Flavio, more questions inline: -----Original Message----- From: Flavio Junqueira [mailto:f...@yahoo-inc.com] Sent: Sunday, July 26, 2009 12:49 PM To: zookeeper-user@hadoop.apache.org Subject: Re: Zookeeper WAN Configuration
Todd, Answers inline: On Jul 26, 2009, at 11:05 AM, Todd Greenwood wrote: > Flavio, thank you for the suggestion. > > I have looked at the documention (relevant snippets pasted in > below), and looked at the presentations > (http://wiki.apache.org/hadoop/ZooKeeper/ZooKeeperPresentations > ), > but I still have some questions about WAN configuration: > > --------------------------------------------------------------- > WAN > ---- > A <-> B > A <-> C > A <-> D > > A is a central processing hub (DC). > B-D are remote colo edge nodes (PODS). > Each POD contains (m) ZK Servers with (q) client connections. > --------------------------------------------------------------- > > What are the advantages and disadvantages to co-locating ZK Servers > across a WAN? Could you correct my admitedly naïve assumtions here? > > 1. ZK Servers within a POD would significantly improve read/write > performance within a given POD, v.s. clients within the POD opening > connections to the DC. > I'm assuming that you're setting the weight of ZooKeeper servers in PODs to zero, which means that their votes when ordering updates do not count. [Todd] Correct. If my assumption is correct, then you should see a significant improvement in read performance. I would say that write performance wouldn't be very different from clients in PODs opening a direct connection to DC. [Todd] So the Leader, knowing that machine(s) have a voting weight of zero, doesn't have to wait for their responses in order to form a quorum vote? Does the leader even send voting requests to the weight zero followers? > 2. ZK Servers within a POD would provide local file transacted > storage of writes, obviating the need to write that code ourselves. > Yes, local zk servers in PODs receive all updates and process them as any other zk server. > 3. ZK Servers within the POD would be resilient to network > connectivity failure between the POD and the DC. Once connectivity > re-established, the ZK Servers in the POD would sync with the ZK > servers in the DC, and, from the perspective of a client within the > POD, everything just worked, and there was no network failure. > We want to have servers switching to read-only mode upon network partitions, but this is a feature under development. We don't have plans for implementing any model of eventual consistency that would allow updates even when not being able to form a quorum, and I personally believe that it would be a major change, with major implications not only to the code base, but also to the semantics of our API. [Todd] What is the current (3.2) behaviour in the case of a network failure that prevents connectivity between ZK Servers in a pod? Assuming the pod is composed of weight=0 followers...are the clients connected to these zookeeper servers still able to read? do they get exceptions on write? do the clients hang if it's a synchronous call? > 4. A WAN topology of co-located ZK servers in both the DC and (n) > PODs would not significantly degrade the performance of the > ensemble, provided large blobs of traffic were not being sent across > the network. If the zk servers in the PODs are assigned weight zero, then I don't see a reason for having lower performance in the scenario you describe. If weights are greater than zero for zk servers in PODs, then your performance might be affected, but there are ways of assigning weights that do not require receiving votes from all co- locations for progress. [Todd] Great, we'll proceed with hierarchical configuration w/ ZK Servers in pods having a voting weight of zero. Could you provide a pointer to a configuration that shows this? The docs are a bit lean in this regard... -Flavio
