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From Erick Erickson <erickerick...@gmail.com>
Subject Re: If the leader dies, will the data be lost?
Date Mon, 06 Apr 2020 11:42:14 GMT
You’ve got the sequence, that’s it exactly.

I don’t quite understand the second part of the question, but let me address data loss.

Before the leader goes down, the sequence of an update is this.
- the doc comes in to the leader (TL)
- the doc is forwarded to all the other tlog replicas (TF) and written to _their_ tlogs
- all the TF replicas ack back to TL
- TL acks back to the client

So, upon getting success back from the update request, all TLOG replicas have the
docs in their local tlog files. So when the leadership changes, the new leader
has all the docs to replay, thus no data loss.

At that point, the old leader’s tlogs are irrelevant. When it  comes back online,
the sequence is:

- synch from the new leader, including any tlogs. This effectively erases the old tlogs
- start writing any new docs into the local tlog

The old leader then remains a follower until some event changes things again.

Best,
Erick

> On Apr 6, 2020, at 1:53 AM, Taisuke Miyazaki <miyazakitaisuke@lifull.com> wrote:
> 
> Hi,
> Using solr 7.5.0 on solr cloud, and replica type is tlog.
> 
> If a leader dies, how is the re-election of the leader and the
> synchronization of the replicas done?
> 
> In my opinion.
> Leader dies→ New tlog replica tries to become Leader→ Replays tlogs not
> reflected in the index→ Becomes Leader
> Is this the right fit first?
> 
> Also, when another leader is elected, does it create a tlog that is only
> available to the old leader? (I'm worried about data being lost if the
> tlogs aren't synchronized.)


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