The ability to copy, move, and rename files and directories; to be able to create an object, then delete it and then add a new one at the same path—those are operations which we perform on files and directories on our computers all the time, and operations we tend to take for granted. And Subversion would like you to think they are granted. Subversion's file management support is quite liberating, affording almost as much flexibility for versioned files that you'd expect when manipulating your unversioned ones. But that flexibility means that across the lifetime of your repository, a given versioned resource might have many paths, and a given path might represent serveral entirely different versioned resources.
Subversion is pretty smart about noticing when an object's version history includes such “changes of address”. For example, if you ask for all the logs 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. Clearly the thing you deleted and the thing you later added aren't the same thing, they merely happen to have had the same path, which we'll call /trunk/object. 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 lived at that path? Clearly, Subversion needs a hint about what you are really asking.
And thanks to moves, versioned resource history can get far more twisted than that, even. 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. [32] 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.
When scenarios like these occur, attempting to instruct Subversion to work with these re-used paths can be a little like instructing a motorist in Chicago's West Suburbs to drive east down Roosevelt Road and turn left onto Main Street. In a mere twenty minutes, you can cross “Main Street” in Wheaton, Glen Ellyn, and Lombard. And no, they aren't the same street. Our motorist—and our Subversion—need a little more detail in order to do the right thing.
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 a revision provided to Subversion for the sole purpose of identifying a unique line of history. Because at most one versioned resource 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, [33] we can think of “Main Street” as our path and “Wheaton” as our peg revision. These two pieces of information identify a unique path which can travelled (north or south on Main Street), and will keep us from travelling 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.
Subversion performs a fairly straightforward algorithm when using peg and operative revisions to find real stuff to work on. First, the path associated with the peg revision is located in that revision in the repository. From there, Subversion begins stepping backwards through historical predecessors of the object located at that path and peg revision. Each of these predecessors represents an earlier version of the object, and each stores a record of which revision it was created in, and at what path. So, while traversing the set of predecessors, Subversion notices whenever one of them is the youngest version of the object to exist as of one of the operative revisions, and if so, maps that operative revision to the created path/created revision pair of the predecessor. The algorithm terminates either when all the operative revisions have been mapped to real object locations, or once there are no more predecessors to traverse, in which case any unmapped operative revisions are marked as invalid for the object being operated on.
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 are we 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 subversion/libsvn_client/ra.c:775: (apr_err=20014) 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 subversion/libsvn_client/ra.c:775: (apr_err=20014) svn: Unable to find repository location for 'concept/IDEA' in revision 1
And when executed, has the expected results. Peg revisions generally default to a value of BASE (the revision currently present in the working copy) when applied to working copy paths, and of HEAD when applied to URLs.
Let's ask the other question, then—in revision 1, what were the contents of whatever file occupied the address concepts/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.
This appears to be the right output. 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 beed 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.