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From Shawn Heisey <s...@elyograg.org>
Subject Re: 3 general questions about SolrCloud
Date Mon, 15 Apr 2013 17:59:59 GMT
On 4/15/2013 9:58 AM, SuoNayi wrote:
> 1. What's the model between the master and replicas in one shard?
> If the replicas are able to catch up with the master when the master
> receives a update request it will scatter the request to all the active
> replicas and expect responses before the request get executed
> by itself.This is called push model,right?
> When a new replica is present it will download the whole index from
> the master can this be called pull model?but when the master pushes
> updates to it how the replica behaves,continuing to download the whole
> index while keeping a track of the updates in a log(tlog) ?

There is no master.  SolrCloud is fully distributed.  One replica on 
each shard is elected leader, but that is not a permanent designation.

> 2.What's the main use of the transaction log?
> Is it only used to serve the NRT get requests and not related with data sync
> between the master and replica?

The transaction log is used to replay transactions when a node starts 
up.  If the differences between the leader replica and a replica that 
just started are small enough, the transaction log will be used to bring 
them back into sync.  If they are too different, the one that just 
started will replicate the full index from the leader.  I am pretty sure 
that the _version_ field present on every document is used to determine 
whether replicas are in sync, not the index version.

> 3.Will the leader election be related with index version?
> If a shard has 3 replicas and when the master goes down how to choose the master,
> do they compare the lastest index version for each one or only consider the earliest
presence time?

Here is my understanding about leader elections, I hope it's right! 
Leader elections only take place when the leader goes down.  Once a 
replica is elected leader, it will remain leader unless it goes down. 
The other replicas can go up and down and the leader will retain that 
role.  I do not think the index version is used at all, the _version_ 
field in the index is probably used instead.

Thanks,
Shawn


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