Its looks amazing, was looking for something like this. Couple of questions
1. How is the data replicated, is it writing synchronously to both primary and backups. What if backup is down 2. what happens in network partition ? Looks like it dynamically distributes data based on the number of nodes in the system. I think in multicast it can discover other nodes, but what happens in tcp. Does not look like its following any consensus protocol like paxos/zab. I just skimmed through the doc, could not get the internal details. Would love to know more about how it ensures data consistency. Thanks, Kishore G On Sat, Apr 27, 2013 at 8:42 PM, Ted Dunning <[email protected]> wrote: > I have just been trying out Hazelcast on another project and have been very > impressed at the simplicity. Very nice. > > > > > On Sat, Apr 27, 2013 at 8:38 PM, kishore g <[email protected]> wrote: > > > Hi Jacques, > > > > Just added a recipe for service discovery using Helix. More details here > > http://helix.incubator.apache.org/recipes/service_discovery.html and see > > the sample code here > > > > > https://github.com/apache/incubator-helix/tree/master/recipes/service-discovery/src/main/java/org/apache/helix/servicediscovery > > > > Note there is no need to run a separate Helix controller. I have listed > > some benefits on the recipe page. There are some more features that drill > > can benefit in terms of operation, for example you can execute commands > on > > each drill bit node and add custom message handlers. Helix comes with the > > messaging service using which you can command the nodes to perform adhoc > > tasks. There is also rest admin interface that provide cluster state and > > also perform admin operations. > > > > Thanks, > > Kishore G > > > > > > > > > > On Tue, Apr 23, 2013 at 2:34 PM, Jacques Nadeau <[email protected]> > > wrote: > > > > > The concept of role determination by ZK is interesting but I'm not > > > sure that level of complexity is needed when nodes are fairly static > > > in their roles. > > > > > > Thanks for the information. I need to think more about this. > > > > > > J > > > > > > > > > > > > On Mon, Apr 22, 2013 at 11:09 AM, kishore g <[email protected]> > wrote: > > > > Hi Jacques, > > > > > > > > Thanks for the pointer, had a quick look and it is indeed very > simple. > > > You > > > > just have the need for service discovery. The slide pointed that > there > > > was > > > > some need for partition and resource management but looks like the > > actual > > > > requirement is quite different. > > > > > > > > While this can still be done through Helix, I dont see much value in > > > using > > > > it if requirement stays the same. > > > > > > > > Few things to ensure > > > > 1) You are not setting any watchers but instead reading all zookeeper > > > > znodes every X seconds, this is good to avoid herd effect during > start > > up > > > > of nodes but might need some tuning when you have large number of > > nodes. > > > I > > > > dint check the curator library if its using zk async library (you > might > > > > want to make sure it uses that). > > > > 2) Not clear how you plan to handle error scenarios, what if the node > > > fails > > > > to start up or is flapping, how will you know that a node is not part > > of > > > > the cluster. Do you plan to have list of nodes else where and compare > > the > > > > two. > > > > 3) How do you plan to blacklist a node that is behaving badly, do you > > > > envision providing an admin api later that will allow one to > > > disable/enable > > > > such nodes. > > > > 4) Do you envision each node having multiple service names, for > example > > > if > > > > you a using sparrow i am assuming few nodes will be schedulers and > > others > > > > workers, is it possible for a node to be both scheduler and worker. > If > > > yes, > > > > how will a node know if it has to be scheduler/worker/both. > > > > > > > > The reason i bring up these points, the way it is designed right now, > > the > > > > nodes own the configuration(host,port, service types etc) and when > they > > > > start up they simply put that information in zk and make it available > > for > > > > others to discover. Helix advocates a different methodology, the node > > > > simply starts up and does not know what it has to do, all > > > > actions/configuration come from outside via transitions. Which means > > all > > > > nodes start up with exactly the same configuration just an id and > > > zookeeper > > > > address. So it really depends on how much configuration you have and > if > > > you > > > > want that to be dynamically changed or you are ok with pushing the > > config > > > > to each node and restarting it. It kind of falls in operability space > > and > > > > its probably too early to have a clear picture about that but it > makes > > > > quite a difference over the long run. > > > > > > > > Hope this helps and thanks again for your time. > > > > > > > > Thanks, > > > > Kishore G > > > > > > > > > > > > > > > > On Sun, Apr 21, 2013 at 8:10 PM, Jacques Nadeau <[email protected]> > > > wrote: > > > > > > > >> Hey Kishore, > > > >> > > > >> I'm really excited about Helix. It is great to see the toolbox > > > >> starting to be filled with such powerful tools. Some random > thoughts > > > >> with regards to Helix/Curator/etc. > > > >> > > > >> It seems like we're trying to avoid even supporting a number of > things > > > >> that the Helix framework provides. We really want to avoid a master > > > >> node. We hope to avoid the concept of particular nodes holding > > > >> specific resources. (As a query engine, we don't currently have the > > > >> concept of things like regions.) We're trying to build upon > Berkeley's > > > >> Sparrow work and avoid the concept of centralized scheduling. The > > > >> driving node for a particular query is the only entity responsible > for > > > >> pushing a query to completion and has direct RPC interaction with > its > > > >> 'children'. > > > >> > > > >> Our current use of zookeeper is strictly for the purpose of service > > > >> registration and membership information. If you want to see the > (lack > > > >> of) complexity of our use right now, you can look here: > > > >> > > > >> > > > > > > https://github.com/apache/incubator-drill/tree/execwork/sandbox/prototype/exec/java-exec/src/main/java/org/apache/drill/exec/coord > > > >> > > > >> Thoughts? > > > >> > > > >> Jacques > > > >> > > > >> On Sun, Apr 21, 2013 at 2:05 PM, kishore g <[email protected]> > > wrote: > > > >> > Thanks Ted for making a case. I am pretty sure there were valid > > > points. > > > >> > > > > >> > I did not get the zero-conf option, is the case that Helix needs > to > > be > > > >> run > > > >> > as a separate service. Helix can be used in both modes as a > service > > > and > > > >> > also a library. We have deployed it in both modes and we have seen > > the > > > >> need > > > >> > for it within LinkedIn. > > > >> > > > > >> > It would be really great if I can get the actual requirements and > do > > > >> > another pass evaluating. > > > >> > > > > >> > Thanks and appreciate your time in answering my questions. > > > >> > > > > >> > Thanks, > > > >> > Kishore G > > > >> > > > > >> > > > > >> > On Sun, Apr 21, 2013 at 10:35 AM, Ted Dunning < > > [email protected]> > > > >> wrote: > > > >> > > > > >> >> Kishore, > > > >> >> > > > >> >> I made the case for Helix and the group seems to have strongly > > > >> gravitated > > > >> >> to the lower level that Curator provides. > > > >> >> > > > >> >> One feature that would have improved the case for Helix would > have > > > been > > > >> >> viable zero-conf operation as an option. > > > >> >> > > > >> >> The game isn't over, however, and if you would like to get > involved > > > >> here on > > > >> >> Drill, it might help to have another point of view. > > > >> >> > > > >> >> > > > >> >> > > > >> >> > > > >> >> On Sun, Apr 21, 2013 at 9:08 AM, kishore g <[email protected]> > > > wrote: > > > >> >> > > > >> >> > Hi Michael, > > > >> >> > > > > >> >> > Thanks for the update. Here are my thoughts, though cant resist > > > >> telling > > > >> >> > good things about Helix since I am the author :-). > > > >> >> > > > > >> >> > Here is how I see zk v/s curator v/s helix. > > > >> >> > > > > >> >> > Zk is amazing for co-ordination and maintaining cluster data > like > > > >> >> > configuration, etc. It provides the concept of ephemeral which > > can > > > be > > > >> >> used > > > >> >> > for liveness detection of a process. However there are lot of > > > corner > > > >> >> cases > > > >> >> > that is non trivial to code. Curator is a library that makes it > > > easy > > > >> to > > > >> >> use > > > >> >> > those apis, it provides the recipes in terms of leader > election, > > > >> barrier, > > > >> >> > etc. Helix provides a much higher abstraction where it treats > > > various > > > >> >> > components of a distributed system as first class citizens and > > > allows > > > >> >> > system builders to think in terms of nodes, resources, > > partitions, > > > >> state > > > >> >> > machine etc. Helix underneath uses zkclient(something like > > > curator) to > > > >> >> make > > > >> >> > it easy to interact with zookeeper. We had plans to use curator > > but > > > >> Helix > > > >> >> > needed really good performance in terms of start up/fail over > > time > > > and > > > >> >> when > > > >> >> > we have 1000's of partitions. We had to use low level apis of > zk > > to > > > >> >> achieve > > > >> >> > that. > > > >> >> > > > > >> >> > From my experience, while building distributed systems cluster > > > >> management > > > >> >> > starts out very simple and one will be able to do a prototype > > very > > > >> >> quickly. > > > >> >> > But over time, things get complicated and need many more > > features. > > > At > > > >> >> > LinkedIn we started in a similar way where we simply used some > > > >> ephemeral > > > >> >> > nodes to know whether we have a lock or not. But over time, lot > > of > > > >> things > > > >> >> > like controlling the assignment from outside, evenly > distributing > > > >> locks, > > > >> >> > hand over of locks gracefully, restricting which nodes can own > a > > > >> >> partition, > > > >> >> > cluster expansion, throttling of any cluster wide operations > etc > > > got > > > >> >> > complicated and we ended up having to implement one solution > for > > > each > > > >> >> > feature. For every feature, we took lot of time to flush out > > issues > > > >> with > > > >> >> zk > > > >> >> > interaction and we had huge scaling issues when we tried with > > > 1000's > > > >> of > > > >> >> > partitions and lot of ephemerals, it was a night mare to debug. > > > Over > > > >> >> time, > > > >> >> > most systems come up with a state machine for example you can > see > > > >> hbase > > > >> >> > master, yarn ( job tracker, task tracker). Its kind of obvious > > that > > > >> >> having > > > >> >> > a state machine is the right way to build a large distributed > > > system > > > >> and > > > >> >> > allows you to have right level of abstraction and is a much > > cleaner > > > >> >> design. > > > >> >> > What Helix did was to generalize this concept and allows one to > > > >> configure > > > >> >> > the state machine. > > > >> >> > > > > >> >> > All other features were basically built on top of states and > > > >> transitions. > > > >> >> > For example, we had some tasks that needs to be distributed > among > > > the > > > >> >> > nodes. when a node dies it should be taken up by another node, > > > this is > > > >> >> > simple using a ephemeral nodes. But lets say you want to limit > > the > > > max > > > >> >> > tasks a node can handle, with Helix is modelled as a constraint > > and > > > >> you > > > >> >> can > > > >> >> > specify how many tasks can run on a node, process etc that is > > > >> completely > > > >> >> > controlled from outside without having to change the > application > > > >> >> > code. Similarly when the dead node comes back other nodes have > to > > > >> >> > gracefully hand over their tasks. Its not trivial to achieve > > this. > > > >> >> > > > > >> >> > There are lot of other things we have encountered while > building > > > >> >> > distributed systems and we have always been able to add them to > > > Helix > > > >> >> such > > > >> >> > that other systems can benefit from it. For example, I recently > > > >> presented > > > >> >> > how to test and debug large scale distributed systems. It > > basically > > > >> comes > > > >> >> > with tools which parses zk transaction logs and provides the > > exact > > > >> >> sequence > > > >> >> > of steps that lead to a failure. More details here > > > >> >> > > > http://www.slideshare.net/KishoreGopalakrishna/data-driven-testing > > > >> >> > > > > >> >> > To summarize, > > > >> >> > > > > >> >> > So its not really zk v/s curator v/s helix. Its basically the > > > level of > > > >> >> > abstraction one wants. One can build Helix using curator which > > > uses zk > > > >> >> > underneath. So it basically boils down to what is the system > you > > > are > > > >> >> > building and how complex can it get. > > > >> >> > > > > >> >> > There are definitely some use cases where Helix is not needed > and > > > is > > > >> >> > probably over kill but Apache Drill looks like a project that > > will > > > get > > > >> >> > pretty big and I am sure you will see all the requirements we > saw > > > over > > > >> >> > time. > > > >> >> > > > > >> >> > Hope this helps. As I mentioned earlier, i will be happy to > > provide > > > >> more > > > >> >> > details and contribute. > > > >> >> > > > > >> >> > thanks, > > > >> >> > Kishore G > > > >> >> > > > > >> >> > > > > >> >> > > > > >> >> > > > > >> >> > > > > >> >> > > > > >> >> > > > > >> >> > > > > >> >> > > > > >> >> > > > > >> >> > > > > >> >> > > > > >> >> > > > > >> >> > > > > >> >> > > > > >> >> > > > > >> >> > > > > >> >> > On Sun, Apr 21, 2013 at 1:57 AM, Michael Hausenblas < > > > >> >> > [email protected]> wrote: > > > >> >> > > > > >> >> > > > > > >> >> > > At the time I put the slides together, Helix was indeed > > > considered. > > > >> >> > > AFAICT, currently we seem to have settled on Netflix Curator > > [1], > > > >> >> > however. > > > >> >> > > I wouldn't exclude the possibility that we may utilise Helix > in > > > >> future; > > > >> >> > > personally, I think it's a great thing. Would be very > > > interested in > > > >> >> your > > > >> >> > > experiences with it (also, re Zk vs. Curator vs. Helix). > > > >> >> > > > > > >> >> > > Cheers, > > > >> >> > > Michael > > > >> >> > > > > > >> >> > > [1] https://github.com/Netflix/curator/wiki > > > >> >> > > > > > >> >> > > -- > > > >> >> > > Michael Hausenblas > > > >> >> > > Ireland, Europe > > > >> >> > > http://mhausenblas.info/ > > > >> >> > > > > > >> >> > > On 21 Apr 2013, at 08:39, kishore g <[email protected]> > > wrote: > > > >> >> > > > > > >> >> > > > Hello, > > > >> >> > > > > > > >> >> > > > I was reading the slide deck from Hadoop summit > > > >> >> > > > > > > >> >> > > > > > >> >> > > > > >> >> > > > >> > > > > > > http://www.slideshare.net/Hadoop_Summit/understanding-the-value-and-architecture-of-apache-drill > > > >> >> > > > > > > >> >> > > > On slide 27, there is mention of using Helix for partition > > and > > > >> >> resource > > > >> >> > > > management. I could not find much details on > > > >> >> > > > https://issues.apache.org/jira/browse/DRILL-53 > > > >> >> > > > > > > >> >> > > > Can some one provide more details on this, we might be able > > to > > > >> >> > > contribute. > > > >> >> > > > > > > >> >> > > > thanks, > > > >> >> > > > Kishore G > > > >> >> > > > > > >> >> > > > > > >> >> > > > > >> >> > > > >> > > > > > >
