Xaprb

This is the last day I’m taking off work to hack on mk-table-sync, and I thought it was time for (yet another) progress report. Here’s what I have done so far:

• All the code, except for a tiny bit of “glue” and “setup” code, is in modules.
• Lots more tests for the modules.
• A new sync algorithm (I still haven’t rewritten the top-down and bottom-up, which are designed for network efficiency more than MySQL efficiency, and are very complicated). This algorithm is called “Chunk” and is based on the chunking module I’m re-using from two of the other tools. This allows syncing the table a bit at a time to avoid locking it so much.
• The tool chooses its own parameters, including choosing the sync algorithm automatically by examining indexes.
• Proper exit codes, as well as several other smaller issues requested via bug reports.
• The tool now syncs entire servers. That is, you don’t have to specify a table. It’ll find all the tables and just sync them.
• The tool can sync many servers. You give it five servers, it will treat the first as the source, and sync every table in the source to each of the four remaining servers in turn.
• It can work via replication. It can discover a master’s slaves via SHOW SLAVE HOSTS and sync each slave to the master. You can also point it at a slave and it’ll discover the master, connect to it, and sync the slave to the master.
• It integrates with mk-table-checksum’s results. If you’ve given the –replicate option to mk-table-checksum, the slave’s results are stored in a table. It can read that table and sync anything marked as different. This can be combined with sync-to-master and auto-discover-slaves functionality.
• Lots of other bugs and problems are gone simply because I’m using the modules I wrote for other tools. This includes issues with table parsing, identifier quoting, etc etc. As an aside, I have to roll my own for almost everything, because I can’t rely on things like DBI’s quote_identifier() function — it does not work in earlier versions, which are amazingly common in the real world.

Whew! So what isn’t done yet?

• Bi-directional syncing.
• The Nibble sync algorithm. It will be preferred over Chunk and can be used in more cases.
• Documentation.
• Full support for wide characters. (This is non-trivial in Perl. I need to research it. A partial solution might not be hard, but I’m worried about the versions included in, for example, RHEL 3, which is very widely used.)
• Updating other tools to work right with the changes to shared code.
• Locking and transaction code. The tool will ultimately use FOR UPDATE/LOCK IN SHARE MODE automatically on InnoDB tables instead of locking them, for example.

Here’s a sample of what it can do, using a replication sandbox I set up with Giuseppe’s MySQL Sandbox. The sandbox contains a copy of the Sakila sample database. I’ll just mangle a few films on the slaves:

baron@kanga:~$ cd rsandbox_5_0_45/
baron@kanga:~/rsandbox_5_0_45$ ./s1
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 6
Server version: 5.0.45-log MySQL Community Server (GPL)

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

slave1 [localhost] {msandbox} ((none)) > update sakila.film set title='academy dinosaur2' limit 12;
Query OK, 12 rows affected, 12 warnings (0.07 sec)
Rows matched: 12  Changed: 12  Warnings: 0

slave1 [localhost] {msandbox} ((none)) > Bye
baron@kanga:~/rsandbox_5_0_45$ ./s2
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 6
Server version: 5.0.45-log MySQL Community Server (GPL)

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

slave2 [localhost] {msandbox} ((none)) > update sakila.film set title='academy dinosaur2' limit 1;
Query OK, 1 row affected, 1 warning (0.05 sec)
Rows matched: 1  Changed: 1  Warnings: 0

slave2 [localhost] {msandbox} ((none)) > Bye

OK, now I’ve messed up the first 12 films on one slave, and the first 1 on another. I could just go ahead and sync them right away, but first I’ll do a table checksum to demonstrate that functionality:

baron@kanga:~/rsandbox_5_0_45$ mk-table-checksum --replicate=test.checksum --port=16045 127.0.0.1 -q

And now I’ll tell the sync tool to go fix the differences the checksum revealed:

baron@kanga:~/rsandbox_5_0_45$ mk-table-sync  --replicate=test.checksum h=127.0.0.1,P=16045 -vx
# Syncing P=16046,h=127.0.0.1
# DELETE INSERT UPDATE ALGORITHM DATABASE.TABLE
#      0      0     12 Chunk     sakila.film
#      0      0      0 Chunk     sakila.film_text
# Syncing P=16047,h=127.0.0.1
# DELETE INSERT UPDATE ALGORITHM DATABASE.TABLE
#      0      0      0 Chunk     sakila.film
#      0      0      0 Chunk     sakila.film_text
baron@kanga:~/rsandbox_5_0_45$ 

Pretty easy, huh? Take a look at the output: the first thing it did was fix the 12 films I changed. sakila.film has a trigger that updates sakila.film_text, so that table got changed too. The checksum tool caught this difference, but the differences were gone by the time the sync tool examined them, again due to the trigger. On the second slave, no differences were found at all, because the changes to the first slave were made on the master, automatically fixing the second slave. (This won’t always be the case, but it worked in this example).

While I’d love to continue building the perfect beast, I’m going to have to call it quits around noon today and start cleaning up, writing the documentation, and getting ready to release the code. I’m not sure how much I’ll finish in that time.

By the way, anyone who wants to is welcome to get the code from the Maatkit SVN repository! I never make a big deal out of that because I generally assume people want to run released code, but SVN is there if you want it…

Si Chen wrote recently about the growth limits of open-source projects. He points out that as a project becomes larger, it gets harder to maintain. I can only agree. As the MySQL Toolkit project has grown, it’s become significantly more work to maintain, document, and enhance. (This is why I’m asking you to sponsor me for a week off my regular job to work on MySQL Table Sync, by the way. Please toss some money in the hat.)

Rewriting code so it’s testable is a major focus for me now. Some of these tools have gotten complicated enough that I can’t keep track of all the code. In other words, they’re collapsing under their own weight.

Back in the project’s humble beginnings, it seemed adequate to just copy and paste a few lines here and there; after all, these are just scripts, right? Right. So I’ll just copy a few lines of code that do command-line option parsing and help screens. Hey, it turns out that several of the tools can connect to more than one server, so simple -u, -h and -p options won’t do; so I invent a DSN-like notation that lets the tools connect to an arbitrary number of servers. Copy and paste that code, too. It’s only ten lines — no big deal. Pretty soon I find out that many of the standard Perl modules aren’t available, for a lot of people. And even when they’re available, people have old versions and can’t upgrade, so I can’t rely on basic things like the quote_identifier() function in DBI modules; time to write my own. Well, that’s only a single line! Surely that’s okay to copy and paste.

As Kurt Vonnegut says, “So it goes.” This is the death not only of quality, but of maintainability and extensibility. The Right Answer ™ is to write everything as modules, with proper test suites, and then make the scripts as minimalistic as possible — essentially gluing the modules together with a few lines of harder-to-test code. That’s how I’m used to working, too, but for some reason I can’t explain, it seemed okay not to work that way with this project. That has turned out to be a big mistake, which I’m slowly correcting out of necessity.

But it turns out it’s not that simple, either. I’ve gotten a lot of emails, phone calls from friends, and bug reports about how hard it is to install or update Perl, or get a CPAN module, on many systems. It turns out that a lot of companies are rightfully suspicious about CPAN (I have a tolerate-hate relationship with it myself), and won’t let my consultant friends install or upgrade any module without a lot of red tape. OK, you say, so bundle and distribute the modules the toolkit needs, and they can be installed locally with the toolkit. That sounds nice, but it’s even worse for a variety of reasons. Just to mention one: did you know that it can be a pain in the butt even to set @INC so a module sitting in the same directory with the script will be found by the script? (Please don’t tell me how easy it is, or I’ll let you respond to the next person trying to get it to work on an obscure platform with a Perl installation from the middle ages). Okay, I’ll mention two reasons: some Perl modules have to be compiled and customized just for the operating system you’re installing them on, or they’ll segfault (of all things)! Don’t get me wrong, I don’t think the grass is greener on the other side; no way do I want to try writing these things in C or Java. Perl is about as portable as it gets.

The net result is that I have to do a lot of little tricks to make these things standalone programs, as much as humanly possible. I’m trying to reduce dependencies on external modules, even those that are part of core Perl. I’m re-inventing functionality because it’s not available in all versions. I’m writing modules that can be tested, but I’m not shipping them as separate modules; I’m basically using sed to copy-and-paste the module’s code into the scripts.

Why am I doing all this work?

Because it’s less work than not doing it.

But it is significantly more work than just whacking together some “scripts” and uploading them. That’s why there is a critical mass beyond which it gets harder to grow a project. The solution to this is to find a way to do things differently, work smarter, not harder. The challenge is to switch the fight against the demons of bad code and maintainability so it’s on my terms. In other words, don’t fight against these characteristics of growth; make them work for me. I won’t say I’ve learned that lesson completely, but I’m starting. That’s why I’m automating basically everything about this project (though for some reason I can’t get WWW::Mechanize to stay logged into Sourceforge, so I’m having a hard time automating part of the release process).

I’m also considering ways to provide this toolkit without taking so much out of my own pocket. What started out as me developing tools for my employer, and them graciously agreeing to let me make them available for Sourceforge, has gone far beyond my employer’s needs now. I can’t ask my employer to carry the weight, so it has fallen to me for a while now. That’s okay for some period while I work out how to do it differently, but not indefinitely. Among other things, it cuts into time I want to spend with my wife. Charging for support has definitely crossed my mind, as has some kind of community/enterprise split (such as the one Zmanda does). I don’t want to go there yet — so I’m just asking for a week of sponsored time off work, to begin with.

By the way, the process of replacing copy/pasted code isn’t without its hitches. I just found and fixed a bug in MySQL Table Checksum that I caused by moving the DSN parsing code to a module. And someone else just reported a different bug in another tool, where it turns out the copy/pasted code wasn’t quite identical and I changed the functionality by moving it to the module. Release early, release often. Rely on users to find bugs and report them. So it goes.