MaxScale/Documentation/Monitors/MariaDB-Monitor.md

MariaDB Monitor

Up until MariaDB MaxScale 2.2.0, this monitor was called MySQL Monitor.

Overview

MariaDB Monitor monitors a Master-Slave replication cluster. It probes the state of the backends and assigns server roles such as master and slave, which are used by the routers when deciding where to route a query. It can also modify the replication cluster by performing failover, switchover and rejoin. Backend server versions older than MariaDB/MySQL 5.5 are not supported. Failover and other similar operations require MariaDB 10.0.2 or later.

Master selection

Only one backend can be master at any given time. A master must be running (successfully connected to by the monitor) and its read_only-setting must be off. A master may not be replicating from another server in the monitored cluster unless the master is part of a multimaster group. Master selection prefers to select the server with the most slaves, possibly in multiple replication layers. Only slaves reachable by a chain of running relays or directly connected to the master count. When multiple servers are tied for master status, the server which appears earlier in the servers-setting of the monitor is selected.

Servers in a cyclical replication topology (multimaster group) are interpreted as having all the servers in the group as slaves. Even from a multimaster group only one server is selected as the overall master.

After a master has been selected, the monitor prefers to stick with the choice even if other potential masters with more slave servers are available. Only if the current master is clearly unsuitable does the monitor try to select another master. An existing master turns invalid if:

1. It is unwritable (read_only is on).
2. It has been down for more than failcount monitor passes and has no running slaves. Running slaves behind a downed relay count.
3. It did not previously replicate from another server in the cluster but it is now replicating.
4. It was previously part of a multimaster group but is no longer, or the multimaster group is replicating from a server not in the group.

Cases 1 and 2 cover the situations in which the DBA, an external script or even another MaxScale has modified the cluster such that the old master can no longer act as master. Cases 3 and 4 are less severe. In these cases the topology has changed significantly and the master should be re-selected, although the old master may still be the best choice.

The master change described above is different from failover and switchover described in section Failover, switchover and auto-rejoin. A master change only modifies the server roles inside MaxScale but does not modify the cluster other than changing the targets of read and write queries. Failover and switchover perform a master change on their own.

As a general rule, it's best to avoid situations where the cluster has multiple standalone servers, separate master-slave pairs or separate multimaster groups. Due to master invalidation rule 2, a standalone master can easily lose the master status to another valid master if it goes down. The new master probably does not have the same data as the previous one. Non-standalone masters are less vulnerable, as a single running slave or multimaster group member will keep the master valid even when down.

Configuration

A minimal configuration for a monitor requires a set of servers for monitoring and a username and a password to connect to these servers.

[MyMonitor]
type=monitor
module=mariadbmon
servers=server1,server2,server3
user=myuser
passwd=mypwd

From MaxScale 2.2.1 onwards, the module name is mariadbmon instead of mysqlmon. The old name can still be used.

The user requires the REPLICATION CLIENT privilege to successfully monitor the state of the servers. SUPER privilege is required for cluster manipulation features such as failover.

MariaDB [(none)]> grant replication client on *.* to 'maxscale'@'maxscalehost';
Query OK, 0 rows affected (0.00 sec)

Common Monitor Parameters

For a list of optional parameters that all monitors support, read the Monitor Common document.

MariaDB Monitor optional parameters

These are optional parameters specific to the MariaDB Monitor. Failover, switchover and rejoin-specific parameters are listed in their own section.

detect_replication_lag

Deprecated and unused as of MaxScale 2.3. Can be defined but is ignored.

detect_stale_master

Allow previous master to be available even in case of stopped or misconfigured replication.

Starting from MaxScale 2.0.0 this feature is enabled by default. It is disabled by default in MaxScale 1.4.3 and below.

This allows services that depend on master and slave roles to continue functioning as long as the master server is available. This is a situation which can happen if all slave servers are unreachable or the replication breaks for some reason.

detect_stale_master=true

detect_stale_slave

Treat running slaves servers without a master server as valid slave servers.

This feature is enabled by default.

If a slave server loses its master server, the replication is considered broken. With this parameter, slaves that have lost their master but have been slaves of a master server can retain their slave status even without a master. This means that when a slave loses its master, it can still be used for reads.

If this feature is disabled, a server is considered a valid slave if and only if it has a running master server monitored by this monitor.

detect_stale_slave=true

mysql51_replication

Deprecated and unused as of MaxScale 2.3. Can be defined but is ignored.

multimaster

Deprecated and unused as of MaxScale 2.3. Can be defined but is ignored.

ignore_external_masters

Ignore any servers that are not monitored by this monitor but are a part of the replication topology. This option was added in MaxScale 2.1.12 and is disabled by default.

MaxScale detects if a master server replicates from an external server. When this is detected, the server is assigned the Slave and Slave of External Server labels and will be treated as a slave server. Most of the time this topology is used when MaxScale is used for read scale-out without master servers, a Galera cluster with read replicas being a prime example of this setup. Sometimes this is not the desired behavior and the external master server should be ignored. Most of the time this is due to multi-source replication.

When this option is enabled, all servers that have the Master, Slave, Slave of External Server, Running labels will instead get the Master, Running labels.

detect_standalone_master

Detect standalone master servers. This feature takes a boolean parameter and is enabled by default.

This setting controls whether a standalone server can be a master. A standalone server is a server from which no other server in the cluster is attempting to replicate from. In most cases this should be left on.

failcount

Number of consecutive monitor passes a master server must be down before it is considered failed. At this point, automatic failover is performed if enabled (auto_failover=true). If automatic failover is not on, the monitor will try to search for another server to fultill the master role. See section Master selection for more details. Changing the master may break replication as queries could be routed to a server without previous events. To prevent this, avoid having multiple valid master servers in the cluster.

The default value is 5 failures.

allow_cluster_recovery

Deprecated and unused as of MaxScale 2.3. Can be defined but is ignored.

enforce_read_only_slaves

This feature is disabled by default. If set to ON, the monitor attempts to set the server read_only flag to ON on any slave server with read_only OFF. The flag is checked at every monitor iteration. The monitor user requires the SUPER-privilege for this feature to work. While the read_only-flag is ON, only users with the SUPER-privilege can write to the backend server. If temporary write access is required, this feature should be disabled before attempting to disable read_only. Otherwise the monitor would quickly re-enable it.

maintenance_on_low_disk_space

This feature is enabled by default. If a running server that is not the master or a relay master is out of disk space (as defined by the general monitor setting disk_space_threshold) the server is set to maintenance mode. Such servers are not used for router sessions and are ignored when performing a failover or other cluster modification operation.

Cluster manipulation operations

Starting with MaxScale 2.2.1, MariaDB Monitor supports replication cluster modification. The operations implemented are:

• failover, which replaces a failer master with a slave
• switchover, which swaps a running master with a slave
• rejoin, which directs servers to replicate from the master
• reset-replication (added in MaxScale 2.3.0), which deletes binary logs and resets gtid:s

See operation details for more information on the implementation of the commands.

The cluster operations require that the monitor user (user) has the following privileges:

• SUPER, to modify slave connections and set globals such as read_only
• REPLICATION CLIENT, to list slave connections
• RELOAD, to flush binary logs
• PROCESS, to check if the event_scheduler process is running
• SHOW DATABASES and EVENTS, to list and modify server events

In addition, the monitor needs to know which username and password a slave should use when starting replication. These are given in replication_user and replication_password.

The user can define files with SQL statements which are executed on any server being demoted or promoted by cluster manipulation commands. See the sections on promotion_sql_file and demotion_sql_file for more information.

The monitor can manipulate scheduled server events when promoting or demoting a server. See the section on handle_server_events for more information.

All cluster operations can be activated manually through MaxAdmin/MaxCtrl. See section Manual activation for more details.

Operation details

Failover replaces a failed master with a running slave. It does the following:

1. Select the most up-to-date slave of the old master to be the new master. If the new master has unprocessed relay log items, cancel and try again later.
2. Prepare the new master:
3. Remove the slave connection the new master used to replicate from the old master.
4. Disable the read_only-flag.
5. Enable scheduled server events (if event handling is on).
6. Run the commands in promotion_sql_file.
7. Start replication from external master is one existed.
8. Redirect all other slaves to replicate from the new master:
9. STOP SLAVE and RESET SLAVE
10. CHANGE MASTER TO
11. START SLAVE
12. Check that all slaves are replicating.

Failover may lose events if no slave managed to replicate the events before the master went down.

Switchover swaps a running master with a running slave. It does the following:

1. Prepare the old master for demotion:
2. Stop any external replication.
3. Enable the read_only-flag to stop writes.
4. Disable scheduled server events (if event handling is on).
5. Flush the binary log (FLUSH LOGS) so that all events are on disk.
6. Run the commands in demotion_sql_file.
7. Wait for all slaves to catch up with the old master by repeatedly querying their gtid:s.
8. Promote new master and redirect slaves as in failover steps 2 and 3. Also redirect the demoted old master.
9. Check that all slaves are replicating.

Rejoin joins a standalone server to the cluster or redirects a slave replicating from a server other than the master. A standalone server is joined by:

1. Run the commands in demotion_sql_file.
2. Enable the read_only-flag.
3. Disable scheduled server events (if event handling is on).
4. Start replication: CHANGE MASTER TO and START SLAVE.

A server which is replicating from the wrong master is redirected simply with STOP SLAVE, RESET SLAVE, CHANGE MASTER TO and START SLAVE commands.

Reset-replication (added in MaxScale 2.3.0) deletes binary logs and resets gtid:s. This destructive command is meant for situations where the gtid:s in the cluster are out of sync while the actual data is known to be in sync. The operation proceeds as follows:

1. Reset gtid:s and delete binary logs on all servers:
2. Stop (STOP SLAVE) and delete (RESET SLAVE ALL) all slave connections.
3. Enable the read_only-flag.
4. Disable scheduled server events (if event handling is on).
5. Delete binary logs (RESET MASTER).
6. Set the sequence number of gtid_slave_pos to zero. This also affects gtid_current_pos.
7. Prepare new master:
8. Disable the read_only-flag.
9. Enable scheduled server events (if event handling is on).
10. Direct other servers to replicate from the new master as in the other operations.

Manual activation

Cluster operations can be activated manually through the REST API, MaxCtrl or MaxAdmin. The commands are only performed when MaxScale is in active mode. All commands require the monitor instance name as the first parameter. Failover selects the new master server automatically and does not require additional parameters. Rejoin requires the name of the joining server as second parameter. Replication reset accepts the name of the new master server as second parameter. If not given, the current master is selected.

Switchover takes one to three parameters. If only the monitor name is given, switchover will autoselect both the slave to promote and the current master as the server to be demoted. If two parameters are given, the second parameter is interpreted as the slave to promote. If three parameters are given, the third parameter is interpreted as the current master. The user-given current master is compared to the master server currently deduced by the monitor and if the two are unequal, an error is given.

Example commands are below:

call command mariadbmon failover MyMonitor
call command mariadbmon rejoin MyMonitor OldMasterServ
call command mariadbmon reset-replication MyMonitor
call command mariadbmon reset-replication MyMonitor NewMasterServ
call command mariadbmon switchover MyMonitor
call command mariadbmon switchover MyMonitor NewMasterServ
call command mariadbmon switchover MyMonitor NewMasterServ OldMasterServ

The commands follow the standard module command syntax. All require the monitor configuration name (MyMonitor) as the first parameter. For switchover, the last two parameters define the server to promote (NewMasterServ) and the server to demote (OldMasterServ). For rejoin, the server to join (OldMasterServ) is required. Replication reset requires the server to promote (NewMasterServ).

It is safe to perform manual operations even with automatic failover, switchover or rejoin enabled since the automatic operations cannot happen simultaneously with the manual one.

If a switchover or failover fails, automatic failover is disabled to prevent master changes to a possibly malfunctioning cluster. Automatic failover can be turned on manually via the REST API or MaxAdmin. Example commands are listed below.

maxadmin alter monitor MariaDB-Monitor auto_failover=true
maxctrl alter monitor MariaDB-Monitor auto_failover true

When a cluster modification is iniated via the REST-API, the URL path is of the form:

/v1/maxscale/modules/mariadbmon/<operation>?<monitor-instance>&<server-param1>&<server-param2>

• <operation> is the name of the command: failover, switchover, rejoin or reset-replication.
• <monitor-instance> is the monitor section name from the MaxScale configuration file.
• <server-param1> and <server-param2> are server parameters as described above for MaxAdmin. Only switchover accepts both, failover doesn't need any and both rejoin and reset-replication accept one.

Given a MaxScale configuration file like

[Cluster1]
type=monitor
module=mariadbmon
servers=server1, server2, server3, server 4
...

with the assumption that server2 is the current master, then the URL path for making server4 the new master would be:

/v1/maxscale/modules/mariadbmon/switchover?Cluster1&server4&server2

Example REST-API paths for other commands are listed below.

/v1/maxscale/modules/mariadbmon/failover?Cluster1
/v1/maxscale/modules/mariadbmon/rejoin?Cluster1&server3
/v1/maxscale/modules/mariadbmon/reset-replication?Cluster1&server3

Automatic activation

Failover can activate automatically if auto_failover is on. The activation begins when the master has been down at least failcount monitor iterations. Before modifying the cluster, the monitor checks that all prerequisites for the failover are fulfilled. If the cluster does not seem ready, an error is printed and the cluster is rechecked during the next monitor iteration.

Switchover can also activate automatically with the switchover_on_low_disk_space-setting. The operation begins if the master server is low on disk space but otherwise the operating logic is quite similar to automatic failover.

Rejoin stands for starting replication on a standalone server or redirecting a slave replicating from the wrong master (any server that is not the cluster master). The rejoined servers are directed to replicate from the current cluster master server, forcing the replication topology to a 1-master-N-slaves configuration.

A server is categorized as standalone if the server has no slave connections, not even stopped ones. A server is replicating from the wrong master if the slave IO thread is connected but the master server id seen by the slave does not match the cluster master id. Alternatively, the IO thread may be stopped or connecting but the master server host or port information differs from the cluster master info. These criteria mean that a STOP SLAVE does not yet set a slave as standalone.

With auto_rejoin active, the monitor will try to rejoin any servers matching the above requirements. Rejoin does not obey failcount and will attempt to rejoin any valid servers immediately. When activating rejoin manually, the user-designated server must fulfill the same requirements.

Limitations and requirements

Switchover and failover only understand simple topologies. They will not work if the cluster has multiple masters, relay masters, or if the topology is circular. The server cluster is assumed to be well-behaving with no significant replication lag and all commands that modify the cluster complete in a few seconds (faster than backend_read_timeout and backend_write_timeout).

The backends must all use GTID-based replication, and the domain id should not change during a switchover or failover. Master and slaves must have well-behaving GTIDs with no extra events on slave servers.

Switchover requires that the cluster is "frozen" for the duration of the operation. This means that no data modifying statements such as INSERT or UPDATE are executed and the GTID position of the master server is stable. When switchover begins, the monitor sets the global read_only flag on the old master backend to stop any updates. read_only does not affect users with the SUPER-privilege so any such user can issue writes during a switchover. These writes have a high chance of breaking replication, because the write may not be replicated to all slaves before they switch to the new master. To prevent this, any users who commonly do updates should not have the SUPER-privilege. For even more security, the only SUPER-user session during a switchover should be the MaxScale monitor user.

When mixing rejoin with failover/switchover, the backends should have log_slave_updates on. The rejoining server is likely lagging behind the rest of the cluster. If the current cluster master does not have binary logs from the moment the rejoining server lost connection, the rejoining server cannot continue replication. This is an issue if the master has changed and the new master does not have log_slave_updates on.

External master support

The monitor detects if a server in the cluster is replicating from an external master (a server that is not monitored by the monitor). If the replicating server is the cluster master server, then the cluster itself is considered to have an external master.

If a failover/switchover happens, the new master server is set to replicate from the cluster external master server. The usename and password for the replication are defined in replication_user and replication_password. The address and port used are the ones shown by SHOW ALL SLAVES STATUS on the old cluster master server. In the case of switchover, the old master also stops replicating from the external server to preserve the topology.

After failover the new master is replicating from the external master. If the failed old master comes back online, it is also replicating from the external server. To normalize the situation, either have auto_rejoin on or manually execute a rejoin. This will redirect the old master to the current cluster master.

Configuration parameters

auto_failover

Enable automated master failover. This parameter expects a boolean value and the default value is false.

When automatic failover is enabled, traditional MariaDB Master-Slave clusters will automatically elect a new master if the old master goes down and stays down a number of iterations given in failcount. Failover will not take place when MaxScale is configured as a passive instance. For details on how MaxScale behaves in passive mode, see the documentation on failover_timeout below.

If an attempt at failover fails or multiple master servers are detected, an error is logged and automatic failover is disabled. If this happens, the cluster must be fixed manually and the failover needs to be re-enabled via the REST API or MaxAdmin.

The monitor user must have the SUPER and RELOAD privileges for failover to work.

auto_rejoin

Enable automatic joining of server to the cluster. This parameter expects a boolean value and the default value is false.

When enabled, the monitor will attempt to direct standalone servers and servers replicating from a relay master to the main cluster master server, enforcing a 1-master-N-slaves configuration.

For example, consider the following event series.

1. Slave A goes down
2. Master goes down and a failover is performed, promoting Slave B
3. Slave A comes back

Slave A is still trying to replicate from the downed master, since it wasn't online during failover. If auto_rejoin is on, Slave A will quickly be redirected to Slave B, the current master.

switchover_on_low_disk_space

This feature is disabled by default. If set to on, when the disk space of a server is exhausted, it will cause the server to be put in maintenance mode. If the server is the current master, then a switchover will also be triggered.

In order for this parameter to have any effect, disk_space_threshold must have been specified for the server or the monitor, and disk_space_check_interval for the monitor.

switchover_on_low_disk_space=true

Note that once the server has been put in maintenance mode, the disk space situation will no longer be monitored and the server will thus not automatically be taken out of maintanance mode even if disk space again would become available.

replication_user and replication_password

The username and password of the replication user. These are given as the values for MASTER_USER and MASTER_PASSWORD whenever a CHANGE MASTER TO command is executed.

Both replication_user and replication_password parameters must be defined if a custom replication user is used. If neither of the parameters is defined, the CHANGE MASTER TO command will use the monitor credentials for the replication user.

The credentials used for replication must have the REPLICATION SLAVE privilege.

replication_password uses the same encryption scheme as other password parameters. If password encryption is in use, replication_password must be encrypted with the same key to avoid erroneous decryption.

failover_timeout and switchover_timeout

Time limit for the cluster failover and switchover in seconds. The default values are 90 seconds.

If no successful failover/switchover takes place within the configured time period, a message is logged and automatic failover is disabled. This prevents further automatic modifications to the misbehaving cluster.

failover_timeout also controls how long a MaxScale instance that has transitioned from passive to active will wait for a failover to take place after an apparent loss of a master server. If no new master server is detected within the configured time period, failover will be initiated again.

verify_master_failure and master_failure_timeout

Enable additional master failure verification for automatic failover. verify_master_failure is a boolean value (default: true) which enables this feature and master_failure_timeout defines the timeout in seconds (default: 10).

Failure verification is performed by checking whether the slave servers are still connected to the master and receiving events. An event is either a change in the Gtid_IO_Pos-field of the SHOW SLAVE STATUS output or a heartbeat event. Effectively, if a slave has received an event within master_failure_timeout seconds, the master is not considered down when deciding whether to failover, even if MaxScale cannot connect to the master. master_failure_timeout should be longer than the Slave_heartbeat_period of the slave connection to be effective.

If every slave loses its connection to the master (Slave_IO_Running is not "Yes"), master failure is considered verified regardless of timeout. This allows faster failover when the master properly disconnects.

For automatic failover to activate, the failcount requirement must also be met.

servers_no_promotion

This is a comma-separated list of server names that will not be chosen for master promotion during a failover or autoselected for switchover. This does not affect switchover if the user selects the server to promote. Using this setting can disrupt new master selection for failover such that an nonoptimal server is chosen. At worst, this will cause replication to break. Alternatively, failover may fail if all valid promotion candidates are in the exclusion list.

servers_no_promotion=backup_dc_server1,backup_dc_server2

promotion_sql_file and demotion_sql_file

These optional settings are paths to text files with SQL statements in them. During promotion or demotion, the contents are read line-by-line and executed on the backend. Use these settings to execute custom statements on the servers to complement the built-in operations.

Empty lines or lines starting with '#' are ignored. Any results returned by the statements are ignored. All statements must succeed for the failover, switchover or rejoin to continue. The monitor user may require additional privileges and grants for the custom commands to succeed.

When promoting a slave to master during switchover or failover, the promotion_sql_file is read and executed on the new master server after its read-only flag is disabled. The commands are ran before starting replication from an external master if any.

demotion_sql_file is ran on an old master during demotion to slave, before the old master starts replicating from the new master. The file is also ran before rejoining a standalone server to the cluster, as the standalone server is typically a former master server. When redirecting a slave replicating from a wrong master, the sql-file is not executed.

Since the queries in the files are ran during operations which modify replication topology, care is required. If promotion_sql_file contains data modification (DML) queries, the new master server may not be able to successfully replicate from an external master. demotion_sql_file should never contain DML queries, as these may not replicate to the slave servers before slave threads are stopped, breaking replication.

promotion_sql_file=/home/root/scripts/promotion.sql
demotion_sql_file=/home/root/scripts/demotion.sql

handle_server_events

This setting is on by default. If enabled, the monitor will attempt to enable and disable server events during a switchover, failover or rejoin. When a server is being demoted, any events with "ENABLED" status are set to "SLAVESIDE_DISABLED". The reverse applies to a server being promoted to master. When a standalone server is rejoined to the cluster, its events are also disabled since it is now a slave. The monitor does not check whether the same events were disabled and enabled during a switchover or failover/rejoin. All events with the expected status are altered.

The monitor does not enable or disable the event scheduler itself. For the events to run on the new master server, the scheduler should be enabled by the admin. Enabling it in the server configuration file is recommended.

Events running at high frequency may cause replication to break in a failover scenario. If an old master which was failed over restarts, its event scheduler will be on if set in the server configuration file. Its events will also remember their "ENABLED"-status and run when scheduled. This may happen before the monitor rejoins the server and disables the events. This should only be an issue for events running more often than the monitor interval or events that run immediately after the server has restarted.

Troubleshooting

Before performing failover or switchover, the MariaDB Monitor first checks that prerequisites are fulfilled, printing any found errors. This should catch and explain most issues with failover or switchover not working. If the operations are attempted and still fail, then most likely one of the commands the monitor issued to a server failed or timed out. To find out exactly what the queries sent to the servers are, start MaxScale with --debug=enable-statement-logging. This setting prints all queries sent to the backends by monitors and authenticators.

A typical reason for failure is that a command such as STOP SLAVE takes longer than the backend_read_timeout of the monitor, causing the connection to break. In this case , increasing the timeout settings of the monitor should help. Another settings to look at are query_retries and query_retry_timeout. These are general MaxScale settings described in the Configuration guide. Setting query_retries to 2 is a reasonable first try.

Using the MariaDB Monitor With Binlogrouter

Since MaxScale 2.2 it's possible to detect a replication setup which includes Binlog Server: the required action is to add the binlog server to the list of servers only if master_id identity is set.

For addition information read the Replication Proxy tutorial.

Example 1 - Monitor script

Here is an example shell script which sends an email to an admin@my.org when a server goes down.

#!/usr/bin/env bash

#This script assumes that the local mail server is configured properly
#The second argument is the event type
event=${$2/.*=/}
server=${$3/.*=/}
message="A server has gone down at `date`."
echo $message|mail -s "The event was $event for server $server." admin@my.org

Here is a monitor configuration that only triggers the script when a master or a slave server goes down.

[Database Monitor]
type=monitor
module=mariadbmon
servers=server1,server2
script=mail_to_admin.sh
events=master_down,slave_down

When a master or a slave server goes down, the script is executed, a mail is sent and the administrator will be immediately notified of any possible problems. This is just a simple example showing what you can do with MaxScale and monitor scripts.