as you may have noticed, I haven't been active in the ZooKeeper project
anymore for a couple of months. I'm a full time student again since march so
that any further activity in Hadoop/ZooKeeper would need to be auto-motivated.
Since I don't want to just fade away and I'll still give a talk about
ZooKeeper on the BerlinBuzzWords conf (Berlin, june 6/7), I listed the reasons
why I wouldn't like to work on the current ZooKeeper code base anymore.
I plan the following structure for my talk:
1) theoretical model / protocol of ZooKeeper
2) practical applications, projects using ZooKeeper
3) shortcomings of the current ZooKeeper code base
A tentative brain dump of part three is listed below. I appreciate any
comments that could help me to give a balanced presentation of the ZooKeeper
If I'd need a ZooKeeper implementation right now I'd probably do a minimal-
feature rewrite in Scala + Akka. I do appreciate ZooKeeper as an invaluable
proof-of-concept implementation and pioneer. But as in american history there
should come others after the pioneers that don't look like Clint Eastwood
anymore and build more tidy things.
* The code is tightly coupled
* most so called "Unit-Tests" are actualy integration tests. They run the
whole application and test one specific functionality.
* no uniform configuration: command line parameters, system properties,
configuration file (java properties)
* configuration properties copied to static class members
* feature bloat on fragile foundation: e.g. chroot + automatic resubscribtion
does not work
* implementation unlike specification: allowed characters in path
* still on ant instead of maven (depends how you see ant vs. maven)
* circular object dependencies (e.g. ZooKeeper <-> ClientCnxn)
* methods with +100 lines of code and nested conditions depth well over 5
* general attitude against refactoring, no knowledge or appreciation of
"effective java" (Josh Bloch) or "clean code" (Robert C. Martin)
* magic numbers instead of enum
* still bound to inline copy of jute (HadoopIO, avro predecessor)
* even hand coded (de)serialization in leader election
* no client-only jar. Every client gets the full server code.
* unhandy API triggered (at least) two client API wrappers: zkClient, cages
* insane amounts of code duplication
* horrible, fragile thread programming: plenty of "XYZ extends Threads"
- implements runnable
- or better: executor framework
- or much better: actors (see Akka)
-> leads to fear of refactoring, because nobody understands all
Thomas Koch, http://www.koch.ro