C.8. Troubleshooting Data Differences

It can sometimes become necessary to identify table and data differences due to unexpected behaviour or failures. There are a number of third party tools that can help identify and fix however a lot of them assume native replication is in place, the following explains the recommended methods for troubleshooting a Tungsten Environment based on MySQL as the source and target technologies.

C.8.1. Identify Structural Differences

If you suspect that there are differences to a table structure, a simple method to resolve this will be to compare schema DDL.

Extract DDL on the Master node, specifying the schema in place of {DB}:

shell> mysqldump -u root -p --no-data -h localhost --databases {DB} >master.sql

Repeat the same on the Slave node:

shell> mysqldump -u root -p --no-data -h localhost --databases {DB} >slave.sql

Now, using diff, you can compare the results

shell> diff master.sql slave.sql

Using the output of diff, you can then craft the necessary SQL statements to re-align your structure

C.8.2. Identify Data Differences

It is possible to use pt-table-checksum from the Percona Toolkit to identify data differences, providing you use the syntax described below for bypassing the native replication checks. First of all, it is advisable to familiarise yourself with the product by reading through the providers own documentation here:

https://www.percona.com/doc/percona-toolkit/2.2/pt-table-checksum.html

Once you are ready, ensure you install the latest version to the persona toolkit on all nodes, next execute the following on the Master node:

shell> pt-table-checksum --set-vars innodb_lock_wait_timeout=500 \
--recursion-method=none \
--ignore-databases=mysql \
--ignore-databases-regex=tungsten* \
h=localhost,u=tungsten,p=secret

On first run, this will create a database called percona, and within that database a table called checksums. The process will gather checksum information on every table in every database excluding the mysql and tungsten related schemas. You can now execute the following SQL Statement on the slave to identify tables with data differences:

SELECT db, tbl, SUM(this_cnt) AS total_rows, COUNT(*) AS chunks
FROM percona.checksums
WHERE (
 master_cnt <> this_cnt
 OR master_crc <> this_crc
 OR ISNULL(master_crc) <> ISNULL(this_crc))
GROUP BY db, tbl;

This SELECT will return any tables that it detects are different, it won't show you the differences, or indeed how many, this is just a basic check. To identify and fix the changes, you could use pt-table-sync, however this product would by default assume native replication and also try and fix the problems for you. In a tungsten environment this would not be recommended, however by using the --print switch you can gather the SQL needed to be executed to fix the mistakes. You should run this, and review the output to determine whether you want to manually patch the data together or consider using tungsten_provision_slave to retrovision a node in the case of large quantities of differences.

To use pt-table-sync, first identify the tables with differences on each slave, in this example, the SELECT statement above identified that there was a data difference on the departments table within the employees database on db2. Execute the pt-table-sync script on the master, passing in the database name, table name and the slave host that the difference exists on:

shell> pt-table-sync --databases employees --tables departments --print h=db1,u=tungsten,p=secret,P=13306 h=db2

The first h= option should be the Master, also the node you run the script from, the second h= option relates to the slave that the difference exists on. Executing the script will output SQL statements that can be used to patch the data, for example the above statement produces the following output:

UPDATE `employees`.`departments`
SET `dept_name`='Sales'
WHERE `dept_no`='d007'
LIMIT 1
/*percona-toolkit src_db:employees src_tbl:departments src_dsn:P=13306,h=db1,p=...,u=tungsten
dst_db:employees dst_tbl:departments dst_dsn:P=13306,h=db2,p=...,u=tungsten
lock:0 transaction:1 changing_src:0 replicate:0 bidirectional:0 pid:24524 user:tungsten host:db1*/;

The UPDATE staments could now be issued directly on the slave to correct the problem.

Warning

Generally, changing data directly on a slave is not recommended, but every environment is different. before making any changes like this ALWAYS ensure you have a FULL backup, and it would be recommended to shun the slave node (if in a clustered environment) before making any changes so as not to cause any potential interruption to connected clients