This tutorial provides a basic Node.js programmer’s introduction to working with gRPC.
By walking through this example you’ll learn how to:
It assumes that you have read the Overview and are familiar with protocol buffers. Note that the example in this tutorial uses the proto3 version of the protocol buffers language. You can find out more in the proto3 language guide and see the release notes for the new version in the protocol buffers GitHub repository.
Our example is a simple route mapping application that lets clients get information about features on their route, create a summary of their route, and exchange route information such as traffic updates with the server and other clients.
With gRPC we can define our service once in a .proto file and implement clients and servers in any of gRPC’s supported languages, which in turn can be run in environments ranging from servers inside Google to your own tablet - all the complexity of communication between different languages and environments is handled for you by gRPC. We also get all the advantages of working with protocol buffers, including efficient serialization, a simple IDL, and easy interface updating.
The example code for our tutorial is in
grpc/grpc/examples/node/dynamic_codegen/route_guide.
As you’ll see if you look at the repository, there’s also a very similar-looking
example in
grpc/grpc/examples/node/static_codegen/route_guide.
We have two versions of our route guide example because there are two ways to
generate the code needed to work with protocol buffers in Node.js - one approach
uses Protobuf.js to dynamically generate the code at runtime, the other uses
code statically generated using the protocol buffer compiler protoc. The
examples behave identically, and either server can be used with either client.
As suggested by the directory name, we’ll be using the version with dynamically
generated code in this document, but feel free to look at the static code
example too.
To download the example, clone the grpc repository by running the following
command:
$ git clone -b v1.17.1 https://github.com/grpc/grpc
$ cd grpc
Then change your current directory to examples/node:
$ cd examples/node
You also should have the relevant tools installed to generate the server and client interface code - if you don’t already, follow the setup instructions in the Node.js quick start guide.
Our first step (as you’ll know from the Overview) is to
define the gRPC service and the method request and response types using
protocol
buffers. You can
see the complete .proto file in
examples/protos/route_guide.proto.
To define a service, you specify a named service in your .proto file:
service RouteGuide {
...
}
Then you define rpc methods inside your service definition, specifying their
request and response types. gRPC lets you define four kinds of service method,
all of which are used in the RouteGuide service:
// Obtains the feature at a given position.
rpc GetFeature(Point) returns (Feature) {}
stream
keyword before the response type.// Obtains the Features available within the given Rectangle. Results are
// streamed rather than returned at once (e.g. in a response message with a
// repeated field), as the rectangle may cover a large area and contain a
// huge number of features.
rpc ListFeatures(Rectangle) returns (stream Feature) {}
stream keyword before the request type.// Accepts a stream of Points on a route being traversed, returning a
// RouteSummary when traversal is completed.
rpc RecordRoute(stream Point) returns (RouteSummary) {}
stream
keyword before both the request and the response.// Accepts a stream of RouteNotes sent while a route is being traversed,
// while receiving other RouteNotes (e.g. from other users).
rpc RouteChat(stream RouteNote) returns (stream RouteNote) {}
Our .proto file also contains protocol buffer message type definitions for all
the request and response types used in our service methods - for example, here’s
the Point message type:
// Points are represented as latitude-longitude pairs in the E7 representation
// (degrees multiplied by 10**7 and rounded to the nearest integer).
// Latitudes should be in the range +/- 90 degrees and longitude should be in
// the range +/- 180 degrees (inclusive).
message Point {
int32 latitude = 1;
int32 longitude = 2;
}
The Node.js library dynamically generates service descriptors and client stub
definitions from .proto files loaded at runtime.
To load a .proto file, simply require the gRPC proto loader library and use its
loadSync() method, then pass the output to the gRPC library’s loadPackageDefinition method:
var PROTO_PATH = __dirname + '/../../../protos/route_guide.proto';
var grpc = require('grpc');
var protoLoader = require('@grpc/proto-loader');
// Suggested options for similarity to existing grpc.load behavior
var packageDefinition = protoLoader.loadSync(
PROTO_PATH,
{keepCase: true,
longs: String,
enums: String,
defaults: true,
oneofs: true
});
var protoDescriptor = grpc.loadPackageDefinition(packageDefinition);
// The protoDescriptor object has the full package hierarchy
var routeguide = protoDescriptor.routeguide;
Once you’ve done this, the stub constructor is in the routeguide namespace
(protoDescriptor.routeguide.RouteGuide) and the service descriptor (which is
used to create a server) is a property of the stub
(protoDescriptor.routeguide.RouteGuide.service);
First let’s look at how we create a RouteGuide server. If you’re only
interested in creating gRPC clients, you can skip this section and go straight
to Creating the client (though you might find it interesting
anyway!).
There are two parts to making our RouteGuide service do its job:
You can find our example RouteGuide server in
examples/node/dynamic_codegen/route_guide/route_guide_server.js.
Let’s take a closer look at how it works.
As you can see, our server has a Server constructor generated from the
RouteGuide.service descriptor object
var Server = new grpc.Server();
In this case we’re implementing the asynchronous version of RouteGuide,
which provides our default gRPC server behaviour.
The functions in route_guide_server.js implement all our service methods.
Let’s look at the simplest type first, getFeature, which just gets a Point
from the client and returns the corresponding feature information from its
database in a Feature.
function checkFeature(point) {
var feature;
// Check if there is already a feature object for the given point
for (var i = 0; i < feature_list.length; i++) {
feature = feature_list[i];
if (feature.location.latitude === point.latitude &&
feature.location.longitude === point.longitude) {
return feature;
}
}
var name = '';
feature = {
name: name,
location: point
};
return feature;
}
function getFeature(call, callback) {
callback(null, checkFeature(call.request));
}
The method is passed a call object for the RPC, which has the Point parameter
as a property, and a callback to which we can pass our returned Feature. In
the method body we populate a Feature corresponding to the given point and
pass it to the callback, with a null first parameter to indicate that there is
no error.
Now let’s look at something a bit more complicated - a streaming RPC.
listFeatures is a server-side streaming RPC, so we need to send back multiple
Features to our client.
function listFeatures(call) {
var lo = call.request.lo;
var hi = call.request.hi;
var left = _.min([lo.longitude, hi.longitude]);
var right = _.max([lo.longitude, hi.longitude]);
var top = _.max([lo.latitude, hi.latitude]);
var bottom = _.min([lo.latitude, hi.latitude]);
// For each feature, check if it is in the given bounding box
_.each(feature_list, function(feature) {
if (feature.name === '') {
return;
}
if (feature.location.longitude >= left &&
feature.location.longitude <= right &&
feature.location.latitude >= bottom &&
feature.location.latitude <= top) {
call.write(feature);
}
});
call.end();
}
As you can see, instead of getting the call object and callback in our method
parameters, this time we get a call object that implements the Writable
interface. In the method, we create as many Feature objects as we need to
return, writing them to the call using its write() method. Finally, we call
call.end() to indicate that we have sent all messages.
If you look at the client-side streaming method RecordRoute you’ll see it’s
quite similar to the unary call, except this time the call parameter
implements the Reader interface. The call’s 'data' event fires every time
there is new data, and the 'end' event fires when all data has been read. Like
the unary case, we respond by calling the callback
call.on('data', function(point) {
// Process user data
});
call.on('end', function() {
callback(null, result);
});
Finally, let’s look at our bidirectional streaming RPC RouteChat().
function routeChat(call) {
call.on('data', function(note) {
var key = pointKey(note.location);
/* For each note sent, respond with all previous notes that correspond to
* the same point */
if (route_notes.hasOwnProperty(key)) {
_.each(route_notes[key], function(note) {
call.write(note);
});
} else {
route_notes[key] = [];
}
// Then add the new note to the list
route_notes[key].push(JSON.parse(JSON.stringify(note)));
});
call.on('end', function() {
call.end();
});
}
This time we get a call implementing Duplex that can be used to read and
write messages. The syntax for reading and writing here is exactly the same as
for our client-streaming and server-streaming methods. Although each side will
always get the other’s messages in the order they were written, both the client
and server can read and write in any order — the streams operate completely
independently.
Once we’ve implemented all our methods, we also need to start up a gRPC server
so that clients can actually use our service. The following snippet shows how we
do this for our RouteGuide service:
function getServer() {
var server = new grpc.Server();
server.addProtoService(routeguide.RouteGuide.service, {
getFeature: getFeature,
listFeatures: listFeatures,
recordRoute: recordRoute,
routeChat: routeChat
});
return server;
}
var routeServer = getServer();
routeServer.bind('0.0.0.0:50051', grpc.ServerCredentials.createInsecure());
routeServer.start();
As you can see, we build and start our server with the following steps:
Server constructor from the RouteGuide service descriptor.Server constructor with
the method implementations.bind() method.start() on the instance to start the RPC server.In this section, we’ll look at creating a Node.js client for our RouteGuide
service. You can see our complete example client code in
examples/node/dynamic_codegen/route_guide/route_guide_client.js.
To call service methods, we first need to create a stub. To do this, we just need to call the RouteGuide stub constructor, specifying the server address and port.
new example.RouteGuide('localhost:50051', grpc.credentials.createInsecure());
Now let’s look at how we call our service methods. Note that all of these methods are asynchronous: they use either events or callbacks to retrieve results.
Calling the simple RPC GetFeature is nearly as straightforward as calling a
local asynchronous method.
var point = {latitude: 409146138, longitude: -746188906};
stub.getFeature(point, function(err, feature) {
if (err) {
// process error
} else {
// process feature
}
});
As you can see, we create and populate a request object. Finally, we call the method on the stub, passing it the request and callback. If there is no error, then we can read the response information from the server from our response object.
console.log('Found feature called "' + feature.name + '" at ' +
feature.location.latitude/COORD_FACTOR + ', ' +
feature.location.longitude/COORD_FACTOR);
Now let’s look at our streaming methods. If you’ve already read Creating the
server some of this may look very familiar - streaming RPCs are
implemented in a similar way on both sides. Here’s where we call the server-side
streaming method ListFeatures, which returns a stream of geographical
Features:
var call = client.listFeatures(rectangle);
call.on('data', function(feature) {
console.log('Found feature called "' + feature.name + '" at ' +
feature.location.latitude/COORD_FACTOR + ', ' +
feature.location.longitude/COORD_FACTOR);
});
call.on('end', function() {
// The server has finished sending
});
call.on('error', function(e) {
// An error has occurred and the stream has been closed.
});
call.on('status', function(status) {
// process status
});
Instead of passing the method a request and callback, we pass it a request and
get a Readable stream object back. The client can use the Readable’s
'data' event to read the server’s responses. This event fires with each
Feature message object until there are no more messages. Errors in the 'data'
callback will not cause the stream to be closed. The 'error' event
indicates that an error has occurred and the stream has been closed. The
'end' event indicates that the server has finished sending and no errors
occured. Only one of 'error' or 'end' will be emitted. Finally, the
'status' event fires when the server sends the status.
The client-side streaming method RecordRoute is similar, except there we pass
the method a callback and get back a Writable.
var call = client.recordRoute(function(error, stats) {
if (error) {
callback(error);
}
console.log('Finished trip with', stats.point_count, 'points');
console.log('Passed', stats.feature_count, 'features');
console.log('Travelled', stats.distance, 'meters');
console.log('It took', stats.elapsed_time, 'seconds');
});
function pointSender(lat, lng) {
return function(callback) {
console.log('Visiting point ' + lat/COORD_FACTOR + ', ' +
lng/COORD_FACTOR);
call.write({
latitude: lat,
longitude: lng
});
_.delay(callback, _.random(500, 1500));
};
}
var point_senders = [];
for (var i = 0; i < num_points; i++) {
var rand_point = feature_list[_.random(0, feature_list.length - 1)];
point_senders[i] = pointSender(rand_point.location.latitude,
rand_point.location.longitude);
}
async.series(point_senders, function() {
call.end();
});
Once we’ve finished writing our client’s requests to the stream using write(),
we need to call end() on the stream to let gRPC know that we’ve finished
writing. If the status is OK, the stats object will be populated with the
server’s response.
Finally, let’s look at our bidirectional streaming RPC routeChat(). In this
case, we just pass a context to the method and get back a Duplex stream
object, which we can use to both write and read messages.
var call = client.routeChat();
The syntax for reading and writing here is exactly the same as for our client-streaming and server-streaming methods. Although each side will always get the other’s messages in the order they were written, both the client and server can read and write in any order — the streams operate completely independently.
Build client and server:
$ npm install
Run the server, which will listen on port 50051:
$ node ./dynamic_codegen/route_guide/route_guide_server.js --db_path=./dynamic_codegen/route_guide/route_guide_db.json
Run the client (in a different terminal):
$ node ./dynamic_codegen/route_guide/route_guide_client.js --db_path=./dynamic_codegen/route_guide/route_guide_db.json