One of my favorite aspects of gRPC isn’t the performance. It’s the well-defined contracts Protobuf provides.

I’ve shared a lot about the benefits of gRPC, and most of that focuses on what makes it fast and reliable. One of the biggest benefits has nothing to do with performance. It’s maintainability and consistency.

But that maintainability and consistency rely heavily on how well you manage the Protobuf contract.

Protobuf Contracts Can Be Mismanaged

Protobuf and gRPC naturally gravitate toward well-managed contracts, but like anything, we can still overcomplicate or mismanage things.

The most common mistake I’ve seen is every application keeping its own copy of the .proto file and generating code from it independently.

When changes happen, they have to be made across every copy of the .proto file. Those copies can drift very quickly.

If nobody introduces a conflicting change, it might work for a while. But conflicts happen eventually.

Imagine two teams independently modify their copies of the .proto file.

One adds:

string email = 3;

Another adds:

string phone_number = 3;

Now both have assigned different meanings to the same field number. And because both assigned a string to field 3, this doesn’t fail in predictable ways.

One application could send an email address, while another is expecting a phone number. Hopefully, you find that drift in testing, not in production.

The easiest way to avoid this scenario is to maintain a single source of truth for the contract.

One Contract, One Source of Truth

At sufficient scale, you may need a schema registry or platform that manages Protobuf/gRPC contracts. But if you only have a few gRPC contracts to manage, I have a pretty simple solution that works well.

First, keep the .proto files in one source-controlled home. That’s the source of truth.

Make any updates there via pull requests. Changes get reviewed and tracked, and because everyone is modifying the same .proto file, it’s much easier to catch conflicts.

Second, generate code from that contract. Bonus points if you automate this as part of the CI process. Then commit and publish the generated code as versioned libraries for the languages you support.

With this approach, applications don’t maintain their own copies of the .proto definition or generate code independently. They import a library/package.

You don’t have to worry about which version of the .proto file was used. You don’t have to worry about the protoc version or which plugin options were used. It becomes a normal software dependency.

All you need to worry about is which version you are using.

Contract changes follow a pretty simple process:

Pull Request -> Review -> Merge -> Code Generation -> Publish -> Dependency Update -> Profit.

Consistency Becomes the Easy Path

The best part of this approach is that consistency becomes the easy path.

If five services consume the same gRPC service, five teams don’t need to understand how to generate the client correctly.

They don’t need to manage a .proto file. They don’t need to install the right version of protoc. They import a library/package.

This approach also makes contract adoption visible.

If version v1.8.0 adds a new capability, you can easily see which applications have upgraded and which are still using older versions. This approach turns contract management into dependency management. And every team has to manage software dependencies anyway.

You Still Need to Manage Compatibility

While this approach makes contract and implementation management much easier, it’s not foolproof.

Breaking changes can still be introduced. Consumers can still sit on old versions.

Adding new fields is generally straightforward. But removing or changing existing fields requires coordination (and should be avoided if possible).

It’s also important to recognize that producers/servers and consumers/clients evolve independently. If you only added a field or two, and the consumer doesn’t need it, running an older version might be fine. But over time, letting consumers run significantly different versions can cause functionality to drift across applications.

Good dependency management and automation can make this much easier to manage, though.

Overall, if you keep backward compatibility in mind, this approach works well. Still, if you let those practices slip, you’ll find yourself coordinating version changes across multiple applications, which is time-consuming and often challenging.

Final Thoughts

I’ve seen entire projects lose months because two teams understood the same contract differently.

Protobuf/gRPC doesn’t make that impossible. But if you structure things well, it can make it much harder. And that is one of my favorite things about Protobuf/gRPC.

It’s fast and efficient, which is great. But a well-managed .proto contract makes integration between systems consistent and much easier to maintain. Don't treat a contract as documentation; treat it as code. Code that can be versioned, distributed, and managed like any other dependency.