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Comment by teeray | Hacker News Reader

The purpose of a git commit message is to answer the question “why does this commit exist?” That is the principal question you should be answering when you type `git commit`. This is the question you will be asking yourself when you find that commit in `git blame` or if it shows up in `git bisect`. Try to help your future self out.

The changelog, on the other hand, answers the question “why does the customer care about these changes?” It could be the same reason as why the commit exists, but the question is different. If the customer doesn’t care, maybe it doesn’t even need an entry. Maybe why they care is slightly different than the reason the commit(s) exist.

This is why I advocate for a hand-updated CHANGELOG.md. It’s a very small amount of writing that forces developers to consider how their PRs will impact customers that an auto-generator will never be able to do well.

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We autogenerate our CHANGELOG.md extract from Changelog-[section] our git commit messages (Release Captain massages them if necessary), and have a GH bot which checks that you have a Changelog- note somewhere in your PR commits (can be Changelog-None: ....).

This avoid merge hell on the CHANGELOG.md file.

> This avoid merge hell on the CHANGELOG.md file.

Right, if you don't do something like this you can be in merge hell very quickly...

Setting `CHANGELOG.md merge=union` in .gitattributes mostly eliminates spurious merges conflicts on the changelog.

Oh wow, thanks for the tip! Had no idea this existed.

Me too! Awesome, thanks!

Doesn't make sense to automatically do this, some changes can be mutually exclusive

I do hand-updated CHANGELOG.md files, for all my projects.

Sometimes, my Git commits can be kind of verbose (and I'll occasionally put in smartass comments).

I wouldn't dream of mining my Git logs for CHANGELOG entries.

Actually, I hand-run many things that a lot of developers automate.

Part of it is because I have control issues...

A couple jobs ago, I was at a company that had a "mandatory" squash / rebase / merge workflow so history would be clean. On top of that, they forced all their developers to update a change log as part of the merge. That file was a source of contention / merge conflict for nearly every PR, often requiring additional rounds of rebasing. On top of that, it was full of information that could've been gathered from the git log. Big waste of time, in my opinion.

We had something kind of similar, but designed to avoid merge contention. We had each PR include a randomly-generated number, and a `changelogs/` folder where you could add a `${number}.md` that was either blank or had a message that would be added to the changelog. After you made the PR, you could run a bash script to edit the PR to contain the number and generate the `${number}.md` file.

It felt kind of silly and I don't know if anyone actually looked at the changelogs, but it took 2 minutes out of my day and worked well.

> The purpose of a git commit message is to answer the question “why does this commit exist?”

Why? Every time I have asked this people say because you'll search the logs (which I have never done in my life) or because "it's good practice"

Doesn't mean others are in the same boat as you. The team I work on frequently write detailed commits to the point where I can (and have several times) successfully searched information from years ago on a block of code with the ticket number and the developers (even myself) reasoning at the time.

Maybe it depends on the project but I've found myself doing this so often that I won't stop, it's such a small task that has given me so much benefit. If it doesn't help anyone then nothing was lost.

Why are you searching a year or more back? Is this using git blame?

To explain why a certain block of code exists. When the code came into existence is rarely that important. You just want to know why.

Why does the code fence against a particular circumstance you didn't think should be possible? Why does it call out to something you think is unrelated? Those questions can be answered by a proper commit message.

I set a breakpoint and run the test suite for that answer

That answers what the code does, but not why it does what it does.

So you never need to understand when or why a bug was introduced? Or you never need to understand why the current behavior as is it is?

Why don’t you put that as a comment on the code?

Comments are for why code is written the way it is, commits are for why the code is written in the first place.

You _could_ tag every line of code with

  // JIRA-123 The PM wants this to be blue

but if you did it would become unreadable and wouldn't be kept up to date.

Because if 37 changes are done to the same 15 line function over time, the amount of comment material will dwarf the function. And most of it will pertain to historic versions of the function which are not what actually appears below the comment; a comment made 13 revisions ago makes sense for the 13-revision-old version of the function.

You just update the comment?

Sometimes, in some kinds of projects, the commit messages looks like this:

  JIRA: #1234

  Adjusted the FOOBAR parameter from 42 to 73.

To know the "why", you have to read the ticket; you will not find anything in the git log.

Oh jeez, please do not make me run “git log” and then open a hundred tabs in an old bug tracker that may or may not still exist to figure out when a problem may have been introduced. I want code reviewers to insist on at least somewhat useful messages for us to skim at 3 AM.

Never looking at git history is like, not reading comments or something. It's an incredibly valuable resource for understanding why the existing code is the way it is.

You didn't answer why. My code passes all the unit test and we almost always have real code using it immediately. The function works. Why am I reading it? The only thing I read are bug reports (usually a spec problem, not normally a logic bug) and new features, or test outputs

If someone complains that our system did something weird, sometimes it’s a mistake we can just fix, but sometimes it’s a non-obvious consequence of some requirement (which I might not have been aware of) that we have to explain to our consumer. It also helps a lot to tell whether it has been like that for a week or five years.

I can’t even imaging having a spec that fully answers anything like this; Microsoft tried for that level of detail but I found they couldn’t keep it up to date.

Because you'd want to know why code does what it does.

If a bug pops up, you'd want to know why some code exists and if the original premise of the code is as it should be or needs revision.

I think it's very rare that a developer receives all that information up front in a team. Eg. Old codebase, lost knowledge or people leaving.

I think you just haven't discovered you can? I don't know of anyone arbitrarily looking through logs, but it's incredibly useful with git blame when you get to a section of code and don't understand it or typically it's done in an odd or unintuitive way. The blame shows who wrote it, information as to what they were working on via the actual commit message, and branch information.

If the person still works with you, you can just ask them about it.

If they don't or you don't want to bother them, the commit message tells you what the change was and often times why to give you a better context.

All the teams I've worked on included information like feature, bug, defect, and the associated ticket number in the branch name, so you have the information at hand to go look at the ticket directly and see what requirements were needed.

I'm starting to understand why. It appears my workflow has the information other people would want from a git log elsewhere (test, specs, examples, etc)

Let's say you have some code written 2 years ago. 2 months ago someone made a bug fix in the code replacing a few of the lines. How would you know from test, specs or examples which specific lines were modified and by whom? I don't get it.

Why would I want to do that? Usually if there's a problem I'll either write a new test or see if someone modified a test I thought was covering the case

Usually people 'own' a file or part of the system so that wouldn't really be happening anyway

As I look at the top ten committers to our key service, two of us are still here, the other eight (including the lead from day one) switched teams or left the company over the last five years. Unfortunately I don’t think 27% turnover per year is unusually high in tech, so ownership isn’t a good replacement for written records.

> Why would I want to do that?

Because sometimes we can learn from history. If a mistake was made at some point it can be good to understand why. Of course you can adopt the mindset that you don't care what ended up causing a bug, but learning from mistakes is a good thing.

> see if someone modified a test I thought was covering the case

How do you see if someone modified the test then? I feel like we are maybe misunderstanding eachother because to me, and seemingly most other commenter here, this is such an obvious no-brainer that it seems something is lost in the communication.

> Because sometimes we can learn from history. If a mistake was made at ...

A commit message helping me learn anything that I didn't get from a code comment sounds like a stretch. I'm doubting this

> How do you see if someone modified the test then? I feel like we are maybe misunderstanding eachother

Clearly. I was suggesting I might see commit messages if I'm using git blame to find out if a case was removed from a set of test or if it never existed in the first place. But I don't see how messages would help at all in anything I do. What I'm looking for is far too specific to be in a commit message and this whole thread about using commit messages to learn sounds nonsensical.

That's right; for log messages to be useful, they have to be very disciplined. They have to stick to a certain format and then if the information you're looking for is of the kind which is provided by that schema, then they are useful.

As an example, say log messages are strictly required to contain a bug database ticket number (even if they are not fixes: bug database tracks tasks too) then that is useful; you can quickly search the git log for a bug number to find all of its commits.

I work on several legacy codebases which were:

- written by people who were not me

- written by people who have since left the company

- the original documentation has rotted away through wiki replacements, issue tracker replacements, or being lost via people turnover or system replacement.

The result in many cases is the code is the _only_ documentation of the system behaviour, so seeing what was introduced together is important context to understanding why it is the way it is. I probably run git blame more than git commit at this point and there's a real QoL difference between the good commit messages and the "changes for ticket12345" commit messages.

If you have never needed to find out why a particular piece of code is the way it is, then you have been very lucky to work in very clean code bases. In my own experience, this has been an infrequent but inevitable part of work - maybe once a month or so, I have had to understand whether a particular piece of code, that seems wrong, had a good reason for existing or not.

Sometimes it turned out to be a mistake in the original commit, or working around a limitation that no longer exists, other times it has saved me from re-introducing a bug that someone had spent effort fixing.

ArrayBoundCheck didn't say they never had to find out why a particular piece of code is the way it is, just that they didn't have to search through log messages.

For isntance, when you use "git blame" and similar tools, the log messages are not involved. You might end up reading the log message of the commit that was responsible for a change, but you didn't search log messages to get there.

In a project with poor commit messages, they will be of little use; reading them won't produce much value, let a lone searching.

Anyway, that seems like the best possible interpretation of the user's comment, anyway.

tsimionescu didn't say anything about searching the logs.

The commit message explains (/should explain) why the commit was made. That is useful even if you arrive at the commit via git blame, bisect, etc.

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