The Ultimate Guide to High Availability with MariaDB

Avrorouter

Avrorouter

The avrorouter is a MariaDB 10.0 binary log to Avro file converter. It consumes binary logs from a local directory and transforms them into a set of Avro files. These files can then be queried by clients for various purposes.

This router is intended to be used in tandem with the Binlog Server. The Binlog Server can connect to a master server and request binlog records. These records can then consumed by the avrorouter directly from the binlog cache of the Binlog Server. This allows MariaDB MaxScale to automatically transform binlog events on the master to local Avro format files.

The avrorouter can also consume binary logs straight from the master. This will remove the need to configure the Binlog Server but it will increase the disk space requirement on the master server by at least a factor of two.

The converted Avro files can be requested with the CDC protocol. This protocol should be used to communicate with the avrorouter and currently it is the only supported protocol. The clients can request either Avro or JSON format data streams from a database table.

Direct Replication Mode

MaxScale 2.4.0 added a direct replication mode that connects the avrorouter directly to a MariaDB server. This mode is an improvement over the binlogrouter based replication as it provides a more space-efficient and faster conversion process.

To enable the direct replication mode, add either the servers or the cluster parameter to the avrorouter service. The avrorouter will then use one of the servers as the replication source. In this mode the source parameter is ignored as there is no need for a source service.

Here is a minimal avrorouter direct replication configuration:

[maxscale]
threads=auto

[server1]
type=server
address=127.0.0.1
port=3306
protocol=MariaDBBackend

[cdc-service]
type=service
router=avrorouter
servers=server1
user=maxuser
password=maxpwd

[cdc-listener]
type=listener
service=cdc-service
protocol=CDC
port=4001

In direct replication mode, the avrorouter stores the latest replicated GTID in the last_gtid.txt file located in the avrodir (defaults to /var/lib/maxscale). To reset the replication process, stop MaxScale and remove the file.

Configuration

For information about common service parameters, refer to the Configuration Guide.

Router Parameters

gtid_start_pos

The GTID where avrorouter starts the replication from in direct replication mode. The parameter value must be in the MariaDB GTID format e.g. 0-1-123 where the first number is the replication domain, the second the server_id value of the server and the last is the GTID sequence number.

This parameter has no effect in the traditional mode. If this parameter is defined, the replication will start from the implicit GTID that the master first serves.

server_id

The server_id used when replicating from the master in direct replication mode. The default value is 1234.

source

The source for the binary logs. This is an optional parameter and is ignored if the router is configured in direct replication mode.

Note: If the source parameter is defined the values for binlogdir and filestem are only read from the source service. This means that the parameters defined for the avrorouter are ignored and the ones in the binlogrouter are used.

The value of this parameter should be the name of a Binlog Server service. The filestem and binlogdir parameters of this service will be read from the router_options and they will be used as the source for the binary logs. This removes the need to manually enter the right binlogdir and filestem options for the avrorouter.

Here is an example of two services. The first service (replication-router) is responsible for downloading the binary logs from the master and the second service (avro-router) will convert the binary logs into Avro format files and store them in /var/lib/mysql.

[replication-router]
type=service
router=binlogrouter
router_options=server-id=4000,binlogdir=/var/lib/mysql,filestem=binlog
user=maxuser
password=maxpwd

[avro-router]
type=service
router=avrorouter
source=replication-router

codec

The compression codec to use. By default, the avrorouter does not use compression.

This parameter takes one of the following two values; null or deflate. These are the mandatory compression algorithms required by the Avro specification. For more information about the compression types, refer to the Avro specification.

match and exclude

These regular expression settings filter events for processing depending on table names. Avrorouter does not support the options-parameter for regular expressions.

binlogdir

The location of the binary log files. This is the first mandatory parameter and it defines where the module will read binlog files from. Read access to this directory is required.

If used in conjunction with the Binlog Server, the value of this option should be the same for both the Binlog Server and the avrorouter if the source parameter is not used.

avrodir

The location where the Avro files are stored. This is the second mandatory parameter and it governs where the converted files are stored. This directory will be used to store the Avro files, plain-text Avro schemas and other files needed by the avrorouter. The user running MariaDB MaxScale will need both read and write access to this directory.

The avrorouter will also use the avrodir to store various internal files. These files are named avro.index and avro-conversion.ini. By default, the default data directory, /var/lib/maxscale/, is used. Before version 2.1 of MaxScale, the value of binlogdir was used as the default value for avrodir.

filestem

The base name of the binlog files. The default value is "mysql-bin". The binlog files are assumed to follow the naming schema <filestem>.<n> where <n> is the binlog number and <filestem> is the value of this router option.

For example, with the following router option:

filestem=mybin,binlogdir=/var/lib/mysql/binlogs/

The first binlog file the avrorouter would look for is /var/lib/mysql/binlogs/mybin.000001.

start_index

The starting index number of the binlog file. The default value is 1. For the binlog mysql-bin.000001 the index would be 1, for mysql-bin.000005 the index would be 5.

If you need to start from a binlog file other than 1, you need to set the value of this option to the correct index. The avrorouter will always start from the beginning of the binary log file.

Note: MaxScale version 2.2 introduces MariaDB GTID support in Binlog Server: currently, if used with Avrorouter, the option mariadb10_master_gtid must be set to off in the Binlog Server configuration in order to correclty read the binlog files.

Example configuration
[replication-router]
type=service
router=binlogrouter
router_options=server-id=4000,binlogdir=/var/lib/mysql,filestem=binlog
user=maxuser
password=maxpwd

[avro-router]
type=service
router=avrorouter
binlogdir=/var/lib/mysql
filestem=binlog
avrodir=/var/lib/maxscale

These options control how large the Avro file data blocks can get. Increasing or lowering the block size could have a positive effect depending on your use case. For more information about the Avro file format and how it organizes data, refer to the Avro documentation.

The avrorouter will flush a block and start a new one when either group_trx transactions or group_rows row events have been processed. Changing these options will also allow more frequent updates to stored data but this will cause a small increase in file size and search times.

It is highly recommended to keep the block sizes relatively large to allow larger chunks of memory to be flushed to disk at one time. This will make the conversion process noticeably faster.

group_trx

Controls the number of transactions that are grouped into a single Avro data block. The default value is 1 transaction.

group_rows

Controls the number of row events that are grouped into a single Avro data block. The default value is 1000 row events.

block_size

The Avro data block size in bytes. The default is 16 kilobytes. Increase this value if individual events in the binary logs are very large. The value is a size type parameter which means that it can also be defined with an SI suffix. Refer to the Configuration Guide for more details about size type parameters and how to use them.

Module commands

Read Module Commands documentation for details about module commands.

The avrorouter supports the following module commands.

avrorouter::convert SERVICE {start | stop}

Start or stop the binary log to Avro conversion. The first parameter is the name of the service to stop and the second parameter tells whether to start the conversion process or to stop it.

avrorouter::purge SERVICE

This command will delete all files created by the avrorouter. This includes all .avsc schema files and .avro data files as well as the internal state tracking files. Use this to completely reset the conversion process.

Note: Once the command has completed, MaxScale must be restarted to restart the conversion process. Issuing a convert start command will not work.

WARNING: You will lose any and all converted data when this command is executed.

Files Created by the Avrorouter

The avrorouter creates two files in the location pointed by avrodir: avro.index and avro-conversion.ini. The avro.index file is used to store the locations of the GTIDs in the .avro files. The avro-conversion.ini contains the last converted position and GTID in the binlogs. If you need to reset the conversion process, delete these two files and restart MaxScale.

Resetting the Conversion Process

To reset the binlog conversion process, issue the purge module command by executing it via MaxAdmin and stop MaxScale. If manually created schema files were used, they need to be recreated once MaxScale is stopped. After stopping MaxScale and optionally creating the schema files, the conversion process can be started by starting MaxScale.

Stopping the Avrorouter

The safest way to stop the avrorouter when used with the binlogrouter is to follow the following steps:

  • Issue STOP SLAVE on the binlogrouter
  • Wait for the avrorouter to process all files
  • Stop MaxScale with systemctl stop maxscale

This guarantees that the conversion process halts at a known good position in the latest binlog file.

Example Client

The avrorouter comes with an example client program, cdc.py, written in Python 3. This client can connect to a MaxScale configured with the CDC protocol and the avrorouter.

Before using this client, you will need to install the Python 3 interpreter and add users to the service with the cdc_users.py script. Fore more details about the user creation, please refer to the CDC Protocol and CDC Users documentation.

Read the output of cdc.py --help for a full list of supported options and a short usage description of the client program.

Avro Schema Generator

If the CREATE TABLE statements for the tables aren't present in the current binary logs, the schema files must be generated with a schema file generator. There are currently two methods to generate the .avsc schema files.

Simple Schema Generator

The cdc_one_schema.py generates a schema file for a single table by reading a tab separated list of field and type names from the standard input. This is the recommended schema generation tool as it does not directly communicate with the database thus making it more flexible.

The only requirement to run the script is that a Python interpreter is installed.

To use this script, pipe the output of the mysql command line into the cdc_one_schema.py script:

mysql -ss -u <user> -p -h <host> -P <port> -e 'DESCRIBE `<database>`.`<table>`'|./cdc_one_schema.py <database> <table>

Replace the <user>, <host>, <port>, <database> and <table> with appropriate values and run the command. Note that the -ss parameter is mandatory as that will generate the tab separated output instead of the default pretty-printed output.

An .avsc file named after the database and table name will be generated in the current working directory. Copy this file to the location pointed by the avrodir parameter of the avrorouter.

Alternatively, you can also copy the output of the mysql command to a file and feed it into the script if you cannot execute the SQL command directly:

# On the database server
mysql -ss -u <user> -p -h <host> -P <port> -e 'DESCRIBE `<database>`.`<table>`' > schema.tsv
# On the MaxScale server
./cdc_one_schema.py <database> <table> < schema.tsv

If you want to use a specific Python interpreter instead of the one found in the search path, you can modify the first line of the script from #!/usr/bin/env python to #!/path/to/python where /path/to/python is the absolute path to the Python interpreter (both Python 2 and Python 3 can be used).

Python Schema Generator

usage: cdc_schema.py [--help] [-h HOST] [-P PORT] [-u USER] [-p PASSWORD] DATABASE

The cdc_schema.py executable is installed as a part of MaxScale. This is a Python 3 script that generates Avro schema files from an existing database.

The script will generate the .avsc schema files into the current directory. Run the script for all required databases copy the generated .avsc files to the directory where the avrorouter stores the .avro files (the value of avrodir).

Go Schema Generator

The cdc_schema.go example Go program is provided with MaxScale. This file can be used to create Avro schemas for the avrorouter by connecting to a database and reading the table definitions. You can find the file in MaxScale's share directory in /usr/share/maxscale/.

You'll need to install the Go compiler and run go get to resolve Go dependencies before you can use the cdc_schema program. After resolving the dependencies you can run the program with go run cdc_schema.go. The program will create .avsc files in the current directory. These files should be moved to the location pointed by the avrodir option of the avrorouter if they are to be used by the router itself.

Read the output of go run cdc_schema.go -help for more information on how to run the program.

Examples

The Avrorouter Tutorial shows you how the Avrorouter works with the Binlog Server to convert binlogs from a master server into easy to process Avro data.

Here is a simple configuration example which reads binary logs locally from /var/lib/mysql/ and stores them as Avro files in /var/lib/maxscale/avro/. The service has one listener listening on port 4001 for CDC protocol clients.

[avro-converter]
type=service
router=avrorouter
router_options=binlogdir=/var/lib/mysql/,
        filestem=binlog,
        avrodir=/var/lib/maxscale/avro/

[avro-listener]
type=listener
service=avro-converter
protocol=CDC
port=4001

Here is an example how you can query for data in JSON format using the cdc.py Python script. It queries the table test.mytable for all change records.

cdc.py --user=myuser --password=mypasswd --host=127.0.0.1 --port=4001 test.mytable

You can then combine it with the cdc_kafka_producer.py to publish these change records to a Kafka broker.

cdc.py --user=myuser --password=mypasswd --host=127.0.0.1 --port=4001 test.mytable |
cdc_kafka_producer.py --kafka-broker 127.0.0.1:9092 --kafka-topic test.mytable

For more information on how to use these scripts, see the output of cdc.py -h and cdc_kafka_producer.py -h.

Building Avrorouter

To build the avrorouter from source, you will need the Avro C library, liblzma, the Jansson library and sqlite3 development headers. When configuring MaxScale with CMake, you will need to add -DBUILD_CDC=Y to build the CDC module set.

The Avro C library needs to be build with position independent code enabled. You can do this by adding the following flags to the CMake invocation when configuring the Avro C library.

-DCMAKE_C_FLAGS=-fPIC -DCMAKE_CXX_FLAGS=-fPIC

For more details about building MaxScale from source, please refer to the Building MaxScale from Source Code document.

Router Diagnostics

The router_diagnostics output for an avrorouter service contains the following fields.

  • infofile: File where the avrorouter stores the conversion process state.
  • avrodir: Directory where avro files are stored
  • binlogdir: Directory where binlog files are read from
  • binlog_name: Current binlog name
  • binlog_pos: Current binlog position
  • gtid: Current GTID
  • gtid_timestamp: Current GTID timestamp
  • gtid_event_number: Current GTID event number

Limitations

The avrorouter does not support the following data types, conversions or SQL statements:

The avrorouter does not do any crash recovery. This means that the avro files need to be removed or truncated to valid block lengths before starting the avrorouter.

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