Category: Golang

My gRPC Annoyances

We have been using gRPC in Thanos since Thanos inception – it has served us great and it has a ton of useful functionality, it solves a lot of problems, it is easy to use, and so on. However, I feel like some stuff is lacking, especially performance that will most likely never be fixed (or, I should say, changed). The framework just does not solve 100% of the things that the Thanos project needs right now. Let’s go through the list of my pet peeves.

Sophisticated compression

A huge part of network traffic between servers in Thanos is used for sending sets of labels, essentially a map<string, string>. Typically, each map only differs by two values between consecutive maps.

Also, we really need streaming to avoid having to buffer the whole response in memory beforehand. Strictly speaking, streaming might not be required because we buffer everything on the querier side either way right now, but there needs to be a way of building the message incrementally.

However, because gRPC is message-based, it means that it is impossible to have a compression scheme that encompasses multiple messages. What I would like to have is some string table that is shared over the whole stream. This also might be in part because gRPC uses HTTP as its transport and there it is impossible to negotiate compression parameters “out of band”. While researching this topic, I stumbled upon https://github.com/WICG/compression-dictionary-transport. Perhaps this will get through at some point and gRPC will be able to leverage this work.

Compression helps a little bit here but not a lot. The gRPC codec interface has []byte as an input, meaning that the input needs to be a contiguous array of bytes in memory. Generally speaking with compression, repeated sequences of bytes have references to them so it would be possible to avoid allocating memory for those repeated sequences if the protobuf unmarshaling interface didn’t need a []byte and would instead be a io.Reader. https://github.com/grpc/grpc-go/issues/499 misses a huge point – the unmarshaling, I believe, still could accept an io.Reader instead of []byte. The point about Marshal() is valid, though.

Fortunately, there is some recent movement to fix this: https://github.com/grpc/grpc-go/issues/6619.

Mirages of ownership

gRPC-Go presents an interesting conundrum to its users – when the user-written code returns from a function that serves a remote procedure call, gRPC-Go takes that variable and marshals it at some point in the future. This is done for performance purposes but it also means that the user is giving away the ownership of the data and that the data must always be not changed after returning. This could become a foot gun in case slices or maps are used because the value of variables of such types are references, making it easy to mutate them accidentally.

Here’s how the “hello world” example looks in grpc-go:

// SayHello implements helloworld.GreeterServer
func (s *server) SayHello(ctx context.Context, in *pb.HelloRequest) (*pb.HelloReply, error) {
	log.Printf("Received: %v", in.GetName())
	return &pb.HelloReply{Message: "Hello " + in.GetName()}, nil
}

In this case, grpc-go takes ownership of &pb.HelloReply{Message: "Hello " + in.GetName()}. This obviously presents no problems but what if there were some slice or map?

type server struct {
  names []string
  ...
}

// Constantly running in the background.
func (s *server) updateNames() {
  for {
    for i := 0; i < len(s.names); i++ {
      s.names[i] = generateRandomName()
    }
    time.Sleep(1*time.Second)
  }
}

// SayHello implements helloworld.GreeterServer
func (s *server) SayHello(ctx context.Context, srv pb.HelloServer) (error) {
        return srv.SendMsg(&pb.HelloReply{Names: s.names})
}

Now this is a bit bad because grpc-go might be trying to marshal s.names while it is being updated in the background. You can find some more context here.

Of course, this is a contrived example and you might think how this could even happen in practice. I had a botched attempt at adding pooling to the marshaling side in Thanos: https://github.com/thanos-io/thanos/pull/4609/files. Fortunately, SendMsg now includes a helpful text:

	// It is not safe to modify the message after calling SendMsg. Tracing
	// libraries and stats handlers may use the message lazily.

Hopefully, all of these problems will be fixed at some point. I am confident that the Codec interface will soon change for the better but the compression stuff will take a longer time. It would allow us to really reduce the memory and CPU usage of Thanos.

Perhaps we will also look at other RPC frameworks in the future. dRPC is a recent project that piqued my interest. I have only dabbled with it for a few hours so I don’t have any opinion on it so far. That’s something for future posts!

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Perils of /metrics data assertions

In the Thanos project the e2e tests are written in part using assertions on the metrics data. I have encountered a few challenges with those assertions that I wanted to share with you.

Always check for the maximum possible value

Writing tests on /metrics data means your testing harness continuously checks /metrics data to see whether some metric equals some value. If the metric takes some time to reach the maximum value, you might erroneously write an equals check that checks for a smaller value. Later on, this will lead to a flaky test situation.

To fix this, I’d suggest running the flaky test locally and trying to write an assertion for a bigger value. Now try to run the same test again and see what happens. Also, look at the logic inside of your code – perhaps it would be possible to calculate the maximum value without using a hard-coded constant?

Check for specific metrics

By default, the end-to-end testing framework only accepts a metric name and sums up all matching series. I would encourage you to be maximally specific because metrics are not usually transactional. In other words, the user (in this case, the e2e testing framework) might see some results that are in the middle of being updated. For instance, if the state is constantly fluctuating then the sum will change but the sum of all matching metrics might never be equal to the desired value. Imagine that we have some code like this (pseudo-code):

foometric.set(123)
// <--- E2E testing framework request for /metrics page comes in here.
barmetric.set(5000)

If the metrics weren’t set previously then the sum can either be 123 or 5123. Hence, the best practice is usually to be as specific as possible with the label matchers. You can use this function to do that.

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