive, so
 isolating these requests will stop them from interfering with other users ongoing
 syncs.
 
+Example `nginx` configuration snippet that handles the cases above. This is just an
+example and probably requires some changes according to your particular setup:
+
+```nginx
+# Choose sync worker based on the existence of "since" query parameter
+map $arg_since $sync {
+    default synapse_sync;
+    '' synapse_initial_sync;
+}
+
+# Extract username from access token passed as URL parameter
+map $arg_access_token $accesstoken_from_urlparam {
+    # Defaults to just passing back the whole accesstoken
+    default   $arg_access_token;
+    # Try to extract username part from accesstoken URL parameter
+    "~syt_(?<username>.*?)_.*"           $username;
+}
+
+# Extract username from access token passed as authorization header
+map $http_authorization $mxid_localpart {
+    # Defaults to just passing back the whole accesstoken
+    default                              $http_authorization;
+    # Try to extract username part from accesstoken header
+    "~Bearer syt_(?<username>.*?)_.*"    $username;
+    # if no authorization-header exist, try mapper for URL parameter "access_token"
+    ""                                   $accesstoken_from_urlparam;
+}
+
+upstream synapse_initial_sync {
+    # Use the username mapper result for hash key
+    hash $mxid_localpart consistent;
+    server 127.0.0.1:8016;
+    server 127.0.0.1:8036;
+}
+
+upstream synapse_sync {
+    # Use the username mapper result for hash key
+    hash $mxid_localpart consistent;
+    server 127.0.0.1:8013;
+    server 127.0.0.1:8037;
+    server 127.0.0.1:8038;
+    server 127.0.0.1:8039;
+}
+
+# Sync initial/normal
+location ~ ^/_matrix/client/(r0|v3)/sync$ {
+	proxy_pass http://$sync;
+}
+
+# Normal sync
+location ~ ^/_matrix/client/(api/v1|r0|v3)/events$ {
+	proxy_pass http://synapse_sync;
+}
+
+# Initial_sync
+location ~ ^/_matrix/client/(api/v1|r0|v3)/initialSync$ {
+	proxy_pass http://synapse_initial_sync;
+}
+location ~ ^/_matrix/client/(api/v1|r0|v3)/rooms/[^/]+/initialSync$ {
+	proxy_pass http://synapse_initial_sync;
+}
+```
+
 Federation and client requests can be balanced via simple round robin.
 
 The inbound federation transaction request `^/_matrix/federation/v1/send/`