Using CVS for Web Development

by Philip Greenspun (philg@mit.edu)

ArsDigita : ArsDigita Systems Journal : One article

If you have a very clear publishing objective, specs that never change, and one very smart developer, you don't need version control. If you have evolving objectives, changing specifications, and multiple contributors, you need version control.

The Solution

  1. three Web servers (can be on one physical computer)
  2. two Oracle users/tablespaces (can be in one Oracle instance)
  3. one Concurrent Versions System (CVS) root
  4. two people trained to understand CVS
Let's go through these item by item.

Item 1: Three Web Servers

Suppose that your overall objective is to serve a Web service accessible at "foobar.com". You need a production server, rooted at /web/foobar (Server 1). You don't want your programmers making changes on the live production site. That's sort of the whole point of this document. So you need a development server, rooted at /web/foobar-dev/ (Server 2). You might think that this is enough. When everyone is happy with the dev server, have a code freeze, test a bit, then copy the dev code over to the production directory and restart.

What's wrong with the two-server plan? Nothing if you are running photo.net circa 1997. The development team consisted of me and Jin. The testing team... me and Jin! Note that there was no possibility of simultaneous development and testing. ArsDigita.com customers, however, usually have enough budget to pay for four or five programmers plus 20 or 30 internal staffers who may be updating content, testing changes, and sometimes contributing code. For a complex site, the publisher may wish to spend a week testing before launching a revision. It isn't acceptable to idle authors and developers while a handful of testers bangs away at the development server. The solution? A staging server, rooted at /web/foobar-staging/ (Server 3).

Here's how the three are used:

  1. developers work continuously in /web/foobar-dev/
  2. when the publisher is mostly happy with the development site, a named version is created and installed at /web/foobar-staging
  3. the testers bang away at the /web/foobar-staging server
  4. when the testers and publishers sign off on the staging server's performance, the site is released to /web/foobar/ (production)
  5. any fixes made to the staging server are merged back into the development server

Item 2: two Oracle users/tablespaces

Atlantic City (New Jersey) Suppose that you have a working production site. You could connect your /web/foobar-dev/ to the production Oracle user. After all, Oracle's raison d'être is concurrency control. It will be happy to run eight simultaneous connections to your production site plus two or three to the development server. The fly in this ointment is that one of your developers might get a little sloppy and write a program that sends drop table users rather than drop table users_experimental_extra_table to the database.

So it would seem that we'll need at least one new Oracle playground. Here are the steps:

  1. create a new Oracle user and tablespace, named "foobardev" (assuming the production user is "foobar")
  2. import a recent Oracle export.dmp file to populate your tablespace with what was on the production site (if you're following the tenets of the ArsDigita Server Architecture you'll always have one from the previous night anyway). Cry with pain as you discover that Oracle imports don't work with LOB columns unless you're importing into an installation that has a tablespace with the same name as the one from which the tables were exported.
  3. every time you alter a table, add a table, or populate a new table, record the operation in /web/foobar-dev/www/doc/sql/patches.sql
  4. when you're ready to move from staging to production, hastily apply all the data model modifications from patches.html to the production Oracle user
Shouldn't we have three Oracle users? One for dev, one for staging, one for production? No. It usually isn't worth it. Adding a column to a relational database table seldom breaks queries. Until Oracle 8.1.5, you weren't able to drop a column. And anyway the radical data model changes tend to take place when a site has yet to be launched.

The bottom line is that it takes work to keep three Oracle users' objects in sync. It is half as much work to sync two and almost as useful. How to deploy these two Oracle users? Park one behind the production server. Use the other one behind the dev and staging servers.

Item 3: one Concurrent Versions System (CVS) root

Rhya Fisher at Harvard Bookstore. Cambridge, MA 1998. The Concurrent Versions System (CVS) is a powerful file system-based tool that can do the following things: CVS is free and open-source.

CVS does all of this via its repository or "CVS root". This is a directory, typically /usr/local/cvsroot/. Most Unix machines don't have enough space in the /usr partition to store all Web content. Remember that the CVS root will be at least as large as all of the files under source control. Thus we will use /cvsweb as our CVS root and, if need be, migrate it to a separate disk subsystem.

Create a project from your development Web sources (from /web/foobar-dev/) so that they will end up at /cvsweb/foobar/.

Item 4: Two Trained CVS Users

Don't plan to teach all of your contributors the arcana of CVS. The ones who use GNU Emacs will need to learn to type c-x c-q and c-c c-c to contribute change comments. But the contributors who use primitive tools (FTP, HTTP PUT, vi) can remain blissfully unaware of the fact that CVS is in use.

Who is really using CVS then? A cron job. Every day just before midnight the cron job should check in all changes from the dev server to the main branch, with the change comment 'nightly check-in YYYY-MM-DD'. The cron job should notify the Release Master if any files that are in the repository have been deleted so that he or she can decide whether the removal was a mistake or if typing cvs remove is warranted (the files don't really go away; they go into an "attic").

One person is designated the Release Master. Normally this person does nothing. When the publisher is happy with the behavior of the development server, the Release Master creates a CVS branch named "199909Launch" or whatever. The Release Master updates the staging server from CVS with this branch. Development proceeds with checkins to the main CVS branch. These won't affect updates from the 199909Launch branch.

Once the staging server has been thoroughly tested, the Release Master checks in any changes that have been made. The check-in happens twice, once to the 199909Launch branch (there won't be any conflicts since nobody has been touching this) and once to the main branch (conflicts may need to be resolved).

When the publisher decides to go live, the Release Master takes the following steps:

  1. manually update the /parameters/foobar.ini file as necessary
  2. update production server from the CVS branch 199909Launch
  3. apply any data model changes (quickly!) from /doc/patches.html.
If there are significant data model changes, do this in the middle of the night and consider bringing up a "comebacklater" server for a few minutes!

If the Release Master is doing all of this hard work, why do we need to train anyone else in CVS? A Web service is 24x7 but one person can't work 24x7. So we need a Release Apprentice for each Web service who knows everything that there is to know about this system.

Exactly which directories do we control?

A programmer's intuitions about which directories to control will generally be 180-degrees off. For example, a programmer might think that it isn't worth controlling graphics files. After all, CVS can't really do much with these besides compare them byte by byte and tag them with dates.

The ArsDigita Community System generally contains the following under /web/foobar:

The bottom line is that it would be nice to just say "all of /web/foobar-dev" but we can't do this unless we're careful with the auxconfigdir (/parameters) and make sure to keep user-uploaded files out of the /web/foobar/ directory.

Do you need a farm of big fancy servers to implement this?

A farm in Alberta, on the way to Calgary from Montana How big and how many computers do you need to adopt the procedures described in this document? Three Web servers, two Oracle users, the CVS package, ... Sounds complicated. Actually you can run it all on a $2000 Linux box.

If you're worried about your developers being sloppy and editing files in /web/foobar/ when they thought they were in /web/foobar-dev/ remember that you can always use cvs update to revert the production site to the most recent approved version.

Suppose that you've ample money for server hardware, co-location fees, and sysadmin resources. You probably want to split the production machine out and only give the Release Master and Release Apprentice access to that box. Let the developers and staging/testing folks fight it out on a development server.

Why not one development area per developer?

Professor Hal Abelson and daughter Amanda, taken for the back cover of the book they wrote together on the LOGO computer language Classically, CVS is used by C developers and each C programmer works from his or her own directory. This makes sense because there is no persistence in the C world. You compile your code, run a binary that builds data structures in RAM and when the program terminates it doesn't leave anything behind (except maybe a core file). Checking out a CVS tree and working on it isn't a big deal.

Compare this to the world of db-backed Web servers. If you want to check out a copy of the tree and play with it, you have to create an Oracle user and tablespace, import a recent Oracle export.dmp file to populate your tablespace with what was on the production site, find a free IP address or port and set up a Web server, and then keep your Oracle table definitions in sync with any alterations other developers may be making.

In the C world, developers live to satisfy themselves. More than likely, not another soul on the planet will ever run the code that they are authoring. So it is fine for them to work alone. In the Web world, developers always work with the publisher and users. Those collaborators will need to be alerted to this new server so that they can offer criticism and advice. They might need special passwords or firewall access since most publishers don't like to let the public see their unfinished development efforts.

In the C world, you've got the luxury of one or two years between product releases. All the work is done by people with at least four years of training. In the Web world, a significant new release may need to be produced in four weeks. Much of the work may be done by people with no formal training of any kind, e.g., designers and content authors editing templates or static .html pages. Given the chronic shortage of personnel in this industry, do you want to limit yourself to being able to hire only those who've been through a CVS training course? To those who are formally minded enough to read the CVS man pages? Remember that most of the contributors on your site will not be programmers.

The bottom line? It is just too much work to set up each contributor with his or her own little server.

Good Things About This System

To end this article on a positive note, let's summarize the good things about this system:

More

asj-editors@arsdigita.com

Reader's Comments

If you are setting up a new cvs server, spend a few extra minutes to configure CVS using the client-server ("pserver") mode, instead of the older file system mode. This will save you pain later and may keep you out of hot water. Pain, because moving the repository (your old one dies, your company IPO's and your boss wants to buy a big fancy server farm, you want to hide the repository behind a firewall) is matter of changing an environment variable. You get immedieate access control (developers can be protected from updating the production environment). CVS in file system mode can "hang" because it leaves a lock file around for each file and directory. Then you need a cvs guru to dive in and fix it. One note: you can't live in a mixed environment. It is either one mode or the other.

An expert tip on using client server: CVS uses gzip for compressing data across the network. The default setting is -z3 which is a pitiful waste of time. Recompile CVS to use -z9 by default (the network is the bottleneck, not CPU resources), or add it to everyone's .cvsrc configuration file (it lives in the users' home directory).

I've had some extremely painful experiences with CVS and large binary files. (Large is +32Mb) When CVS checks a file out of the repository, even if it is doing nothing more than a straight copy (no diff'ing, merging, etc.) the program brings the whole file into contiguous memory. This bloats the CVS process resident set size to at least the size of the file, +6Mb for the program, give or take. The process is inefficient, so subsequent large files don't reusue the space well. CVS bloats even more. Make sure that your server is configured with a lot of swap space (it should have a lot of memory anyway). Even so, performance will drag down into the ground until CVS is finished (could be 30 minutes for a large working set), then things will "mysteriously" return to normal.

-- Ken Mayer, July 23, 1999

Your proposed once per day automatic check-in of everything is a nice idea for a group such as your ArsDigita companay with it's fairly non standard mission statement.

In more mundane companies however you usually have at least one mid-lewel manager who will see the amount of code checked-in every day as a measurement of individual emplye efficiency, and wrech all sorts of havoc with this misguided "knowledge".
I'm sure some of you have expierenced mid-level managers who were too dump to even figure out how to do this, but I have never been that unlucky ;-)

Apart from this your proposed method sounds remarkely similar to what I have been doing for various db backed websites over the last few years. It has proven itself to me to be a great time saver and I don't even want to calculate how many near disasters with their associated all night fix-up sessions it has saved me or my co-workers from.

The pserver is surely the only way to share CVS among a group of people without running into all sorts of non-interesting problems with nfs etc. You can also tunnel it through ssh for secure over-the-net operations.

-- Kristian Sørensen, July 24, 1999

Regarding putting the stuff in /parameters - the .ini files - under CVS, and requiring different .ini files for your three servers: this is a darn good reason to use Tcl configuration files in AOLserver 3.0 instead of .ini files. Then config file can use Tcl to determine whether it's a production, dev, or staging server (based on an environment variable, or the server home, etc.), and use the appropriate config values.

-- Rob Mayoff, February 26, 2000
Although my company does not use CVS, we have used Microsoft's Visual Source Safe and Intersolv's PVCS Version Manager. Both were a pain to setup and have people use them. All complains usually go away after the first time that version control saves your day after some screwup.

As for the managers, they usually don't care. Obviously some misguided soul is going to use this tool to gather information on who worked on what and for how long, but around the office 99% of the people are interested in it because it saves us from many headaches.

I don't think I ever want to work on a project without some kind of version control.

-- Pedro Vera-Perez, March 14, 2000

When dealing with large teams of developers using CVS can be a real headache. One alternative would be BitKeeper which solves most (if not all) CVS's problems. It was written by the guys that did SUN's TeamWare's source management system.

-- Petru Paler, April 18, 2000

The Mozilla project is doing well with CVS and a quite large number of developers.

Also, CVS pserver mode is rather unsafe, but it works really well over SSH. I made a small Perl script that is used as a shell for accounts that I only want them to do CVS that checks if the user is going to run CVS, and shows them a message if not.

#! /usr/bin/perl -Tw

use strict;

delete $ENV{ENV};
$ENV{PATH}='/usr/bin:/bin:/usr/sbin:/sbin';

if($ARGV[1] ne 'cvs server') {
        print STDERR "This account can only be used for CVS access\n";
        exit(1);
}

exec("cvs server");


-- Pierre Phaneuf, November 6, 2000
The whole purpose of having a source control system is so multiple developers can work on the same set of source files. Suggesting only 2 persons need to know CVS while others still check stuff in with FTP shows that the author does not appreciate the full potential of CVS or a similar source control system.

-- jay teo, March 9, 2001
I have to disagree with your comments on having one development area per developer. I think its mandatory and would defeat the whole purpose of CVS. Why would you have two developers working in the same directory at the same time? The file system doesn't allow for two users to edit the same file at the same time.

Also there is no need to create another instance of Oracle for this purpose. The same development instance can be used for all developers. Database changes happen more infrequently than does code changes.

Where I work, the process of creating a seperate development space has been packaged into an RPM, or you can use whatever packaging system you prefer. There is no need for different IP address, just use a different port. All development areas can reside on the same machine.

-- Thai Nguyen, February 14, 2002

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