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...
Best Answer
The
autoincrement%settings you mentioned take care ofAUTO_INCREMENTvalues, but do not handle otherUNIQUE KEYs. Keep in mind that if a dup key error occurs, replication is stopped until a human intervenes.Depending on how the code (router, proxy, etc) picks which Master to write to, you may have the "critical read" problem. This is where, say, someone posts something, then goes to another web page and expects to see the results of it. But the read goes to another machine and the results have not [yet] arrived because of replication delay.
Automatic failover is a big challenge. It is probably best left to be manual. If the network dies, you could have the "split brain" problem. In this case it is unsafe to failover because machine can't see the other. If they both blindly thing the other is dead, and take control, you could get data conflicts that are 'impossible' to fix.
Some of the issues are avoided by designating one of the Master as active and having the other be
readonly.After all, every write to a Master is also performed on every Slave (including the other Master in a dual-Master setup). That is, there is essentially no "write scaling" by having dual-Master. There is some "read scaling".
What is your goal of picking M-M? Scaling? HA? Disaster protection? Facilitate certain schema changes? Other? Are the two Masters located next to each other? On the same power source?
One of my points is that M-M provides limited benefits. And you may have to do other things to really achieve them. (Separate data centers, careful routing of queries to servers, etc.)
A number of the issues are lessened if you go to one of the "cluster solutions" (Galera or Group Replication). They require 3 servers, preferably in 3 geographic locations. But they handle split brain, automatic failover, and some other issues.