Hi, I started yesterday to work on my idea of an alternative ZooKeeper client interface.[1] Instead of methods on a ZooKeeper class, a user should instantiate an Operation (Create, Delete, ...) and forward it to an Executor which handles session loss errors and alikes.
By doing that, I got shocked by the sheer number of WTF issues I found. I'm sorry for ranting now, but it gets quicker to the poing. - Hostlist as string The hostlist is parsed in the ctor of ClientCnxn. This violates the rule of not doing (too much) work in a ctor. Instead the ClientCnxn should receive an object of class "HostSet". HostSet could then be instantiated e.g. with a comma separated string. - cyclic dependency ClientCnxn, ZooKeeper ZooKeeper instantiates ClientCnxn in its ctor with this and therefor builds a cyclic dependency graph between both objects. This means, you can't have the one without the other. So why did you bother do make them to separate classes in the first place? ClientCnxn accesses ZooKeeper.state. State should rather be a property of ClientCnxn. And ClientCnxn accesses zooKeeper.get???Watches() in its method primeConnection(). I've not yet checked, how this dependency should be resolved better. - Chroot is an attribute of ClientCnxn I'd like to have one process that uses ZooKeeper for different things (managing a list of work, locking some unrelated locks elsewhere). So I've components that do this work inside the same process. These components should get the same zookeeper-client reference chroot'ed for their needs. So it'd be much better, if the ClientCnxn would not care about the chroot. - deleteRecursive does not belong to the other methods DeleteRecursive has been committed to trunk already as a method to the zookeeper class. So in the API it has the same level as the atomic operations create, delete, getData, setData, etc. The user must get the false impression, that deleteRecursive is also an atomic operation. It would be better to have deleteRecursive in some helper class but not that deep in zookeeper's core code. Maybe I'd like to have another policy on how to react if deleteRecursive fails in the middle of its work? - massive code duplication in zookeeper class Each operation calls validatePath, handles the chroot, calls ClientCnxn and checks the return header for error. I'd like to address this with the operation classes: Each operation should receive a prechecked Path object. Calling ClientCnxn and error checking is not (or only partly) the concern of the operation but of an "executor" like class. - stat is returned by parameter Since one can return only one value in java it's the only choice to do so. Still it feels more like C then like Java. However with operator classes one could simply get the result values with getter functions after the execution. - stat calls static method on org.apache.zookeeper.server.DataTree It's a huge jump from client code to the internal server class DataTree. Shouldn't there rather be some class related to the protobuffer stat class that knows how to copy a stat? - Session class? Maybe it'd make sense to combine hostlist, sessionId, sessionPassword and sessionTimeout in a Session class so that the ctor of ClientCnxn won't get too long? I may have missed some items. :-) Once again, please excuse my harsh tone. May I put the above issues in jira and would you accept (backwards compatible) patches for it for 3.4.0? Zookeeper is a fascinating project. Cudos to the devs. I've only looked in the client side code, which is what most users of zookeeper will ever see if they see any zookeeper internal code at all. So it may make sense to make this piece of the project as nice and clean as possible. [1] http://mail-archives.apache.org/mod_mbox/hadoop-zookeeper- dev/201005.mbox/%3c201005261509.54236.tho...@koch.ro%3e Best regards, Thomas Koch, http://www.koch.ro
