Polling engine usage in client server

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+# Polling Engine Usage on gRPC client and Server
+
+_Author: Sree Kuchibhotla (@sreecha) - Sep 2018_
+
+
+This document talks about how polling engine is used in gRPC core (both on client and server code paths).
+
+## gRPC client
+
+### Relation between Call, Channel (sub-channels), Completion queue, `grpc_pollset` 
+- A gRPC Call is tied to a channel (more specifically a sub-channel) and a completion queue for the lifetime of the call.
+- Once a _sub-channel_ is picked for the call, the file-descriptor (socket fd in case of TCP channels) is added to the pollset corresponding to call's completion queue. (Recall that as per [grpc-cq](grpc-cq.md), a completion queue has a pollset by default)
+
+![image](../images/grpc-call-channel-cq.png)
+
+
+### Making progress on Async `connect()` on sub-channels  (`grpc_pollset_set` usecase)
+- A gRPC channel is created between a client and a 'target'. The 'target' may resolve in to one or more backend servers.
+- A sub-channel is the 'connection' from a client to the backend server
+- While establishing sub-cannels (i.e connections) to the backends, gRPC issues async [`connect()`](https://github.com/grpc/grpc/blob/v1.15.1/src/core/lib/iomgr/tcp_client_posix.cc#L296) calls which may not complete right away.  When the `connect()` eventually succeeds, the socket fd is makde 'writable'
+  - This means that the polling engine must be monitoring all these sub-channel `fd`s for writable events and we need to make sure there is a polling thread that monitors all these fds
+  - To accomplish this, the `grpc_pollset_set` is used the following way (see picture below)
+
+![image](../images/grpc-client-lb-pss.png)
+
+## gRPC server
+
+- The listening fd (i.e., the socket fd corresponding to the server listening port) is added to each of the server completion queues. Note that in gRPC we use SO_REUSEPORT option and create multiple listening fds but all of them map to the same listening port
+- A new incoming channel is randomly assigned to some server completion queue (see picture below)
+
+![image](../images/grpc-server-cq-fds.png)
+
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