Re: [Bigdata-developers] Why zookeeper?

[Fred O. <fko...@gm...>]

Bryan,

Services (started by per-instance configuration) can register with the
lookup service with their physical ID, and include the logical ID of
their group (quorum), if any, in their service entries, allowing them
to be discovered by logical ID. I can imagine a new stateless service
whose sole function is binding logical IDs to unbound physical
services, taking into account hardware configuration, rack/network
awareness, etc.

Now that we have groups of physical services mapped to logical
services, there is still the need for leader election. I'm willing to
accept using zookeeper for that limited purpose, though I think I'd
prefer some embedded paxos implementation if I had one on hand.
Leaders can identify themselves with service entries in their
registrations, allowing service leaders to be easily discovered.
(Timing and failure scenarios would have to be considered carefully.
If the idea of registering as a service leader doesn't work, then
clients can always ask a service who its leader is.)  It is important
to me that clients need not consult zookeeper directly, limiting the
number of processes being coordinated to the smallest possible number,
decreasing complexity.

Once service leaders have been established, each leader can
communicate directly with its group members to complete
initialization, identify and share the most recent updates if needed,
etc. I think that using zookeeper as an information conduit when
service to service communication still drives up the complexity
unnecessarily.

Fred


On Mon, Jul 26, 2010 at 6:21 PM, Bryan Thompson <br...@sy...> wrote:
> Fred,
>
> Can you provide some examples for the HA enabled per-instance configuration?  I would like the instance based configuration to be compatible with the rules-based approach.  In addition, the services will need to publish certain metadata about the logical service instances into zookeeper for the HA quorums.    I've attach a current copy of the HA/zookeeper integration document which specifies the zookeeper paths that are used by the quorum.  Everything is organized under the zpath of the logical service.  Take a look and then let's see if we can bring this down to some concrete points for both mechanisms to operate.