MySQL master master replication, safe to delete rows on both masters

deletemulti-masterMySQLmysql-5.1replication

We have a MySQL (v5.1.61) cluster with two masters (and two slaves). We have a table logs with a primary key column logID. This log table grows rather huge, and we'd like to delete data from it. So I've written a script that does that. — Is it safe to run this script on both masters? It deletes data only. What I'm wondering is what will the MySQL replication module do, if the row it's about to delete has already been deleted? Will replication break, or is it safe to delete rows on both masters? Does it matter if we use row based or statement based replication?

Example: The log data deletion script deletes row 123 on Master A. At the same time, the script deletes that same row on Master B. Then master B reads its relay log, and sees: "I should delete row 123, because it was deleted on Master A and I'm its slave. Let's delete it… … …but it's already gone!" — now what will Master B do? Will it halt replication, as if there was a duplicate key error? Or will it think "Fine, I need do nothing", and everything is okay.

Best Answer

Your question already has the key to your answer. It does depend on row based or statement based replication.

STATEMENT-BASED REPLICATION

If you run DELETE FROM tblname WHERE blahblahblah; and the rows matching blahblahblah do not exist, no big deal. Replication will just carry on. It will just take you a lot longer to realize your data draft on the Slave (or in your case, either Master) if there were slight differences at all.

ROW-BASED REPLICATION

If you run DELETE FROM tblname WHERE blahblahblah; and the rows matching blahblahblah do not exist, that can break replication because the exact row info is embedded in the relay logs on slave. That row is expected to exist for the delete to happen. There is an error code for it:

sh-4.1# perror 1032
MySQL error code 1032 (ER_KEY_NOT_FOUND): Can't find record in '%-.192s'

You will likely see error log messages like these:

2014-02-27 22:03:00 4070 [ERROR] Slave SQL: Could not execute Update_rows event on table mydb.mytable; Can't find record in 'mytable', Error_code: 1032; handler error HA_ERR_KEY_NOT_FOUND; the event's master log mysql-bin.000166, end_log_pos 93114177, Error_code: 1032

2014-03-17 10:50:15 11596 [ERROR] Slave SQL: Could not execute Delete_rows event on table mydb.mytable; Can't find record in 'mytable', Error_code: 1032; handler error HA_ERR_KEY_NOT_FOUND; the event's master log mysql-bin.000337, end_log_pos 427881, Error_code: 1032

Here are other links that discuss this

For more insights, please read Advantages and Disadvantages of Statement-Based and Row-Based Replication from the MySQL Documentation.

Question 1

Does the DML operations committed by db2 during the replication process gets included in its own bin-log?

Answer to Question 1

Yes it will, provided you have this in /etc/my.cnf on both db1 and db2

[mysqld]
log-slave-updates

If you do not have this, add it and restart mysql

Question 2

Would the resulting bin-log in db2 be exactly the same with the bin-log of db1, to the letter?

Answer to Question 2

Yes. Make sure the clocks on both DB servers are synchronized

Question 3

What happens to the entries in db2 relay-log once they are committed to the database during the replication process, are they discarded? What role does the relay-log info log has in this?

Answer to Question 3

In MySQL Replication, the IO Thread of a Slave will read its Master's bin-log entries and store them in a FIFO queue. For each relay log in a slave, when every entry in the currently processed relay is executed it is rotated out and discarded. If relay logs are piling up, this quickly indicates that the SQL thread died because of any SQL error. Just do SHOW SLAVE STATUS\G to find out what stopped the SQL thread. The IO Thread would conitnue collecting completed SQL statements from its Master.

Question 4

How does db1 know where in the bin-log of db2 (somehow dependent on the answer of Question 2), it will start the replication process?

Answer to Question 4

When you do SHOW SLAVE STATUS\G, look for the following lines:

Master_Log_File : The latest binary log whose most recently command was copied to the Slave
Read_Master_Log_Pos : The latest position of the latest binary log whose most recently command was copied to the Slave
Relay_Master_Log_File : The latest binary log whose most recently command was executed on the Slave
Exec_Master_Log_Pos : The latest binary log whose most recently command was executed on the Slave
Relay_Log_Space : The sum total (in bytes) of all relay logs. By default, each relay log is the default size of a binary log (1G). If Relay_Log_Space starts to significant exceed 1G, this indicates one of two things:
- SQL thread died due to SQL Error
- SQL thread is busy with a long-running query

Question 4.1

If you enable log-slave-updates on both databases i.e. dB1 & dB2, then that would mean all items from the binary log of dB1, which was successfully replicated by dB2 will be written into dB2's binary log and vice-versa. Would not this result to some sort of infinite circular replication or duplications of entries on both databases, if it's possible at all, considering the possible key-collision issues that would arise? What I'm trying to say is, How would dB1 know once it checks on the binary log of dB2 that, "I should not replicate those entries in there because they all just came from me"?

Answer to Question 4.1

You must have log-slave-updates available on both DB servers in order to have an audit trail that the SQL executed on on DB server made it to the other. If you don't, you would have to do your due diligence to compare the data explicitly. Such ways would include:

Running CHECKSUM TABLE on every table you have in both DB servers to compare their contents.
Using pt-table-checksum, which is an automated version of running CHECKSUM TABLE between Master and one or more Slaves

You need not worry about infinite circular replication unless you are dealing with more that two masters. There have been rare times when someone with, let's say four Masters, removes one of the four servers from circular rep cluster. Let's suppose the the server_id is 13. It is remotely, but still, possible for binary log entries whose server_id belongs to the server that removed to be inside the relay logs on other servers. Only in such a scenario would you worry about infinite circular replication.

To circumvent such situations, MySQL 5.5 has a new option for the CHANGE MASTER TO command called IGNORE_SERVER_IDS. You would do the following to repair things on all the remaining servers:

STOP SLAVE;
CHANGE MASTER TO IGNORE_SERVER_IDS = (13);
START SLAVE;

In fact, here is what the MySQL Documentation says on this:

IGNORE_SERVER_IDS was added in MySQL 5.5. This option takes a comma-separated list of 0 or more server IDs. Events originating from the corresponding servers are ignored, with the exception of log rotation and deletion events, which are still recorded in the relay log.

In circular replication, the originating server normally acts as the terminator of its own events, so that they are not applied more than once. Thus, this option is useful in circular replication when one of the servers in the circle is removed. Suppose that you have a circular replication setup with 4 servers, having server IDs 1, 2, 3, and 4, and server 3 fails. When bridging the gap by starting replication from server 2 to server 4, you can include IGNORE_SERVER_IDS = (3) in the CHANGE MASTER TO statement that you issue on server 4 to tell it to use server 2 as its master instead of server 3. Doing so causes it to ignore and not to propagate any statements that originated with the server that is no longer in use.

Question 5

On INSERT queries on the master, what form of the query is written into the binary log? Is it the 'raw' form of the query, or the one which already has the auto-generated value of the auto-increment key?

Answer to Question 5

Whichever form is presented. Here is what I mean: The raw form would usually not include the auto_increment column expressed explicitly. On the other hand, it you import a mysqldump into a DB server with binary logging, the rows being inserted would explicitly be given. Either version of INSERT would be allowed execution in mysqld. In like fashion, either version of INSERT would be recorded AS IS...

MySQL Replication not proceeding

This turned out to not be a mysql issue at all. The network team recently installed a new security device that will block packets on certain rules. A legitimate database write contained a sequence of characters the device deemed nefarious.

The overall connection handshake for replication was able to make it fine but then it just sat there asking the master for the next log entry whose packets were never making it back.

As far as the broken slave was concerned, it was up to date b/c it had executed the most recent event in the relay logs it had received.