SVNBOOK Chap3 Peg and Operative Revisions

Peg and Operative Revisions

We copy, move, rename, and completely replace files and directories on our computers all the time. And your version control system shouldn't get in the way of your doing these things with your version-controlled files and directories, either. Subversion's file management support is quite liberating, affording almost as much flexibility for versioned files as you'd expect when manipulating your unversioned ones. But that flexibility means that across the lifetime of your repository, a given versioned object might have many paths, and a given path might represent several entirely different versioned objects. This introduces a certain level of complexity to your interactions with those paths and objects.

Subversion is pretty smart about noticing when an object's version history includes such “changes of address.” For example, if you ask for the revision history log of a particular file that was renamed last week, Subversion happily provides all those logs—the revision in which the rename itself happened, plus the logs of relevant revisions both before and after that rename. So, most of the time, you don't even have to think about such things. But occasionally, Subversion needs your help to clear up ambiguities.

The simplest example of this occurs when a directory or file is deleted from version control, and then a new directory or file is created with the same name and added to version control. The thing you deleted and the thing you later added aren't the same thing. They merely happen to have had the same path—/trunk/object, for example. What, then, does it mean to ask Subversion about the history of /trunk/object? Are you asking about the thing currently at that location, or the old thing you deleted from that location? Are you asking about the operations that have happened to all the objects that have ever lived at that path? Subversion needs a hint about what you really want.

And thanks to moves, versioned object history can get far more twisted than even that. For example, you might have a directory named concept, containing some nascent software project you've been toying with. Eventually, though, that project matures to the point that the idea seems to actually have some wings, so you do the unthinkable and decide to give the project a name. [17] Let's say you called your software Frabnaggilywort. At this point, it makes sense to rename the directory to reflect the project's new name, so concept is renamed to frabnaggilywort. Life goes on, Frabnaggilywort releases a 1.0 version and is downloaded and used daily by hordes of people aiming to improve their lives.

It's a nice story, really, but it doesn't end there. Entrepreneur that you are, you've already got another think in the tank. So you make a new directory, concept, and the cycle begins again. In fact, the cycle begins again many times over the years, each time starting with that old concept directory, then sometimes seeing that directory renamed as the idea cures, sometimes seeing it deleted when you scrap the idea. Or, to get really sick, maybe you rename concept to something else for a while, but later rename the thing back to concept for some reason.

In version 1.1, Subversion introduced a way for you to tell it exactly which Main Street you meant. It's called the peg revision, and it is provided to Subversion for the sole purpose of identifying a unique line of history. Because at most, one versioned object may occupy a path at any given time—or, more precisely, in any one revision—the combination of a path and a peg revision is all that is needed to refer to a specific line of history. Peg revisions are specified to the Subversion command-line client using at syntax, so called because the syntax involves appending an “at sign” (@) and the peg revision to the end of the path with which the revision is associated.

But what of the --revision (-r) of which we've spoken so much in this book? That revision (or set of revisions) is called the operative revision (or operative revision range). Once a particular line of history has been identified using a path and peg revision, Subversion performs the requested operation using the operative revision(s). To map this to our Chicagoland streets analogy, if we are told to go to 606 N. Main Street in Wheaton, [18] we can think of “Main Street” as our path and “Wheaton” as our peg revision. These two pieces of information identify a unique path that can be traveled (north or south on Main Street), and they keep us from traveling up and down the wrong Main Street in search of our destination. Now we throw in “606 N.” as our operative revision of sorts, and we know exactly where to go.

The peg revision algorithm

The Subversion command-line performs the peg revision algorithm any time it needs to resolve possible ambiguities in the paths and revisions provided to it. Here's an example of such an invocation:

  $ svn command -r OPERATIVE-REV item@PEG-REV

If OPERATIVE-REV is older than PEG-REV, then the algorithm is as follows:

• Locate item in the revision identified by PEG-REV. There can be only one such object.
• Trace the object's history backwards (through any possible renames) to its ancestor in the revision OPERATIVE-REV.
• Perform the requested action on that ancestor, wherever it is located, or whatever its name might be or have been at that time.

But what if OPERATIVE-REV is younger than PEG-REV? Well, that adds some complexity to the theoretical problem of locating the path in OPERATIVE-REV, because the path's history could have forked multiple times (thanks to copy operations) between PEG-REV and OPERATIVE-REV. And that's not all—Subversion doesn't store enough information to performantly trace an object's history forward, anyway. So the algorithm is a little different:

• Locate item in the revision identified by OPERATIVE-REV. There can be only one such object.
• Trace the object's history backwards (through any possible renames) to its ancestor in the revision PEG-REV.
• Verify that the object's location (path-wise) in PEG-REV is the same as it is in OPERATIVE-REV. If that's the case, then at least the two locations are known to be directly related, so perform the requested action on the location in OPERATIVE-REV. Otherwise, relatedness was not established, so error out with a loud complaint that no viable location was found. (Someday, we expect that Subversion will be able to handle this usage scenario with more flexibility and grace.)

Note that even when you don't explicitly supply a peg revision or operative revision, they are still present. For your convenience, the default peg revision is BASE for working copy items and HEAD for repository URLs. And when no operative revision is provided, it defaults to being the same revision as the peg revision.

Say that long ago we created our repository, and in revision 1 added our first concept directory, plus an IDEA file in that directory talking about the concept. After several revisions in which real code was added and tweaked, we, in revision 20, renamed this directory to frabnaggilywort. By revision 27, we had a new concept, a new concept directory to hold it, and a new IDEA file to describe it. And then five years and twenty thousand revisions flew by, just like they would in any good romance story.

Now, years later, we wonder what the IDEA file looked like back in revision 1. But Subversion needs to know if we are asking about how the current file looked back in revision 1, or if we are asking for the contents of whatever file lived at concepts/IDEA in revision 1. Certainly those questions have different answers, and because of peg revisions, you can ask either of them. To find out how the current IDEA file looked in that old revision, you run:

  $ svn cat -r 1 concept/IDEA 
  svn: Unable to find repository location for 'concept/IDEA' in revision 1

Of course, in this example, the current IDEA file didn't exist yet in revision 1, so Subversion gives an error. The command above is shorthand for a longer notation which explicitly lists a peg revision. The expanded notation is:

  $ svn cat -r 1 concept/IDEA@BASE
  svn: Unable to find repository location for 'concept/IDEA' in revision 1

And when executed, it has the expected results.

The perceptive reader is probably wondering at this point if the peg revision syntax causes problems for working copy paths or URLs that actually have at signs in them. After all, how does svn know whether news@11 is the name of a directory in my tree, or just a syntax for “revision 11 of news”? Thankfully, while svn will always assume the latter, there is a trivial workaround. You need only append an at sign to the end of the path, such as news@11@. svn only cares about the last at sign in the argument, and it is not considered illegal to omit a literal peg revision specifier after that at sign. This workaround even applies to paths that end in an at sign—you would use filename@@ to talk about a file named filename@.

Let's ask the other question, then—in revision 1, what were the contents of whatever file occupied the address concept/IDEA at the time? We'll use an explicit peg revision to help us out.

  $ svn cat concept/IDEA@1
  The idea behind this project is to come up with a piece of software
  that can frab a naggily wort.  Frabbing naggily worts is tricky
  business, and doing it incorrectly can have serious ramifications, so
  we need to employ over-the-top input validation and data verification
  mechanisms.

Notice that we didn't provide an operative revision this time. That's because when no operative revision is specified, Subversion assumes a default operative revision that's the same as the peg revision.

As you can see, the output from our operation appears to be correct. The text even mentions frabbing naggily worts, so this is almost certainly the file which describes the software now called Frabnaggilywort. In fact, we can verify this using the combination of an explicit peg revision and explicit operative revision. We know that in HEAD, the Frabnaggilywort project is located in the frabnaggilywort directory. So we specify that we want to see how the line of history identified in HEAD as the path frabnaggilywort/IDEA looked in revision 1.

  $ svn cat -r 1 frabnaggilywort/IDEA@HEAD
  The idea behind this project is to come up with a piece of software
  that can frab a naggily wort.  Frabbing naggily worts is tricky
  business, and doing it incorrectly can have serious ramifications, so
  we need to employ over-the-top input validation and data verification
  mechanisms.

And the peg and operative revisions need not be so trivial, either. For example, say frabnaggilywort had been deleted from HEAD, but we know it existed in revision 20, and we want to see the diffs for its IDEA file between revisions 4 and 10. We can use the peg revision 20 in conjunction with the URL that would have held Frabnaggilywort's IDEA file in revision 20, and then use 4 and 10 as our operative revision range.

  $ svn diff -r 4:10 http://svn.red-bean.com/projects/frabnaggilywort/IDEA@20
  Index: frabnaggilywort/IDEA
  ===================================================================
  --- frabnaggilywort/IDEA	(revision 4)
  +++ frabnaggilywort/IDEA	(revision 10)
  @@ -1,5 +1,5 @@
  -The idea behind this project is to come up with a piece of software
  -that can frab a naggily wort.  Frabbing naggily worts is tricky
  -business, and doing it incorrectly can have serious ramifications, so
  -we need to employ over-the-top input validation and data verification
  -mechanisms.
  +The idea behind this project is to come up with a piece of
  +client-server software that can remotely frab a naggily wort.
  +Frabbing naggily worts is tricky business, and doing it incorrectly
  +can have serious ramifications, so we need to employ over-the-top
  +input validation and data verification mechanisms.

Fortunately, most folks aren't faced with such complex situations. But when you are, remember that peg revisions are that extra hint Subversion needs to clear up ambiguity.