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32 lines
2.2 KiB
32 lines
2.2 KiB
# Polling Engine Usage on gRPC client and Server |
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_Author: Sree Kuchibhotla (@sreecha) - Sep 2018_ |
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This document talks about how polling engine is used in gRPC core (both on client and server code paths). |
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## gRPC client |
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### Relation between Call, Channel (sub-channels), Completion queue, `grpc_pollset` |
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- A gRPC Call is tied to a channel (more specifically a sub-channel) and a completion queue for the lifetime of the call. |
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- 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) |
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### Making progress on Async `connect()` on sub-channels (`grpc_pollset_set` usecase) |
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- A gRPC channel is created between a client and a 'target'. The 'target' may resolve in to one or more backend servers. |
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- A sub-channel is the 'connection' from a client to the backend server |
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- While establishing sub-channels (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 make 'writable' |
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- 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 |
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- To accomplish this, the `grpc_pollset_set` is used the following way (see picture below) |
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## gRPC server |
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- 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 |
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- A new incoming channel is assigned to some server completion queue picked randomly (note that we currently [round-robin](https://github.com/grpc/grpc/blob/v1.15.1/src/core/lib/iomgr/tcp_server_posix.cc#L231) over the server completion queues) |
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