Good day everyone!
I want to clarify something about the behavior of master-master replication in MySQL, with respect to the logs i.e. bin-log and relay-log, it use in performing replication.
I assume that in this kind of setup, only one of the 2 db servers is processing DML operations i.e. 'active' server. Read requests are distributed to both of the servers.
So when db1 is the active server, DML operations to it are recorded in its bin-log, and db2 would have to fetch it in, placed it in its relay-log and also process it as a part of the replication process.
Question 1: Does the DML operations committed by db2 during the replication process gets included in its own bin-log?
Question 2: Would the resulting bin-log in db2 be exactly the same with the bin-log of db1, to the letter?
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?
The reason I am asking this because I could not get the whole scenario yet when db1 goes down and db2 becomes the active db server.
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?
UPDATE 1:
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"?
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?
UPDATE 2:
Question 5.1: So you're saying that on raw INSERT queries, the actual query itself is the one written on the bin-log. So in essence, when such queries are replicated and committed on the slaves, it is the case that the same tuple would be associated with or will have, different values on its auto_incremented field, across the different databases i.e. slaves?
Example:
Insert this tuple ('Uncle Sam', 'Male', 26), on a table with auto-increment field say, pki
Then, it follows therefore from your answer that pki
may have different values for this tuple across the different databases i.e. master and slaves.
Question 5.2:Assuming above is right, would not this be an issue when load-balancing read queries across databases?
Thank you very much!
Best Answer
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
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: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:
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:In fact, here is what the MySQL Documentation says on this:
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...