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From "Serge Tsv (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-3454) 'java.lang.NullPointerException' is thrown when starting a master db before a slave one
Date Mon, 03 Mar 2008 17:06:51 GMT

    [ https://issues.apache.org/jira/browse/DERBY-3454?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12574612#action_12574612
] 

Serge Tsv commented on DERBY-3454:
----------------------------------

V.Narayanan, thanks for your comments!

I see the following generic use case for receiver/transmitter classes:

1. A receiver/transmitter instance is created.
2. An initConnection method is called on the created instance
3. sendMessage/readMessage calls are executed on the initialized 
     receiver/transmitter instance

For the flow above:
   * If (3) is executed before (2), then I guess this would be a programmatic error 
     and a RE should be called (like IllegalStateException)
   * If (2) fails, I do also consider executing (3) after that as a programmatic error, 
      because in this case a transmitter/receiver hasn't been properly initialized. 
      And there's an attempt to use a transmitter/receiver which has turned up to
      be in an illegal state.

You're right that the transmitter/receiver is an abstraction above the connection.
But I'm just trying to say that it might be wrong (from a programmatic point of view)
to try to use them in an illegal state. And might be better to properly process a (2)
failure and make it so sendMessage/readMessage methods won't be called 
afterwards, instead of relying on an IOException been reported to a user.

> If this had been true the NPE (which is also a RuntimeException)
would have resulted in a XRE04 :).

I think that XRE04 is thrown by an initConnection method. The NPE was caused 
by a tearDown method, and now it's been fixed, and there're no further calls to
sendMessage/readMessage in case of XRE04.

Thanks!

> 'java.lang.NullPointerException' is thrown when starting a master db before a slave one
> ---------------------------------------------------------------------------------------
>
>                 Key: DERBY-3454
>                 URL: https://issues.apache.org/jira/browse/DERBY-3454
>             Project: Derby
>          Issue Type: Bug
>          Components: Replication
>    Affects Versions: 10.4.0.0
>            Reporter: Serge Tsv
>            Assignee: V.Narayanan
>         Attachments: Derby3454.diff, Derby3454.stat
>
>
> The 'java.lang.NullPointerException' exception is thrown when a database is started in
a master mode and is trying to establish a connection to an slave database socket, which is
not available.
> The exception is by the MasterController#startMaster(). First, it tries to setup connection
with a slave database using a transmitter:
>     MasterController#setupConnection
>        -> transmitter = new ReplicationMessageTransmit(); 
>             transmitter.initConnection()
>               -> new InetSocketAddress() -> createSocket() -> connect()
> The connect() method throws a ConnectException, and so fails to create a socketConn instance.
The exception is then wrapped several times an propagated back to the MasterController#startMaster()
method. It's caught there and then a MasterController#teardownNetwork() method is called,
which tries to send a STOP message using the aforementioned transmitter, which hasn't been
able to init a connection. 
> A transmitter simply tries to call socketConn.writeMessage(message), which throws NPE
because socketConn is null.
> Thanks!

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