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https://issues.apache.org/jira/browse/SLING-2939?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13697252#comment-13697252
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Ian Boston commented on SLING-2939:
-----------------------------------
[~egli]
JGroups supports UDP multicast, UDP, TCP and Tunnelling through firewalls.
Configuration documentation and diagrams are at [1]. However once you are on
more than one subnet you can't UDP multicast outside that subnet without router
support, which is the reason for the other protocols. Using the other protocols
to hop between subnets requires that the configuration of the subnets is known.
In this respect JGroups is no different from Zookeeper, in that both require
some level of deployment configuration and complexity to make them work.
I think the main difference is that JGroups expects to be embedded on every
instance in the cluster and to self configure leaders reusing every member of
the cluster to achieve resilience, whereas Zookeeper (like Chubby) requires
centralised Zookeeper servers that are made resilient through replicas.
Probably, Zookeeper is better suited to very large clusters (eg 2500 nodes
upwards), and JGroups is better suited to smaller clusters, although I have no
evidence to back that up, and I am sure both communities would disagree.
The reason I mentioned ElasticSearch is that I know it supports very large
clusters and when you deploy it in AWS, you tell it you are running in AWS. I
havent looked into the detail of precisely what it does, but I have talked to
people who run it over multiple AWS sites, multitennanted, successfully. Which
makes it worth looking at.
Without hard evidence, it might be better to provide a JGroups bundle and a
Zookeeper bundle and find out what the real issues are with them. IIRC the
JGroups code required to do this is minimal, as I did a something simular for
Sling last year. Not certain how much effort is required for Zookeeper.
1 see http://www.jgroups.org/manual/html/user-advanced.html#d0e2251
> 3rd-party based implementation of discovery.api
> -----------------------------------------------
>
> Key: SLING-2939
> URL: https://issues.apache.org/jira/browse/SLING-2939
> Project: Sling
> Issue Type: Task
> Components: Extensions
> Affects Versions: Discovery API 1.0.0
> Reporter: Stefan Egli
> Assignee: Stefan Egli
>
> The Sling Discovery API introduces the abstraction of a topology which
> contains (Sling) clusters and instances, supports liveliness-detection,
> leader-election within a cluster and property-propagation between the
> instances. As a default and reference implementation a resource-based, OOTB
> implementation was created (org.apache.sling.discovery.impl).
> Pros and cons of the discovery.impl
> Although the discovery.impl supports everything required in discovery.api, it
> has a few limitations. Here's a list of pros and cons:
> Pros
> No additional software required (leverages repository for intra-cluster
> communication/storage and HTTP-REST calls for cross-cluster communication)
> Very small footprint
> Perfectly suited for a single clusters, instance and for small, rather
> stable hub-based topologies
> Cons
> Config-/deployment-limitations (aka embedded-limitation): connections
> between clusters are peer-to-peer and explicit. To span a topology, a number
> of instances must (be made) know (to) each other, changes in the topology
> typically requires config adjustments to guarantee high availability of the
> discovery service
> Except if a natural "hub cluster" exists that can serve as connection
> point for all "satellite clusters"
> Other than that, it is less suited for large and/or dynamic topologies
> Change propagation (for topology parts reported via connectors) is
> non-atomic and slow, hop-by-hop based
> No guarantee on order of TopologyEvents sent in individual instances - ie
> different instances might see different orders of TopologyEvents (ie changes
> in the topology) but eventually the topology is guaranteed to be consistent
> Robustness of discovery.impl wrt storm situations depends on robustness
> of underlying cluster (not a real negative but discovery.impl might in theory
> unveil repository bugs which would otherwise not have been a problem)
> Rather new, little tested code which might have issues with edge cases
> wrt network problems
> although partitioning-support is not a requirement per se, similar
> edge-cases might exist wrt network-delays/timing/crashes
> Reusing a suitable 3rd party library
> To provide an additional option as implementation of the discovery.api one
> idea is to use a suitable 3rd party library.
> Requirements
> The following is a list of requirements a 3rd party library must support:
> liveliness detection: detect whether an instance is up and running
> stable leader election within a cluster: stable describes the fact that a
> leader will remain leader until it leaves/crashes and no new, joining
> instance shall take over while a leader exists
> stable instance ordering: the list of instances within a cluster is
> ordered and stable, new, joining instances are put at the end of the list
> property propagation: propagate the properties provided within one
> instance to everybody in the topology. there are no timing requirements bound
> to this but the intention of this is not to be used as messaging but to
> announce config parameters to the topology
> support large, dynamic clusters: configuration of the new discovery
> implementation should be easy and support frequent changes in the (large)
> topology
> no single point of failure: this is obvious, there should of course be no
> single point of failure in the setup
> embedded or dedicated: this might be a hot topic: embedding a library has
> the advantages of not having to install anything additional. a dedicated
> service on the other hand requires additional handling in deployment.
> embedding implies a peer-to-peer setup: nodes communicate peer-to-peer rather
> than via a centralized service. this IMHO is a negative for large topologies
> which would typically be cross data-centers. hence a dedicated service could
> be seen as an advantage in the end.
> due to need for cross data-center deployments, the transport protocol
> must be TCP (or HTTP for that matter)
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