hi amit,

sorry for the late response. this week has been crunch time for a lot of different things.

here are your answers:


1. it is still in prototype phase. we are evaluating different aspects, but there is still some work to do to make it production ready. we also need to get an engineering team to signup to stand behind it.

2. it's a generic pub/sub message bus. in some sense it is really a datacenter solution with extensions for multi-data center operation, so it is perfectly reasonable to use it in a single datacenter setting.

3. yeah, we have removed the hw.bash script. it had some hardcoded assumptions and was a swiss army knife on steroids. he have been breaking it up into simpler scripts.

4. session expiry really represents a fundamental connectivity problem, so both bk and hedwig restart the component that gets the expired session errror.


1. yes.

2. once all subscribers have consumed a message there is a background process that cleans it up.

3. yes there is a replication factor and we ensure replication on writes and there is a recovery tool to recover bookies that fail. we don't have to worry about conflicts because there is only a single writer for a give ledger. because of this we do not need to do quorum reads.


yes, this is something we need to work on. i'll see if i can push out some of our hello world applications. we'd also like to put a JMS API on top so that the API is more familiar (and documented :). i don't want to delay the answers to your other questions, so let me answer that HedwigSubscriber is the class for clients. the other classes are internal. (for cross data center hubs use a special kind of subscriptions to do cross data center updates.)


On 10/05/2010 10:32 PM, amit jaiswal wrote:

In Hedwig talk (http://vimeo.com/13282102), it was mentioned that the primary
use case for Hedwig comes from the distributed key-value store PNUTS in Yahoo!,
but also said that the work is new.

Could you please about the following:

Production readiness / Deployment
1. What is the production readiness of Hedwig / BookKeeper. Is it being used
anywhere (like in PNUTS)?
2. Is Hedwig designed to use as a generic message bus or only for
multi-datacenter operations?
3. Hedwig installation and deployment is done through a script hw.bash, but that
is difficult to use especially in a production environment. Are there any other
packages available that can simplify the deployment of hedwig.
4. How does BK/Hedwig handle zookeeper session expiry?

Data Deletion, Handling data loss, Quorum
1. Does BookKeeper support deletion of old log entries which have been consumed.
2. How does Hedwig handles the case when all subscribers have consumed all the
messages. In the talk, it was said that a subscriber can come back after hours,
days or weeks. Is there any data retention / expiration policy for the data that
is published?
3. How does Hedwig handles data loss? There is a replication factor, and a write
operation must be accepted by majority of the bookies, but how data conflicts
are handled? Is there any possibility of data conflict at all? Is the
replication only for recovery? When the hub is reading data from bookies, does
it reads from all the bookies to satisfy quorum read?

What is the difference between PubSubServer, HedwigSubscriber,
HedwigHubSubscriber. Is there any HelloWorld program that simply illustrates how
to instantiate a hedwig client, and publish/consume messages. (HedwigBenchmark
class is helpful, but was looking something like API documentation).


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