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eng-practices

Google's Engineering Practices documentation

Writing good CL descriptions

A CL description is a public record of what change is being made and why it was made. It will become a permanent part of our version control history, and will possibly be read by hundreds of people other than your reviewers over the years.

Future developers will search for your CL based on its description. Someone in the future might be looking for your change because of a faint memory of its relevance but without the specifics handy. If all the important information is in the code and not the description, it’s going to be a lot harder for them to locate your CL.

First Line

The first line of a CL description should be a short summary of specifically what is being done by the CL, followed by a blank line. This is what most future code searchers will see when they are browsing the version control history of a piece of code, so this first line should be informative enough that they don’t have to read your CL or its whole description just to get a general idea of what your CL actually did.

By tradition, the first line of a CL description is a complete sentence, written as though it were an order (an imperative sentence). For example, say "Delete the FizzBuzz RPC and replace it with the new system.” instead of "Deleting the FizzBuzz RPC and replacing it with the new system.” You don’t have to write the rest of the description as an imperative sentence, though.

Body is Informative

The rest of the description should be informative. It might include a brief description of the problem that’s being solved, and why this is the best approach. If there are any shortcomings to the approach, they should be mentioned. If relevant, include background information such as bug numbers, benchmark results, and links to design documents.

If you include links to external resources consider that they may not be visible to future readers due to access restrictions of retention policies. Where possible include enough context for reviewers and future readers to understand the CL.

Even small CLs deserve a little attention to detail. Put the CL in context.

Bad CL Descriptions

“Fix bug” is an inadequate CL description. What bug? What did you do to fix it? Other similarly bad descriptions include:

Some of those are real CL descriptions. Their authors may believe they are providing useful information, but they are not serving the purpose of a CL description.

Good CL Descriptions

Here are some examples of good descriptions.

Functionality change

Example:

rpc: remove size limit on RPC server message freelist.

Servers like FizzBuzz have very large messages and would benefit from reuse. Make the freelist larger, and add a goroutine that frees the freelist entries slowly over time, so that idle servers eventually release all freelist entries.

The first few words describe what the CL actually does. The rest of the description talks about the problem being solved, why this is a good solution, and a bit more information about the specific implementation.

Refactoring

Example:

Construct a Task with a TimeKeeper to use its TimeStr and Now methods.

Add a Now method to Task, so the borglet() getter method can be removed (which was only used by OOMCandidate to call borglet’s Now method). This replaces the methods on Borglet that delegate to a TimeKeeper.

Allowing Tasks to supply Now is a step toward eliminating the dependency on Borglet. Eventually, collaborators that depend on getting Now from the Task should be changed to use a TimeKeeper directly, but this has been an accommodation to refactoring in small steps.

Continuing the long-range goal of refactoring the Borglet Hierarchy.

The first line describes what the CL does and how this is a change from the past. The rest of the description talks about the specific implementation, the context of the CL, that the solution isn’t ideal, and possible future direction. It also explains why this change is being made.

Small CL that needs some context

Example:

Create a Python3 build rule for status.py.

This allows consumers who are already using this as in Python3 to depend on a rule that is next to the original status build rule instead of somewhere in their own tree. It encourages new consumers to use Python3 if they can, instead of Python2, and significantly simplifies some automated build file refactoring tools being worked on currently.

The first sentence describes what’s actually being done. The rest of the description explains why the change is being made and gives the reviewer a lot of context.

Review the description before submitting the CL

CLs can undergo significant change during review. It can be worthwhile to review a CL description before submitting the CL, to ensure that the description still reflects what the CL does.

Next: Small CLs