Module: src/client_interceptors

Client Interceptors

This module describes the interceptor framework for clients. An interceptor is a function which takes an options object and a nextCall function and returns an InterceptingCall:

var interceptor = function(options, nextCall) {
  return new InterceptingCall(nextCall(options));

The interceptor function must return an InterceptingCall object. Returning new InterceptingCall(nextCall(options)) will satisfy the contract (but provide no interceptor functionality). nextCall is a function which will generate the next interceptor in the chain.

To implement interceptor functionality, create a requester and pass it to the InterceptingCall constructor:

return new InterceptingCall(nextCall(options), requester);

A requester is a POJO with zero or more of the following methods:

start(metadata, listener, next)

  • To continue, call next(metadata, listener). Listeners are described
  • below.

sendMessage(message, next)

  • To continue, call next(message).


  • To continue, call next().

cancel(message, next)

  • To continue, call next().

A listener is a POJO with one or more of the following methods:

onReceiveMetadata(metadata, next)

  • To continue, call next(metadata)

onReceiveMessage(message, next)

  • To continue, call next(message)

onReceiveStatus(status, next)

  • To continue, call next(status)

A listener is provided by the requester's start method. The provided listener implements all the inbound interceptor methods, which can be called to short-circuit the gRPC call.

Three usage patterns are supported for listeners: 1) Pass the listener along without modification: next(metadata, listener). In this case the interceptor declines to intercept any inbound operations. 2) Create a new listener with one or more inbound interceptor methods and pass it to next. In this case the interceptor will fire on the inbound operations implemented in the new listener. 3) Make direct inbound calls to the provided listener's methods. This short-circuits the interceptor stack.

Do not modify the listener passed in. Either pass it along unmodified, ignore it, or call methods on it to short-circuit the call.

To intercept errors, implement the onReceiveStatus method and test for status.code !== grpc.status.OK.

To intercept trailers, examine status.metadata in the onReceiveStatus method.

This is a trivial implementation of all interceptor methods: var interceptor = function(options, nextCall) { return new InterceptingCall(nextCall(options), { start: function(metadata, listener, next) { next(metadata, { onReceiveMetadata: function (metadata, next) { next(metadata); }, onReceiveMessage: function (message, next) { next(message); }, onReceiveStatus: function (status, next) { next(status); }, }); }, sendMessage: function(message, next) { next(message); }, halfClose: function(next) { next(); }, cancel: function(message, next) { next(); } }); };

This is an interceptor with a single method: var interceptor = function(options, nextCall) { return new InterceptingCall(nextCall(options), { sendMessage: function(message, next) { next(message); } }); };

Builders are provided for convenience: StatusBuilder, ListenerBuilder, and RequesterBuilder

gRPC client operations use this mapping to interceptor methods:

grpc.opType.SEND_INITIAL_METADATA -> start grpc.opType.SEND_MESSAGE -> sendMessage grpc.opType.SEND_CLOSE_FROM_CLIENT -> halfClose grpc.opType.RECV_INITIAL_METADATA -> onReceiveMetadata grpc.opType.RECV_MESSAGE -> onReceiveMessage grpc.opType.RECV_STATUS_ON_CLIENT -> onReceiveStatus




<inner> getCall(channel, path [, options])

Get a call object built with the provided options.

Name Type Argument Description
channel grpc.Channel
path string
options grpc.Client~CallOptions <optional>

Options object.

<inner> getInterceptingCall(method_definition, options, interceptors, channel, responder)

Name Type Description
method_definition grpc~MethodDefinition
options grpc.Client~CallOptions
interceptors Array.<Interceptor>
channel grpc.Channel
responder function | EventEmitter

<inner> getLastListener(method_definition, emitter [, callback])

Creates the last listener in an interceptor stack.

Name Type Argument Description
method_definition grpc~MethodDefinition
emitter EventEmitter
callback function <optional>

<inner> resolveInterceptorProviders(providers, method_definition)

Transforms a list of interceptor providers into interceptors.

Name Type Description
providers Array.<InterceptorProvider>
method_definition grpc~MethodDefinition
null | Array.<Interceptor>