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From Andor Molnar <an...@cloudera.com>
Subject Re: Observers: a link to message bus?
Date Wed, 24 Jan 2018 10:15:50 GMT
Hi Bruno,

Unfortunately I have no idea what plugin system does the author of this doc
refer to, but I can think of the following 2 solutions:

1) Fire up some observers in ZK cluster and connect to them via some
standard client. Create a zk node and create a watch for its children.
Producer can create nodes as children of this special node and connected
clients (consumers) can receive them in an ordered manner. This setup will
not affect the overall performance of the cluster, because the 'message
bus' only uses observers to achieve its goal.

2) Write your custom Observer node by implementing ZK protocol that can
receive and parse INFORM messages. These messages are consumed by Observers
too. Obviously this solution could be enormously cumbersome, although
Observers are the least complicated nodes of a ZK ensemble, but I'm not
aware of any existing 'plugin' solution which would achieve similar
forwarder mechanism.


On Wed, Jan 17, 2018 at 2:57 PM, Martin Gainty <mgainty@hotmail.com> wrote:

> ________________________________
> From: Bruno Freudensprung <bfreudensprung@expertsystem.com>
> Sent: Tuesday, January 16, 2018 7:54 AM
> To: user@zookeeper.apache.org
> Subject: Observers: a link to message bus?
> Hi,
> I have a question regarding this part of the Observers Guide (
> https://zookeeper.apache.org/doc/r3.4.11/zookeeperObservers.html):
> ZooKeeper Observers<https://zookeeper.apache.org/doc/r3.4.11/
> zookeeperObservers.html>
> zookeeper.apache.org
> Setting up a ZooKeeper ensemble that uses Observers is very simple, and
> requires just two changes to your config files. Firstly, in the config file
> of every node ...
> "As a link to a message bus : [...] Observers would give a natural
> integration point for this work: a plug-in mechanism could be used to
> attach the stream of proposals an Observer sees to a publish-subscribe
> system"
> I've not been able to find such a plug-in mechanism in the source code of
> ZooKeeper. For the moment the only integration point I can think of is the
> commitRequest method of the  org.apache.zookeeper.server.quorum.ObserverZooKeeperServer
> class. Am I looking in the right direction?
> To be honest: this is for the moment only pure curiosity since Apache
> Kafka seems to be a more "natural" and out-of-the-box option for an ordered
> message bus. However since I might probably need ZooKeeper for its
> distributed lock and leader election features, I was wondering if I could
> use it as an ordered message bus as well.
> MG>if ActiveMQ does indeed support ordered messaging for multiple active
> consumers thru Message Groups as stated:
> MG>Message Groups which is the way to load balance multiple active
> consumers listening to the same queue while preserving message order.
> MG>i have asked why does Zookeeper not support MessageOrderingTechnology
> such as ActiveMQ  as a plugin?
> MG>have you discovered "plugin integration" within the Zookeeper
> Architecture?
> Best regards,
> Bruno.
> MG>Thanks Bruno

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