How to Mock gRPC Endpoints with GripMock

Many backend services need to integrate with other services via a number of protocols or API conventions.

Common ones are: REST, GraphQL, Queues, and gRPC.

Today, we're looking at how we can simplify microservice development by mocking external gRPC endpoints with GripMock.

Using GripMock is a simple 4-step process:

1. Pull in the gRPC definitions you need

We start by creating a new folder: mkdir grpc-mock-server.

Now, create a file with all the gRPC definitions you need. You can do this by copying over the gRPC files with the method definitions you need and removing everything you don't need.

Your should now have something like this:

// hello.proto
syntax = "proto3";

package auth;
option go_package = "github.com/simonme/article-examples/gripmock/hello;hello";

service Greeter {
  rpc SayHello (HelloRequest) returns (HelloReply) {}
}

message HelloRequest {
  string name = 1;
}
    
message HelloReply {
  string message = 1;
}

2. Write stub.json files to define your stubs

Next up we need to define the value our mock server should return.

Create a stubs/hello-stub.json file with the following content:

// stubs/hello-stub.json
{
  "service": "Greeter",
  "method": "SayHello",
  "input": { "matches": { "name": ".*" } },
  "output": { "data": { "message": "Hello mock user!" } }
}

Let's go through this:

• In service we define the gRPC service
• method is the method name we want to stub
• We now create an input matching rule that tells GripMock when this stub should be used
• Finally, we define the output of the gRPC method

If we want to stub multiple methods, we simply create multiple *-stub.json files.

3. Start the Server

Now run this command to start the server:

docker run --rm --name grpc-mock -p 4770:4770 -p 4771:4771 -v /Users/simon/projects/article-examples:/proto tkpd/gripmock --stub=/proto/stubs /proto/hello.proto

Let's break this up:

• docker run --rm run this image and remove the container afterwards
• --name grpc-mock give this container a name
• -p 4770:4770 -p 4771:4771 map ports 4770 and 4771
• -v /Users/simon/projects/article-examples:/proto add the article-examples folder (which contains the proto definition and stubs as a folder (/proto) to the docker container
• tkpd/gripmock which Docker image to use
• --stub=/proto/stubs the stubs will be in the /proto/stubs folder
• /proto/hello.proto this defines the path to the relevant proto file

Starting the GripMock server can take a few seconds. You need to wait until you see Serving gRPC on … in your console.

Starting GripMock
Serving stub admin on http://:4771
grpc server pid: 119
Serving gRPC on tcp://:4770

Closing Remarks

You can find out more about GripMock in their docs.

GripMock Q&A:

• Q: What should my input rule look like if the method has no inputs? A: "input": { "matches": {} }
• Q: How can I run the GripMock container in background? A: Add the -d flag to the docker command somewhere before tkpd/gripmock to run it detached

If you have any other questions (or answers) you can email them to me and I'll add them here.