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Breaking Change Detector

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Protobuf is a great way to represent your APIs and generate stubs in each language you develop with. As such, Protobuf APIs should be stable so as not to break consumers across repositories. Even in a monorepo context, making sure that your Protobuf APIs do not introduce breaking changes is important so that different deployed versions of your services do not have wire incompatibilities.

Prototool exposes a breaking change detector through the prototool break check command. This will check your current Protobuf definitions against a past version of your Protobuf definitions to see if there are any source or wire incompatible changes. Some notes on this command:

  • The breaking change detection operates on a per-package basis, not per-file - definitions can be moved between files within the same Protobuf package without being considered breaking.
  • The breaking change detector can either check against a given git branch or tag, or it can check against a previous state saved with the prototool break descriptor-set command.
  • The breaking change detector understands the concept of beta vs. stable packages, discussed in the V2 Style Guide. By default, the breaking change detector will not check beta packages for breaking changes, and will not allow stable packages to depend on beta packages, however both of these options are configurable in your prototool.yaml file.

The following is the list of changes understood to be breaking:

  • Deleting or renaming a package.
  • Deleting or renaming an enum, enum value, message, message field, service, or service method.
  • Changing the type of a message field.
  • Changing the tag of a message field.
  • Changing the label of a message field, i.e. optional, repeated, required.
  • Moving a message field currently in a oneof out of the oneof.
  • Moving a message field currently not in a oneof into the oneof.
  • Changing the function signature of a method.
  • Changing the stream value of a method request or response.

Over the next few minor versions of Prototool, we may add to this list if other changes are understood to be breaking, for example we may detect file option changes. However, the addition of additional checks is highly unlikely, and we will update the documentation accordingly when we are sure that no additional checks will be added.

A note on reserved fields: prototool break check expects you to deprecate fields instead of removing them and adding them to reserved. See the V2 Style Guide's discussion on the reserved keyword for more details.

Usage

Git

The breaking change detector can either be used to check definitions against a commit on a git branch or tag, or can check against a previously saved state. The only non-flag input argument is the directory to check, similar to other prototool commands.

Assuming your Protobuf definitions are in path/to/proto (which is usually where your prototool.yaml file will be as well):

# Checks against the default git branch of your repository
prototool break check path/to/proto
# Checks against the git branch or tag "dev"
prototool break check path/to/proto --git-branch dev

What this does behind the scenes:

  • Compiles all your Protobuf definitions in path/to/proto into a FileDescriptorSet.
  • Calls git clone --depth 1, optionally with --branch if the --git-branch flag was specified, and makes a clone of the git repository at your current directory into a temporary directory.
  • Compiles all the Protobuf definitions in path/to/proto in this temporary clone into a FileDescriptorSet.
  • Compares the two FileDescriptorSets to see if breaking changes were introduced from the current state to the previous state.
  • Deletes the temporary clone.

For this to work, you must run prototool from the root of a git checkout, and the directory path given to prototool must be relative. You can also specify no directory, as with other prototool commands, and prototool will assume your current directory contains all Protobuf definitions.

Saved State

If not using git, or you would prefer not to rely on git state to compare your Protobuf definitions for breaking changes, you can instead save state to a file and use that for comparsion.

To save your current state:

prototool break descriptor-set path/to/proto -o break_descriptor_set.bin

This is in effect an alias for prototool descriptor-set --include-imports -o FILE, however the -o flag is required.

To check your current definitions against a previous state:

prototool break check path/to/proto --descriptor-set break_descriptor_set.bin

What this does behind the scenes:

  • Compiles all your Protobuf definitions in path/to/proto into a FileDescriptorSet.
  • Deserializes break_descriptor_set.bin into a FileDescriptorSet.
  • Compares the two FileDescriptorSets to see if breaking changes were introduced from the current state to the previous state.

Beta vs. Stable Packages

As described in the V2 Style Guide, prototool understands the concept of beta vs. stable packages.

If a package's last component is vMAJORbetaBETA, where MAJOR and BETA are both greater than 0, prototool break check will understand that this package is a beta package, otherwise the package is understood as a stable pacakge.

The following are examples of beta packages.

package uber.trip.v1beta1;
package uber.user.v1beta2;
package uber.road.v2beta1;

By default, Prototool will not check beta packages for breaking changes, and will also check to make sure not stable packages depend on beta packages. Both of these options are configurable in your prototool.yaml.

break:
  # Include beta packages in breaking change detection.
  include_beta: true
  # Allow stable packages to depend on beta packages.
  # If include_beta is true, this is implicitly set.
  allow_beta_deps: true

Per-package breaking change detection

Some notes on why we chose per-package instead of per-file logic for breaking change detection.

Moving definitions between files within the same Protobuf package does not result in a breaking change on either a source code or wire level. For example, one can move message Foo within package bar.baz.v1 from file bat.proto to bam.proto without any effect on the resulting source code or over-the-wire representation (with a few minor exceptions, see below).

Refactoring file structure isn't just common, it's encouraged in many cases. We've seen a lot of instances of files being named in inconsistent manners, and instances where types logically belong in other files. If we were to enforce breaking changes on a per-file basis, this would encourage inconsistent file structure as schemas evolve, with no benefit (since there is no actual breaking change). We want breaking change detection to prevent actual breakages in the API, but we don't want to limit developers beyond that as it will have the effect of producing APIs that are even more inconsistent, which is against Prototool's main goal.

There are two main exceptions to the "no breaking changes in generated code" statement:

  • In Java, we typically have the java_outer_classname file options match the Protobuf file name. This means that types generated inside this classname would change for Java. However, with java_multiple_files = true set (also standard, and largely the standard across the Protobuf community including the Google Cloud APIs), there are only two public methods that go inside this class: registerAllExtensions and getDescriptor. Both of these methods should not be manually relied on outside of generated code, and it's such an edge case/advanced case that it's not worth enforcing per-file breaking changes given the drawbacks.
  • In C++ and Python (and other languages that rely on per-file imports), the file imports may need to change. For example, importing "bat.pb.h" may need to change to "bam.pb.h". However, since this is something that can be detected at compile-time (and should be detectable at testing time for dynamic languages), and given the drawbacks of per-file breaking change detection, we still prefer per-package breaking change detection.

Future source code location references

For now, unlike other prototool commands, prototool break check only outputs error messages and does not output filename:line:column location references. The problem of referencing your current Protobuf files with the location of a breaking change is harder than it seems, however we aim to implement this in the future. The logic that will likely be implemented is as follows.

  • For deleted enum values, message fields, or service methods, point to the encapsulating enum, message, or service.
  • For renamed fields, fields with a type change or tag change, or service methods whose signature changes, point to the field or service method.
  • For deleted enums, messages, or services, point to 1:1 of the file where this was deleted. Determining the file to put this on will be not technically possible if the file no longer exists or is renamed, so in this case, either do not output a filename and default to <input>, or choose the first file in the package alphabetically if a file is required. If all files were deleted, <input> will likely have to be defaulted to.

Implementation

The breaking change detection is primarily implemented through a series of packages within internal.

  • internal/reflect - This wraps the Protobuf definition at internal/reflect/proto/uber/proto/reflect/v1/reflect.proto, which is intended to contain definitions that represent Protobuf definitions on a per-package basis. This file is intended to be analogous to the descriptor.proto file in Protobuf's source code, however, as of now, reflect.proto only contains the information necessary for breaking change detection. This could be extended in the future and exposed outside of Prototool's internal implementation.
  • internal/extract - This wraps internal/reflect in a manner that makes working with the reflect.proto definitions easier within Golang.
  • internal/breaking - This uses internal/extract to implement the actual breaking change logic.
  • internal/git - This provides the function to do temporary git clones.