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Tom Lord's Decentralized Revision Control System
Bruce Perens writes: "He'll have to change its name, but Tom Lord's arch revision control system is revolutionary. Where CVS is a cathedral, 'arch' is a bazaar, with the ability for branches to live on separate servers from the main trunk of the project's development. Thus, you can create a branch without the authority, or even the cooperation, of the managers of the main tree. A global name-space makes all revision archives worldwide appear as if they are the same repository. Using this system, most of what we do using 'patch' today would go away -- we'd just choose, or merge, branches. Much of the synchronization problem we have with patches is handled by tools that eliminate and/or manage conflicts -- they solve some of the thorny graph topology issues around patch management. Arch also poses its own answer to the 'Linus Doesn't Scale' problem. This is well worth checking out." If you're asking "What about subversion?", well, so is Tom.
I guess I'm wondering why arch uses FTP as its network protocol. The FAQ says that it should be workable behind firewalls since the data is all transferred in passive mode, but this still seems like a huge step backwards.
So, what am I missing? I only got to read a little bit of the site before it got DDOS'd by slashdot.
You cannot apply a technological solution to a sociological problem. (Edwards' Law)
Not only 'what about Subversion' but also 'what about CVS, what about Aegis'. If you include non-free systems then what about Perforce or Bitkeeper.
:-(.
This is getting worse than journalling filesystems
-- Ed Avis ed@membled.com
That sounds like hype. In the real world, selecting the aspects of software we want to compile from on remote sites would have serious implications. The first being security. The second being quality. Linus may not scale, but he has good judgement. That's the fundamental problem.
It sounds like it has a lot of nice features, but then you realize the whole thing is written in sh? One of the nice things about CVS is that the client-server nature allows someone to use pretty much any operating system as a client. Subversion takes this to the next step, by making all connections use the client-server model.
Forcing everyone to use sh is a major hassle. I know that it would work with any "reasonably POSIX" OS, but then developers can't get arch accessibility built into their favorite tools, like NetBeans or whatever.
Creating local branches is pretty cool, though.
Mike
From the article, it looks good.
But let me say that I've sometimes been in the position of having to merge branches. In my first hacking job, I had to take code that had been written by 2 crazy Polish programmers, and merge 37 non-working branches into one branch that worked. It was *not* fun, and I enjoyed a well-deserved beer when it was done.
IMO, a distributed system of archive management that doesn't make ongoing reference to a central tree is a sure recipe for chaos, and poses the risk of making software harder to install/use for the non-skilled, and creating a lot of work in merging disparate branches for the skilled.
You want package xxyzz? OK - go to Jim's store in San Diego. It's easy to set up. Oh, I forgot to tell you, you've gotta get some bits from Lucy's store in Manchester, and Frieda's fixed a few bugs too - get her fixes from Bonn. And don't forget Peter's enhancements - his store is at the Adelaide University site. What? it doesn't compile? What kind of idiot are you? Just hack it till it does compile, then put it together in your own tree!
-- In the beginning was the WORD, and the WORD was UNSIGNED, and the main(){} was without form and void...
It is an important feature of subversion that it will be CVS compatible. I manage a 10+ year old/1+GB CVS repository. CVS has a lot of faults, but I can't throw that version history away. It's too valuable. subversion gives me hope that I'll get something more usable than CVS (we'll see, won't we!) without much pain.
I'm really hoping the subversion developers succeed.
Having said that, I'm all for arch succeeding too. Perhaps it will be better for new projects. Who knows.
This seems like it's worse than CVS. Functionally, I'm quite happy with CVS. The main complaint I have about it is that it isn't self-contained but invokes rcs and other shell commands in mysterious ways. "arch" seems to make things worse, not better in that regard. What I would like to see is something mostly like CVS, but something that is implemented as a clean, self-contained library with a single command line executable (with subcommands) and a built-in HTTP-based server. Until that comes along, I think I'll just stick with CVS.
Subversion was deliberately designed to address CVS's shortcomings, not to break new ground. Our philosophy was essentially conservative: CVS basically works, but has some bugs and maintainability problems. Let's keep the model and fix the problems. Result: Subversion.
The ideal situation is a world where both models have good, free implementations. Then we'll all very quickly find out which model works better. :-)
-Karl
http://www.red-bean.com/kfogel
Adds renaming over top of CVS and some other niceties. Can be used to create patches that contain versioning changes. With Meta-CVS, people can restructure directories in conflicting ways, and then resolve conflicts when they merge the structure.
http://users.footprints.net/~kaz/mcvs.html
This doesn't add anything else; no atomic commits or distributed operation over multiple repositories, etc.
Of course, you can use branches to track foreign code streams, as you can with CVS. The nice thing is that you can rename things on your own branch and keep up with an unrenamed source of patches. Or if the other people are using Meta-CVS, they can give you patches that include restructuring.
Meta-CVS is currently about 1600 physical lines of Common Lisp (with some CLISP extensions and bindings to glibc2) scattered in twenty or so files. A lot is done with little!
I've done SCM for a number of years, professionally evaluated version control product, and helped edit an Anti-Pattern book on the subject. It seems, at least to me, that the majority of version control systems out there have the basis covered when it comes to check-in, check-out, branching, and labeling. The standard features, if you will.
...this worked at least twice for me.)
However, most of the reasons that I've seen companies change version control systems is because of completely different reasons. Here are a few that come to mind:
- A version control system must be fast. I worked at one company where we tried to use Visual SourceSafe over a WAN; it took HOURS to share code. A good VCS should transmit the minimal amount of data.
- A version control system must provide security. All too often management uses the SCM repository as kind of a shared directory (BAD, BAD, BAD) -- and people who have no need to see or modify the code, do... implicitly.
- A version control system should provide extensive auditing and notification capabilities that can be discretely turned on and off. Allow logging the positive, the negative, and letting people know when particular operations happen to a set of files. In once case we attempted to get PVCS to automate scripts on a change to send mail to the PM. Checking in a directory flooded inboxes, since it could audit collections of code.
- There MUST be a recovery mechanism. Ever try to recover a lost SourceSafe password? Yikes. (Gaining re-entry is possible, back stuff up, change your password, do a diff. Copy pattern into the admin record with hex editor. Login as admin with new password. Change admin password.
- Again, there MUST be a recovery mechanism. I love RCS, SCCS, and PVCS for their file-related mechanisms. Why? I've had SCM systems go down hard when the database got munged. Yes, you can recover from a backup, but a lot of work gets lost. With an open file format, you can at least hand fix localized problems.
- That said, good version control systems should allow you to check in collections of files as atomic units, move files and directories, and operate on projects as a whole. Anytime I have twiddle with a repository, thereby breaking past history, something is seriously wrong with the VCS system model.
- Good systems must have an IMPORT / EXPORT capability that PRESERVES HISTORY. The less I feel locked into a solution, the more likely I'll be to try it out. Porting between system is usually painful.
- SCM systems must conform to how the CM manager wants to run things, not the other way around. Let's face it, users can and will make mistakes, and that's okay. Mistakes should be fixable. I'll never use StarTeam because it was too easy for users to check in accidentally branches that couldn't be removed. Tech support argued that version control should reflect the history of the product, where I maintain (and still do) that it should reflect the intended history. If I want to include user errors, that should be my policy, not the tools. My users should be able to reflect upon the project history and know why things changed. Period. You don't use a hack to undo a mistake.
- Branching notation should be clear and to the point. CVS has it's magic numbers, StarTeam has god awful views. Let me choose the numbering scheme, don't play games with odd/even numbering. Version numbers should not be overloaded to carry additional meta-information by the product.
- A good SCM tool should remember tag history. Suppose I accidently move or delete a tag, now I want to put it back. Suppose I want to see where it's been. This case is rare, but anyone who's had a user twiddle with the wrong tags feels this pain as sharp and deep.
- More ADMINISTRATIVE control. My big beef with CVS is when I have to twiddle with the repository structures and permissions directly to accomplish what I want done. No. No. No. There should be a tool (that audit's change) for standard operations.
- An admin should have the ability to define, enforce, and audit user permissions that should be applied cross dimensionally against repository, commands, and elements within the repository.
- Data should be stored in a manner that can be parsed by custom tools. It allows me to write extensions and automation.
- Nothing should be possible in a GUI that is not possible from the command line. The inverse holds true as well. Everything should be automation friendly. Early versions of PVCS pissed me off for this reason. As a SCM manager, I've used both, and I'll take a command line over a GUI any day. My novice users want a GUI, my advanced ones usually revert back to command lines (and integrate it with their editors).
- There must be readable 2 and 3 way diffs.
- A good SCM tool will be able to produce reports, or at least make it possible to export information that can produce reports.
- A good SCM tool should know how to handle binary files efficiently, rather than just storing the whole copy.
- A good SCM system should not put a limitation on comments.
- A good version control system should not try to "do it all" (CCC/Harvest) and do none of it well. When GUI's pop up off screen, or you have to artificially create packages for simple files, something's wrong. Which leads into...
SCM systems should operate the way the users of that system do.
There is a BIG difference between how commercial houses run things verses OpenSource projects.
Commercial groups usually have a smaller set of developers, they are known in advance, and commonly use the locking model. OpenSource models tend to use concurrency a lot more, and operate on the applying diff's procedure. (Yes, I know, exceptions are out there.)
Thus, some tools that feel more natural in some environments get quickly rejected in others. I've yet to see someone produce a readable guide about version control abstracted at a high level bringing all the terminology together. (Incidentally, I'm about to release one; email me for a draft.)
The overall problem in tends to be that people look on the side of the box for features, rather than asking if the features are even applicable for what they're doing.
Worse yet, proper SCM often gets sidestepped in commercial world. Ask: Do you want branching? You get, is it a feature?...yes! Now ask: Do you know when it's appropriate to branch, how to do the branch efficiently, how to graft branches back to the root, or how to physically do it... and you find out this is where a lot of bad CM happens. It isn't fun to inherit a screwed up repository.
The most common downfall of SCM, as I've seen in the commercial world, is a failure of the those running it (quite often over-tasked infrastructure people) failing to understand the product being built with the tool, failure by team leads to communicate repository structure, failure by management as they use the SCM tool as a substitute for communication, and failure by the developers who don't know how to use the tool and when to use the appropriate features.
I'm surprised this one got modded up. The poster clearly knows nothing about the topic; it's just an ignorant flame.
In case anyone's wondering, arch supports and uses write permissions; however, it also allows you to start your OWN server, and people can hook up to it in parallel with the main server, and get all the branches which appear on either.
You can commit all the crashy code you want on your own server, but it won't affect anyone who isn't using your server.
The genius is that your server is hooked up to the original server, live, and you can track the changes they make, merging when and where you like. If the project manager for the original server feels like it (and if you let him), he can track the changes on your server as well. If someone else has started their own branch server, you can merge directly with them as well.
VERY clever.
Although I don't dig the Subversion trashing; Subversion is also very cool for its own purposes. I'm glad Tom took the time to underline the differences, but I'm unhappy that the result is so slanted. It didn't need to be: both arch and Subversion stand on their own as superb projects, and there's even another one coming out of IBM "sometime" which has its own merits.
-Billy
I am getting soooo tired of this notion:
Arch also poses its own answer to the 'Linus Doesn't Scale' problem.
Look people, the "Linus doesn't scale" issue is NOT something that can be solved by replacing the use of 'patch'. Putting the Linux kernel on CVS (or Arch or whatever) would just allow people to commit stupid changes.
The reason Linus doesn't scale is not because he doesn't have enough time to run 'patch'. It's because changes to the kernel MUST be approved.
I'd also like to say, up front, to the Anonymous poster who asked:
Anyone know a good system of incoroprating source control with a databases? Oracle and Postgres would do.
Subversion does. The backend it currently uses is Berkeley DB, but the backend is pluggable. After version 1.0 comes out, expect to see a backend for one of the SQL databases pop up.
Now, on to Tom's comparison to Subversion. Caveat: I am not a Subversion guru. I lurk in the developer mailing list, and I use Subversion myself. Therefore, I may make mistakes about details, but I'm fairly certain I won't provide completely bogus information. I got some reviews on this post from the Subversion dev list, including some comments from Tom, but any mistakes in here are my own, and they're copyrighted mistakes, dammit.
I'm not going to quote whole sections; just enough for context.- Smart Servers vs. Smart Clients. Subversion
clients are also smart, although perhaps not as smart
as Arch. Diffs travel in both directions,
so a minimum of network traffic is used. Many Subversion
operations (status, diffs against the last revision, etc)
are purely client-side opereations.
- Trees in a Database vs. Trees in a File Systems
This is misleading. You *can* get stuff out of the Subversion
database with the standard BDB tools, so Subversion
isn't required. Also, because Subversion is based
on WebDAV, access to the database through a web
server is a freebee; also, Subversion is very Windows
friendly, from many points of view, which should help its
adoption in a corporate setting.
Subversion only stores the differences between two versions
of a file or directory, which is space efficient. The advantage
to being able to access a filesystem-based repository of diffs
is arguable.
- Centralized Control vs. Open Source Best Practices
In practical application, there is no advantage to the ARCH system
over Subversion. Subversion allows per-file/directory sourcing,
so you could create a project that includes sources from any number
of different repositories. (This code is not currently working
in Subversion.)
These are simple mistakes. There is also one statement that is wrong: arch is better able to recover from server disasters The argument was that, because arch is a dumb FS, it is easily mirrored. The implication is that databases aren't easily mirrored. BDB is just as easily mirrored, and most other databases are easily replicated.Other comments pointed out were:
- Subversion does not require Apache. It works over a local
filesystem just fine. If you want network access, you need
Apache.
- Subversion has all of the strengths of Apache. You therefore
get Apache access control (well defined and understood), SSL,
client and server certificates, and interoperability with other
WebDAV clients, among other things.
- With Subversion, you have both client side and server side hooks,
as well as smart diffs.
- Arch has both revision libraries and repositories. The comparison
document doesn't differentiate between them. In some cases, the
comparisons made aren't meaningful. Revision libraries, for example
"... also have to be created and maintained by the user.
So comparing them to accessing past revisions through normal means in
subversion is not a fair, or even really meaningful, comparison." (Daniel Berlin).
- When comparing Arch's repositories to Subversion's there is no
speed advantage. Arch's storage is either diffy (storing only differences),
in which case it is not easily browsed and is no faster (at best) than
Subversion; or the storage isn't diffy, in which case it isn't efficiently
stored (imagine multiple copies of each file for each revision).
- Subversion's choice of BDB as a backend was not accidental. Some of
the tools Subversion got from using BDB are: Hot
backup and replication, all kinds of existing tools that know
about BDB databases (e.g. Python or Perl bindings). A body of -
"community" knowledge. etc (Greg Stein).
I've left out vaporware features, such as the future SQL backend of Subversion 2.0.