Introduction to Version Control
This talk is assuming you are speaking at Wellesley College. You're probably not going to be speaking at Wellesley, so you'll need to make a handful of changes. For instance, it is highly recommended that you browse through the wikipedia page history of your campus's school ahead of time to find a fun edit to use as an example. (Here are examples used at Wellesley and Northeastern and Rutgers.)
Why version control
- I want you to think back to the last time you worked on a big project. Could be for school, could be for some other purpose. (Take a moment to let them think.) How many people worked on that project? (Get answers.) How long did you work on it? (Get answers.) How many separate pieces did it have? (Get answers.)
- Now here's the real question - how did you keep track of the changes that you made?
- Some people will say they emailed around different copies of files.
- Others may say that they had people all working in person and talking about changes.
- Some will probably mention version control systems.
- How well did that work out? (Attendees will typically chuckle.)
- All sorts of problems can crop up when you're working on a project. What if you don't like the changes you made and you want to go back to how things were? What if multiple people want to make changes to the same file at the same time? What if you have some sort of class clown in your group who keeps going through and changing random words to 'Batman'? How do you handle that.
- The answer: version control. It's a system for tracking different versions of a file. The most popular tools let every contributor keep a copy of the project on their own computer, making whatever changes they want, and eventually synchronizing with the rest of the team.
- Okay, so imagine you're working on a project with about 19 million other people, editing 4 million files?
- That's Wikipedia. As you can imagine, they've put a lot of thought into version control. Let's go look at how they do it. (Navigate web browser to http://en.wikipedia.org/wiki/Wellesley_College )
- So one of the great things about Wikipedia is that the system keeps every track of every version of every article. The wiki system is a version control system, too. If you click on the "Edit" tab, you can see the built in way to edit every article. Let's take a look at the history tab here. (click history tab; zoom in so it's large enough for people to see; make it really enormous)
- So here, you can see a little heading for every version of the page, including (mouse over the date) the time and date of that version, who saved that version, and a brief summary of what the changes are. Here, the most recent version says +73 -- that means that 73 characters were added in that revision.
- One standard feature of version control tools is to be able to see each version of the file. On Wikipedia, you can do that by clicking on the date. (click on the most recent edit, taking you to e.g. http://en.wikipedia.org/w/index.php?title=Wellesley_College&oldid=559029901 )
- You can see the box at the top that says, "This is the current revision of the page." The real thing going on here is that we're using an identifier for the page that will never change, even if the page gets edited, because we're looking at the particular revision.
- Let's hit back, and take a look at an older version that has an edit summary. In this case, we see they say they added a note about New Zealand. When we click the date (Click the date)... and load the page, we can see visually that the page seems to have a note about New Zealand. But we don't know just from looking at this version of the page if maybe they made other changes.
- You folks already saw diffs, and like most version control tools, Wikipedia lets you see a diff between the two pages. It's line-based, like a lot of tools used in programming. Let's click back... (back to http://en.wikipedia.org/w/index.php?title=Wellesley_College&action=history ) and instead of clicking on the date, we'll select this revision as the first item selected, and the one right after it as the one we want to diff against. When we click "Compare selected revisions", it takes us to a page like http://en.wikipedia.org/w/index.php?title=Wellesley_College&diff=558216275&oldid=556114296
- On that page, you can see the one + line: the addition of this link to a different school in New Zealand. In particular, this shows us that there are no changes other than the particular one about the new link.
- One key difference between wikipedia's version control system and most others is that Wikipedia's default is to accept contributions. If you make a change, it will instantly go live. In fact, let's make a change now. (Make a minor edit.) Now, someone could come along and revert the change. They could even ban our account, if they think we're acting in bad faith. But this edit will still have been part of the current version for a brief period of time. In most other version control systems, a change has to be reviewed and approved before it goes live.
Different version control tools out there
- So that's the very basics of what version control is. In open source projects, probably the most popular version control tool is called git. It's a command line program, and during the laptop setup process you already installed it hopefully. A lot of projects tracked with git also use Github, which is a website for browsing these git repositories.
- We're going to focus on git and github today, but you might come across other version control systems and websites today and in the future, so I just want to mention a few of them:
- Other systems: Some of the more popular version control systems include Mercurial, Subversion, GNU-Bazaar. There are some proprietary version control systems, too, but we don't bother with them.
- Then there are some code hosting sites that work with different types of version control systems. Github works with git. Google Code works with git, mercurial, and subversion. Bitbucket works with mercurial and git. Again, there are other hosting sites, too many to mention here.
- If you're trying to work on a project but you're having trouble understanding their version control system, check and see which one it is. We have training missions on Openhatch.org for using git and subversion, and can likely help you informally on #openhatch if it's some other system.
- To get back to git, which is again the most popular tool and the one we'll be teaching you about today.
- Git calls your revisions commits. This refers not to changes to individual files, but to the state of the project at a given point. Here's a list of commits (https://github.com/bundestag/gesetze/commits/master, then click on an individual hexadecimal #) which you can see sometimes involve changes to one file, and sometimes to more. You can also see the diff between this commit and the previous one. You get to decide how frequently to bundle your changes into commits. I personally commit frequently and compulsively.
- Another difference is how the version numbers are encoded. In git, the revision number is a checksum of all the important information about the commit: the name and email address of the person who made the change, plus the contents of the files that are part of the commit, plus a timestamp of when the commit was made, plus a few other bits of data. In some systems, the version numbers are just numbers that increase over time. With git, since every person uses their own computer and can make commits whenever they want, the commit IDs can't collide, so they're computed based on the content.
Other things you can use version control for
- A lot of people use version control for code, but as we can see with wikipedia that's not the only use for it. You can use version control on almost anything digital. For instance:
- There have been several efforts to keep track of legislation via version control systems. For instance, Gesetze follows the German Bundestag.
- The White House has put part of their open data initiative on Github and has been accepting issues and pull requests.
- The city of Chicago has started sharing its open data via Github.
- Hundreds of people have put up repositories of poetry, and a friend of mine uses git to keep track of her novel.
- Some educators are sharing syllabi.
- Some scientists are publishing their raw data, as well as the scripts they use for processing and analyzing it, on Github.
Exercises & Follow up
- (Put on projector: https://openhatch.org/missions/git)
- This is the URL for an interactive teaching tool to show you how open source communities use git, and that helps you get used to using it on your own computer. We'll have teaching assistants walking around hoping to help you.
- If you finish that, work through this second URL:
- (put on projector: http://try.github.io/ ) but we really encourage you to work through the training mission first.