Mysql – ACID-compliance of MySQL master-master replication

innodbMySQLreplication

I've created two MySQL 5.1 servers, replicating off each other (log-bin) in a low latency environment (different AZs within an EC2 region). All tables that are being replicated are using the InnoDB storage engine.

It appears to work as expected in a number of scenarios I have tested (not brimming with confidence, though), but I am worried that I cannot find any answers on whether this setup guarantees ACID for both nodes on transactions made to either node.

If not, are there steps I can take to meet this requirement?

I've seen various bits and pieces around the mysql.com site such as:

… For the greatest possible durability and consistency in a replication setup using InnoDB with transactions, you should use innodb_flush_log_at_trx_commit=1, sync_binlog=1 …

but I find it rather worrying that I cannot find a definitive answer (or even guide) on the matter.

Best Answer

MySQL replication is not ACID compliant since it is asynchronous. There are no guarantees that queries executed on one server will be committed on the other. In fact, the master doesn't even know about the slave except that it's pulling binary logs. The binary logs themselves "might" be considered ACID compliant when sync_binlog=1 is on, but one server does not know if the other server has committed the change.

If you truly need something close to ACID compliance in your replication, what you want is synchronous replication, which is provided by products like Tungsten Replicator, Galera, and Percona Xtradb Cluster (which is Galera + Percona Server), and MySQL Cluster. I'm not sure if they fit a strict definition of ACID compliance, but they are certainly a lot closer. MySQL 5.5 has something called semi-synchronous replication, but don't be fooled...it's still asynchronous.

You can improve the reliability of MySQL replication by doing a few things:

Use Row-based Replication (RBR) by setting binlog_format=ROW in my.cnf. This will replicate the entire row for every change made instead of replicating the SQL statement that made the change (as with statement-based replication, [SBR]). That minimizes the chances that a statement executes differently between servers if there are small discrepancies already.
Run pt-table-checksum frequently to verify the consistency between master and slaves. If there are certain tables that are more important, run it on those tables more frequently.
Set up monitoring that checks the output of pt-table-checksum and alerts you when there are discrepancies. You can use pmp-check-pt-table-checksum from the Percona MySQL Nagios Plugins if you use nagios. Otherwise, it's just a simple query to determine this information.
If you have discrepancies, use pt-table-sync to resolve them. Read the documentation carefully, as you can shoot yourself in the foot relatively easily with this tool.

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 – Master Master Setup with Single Master Write

With a single write master <> master setup, I don't need to worry about setting the auto_increment_offset correct?

Yes, you are right. I have setup Master-Master setups for years with one Write Master. I never touched the auto_increment options. Never had an incident.

Does MySQL replicate all databases by default, or do I need to explicitly identify them via binlog-do-db?

Yes, it is by default. Although binlog-do-db exists, I would not recommend it since you are doing Master-Master. If you have additional slaves, it is best to setup filtering on the slave. That way, you have all binlog events to point-in-time recovery and other incidentals. Such incidentals might include Star Topologies (which feature Distribution Masters, a Master that does not house any data, only binlogs Such binlogs could be filter with binlog_do_db to reduce binlog traffic to slaves).

Do I need to do an export from master1 and import to master2, before I start replication ?

You do need to to export from master1 and import to master2.

can I start master 2 to pick up all transactions over the last several months?

The only way you can do that is if you had every binlog since you loaded master1 aznd never erased any of the binlogs. Most people rotate their binlogs, making this impossible.

I need to setup a "replicator" user (with replication perms) on both master1 and master2?

Yes

EPILOGUE

I have setup ucarp over the years as the failover mechanism for the floating IP. Most use Linux Heartbeat, Pacemaker, and others. So, you are in the ballpark of what needs to be done.