The Ultimate Guide to High Availability with MariaDB

Mirror

Mirror

Overview

The mirror router is designed for data consistency and database behavior verification during system upgrades. It allows statement duplication to multiple servers in a manner similar to that of the Tee filter with exporting of collected query metrics.

For each executed query the router exports a JSON object that describes the query results and has the following fields:

Key Description
query The executed SQL if an SQL statement was executed
command The SQL command
session The connection ID of the session that executed the query
query_id Query sequence number, starts from 1
results Array of query result objects

The objects in the results array describe an individual query result and have the following fields:

Key Description
target The target where the query was executed
checksum The CRC32 checksum of the result
rows Number of returned rows
warnings Number of returned warnings
duration Query duration in milliseconds
type Result type, one of ok, error or resultset

Configuration Parameters

main

  • Type: target
  • Mandatory: Yes
  • Dynamic: Yes

The main target from which results are returned to the client. This is a mandatory parameter and must define one of the targets configured in the targets parameter of the service.

If the connection to the main target cannot be created or is lost mid-session, the client connection will be closed. Connection failures to other targets are not fatal errors and any open connections to them will be closed. The router does not create new connections after the initial connections are created.

exporter

  • Type: enum
  • Mandatory: Yes
  • Dynamic: Yes
  • Values: log, file, kafka

The exporter where the data is exported. This is a mandatory parameter. Possible values are:

  • log

  • Exports metrics to MaxScale log on INFO level. No configuration parameters.

  • file

  • Exports metrics to a file. Configured with the file parameter.

  • kafka

  • Exports metrics to a Kafka broker. Configured with the kafka_broker and kafka_topic parameters.

file

  • Type: string
  • Default: No default value
  • Mandatory: No
  • Dynamic: Yes

The output file where the metrics will be written. The file must be writable by the user that is running MaxScale, usually the maxscale user.

When the file parameter is altered at runtime, the old file is closed before the new file is opened. This makes it a convenient way of rotating the file where the metrics are exported. Note that the file name alteration must change the value for it to take effect.

This is a mandatory parameter when configured with exporter=file.

kafka_broker

  • Type: string
  • Default: No default value
  • Mandatory: No
  • Dynamic: Yes

The Kafka broker list. Must be given as a comma-separated list of broker hosts with optional ports in host:port format.

This is a mandatory parameter when configured with exporter=kafka.

kafka_topic

  • Type: string
  • Default: No default value
  • Mandatory: No
  • Dynamic: Yes

The kafka topic where the metrics are sent.

This is a mandatory parameter when configured with exporter=kafka.

on_error

  • Type: enum
  • Default: ignore
  • Mandatory: No
  • Dynamic: Yes
  • Values: ignore, close

What to do when a backend network connection fails. Accepted values are:

  • ignore

  • Ignore the failing backend if it's not the backend that the main parameter points to.

  • close

  • Close the client connection when the first backend fails.

This parameter was added in MaxScale 6.0. Older versions always ignored failing backends.

report

  • Type: enum
  • Default: always
  • Mandatory: No
  • Dynamic: Yes
  • Values: always, on_conflict

When to report the result of the queries. Accepted values are:

  • always

  • Always report the result for all queries.

  • on_conflict

  • Only report when one or more backends returns a conflicting result.

This parameter was added in MaxScale 6.0. Older versions always reported the result.

Example Configuration

[server1]
type=server
address=127.0.0.1
port=3000

[server2]
type=server
address=127.0.0.1
port=3001

[MariaDB-Monitor]
type=monitor
module=mariadbmon
servers=server1,server2
user=maxuser
password=maxpwd
monitor_interval=2s

[Mirror-Router]
type=service
router=mirror
user=maxuser
password=maxpwd
targets=server1,server2
main=server1
exporter=file
file=/tmp/Mirror-Router.log

[Mirror-Listener]
type=listener
protocol=mariadbclient
service=Mirror-Router
port=3306

Limitations

  • Broken network connections are not recreated.

  • Prepared statements are not supported.

  • Contents of non-SQL statements are not added to the exported metrics.

  • Data synchronization in dynamic environments (e.g. when replication is in use) is not guaranteed. This means that result mismatches can be reported when the data is only eventually consistent.

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