Re: Binary Logs and Transactions (with InnoDB and MyISAM)

[Chris Nolan]

Hi David,

David Griffiths wrote:

From reading the docs, a binary log is an efficient representation of all
data-modifying SQL that is run on the master database. I was unable to
figure out what happens if a slave is interrupted while in the middle of
processing a binary log.
When a binary log is applied to a slave database, what happens if the
machine or database dies half way through the log?
 

Basically, the slave will try to "catch up" when it restarts.

For example, with InnoDB, say the following statements are run and stored in
the binary log:
---------------------------
1) INSERT INTO table_a (column_a, column_b, ...) VALUES (...);
2) COMMIT;

3) UPDATE table_a SET column_b = 'some_value' WHERE column_c =
'something_else';
4) COMMIT;
-----------------------------
The slave-machine (also using InnoDB tables) start processing the binary
log. Statement 1 and 2 are processed, but it dies before Statement 3 (UPDATE
table_a...) is executed.
 

If I recall correctly, the binary log uses transactions as it's basic 
units. I'm not even sure if the slave will see statement 3 before 
statement 4, but I know it definitely will not act on it in any way.

If you restart the slave, would it start the binary log back at the
beginning, or is the offset inside the file stored in the database so that
the slave database starts at Statement 3? Or would the whole log be
re-processed (potentially causing problems with inserting rows with unique
keys)?
 

The slave won't try to reperform actions that are already processed. You 
can relax regarding unique attributes.

How does this work with MyISAM?
 

In essentially the same way. As each statement is basically bound with 
BEGIN and COMMIT statements, each statement is processed by the slave 
after it successfully completes on the master.

Thanks,
David
 

Regards,

Chris

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