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zookeeper as configuration persistence medium?
Hi everyone,

We've been using ZooKeeper for service discovery for a couple months now and have been pretty happy.

A bit about our setup: our ZK ensemble comprises 3 nodes and is spread across data centers.  Compared to other ZK instances it is likely very low-traffic: perhaps 200 writes/1000 reads/500 clients per day.  So, small potatoes in terms of traffic.

Currently all nodes stored in our ZK are actively managed/populated by specific clients, such that we would be able to theoretically wipe ZooKeeper and clients would repopulate znodes appropriately.

I wanted to use the community as a sounding board here. Reading through the ZK literature I've found, I haven't seen this use case mentioned explicitly:

Now we are considering storing configuration data in ZooKeeper.  Currently this configuration is stored on the filesystem and read once at startup of a component.

My gut reaction is storing this data in ZooKeeper—using ZK itself as the persistence medium—is a bad idea.  Because it would be storing data that isn't present anywhere else: supposing a ZK crash, we would need to replay transaction logs to get the data back, rather than using our existing backup solutions.

I think a better solution would be to write a component which is responsible for persisting and managing updates to this configuration, and makes it available to interested clients via ZooKeeper.  This would keep all znodes programmatically generated, which I think would lead to a cleaner node space.  It would also help you avoid orphaned nodes, sitting around and not being accessed/cleaned up anymore.  Eventually nobody would know what those orphaned nodes are there for or if anyone is relying on their presence.

Am I on the right track here?  Are there ZK deployments out there being used in this way?  In addition, is there a way to check the last time a znode was used?  Or, a pattern which encourages not orphaning nodes in the first place?

Thanks in advance,
Patrick Braga-Henebry

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