Thanks David & Ron for the clearest explanation!!
Thanks & Regards Arindam [email protected] wrote: ----- To: MarkLogic Developer Discussion <[email protected]> From: David Lee Sent by: [email protected] Date: 10/30/2013 01:46AM Subject: Re: [MarkLogic Dev General] Reg: E-Node and D-Node configuration I would like to make a slight adjustment to this generality. That is in Cloud Computing, Amazon AWS/EC2 in particular. Amazon has "regions" which are composed of 3 or more "zones" ... Each "zone" is as nearly a seperate datacenter as physically possible except for connectivity. That is they are generally built on seperate geographic topology (dont share flood plains, earthquake fault lines etc) and are generally fed by issolated infrastructure ( power, internet etc). Yet ... they are close enough together that the latency between zones is faster than the latency to an SSD drive write cycle. Most zones are within 2ms of each other. Thats fast. While this is not quite as fast as talking to nodes sharing the same rack and bridge on a FIOS channel ... its still *damn fast*. Creating a cluster across amazon zones within the same region can achieve both HA and DR to a large degree. It's not perfect as its *possible* (I belive its happened once in the last 10 years) for multiple zones in a region to fail simultaneously ... but its vastly more resilient than putting all your eggs in the same datacenter yet the latancy between zones in the same region is so small that creating clusters within a zone are extremely fast. Maybe not *quite* as fast as on the same rack but it's a good compromise. Especially if you make a hybrid. That is several nodes in each zone co-located but cross-replicating ("local disk failover") across zones. If you want real DR you should create a replica cluster in a different region and do foreign replication to it. But even without that ... its pretty amazing. The result is a very fault resistant HA system that performs very well. A good compromise. Look forward to a coming announcement with will make such an architecture easier to build. But you dont *have* to wait. MarkLogic 6.0 runs great on Amazon EC2 out of the box. But we have made things simpler still to create a fault tolerant system that is also HA and performing. Nothing is perfect, everything is a compromise, but this combination of multi zone single region clusters is as near as perfect as I can imagine, or anyone else could build on their own. -David ----------------------------------------------------------------------------- David Lee Lead Engineer MarkLogic Corporation [email protected] Phone: +1 812-482-5224 Cell: +1 812-630-7622 www.marklogic.com -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Ron Hitchens Sent: Tuesday, October 29, 2013 3:40 PM To: MarkLogic Developer Discussion Subject: Re: [MarkLogic Dev General] Reg: E-Node and D-Node configuration You can think of a MarkLogic cluster as a single virtual server. A cluster is made up of nodes (E, D or E/D) but the cluster should be thought of as an indivisible unit. D (data) nodes are MarkLogic processes that have forests attached. E (evaluator) nodes are those nodes which run XQuery/XSLT requests on an appserver. In a cluster, all nodes share the same appserver configuration, so any node can be an E node. Typically, when configuring dedicated E and D nodes, you configure things to send requests to only those nodes that you want to act as E's, allowing the others to act only as D's. Communication between nodes in a cluster is basically this: For queries (read-only) no locks are needed (read up on MVCC). Each search operation is fired in parallel to every D node in the cluster (this is the "map" phase). When the last D node has responded, the E node can then merge the results (the "reduce"). So, the lower the latency in communication between nodes, the better the overall throughput. You really don't want any slow links between nodes in the cluster because it can slow down all the E nodes. For update (write), cluster-wide locks must be obtained for documents that are, or might be, updated. All nodes in the cluster must acknowledge the lock(s) before the update(s) can proceed. This basically means that updates can't happen faster than the slowest responding node in the cluster. Oh, and the locks need to be released as well, via inter-node communication. Again, bad for overall performance when communication links between nodes slow down, even with super-fast, beefy hardware. As Mike pointed out, clusters are not database replication. You cluster to improve performance by spreading the immediate work across multiple CPU and disks co-located together. You can add synchronous replication between nodes in a cluster to provide for HA failover in the event a node fails. This has a latency cost, but makes the cluster more robust. You replicate databases asynchronously between clusters to provide for disaster recovery if an entire cluster is lost or becomes unreachable. Hope that helps. --- Ron Hitchens {[email protected]} +44 7879 358212 On Oct 28, 2013, at 10:03 PM, Arindam3 B <[email protected]> wrote: > > Thanks Mike for the great walkthrough. Just trying to understand more on the > xqdp protocol. Can you throw some light on how it operates between enodes n > dnodes? > > Thanks & Regards > Arindam > > -----Michael Blakeley <[email protected]> wrote: ----- > > ======================= > To: MarkLogic Developer Discussion <[email protected]> > From: Michael Blakeley <[email protected]> > Date: 10/28/2013 10:31PM > Subject: Re: [MarkLogic Dev General] Reg: E-Node and D-Node configuration > ======================= > ÿ Hosts within a cluster should have low-latency communications: gigabit > ethernet or better. Ideally they should all be on the same switch and/or > VLAN, with no router hops between hosts. If you try to set up a cluster > across a WAN link you are likely to see poor performance and poor > reliability. You might be trying to handle high availability (HA) and > disaster recovery (DR) with a single cluster: that would be a mistake. > > For high availability, use a single cluster with low-latency communications. > Configure forest replication and host failover to provide the desired degree > of protection against host failures. The docs at > http://docs.marklogic.com/guide/cluster/failoverÿtalk about this as > "local-disk failover". > > For disaster recovery - scenarios where an entire data center goes offline - > use database replication to a different cluster. This can use higher-latency > communications, such as a WAN link. The docs at > http://docs.marklogic.com/guide/database-replicationÿdescribe this. The DR > replica cluster can also implement local-disk failover to provide its own HA. > > -- Mike > > On 28 Oct 2013, at 06:41 , Arindam3 B <[email protected]> wrote: > >> Hi, >> >> I had a query regarding the E-Node and D-Node setup in Marklogic. >> >> In a distributed environment, if I plan to keep the Enodes and DNodes >> separately in different physical locations over the LAN or WAN (across >> geographies), what is the potential risk? >> How does failover work in that scenario? >> I have read that ENodes and DNodes communicate through XQDP protocol, so in >> this case will there be performance issues? >> >> Does Marklogic recommend having ENode and DNode cluster in the same physical >> box? >> If so, then across the network if we have a set of E-D-Nodes, how is the >> network latency reduced while synching the data during replication? >> >> If you can provide me with some information about XQDP protocol it would be >> great!! >> >> Thanks & Regards >> Arindam Bose >> =====-----=====-----===== >> Notice: The information contained in this e-mail >> message and/or attachments to it may contain >> confidential or privileged information. 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