[infinispan-issues] [JBoss JIRA] Commented: (ISPN-1000) PUSH based rehashing

[Manik Surtani (JIRA)]

    [ https://issues.jboss.org/browse/ISPN-1000?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12596804#comment-12596804 ] 

Manik Surtani commented on ISPN-1000:
-------------------------------------

Some design ideas:

{code}
# PUSH based rehashing

The purpose behind this is to provide for a better rehashing scheme that is:
	* more robust
	* has fewer RPC messages
	* performs better

The current rehashing scheme is RPC-heavy and attempts to be non-blocking, resulting in brittleness.

## Overview

* Every node registers a view change listener
* On JOIN, broadcast JOIN message containing optional topology info (if config is topology aware).
* On view change, 
	* determine if current node is affected and kick off a RehashTask
	* If related to a JOIN, RehashTask blocks on JOIN message to grab TopologyInfo (if config is topology-aware).
	* Enter READ_ONLY mode
	* if affected, loop through keyset in data container and non-shared cache store.  For each key:
		* if LAST_OWNER on OLD_CH and key has a new owner in NEW_CH, add to push set
		* if LAST_OWNER is no longer an owner based on NEW_CH, add key to invalidation set (deals with JOINs and LEAVEs in the same way now!)
	* Push state to new owners
	* Apply NEW_CH to local node, exit READ_ONLY mode.
	* If successful, invalidate keys in invalidation set
	
### Determining if node is affected by ViewChange:
* If JOIN:
	* If idx(JOINER) within + or - numOwners of idx(self)
* Or if LEAVE:
	* and idx(LEAVER) within + or - (numOwners + 1) idx(self)

_TODO_: HOw would this affect vnodes?  LEAVER or JOINER will have multiple positions to check, and self will have multiple positions too.  Can this be encapsulated in the ConsistentHash impl?

### READ_ONLY mode:
I think attempting a non-blocking rehash at this stage is way too complex.  We have it with the current scheme and we can see how brittle it is.  IMO we should go for a blocking model for now and make sure rehashing is stable and robust, and consider non-blocking later.

READ_ONLY mode is achieved by using the TransactionLogger.  But we won't actually use this to *log* any transactions for now.  Instead we can use it to block new transactions and non-transactional writes since it exists in all the appropriate places.

### Parallel joiners and leavers
How will this work with parallel joiners and leavers?


{code}

> PUSH based rehashing
> --------------------
>
>                 Key: ISPN-1000
>                 URL: https://issues.jboss.org/browse/ISPN-1000
>             Project: Infinispan
>          Issue Type: Feature Request
>          Components: Distributed Cache
>    Affects Versions: 4.2.0.Final
>            Reporter: Manik Surtani
>            Assignee: Manik Surtani
>              Labels: rehash
>             Fix For: 5.0.0.CR1, 5.0.0.FINAL
>
>
> Current rehash schemes are based on a PULL of state.  Joiners (and new owners after a leave) pull state from their neighbours.  This JIRA is to reimplement this as a PUSH based scheme, where all nodes detect new joiners (or leavers) and analyse their internal state and determine what needs to be pushed where.
> The scheme should be more robust, involving far fewer RPCs and coordination, and would work better for merge views detected when partitions heal.
> Based on Bela's prototype on https://github.com/belaban/infinispan/tree/rebalance-changes

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