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From Oskar Zinger <oska...@yahoo.com>
Subject Re: NullPointerException when Shuting Down Derby
Date Mon, 25 Feb 2013 19:42:52 GMT
Hi Dag,

Imagine a two server system, S1 and S2. One is "designated" primary (S1) and another is secondary
(S2). Here is a scenario, and the sequence of events:

Note: Designated primary will always take control back as a primary server in the cluster


1. (S1) primary starts and starts Derby - right now its stand-alone server
2. (S2) secondary starts and starts Derby - now it will setup replication, will execute startSlave
in a new thread, and execute startMaster
3. (S1) now designated primary gets shutdown or crashes
4. (S2) the secondary server detects this, assumes the role of primary and stops Derby (shutdown
of entire Derby engine - including all databases - NOT using stopMaster / stopSlave), starts
Derby as the new master (primary)
5. (S1) now designated primary comes back and wants to take control back as the primary -
that's where the problem happens - we call it failback, a couple of things happen:
      -- (S1) starts first as a secondary of the cluster - it needs to resync configuration
and database, now (S2) Derby is Primary, (S1) Derby is Secondary
      -- (S1) now sends message to switch roles, (S1) Derby is going to shutdown (NullPointerException)
and restart, (S2) is going to shutdown and restart (cannot setup replication because of NPE
on S1)

Basically, it works the same way as in Step 4, and no NPE. And the strangest thing is - this
is only happening on 1-processor system, its not possible to reproduce on a 2-processor system.

Thanks,
Oskar



________________________________
 From: Dag Wanvik <dag.wanvik@oracle.com>
To: Derby Discussion <derby-user@db.apache.org> 
Sent: Sunday, February 24, 2013 10:42 PM
Subject: Re: NullPointerException when Shuting Down Derby
 



On 31.01.2013 06:13, Oskar Zinger wrote:

This is only happening in a specific scenario when a host application server failbacks, so
what it does is stops a service that manages derby network server, and restarts it.
>
So, is this an attempt to shut down the ex-slave (now the failed
    over master) after the old master has been (re)started? I would
    perhaps be helpful if you can explain the replication scenario in
    some detail, since replication contains much code specific to
    replication.

Thanks,
Dag
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