Open Source Comes to Campus/Resources: Difference between revisions

=== Version Control Demo ===▼

(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!)▼

(FIXME: Go through this, and change the exercises so they start by having us make some edit to the page.)▼

* In open source software, people use version control to keep track of files as they change. To give you a sense of what this is for, how many of you have worked on group programming projects? (A few hands raise up.)▼

* How did you keep track of different people working on different files?▼

** 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.▼

* How well did that work out?▼

** Attendees will typically chuckle.▼

* So, yeah, one common way to do that is to email around copies of a file you're working on. A different way is to use what's called version control, where there is 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.▼

* To talk about version control, I want to show you how version control works for purely text documents. So let's look at the Wikipedia page for Wellesley College.▼

* (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)▼

* (find yourself at http://en.wikipedia.org/w/index.php?title=Wellesley_College&action=history ; 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.▼

* Whew. That was a lot. Are there any questions? I'm happy to go over and review anything here. (Take questions from the whole group, since probably someone else has the same question as any individual person who asks.)▼

* 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''. One fun git repository that's hosted on Github is this one,▼

** (go to https://github.com/bundestag/gesetze)▼

** which is the law of Germany. It's a good example of a version control project, tracked in git, where not everyone is allowed to edit the repository. But we can click around on their Github site and see all the commits.▼

** Again, one difference here is that Wikipedia is the encyclopedia that anyone can edit. For Wikipedia, that means that as soon as we edit a page, it's automatically live. For open source software projects, or the German legal system, there's often a review process -- you can use version control to prepare a diff and share it, and if the maintainer likes it, they ''merge'' it into the main project.▼

** (Click "Commits" -- https://github.com/bundestag/gesetze/commits/master )▼

* Another difference is that in git, revisions are called commits. They track not just changes to one file, but the entire state of the project at that time. So when you click "Browse code" at the most recent commit, it takes you to e.g. https://github.com/bundestag/gesetze/tree/ee428461b034de54f1cdcab524d2808486383677 which shows you the entire project.▼

** Usually you are just interested in the difference between this commit and the previous commit, so Github makes that easy: you just click on this short hexadecimal number, and it shows you a diff.▼

** One more 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.▼

* So, in a nutshell, those are the key notions behind version control, and git. Now's a great time for questions! After the questions, we'll take some time to work through a training mission to give you hands on experience in git and how projects use it.▼

* (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.▼