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 >> > > >> > > >> > >>
