api/faq: add entry on incremental xDS. (envoyproxy#10876)

Also remove some stale comments around delta xDS in ConfigSource.

Fixes envoyproxy#10836

Signed-off-by: Harvey Tuch <htuch@google.com>
Signed-off-by: pengg <pengg@google.com>

api/envoy/api/v2/core/config_source.proto

@@ -57,10 +57,6 @@ message ApiConfigSource {
     // Using the delta xDS gRPC service, i.e. DeltaDiscovery{Request,Response}
     // rather than Discovery{Request,Response}. Rather than sending Envoy the entire state
     // with every update, the xDS server only sends what has changed since the last update.
-    //
-    // DELTA_GRPC is not yet entirely implemented! Initially, only CDS is available.
-    // Do not use for other xDSes.
-    // [#comment:TODO(fredlas) update/remove this warning when appropriate.]
     DELTA_GRPC = 3;
   }
 

api/envoy/config/core/v3/config_source.proto

@@ -58,10 +58,6 @@ message ApiConfigSource {
     // Using the delta xDS gRPC service, i.e. DeltaDiscovery{Request,Response}
     // rather than Discovery{Request,Response}. Rather than sending Envoy the entire state
     // with every update, the xDS server only sends what has changed since the last update.
-    //
-    // DELTA_GRPC is not yet entirely implemented! Initially, only CDS is available.
-    // Do not use for other xDSes.
-    // [#comment:TODO(fredlas) update/remove this warning when appropriate.]
     DELTA_GRPC = 3;
   }
 

api/envoy/config/core/v4alpha/config_source.proto

@@ -59,10 +59,6 @@ message ApiConfigSource {
     // Using the delta xDS gRPC service, i.e. DeltaDiscovery{Request,Response}
     // rather than Discovery{Request,Response}. Rather than sending Envoy the entire state
     // with every update, the xDS server only sends what has changed since the last update.
-    //
-    // DELTA_GRPC is not yet entirely implemented! Initially, only CDS is available.
-    // Do not use for other xDSes.
-    // [#comment:TODO(fredlas) update/remove this warning when appropriate.]
     DELTA_GRPC = 3;
   }
 

docs/root/faq/api/incremental.rst

@@ -0,0 +1,11 @@
+What is the status of incremental xDS support?
+==============================================
+
+The :ref:`incremental xDS <xds_protocol_delta>` protocol is designed to improve efficiency,
+scalability and functional use of xDS updates via two mechanisms:
+
+* Delta xDS. Resource deltas are delivered rather than state-of-the-world.
+* On-demand xDS. Resource can be lazy loaded depending on request contents.
+
+Currently, all xDS protocols (including ADS) support delta xDS. On-demand xDS is supported for
+:ref:`VHDS <config_http_conn_man_vhds>` only.

docs/root/faq/overview.rst

@@ -12,8 +12,8 @@ Build
   build/binaries
   build/boringssl
 
-API versioning
---------------
+API
+---
 .. toctree::
   :maxdepth: 2
 
@@ -23,6 +23,7 @@ API versioning
   api/control_plane
   api/package_naming
   api/why_versioning
+  api/incremental
 
 Performance
 -----------