Version Control with Subversion. Chapter 1. Fundamental Concepts. This chapter is a short, casual introduction to Subversion and. We begin with a discussion of. Subversion, and show some simple examples of. Subversion in use. Even though the examples in this chapter show people sharing. Subversion. can manage any sort of file collection—it's not limited to. A version control system (or revision control system) is a. Of course, merely. What. makes a version control system useful is the fact that it allows. In this section, we'll introduce some fairly high- level. We'll limit our. discussion to modern version control systems—in today's. At the core of the version control system is a repository. The. repository usually stores information in the form of a. Any number of. clients connect to the repository, and. By writing data, a client. Figure 1. 1, “A typical client/server system” illustrates. Figure 1. 1. A typical client/server system. Why is this interesting? Port Manteaux churns out silly new words when you feed it an idea or two. Enter a word (or two) above and you'll get back a bunch of portmanteaux created by jamming. In finance, an exchange rate (also known as a foreign-exchange rate, forex rate, ER, FX rate or Agio) between two currencies is the rate at which one currency will be. Git is the source code version control system that is rapidly becoming the standard for open source projects. It has a powerful distributed model which allows. ![]() Small and Fast. Git is fast. With Git, nearly all operations are performed locally, giving it a huge speed advantage on centralized systems that constantly have to. Lyberty.com's weekly/monthly splash page. (Yes, a splash page is old fashioned, but it's been a tradition here since 1999.). Learn how to use Git with Jenkins. The GitHub plugin should be used if GitHub is the Git server of choice - CloudBees blog. So far, this sounds like the. And indeed, the. repository is a kind of file server, but. What makes the repository special. When a client reads data from the repository, it normally. But what. makes a version control client interesting is that it also has. A version control client can ask historical. What did this directory contain last. Wednesday?” and “Who was the last person to. These are the sorts of questions that are at the heart of any. A version control system's value comes from the fact that it. Most software programs understand. So how does a version control. How does his or her word. The answer is found in the version. A working copy is, quite literally, a local copy of a. VCS- managed data upon which. Working copies appear to other software. The task of. managing the working copy and communicating changes made to. If the primary mission of a version control system is to. But different systems use different strategies to. It's important to understand these different. First, it will help you. Subversion. Beyond. Subversion, since Subversion itself supports a couple of. The problem of file sharing. All version control systems have to solve the same. It's all too easy for users. Consider the scenario shown in. Figure 1. 2, “The problem to avoid”. Suppose we have two coworkers, Harry and Sally. They each. decide to edit the same repository file at the same time. If Harry saves his changes to the repository first, it's. Sally could accidentally. While. Harry's version of the file won't be lost forever (because. Harry made. won't be present in Sally's newer version. Harry's changes to begin. Harry's work is still effectively lost—or at. This is definitely a situation we want. Figure 1. 2. The problem to avoid. The lock- modify- unlock solution. Many version control systems use a. In this model, the repository allows only one person to. This exclusivity policy is managed. Harry must “lock” a file before. If Harry has locked a. Sally cannot also lock it, and therefore cannot make. All she can do is read the file. Harry to finish his changes and release his. After Harry unlocks the file, Sally can take her turn. Figure 1. 3, “The lock- modify- unlock solution”. Figure 1. 3. The lock- modify- unlock solution. The problem with the lock- modify- unlock model is that it's. Locking may cause administrative. Sometimes Harry will lock a file and then forget about it. Meanwhile, because Sally is still waiting to edit the file. And then Harry goes on vacation. Now. Sally has to get an administrator to release Harry's lock. The situation ends up causing a lot of unnecessary delay. Locking may cause unnecessary. What if Harry is editing the beginning of a text file. Sally simply wants to edit the end of the same file? These changes don't overlap at all. They could easily. There's no need for them to take turns in this.Locking may create a false sense of.Suppose Harry locks and edits file A, while Sally.B. But what if A and. B depend on one another, and the changes made to each are. Suddenly A and B don't work. The locking system was powerless to. It's easy for Harry and Sally to. Locking often. becomes a substitute for real communication. The copy- modify- merge solution. Subversion, CVS, and many other version control systems. In this model, each user's client. Users then work simultaneously and. Finally, the. private copies are merged together into a new, final. The version control system often assists with the. Here's an example. Say that Harry and Sally each create. They work concurrently and make changes to the. A within their copies. Sally saves her changes to. When Harry attempts to save his changes. A is. out of date. In other words, file A. So Harry asks his client. A. Chances are. that Sally's changes don't overlap with his own; once he has. Figure 1. 4, “The copy- modify- merge solution” and. Figure 1. 5, “The copy- modify- merge solution (continued)” show. Figure 1. 4. The copy- modify- merge solution. Figure 1. 5. The copy- modify- merge solution (continued)But what if Sally's changes do overlap. Harry's changes? What then? This situation is called a. When Harry asks his client to merge the. A is somehow flagged as being in a state of conflict. Note that software can't. Once Harry has manually resolved the overlapping. Sally—he. can safely save the merged file back to the. The copy- modify- merge model may sound a bit chaotic, but. Users can work in. When they work on. And. the amount of time it takes to resolve conflicts is usually. In the end, it all comes down to one critical factor. When users communicate poorly, both. No system can. force users to communicate perfectly, and no system can. So there's no point in being. Version Control the Subversion Way. We've mentioned already that Subversion is a modern. As we described in. Version Control Basics” (our. Subversion's versioned data, and it's. In this section, we'll begin to. Subversion implements. Subversion implements the concept of a version control. Unlike a working copy, a Subversion repository is an. Subversion's own libraries and tools. As most of a user's. Subversion interactions involve the use of the Subversion. Subversion working. For the finer details of the. Chapter 5, Repository Administration. A Subversion client commits (that is, communicates the. By atomic transaction, we mean. Subversion tries to retain. Each time the repository accepts a commit, this creates a. Each revision is assigned a. The initial revision of a freshly. Figure 1. 6, “Tree changes over time”. Imagine. an array of revision numbers, starting at 0, stretching from. Each revision number has a filesystem tree. Figure 1. 6. Tree changes over time. Addressing the Repository. Subversion client programs use URLs to identify versioned. Subversion repositories. For the. most part, these URLs use the standard syntax, allowing for. URL. http: //svn. Subversion repository URLs aren't limited to only. Because Subversion. URLs used to address the repository. Table 1. 1, “Repository access URLs”. URL schemes map to the available. For more details about. Subversion's server options, see. Chapter 6, Server Configuration. Table 1. 1. Repository access URLs. Schema. Access methodfile: ///Direct repository access (on local disk)http: //Access via Web. DAV protocol to Subversion- aware. Apache serverhttps: //Same as http: //, but with. SSL encapsulation (encryption and authentication)svn: //Access via custom protocol to an. Same as svn: //, but through. SSH tunnel. Subversion's handling of URLs has some notable nuances. For example, URLs containing the file: //. Also, users of the file: // scheme on. Windows platforms will need to use an unofficially. Either of the two. URL path syntaxes will work, where. X is the drive on which the repository. X: /var/svn/reposfile: ///X|/var/svn/repos. Note that a URL uses forward slashes even though the. URL) form of a path on Windows uses backslashes. Also note that when using. X|/. form at the command line, you need to quote the URL (wrap it. Note. You cannot use Subversion's file: // URLs. URLs can. When you attempt to view. URL in a regular web browser, it. However, Subversion's. Repository Layer”), and your browser. The Subversion client will automatically encode URLs as. For example, the URL. ASCII characters. Subversion as if. C3%B1a. If the URL contains spaces, be sure to place it within. There is one notable exception to Subversion's handling of. URLs which also applies to its handling of local paths in many. If the final path component of your URL or. Peg and Operative Revisions”—in order to make. Subversion properly address that resource. In Subversion 1. 6, a new caret (^). About - Git. Branching and Merging. Small and Fast. Distributed. Data Assurance. Staging Area. Free and Open Source. Branching and Merging. The Git feature that really makes it stand apart from nearly every other SCM out there is its branching model. Git allows and encourages you to have multiple local branches that can be entirely independent of each other. The creation, merging, and deletion of those lines of development takes seconds. This means that you can do things like. Frictionless Context Switching. Create a branch to try out an idea, commit a few times, switch back to where you branched from, apply a patch, switch back to where you are experimenting, and merge it in. Role- Based Codelines. Have a branch that always contains only what goes to production, another that you merge work into for testing, and several smaller ones for day to day work. Feature Based Workflow. Create new branches for each new feature you're working on so you can seamlessly switch back and forth between them, then delete each branch when that feature gets merged into your main line. Disposable Experimentation. Create a branch to experiment in, realize it's not going to work, and just delete it - abandoning the work—with nobody else ever seeing it (even if you've pushed other branches in the meantime). Notably, when you push to a remote repository, you do not have to push all of your branches. You can choose to share just one of your branches, a few of them, or all of them. This tends to free people to try new ideas without worrying about having to plan how and when they are going to merge it in or share it with others. There are ways to accomplish some of this with other systems, but the work involved is much more difficult and error- prone. Git makes this process incredibly easy and it changes the way most developers work when they learn it. Small and Fast. Git is fast. With Git, nearly all operations are performed locally, giving it a huge speed advantage on centralized systems that constantly have to communicate with a server somewhere. Git was built to work on the Linux kernel, meaning that it has had to effectively handle large repositories from day one. Git is written in C, reducing the overhead of runtimes associated with higher- level languages. Speed and performance has been a primary design goal of the Git from the start. Let's see how common operations stack up against. Subversion, a common centralized version control system that is similar. CVS or Perforce. Smaller is faster. For testing, large AWS instances were set up in the same availability zone. Git and SVN were installed on both machines, the Ruby repository was copied to. Git and SVN servers, and common operations were performed on both. In some cases the commands don't match up exactly. Here, matching on the lowest. For example, the 'commit' tests also include. Git, though most of the time you would not actually be pushing. All of these times are in seconds. Operation. Git. SVNCommit Files (A)Add, commit and push 1. Commit Images (B)Add, commit and push 1. Diff Current. Diff 1. Diff Recent. Diff against 4 commits back (2. Diff Tags. Diff two tags against each other (v. Log (5. 0)Log of the last 5. Log (All)Log of all commits (2. M of output) 0. 5. Log (File)Log of the history of a single file (array. Update. Pull of Commit A scenario (1. Blame. Line annotation of a single file (array. Note that this is the best case scenario for SVN - a server with no load with an. MB/s bandwidth connection to the client machine. Nearly all of these. SVN if that connection was slower, while. Git times would not be affected. Clearly, in many of these common version control operations, Git is. SVN, even under ideal conditions. One place where Git is slower is in the initial clone operation. Here, Git is downloading the entire history rather than only the latest. As seen in the above charts, it's not considerably slower for an operation. Operation. Git*Git. SVNClone. Clone and shallow clone(*) in Git vs checkout in SVN 2. Size (M)Size of total client side data and files after clone/checkout (in M)1. It's also interesting to note that the size of the data on the client side. Git also has every version of every file for the. This illustrates how efficient it is at compressing. Distributed. One of the nicest features of any Distributed SCM, Git included, is that it's distributed. This means that instead of doing a "checkout" of the current tip of the source code, you do a "clone" of the entire repository. Multiple Backups. This means that even if you're using a centralized workflow, every user essentially has a full backup of the main server. Each of these copies could be pushed up to replace the main server in the event of a crash or corruption. In effect, there is no single point of failure with Git unless there is only a single copy of the repository. Any Workflow. Because of Git's distributed nature and superb branching system, an almost endless number of workflows can be implemented with relative ease. Subversion- Style Workflow. A centralized workflow is very common, especially from people transitioning from a centralized system. Git will not allow you to push if someone has pushed since the last time you fetched, so a centralized model where all developers push to the same server works just fine. Integration Manager Workflow. Another common Git workflow involves an integration manager — a single person who commits to the 'blessed' repository. A number of developers then clone from that repository, push to their own independent repositories, and ask the integrator to pull in their changes. This is the type of development model often seen with open source or Git. Hub repositories. Dictator and Lieutenants Workflow. For more massive projects, a development workflow like that of the Linux kernel is often effective. In this model, some people ('lieutenants') are in charge of a specific subsystem of the project and they merge in all changes related to that subsystem. Another integrator (the 'dictator') can pull changes from only his/her lieutenants and then push to the 'blessed' repository that everyone then clones from again. Data Assurance. The data model that Git uses ensures the cryptographic integrity of every bit. Every file and commit is checksummed and retrieved by its. It's impossible to get anything out of Git. It is also impossible to change any file, date, commit message, or any other. Git repository without changing the IDs of everything after it. This means that if you have a commit ID, you can be assured not only that. Most centralized version control systems provide no such integrity by default. Staging Area. Unlike the other systems, Git has something called the "staging area" or "index". This is an intermediate area where commits can be formatted and reviewed before completing the commit. One thing that sets Git apart from other tools is that it's possible to quickly stage some of your files and commit them without committing all of the other modified files in your working directory or having to list them on the command line during the commit. This allows you to stage only portions of a modified file. Gone are the days of making two logically unrelated modifications to a file before you realized that you forgot to commit one of them. Now you can just stage the change you need for the current commit and stage the other change for the next commit. This feature scales up to as many different changes to your file as needed. Of course, Git also makes it easy to ignore this feature if you don't want that kind of control — just add a '- a' to your commit command in order to add all changes to all files to the staging area. Free and Open Source. Git is released under the GNU General Public License version 2. The Git project chose to use GPLv. However, we do restrict the use of the term "Git" and the. Please. see our trademark policy for.
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