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From Jørgen Løland (JIRA) <j...@apache.org>
Subject [jira] Created: (DERBY-4197) Simplify initiation of replication
Date Wed, 29 Apr 2009 12:23:30 GMT
Simplify initiation of replication

                 Key: DERBY-4197
                 URL: https://issues.apache.org/jira/browse/DERBY-4197
             Project: Derby
          Issue Type: New Feature
          Components: Replication
    Affects Versions:
            Reporter: Jørgen Løland
            Priority: Minor

Starting Derby replication is brittle; you need to perform multiple steps in exact order.
Furthermore, the steps involves freezing the database on the master Derby instance the time
it takes to copy the database to the slave node.

A much simpler way to start replication would be:

1) Make an online backup of the database on the master Derby instance. Enable log archiving.
2) Copy the backup to the slave node while the master Derby instance continues to handle transactions
3) Boot the slave database in slave mode.
4) Master sends all log written since the backup was made, or from a specified log instant
(a number that can be specified as an option to either the startSlave or startMaster command?).

The main implementation effort is in step 4, because this step requires the master derby instance
to read and send log records that are on disk. This differs from the current replication code
that captures log records on the master as they are about to be written to disk (i.e., replication
does not read files). 

Since the master instance is supposed to continue transaction processing, it will write to
the latest log file while the replication thread will have to read log files (including the
currently open one). The Derby code that reads log files is written to handle a single thread
(the recovery thread) only in the current implementation. 

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