L11: Add Ruby Client and Server Interceptors

L11-Ruby-Interceptors.md

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+Title
+----
+* Author(s): Shaun McCormick (@splittingred)
+* Approver: mehrdada / apolcyn
+* Status: In Review
+* Implemented in: Ruby
+* Last updated: Sept 6, 2017
+* Discussion at: TBD (filled after thread exists)
+
+## Abstract
+
+Ruby gRPC client stubs and servers will present an interceptor interface, which will allow configuration of 
+interceptors that will execute on any inbound or outbound operation. The server and client interceptor APIs will
+behave similarly, using a compositional model for interceptors that allow for request-type-specific interception. 
+
+## Background
+
+The Ruby language implementation for gRPC does not currently have an interceptor interface, whereas other languages
+(ex: Go, Java) do. Application concerns and extensibility are currently closed in the Ruby implementation, making
+it difficult to extend the libraries for various application needs. 
+
+### Related Proposals: 
+
+* None
+
+## Proposal
+
+Add both server and client interceptors to the Ruby gRPC implementation. Similar to the Golang implementation,
+interceptors are set at the initialization of the client stub and RPC Server. They provide per-call interception
+for gRPC Ruby services and clients.
+
+### Client Interceptors
+
+This interceptor naively logs outbound requests and the method they call, and injects
+a unique `request_id` metadata value into the metadata sent:
+
+```ruby
+require 'securerandom'
+
+class AppClientInterceptor < GRPC::ClientInterceptor
+  def request_response(request:, call:, method:, metadata:)
+    logger.info "Sending request/response to #{method}"
+    metadata['request_id'] = generate_request_id
+    yield
+  end
+
+  def client_streamer(requests:, call:, method:, metadata:)
+    logger.info "Sending client streamer to #{method}"
+    metadata['request_id'] = generate_request_id
+    yield
+  end
+
+  def server_streamer(request:, call:, method:, metadata:)
+    logger.info "Sending client streamer to #{method}"
+    metadata['request_id'] = generate_request_id
+    yield
+  end
+
+  def bidi_streamer(requests:, call:, method:, metadata:)
+    logger.info "Sending bidi streamer to #{method}"
+    metadata['request_id'] = generate_request_id
+    yield
+  end
+
+  private
+
+  def logger
+    @logger ||= Logger.new(STDOUT)
+  end
+
+  def generate_request_id
+    SecureRandom.uuid
+  end
+end
+```
+
+Each interceptor can choose then to combine functionality for different request types into a protected 
+or private method, or handle each request type differently, based on the needs of the interceptor.
+
+You'll also note that the interceptor must yield back in order to complete the call. Then the interceptor
+can be added to the stub via the initializer, which passes in an array of interceptors:
+
+```ruby
+MyStub.new(
+  '0.0.0.0:0', 
+  :this_channel_is_insecure, 
+  interceptors: [AppClientInterceptor.new]
+)
+```
+
+Interceptors maintain order of insertion by using the FIFO execution order native to Ruby's Array. 
+In other words, if multiple interceptors are added, they will intercept in the order they were set in the 
+passed array.
+
+#### Client Interception API
+
+Client interceptors have four separate methods, one for each request type. Their arguments are similar;
+the only difference being if it is a streamed call, `request` will be `requests` and have an array of the
+requests that were sent. 
+
+For example, the arguments for a unary request/response call:
+
+```ruby
+##
+# @param [Object] request The protobuf request message
+# @param [GRPC::ActiveCall::InterceptableView] call A restricted view of the active call object
+# @param [Method] method The method being called
+# @param [Hash] metadata The call's outbound metadata
+def request_response(request:, call:, method:, metadata:)
+  yield
+end
+```
+
+The call object is a restricted view, which only exposes the `deadline` attribute that was set on the call.
+
+Also, the metadata hash here is able to be updated and will propagate to the server when the call executes.
+This allows client interceptors to inject metadata for calls dynamically.
+
+### Server Interceptors
+
+This interceptor logs incoming requests and the service and method they call:
+
+```ruby
+class ServerRequestLogInterceptor < GRPC::ServerInterceptor
+  def request_response(request:, call:, method:)
+    response = yield
+    log(method: method, response: response)
+    response
+  end
+
+  def client_streamer(call:, method:)
+    response = yield
+    log(method: method, response: response)
+    response
+  end
+
+  def server_streamer(request:, call:, method:)
+    response = yield
+    log(method: method, response: response)
+    response
+  end
+
+  def bidi_streamer(requests:, call:, method:)
+    response = yield
+    log(method: method, response: response)
+    response
+  end
+
+  private
+
+  def log(method:, response:)
+    mth = "#{method.owner.name}.#{method.name}"
+    if response.is_a?(GRPC::BadStatus)
+      logger.info "[GRPC::Ok] (#{mth})"
+    else
+      logger.error "[#{response.class.name}] (#{mth}) #{response.message}"
+    end
+  end
+
+  def logger
+    @logger ||= Logger.new(STDOUT)
+  end
+end
+```
+
+The interceptor can then be loaded via the initializer for the service:
+
+
+```ruby
+server = GRPC::RpcServer.new(
+  interceptors: [ServerRequestLogInterceptor.new] 
+)
+server.add_http2_port('0.0.0.0:0', :this_port_is_insecure)
+server.handle(MyService)
+server.run_till_terminated
+```
+
+This example will produce logs like:
+
+> [GRPC::Ok] (MyService.a_rpc_method)
+
+> [GRPC::Internal] (MyService.a_failing_rpc_method) An error occurred in my service!
+
+### Server Interception API
+
+Server interceptors act as decorators around handlers. Similar to client interceptors, server interceptors
+have four separate methods, one for each request type. 
+
+Unary and server streamer interceptor calls will have a single request,
+`SingleReqView` view of the active call, and the method being requested. Client streamer interceptor calls 
+have only the `MultiReqView` and method passed (similar to the handler). Finally, the bidi call will receive 
+an enumerable of requests that are passed.
+
+For example, the arguments for a unary request/response call:
+
+```ruby
+##
+# @param [Object] request The protobuf request message
+# @param [GRPC::ActiveCall::SingleReqView] call A restricted view of the active call object
+# @param [Method] method The method being called
+def request_response(request:, call:, method:)
+  yield
+end
+```
+
+Trailing metadata is able to be modified as well in interceptors, via the normal call API:
+
+```ruby
+call.output_metadata[:my_key] = 'my_value'
+```
+
+## Rationale
+
+To provide support for intercepting both at the client and server level, which is currently not supported,
+and to bring parity for Ruby with the other language implementations.
+
+Methods per-request-type were done as opposed to a singular interceptor method (such as a `intercept` method
+on each interceptor) in order to clearly distinguish the type of request that is coming in. Interceptors can
+from that consolidate their behavior, but the separation makes for cleaner delineation between the types of 
+requests and makes explicit the differences between each in the arguments.
+
+## Implementation
+
+Implementation is currently finishing review here: https://github.com/grpc/grpc/pull/12100
+
+### General Changes
+
+* Introduce a `GRPC::Interceptor` base class that client and server interceptors derive from, and provide
+  the internal API for an options Hash in the interceptor class itself that derived interceptors can utilize
+* Introduce a new `GRPC::InterceptorRegistry` class that centrally handles the storage of the interceptors
+* Introduce an `GRPC::InterceptionContext` class that centralizes the logic required to recursively handle
+  the interception of calls for both client and server interceptors
+* Refactor `GRPC::Generic::BidiCall` to return the `replies` generated so interceptors can receive it 
+
+### Client Interceptor Changes
+
+* Add an `interceptors` argument that accepts an Array to the `ClientStub` initializer and builds a
+  `GRPC::InterceptorRegistry` instance from it
+* Add an `InterceptableView` view that restricts access to the call to only be the `deadline` attribute
+  and a new `interceptable` method to the `ActiveCall` object that returns the view
+* Add wrapping for each request type (such as `request_response`, `client_streamer`, etc) 
+  in the `ClientStub` class that builds an interception context and wraps the `ActiveCall` call in an
+  interception block
+
+### Server Interceptor Changes
+
+* Add an `interceptors` argument that accepts an Array to the `GRPC::RpcServer` initializer and 
+  builds an `GRPC::InterceptorRegistry` instance from it
+* Update the `handle_*` methods in `GRPC::Generic::RpcDesc` to take in a `GRPC::InterceptionContext` 
+  instance, which then wraps the calls to the handler in an interception block