Replica imaging and bootstrap

Source: https://patroni.readthedocs.io/en/latest/replica_bootstrap.html

Patroni allows customizing creation of a new replica. It also supports defining what happens when the new empty cluster is being bootstrapped. The distinction between two is well defined: Patroni creates replicas only if the initialize key is present in DCS for the cluster. If there is no initialize key - Patroni calls bootstrap exclusively on the first node that takes the initialize key lock.

Bootstrap

PostgreSQL provides initdb command to initialize a new cluster and Patroni calls it by default. In certain cases, particularly when creating a new cluster as a copy of an existing one, it is necessary to replace a built-in method with custom actions. Patroni supports executing user-defined scripts to bootstrap new clusters, supplying some required arguments to them, i.e. the name of the cluster and the path to the data directory. This is configured in the bootstrap section of the Patroni configuration. For example:

bootstrap:
    method: <custom_bootstrap_method_name>
    <custom_bootstrap_method_name>:
        command: <path_to_custom_bootstrap_script> [param1 [, ...]]
        keep_existing_recovery_conf: False
        no_params: False
        recovery_conf:
            recovery_target_action: promote
            recovery_target_timeline: latest
            restore_command: <method_specific_restore_command>

Each bootstrap method must define at least a name and a command. A special initdb method is available to trigger the default behavior, in which case method parameter can be omitted altogether. The command can be specified using either an absolute path, or the one relative to the patroni command location. In addition to the fixed parameters defined in the configuration files, Patroni supplies two cluster-specific ones:

--scope
Name of the cluster to be bootstrapped

--datadir
Path to the data directory of the cluster instance to be bootstrapped

Passing these two additional flags can be disabled by setting a special no_params parameter to True.

If the bootstrap script returns 0, Patroni tries to configure and start the PostgreSQL instance produced by it. If any of the intermediate steps fail, or the script returns a non-zero value, Patroni assumes that the bootstrap has failed, cleans up after itself and releases the initialize lock to give another node the opportunity to bootstrap.

If a recovery_conf block is defined in the same section as the custom bootstrap method, Patroni will generate a recovery.conf before starting the newly bootstrapped instance (or set the recovery settings on Postgres configuration if running PostgreSQL >= 12). Typically, such recovery configuration should contain at least one of the recovery_target_* parameters, together with the recovery_target_action set to promote.

If keep_existing_recovery_conf is defined and set to True, Patroni will not remove the existing recovery.conf file if it exists (PostgreSQL <= 11). Similarly, in that case Patroni will not remove the existing recovery.signal or standby.signal if either exists, nor will it override the configured recovery settings (PostgreSQL >= 12). This is useful when bootstrapping from a backup with tools like pgBackRest that generate the appropriate recovery configuration for you.

Besides that, any additional key/value pairs informed in the custom bootstrap method configuration will be passed as arguments to command in the format --name=value. For example:

bootstrap:
    method: <custom_bootstrap_method_name>
    <custom_bootstrap_method_name>:
        command: <path_to_custom_bootstrap_script>
        arg1: value1
        arg2: value2

Makes the configured command to be called additionally with --arg1=value1 --arg2=value2 command-line arguments.

Note

Bootstrap methods are neither chained, nor fallen-back to the default one in case the primary one fails

As an example, you are able to bootstrap a fresh Patroni cluster from a Barman backup with a configuration like this:

bootstrap:
    method: barman
    barman:
        keep_existing_recovery_conf: true
        command: patroni_barman --api-url https://barman-host:7480 recover
        barman-server: my_server
        ssh-command: ssh postgres@patroni-host

Building replicas

Patroni uses tried and proven pg_basebackup in order to create new replicas. One downside of it is that it requires a running leader node. Another one is the lack of ‘on-the-fly’ compression for the backup data and no built-in cleanup for outdated backup files. Some people prefer other backup solutions, such as WAL-E, pgBackRest, Barman and others, or simply roll their own scripts. In order to accommodate all those use-cases Patroni supports running custom scripts to clone a new replica. Those are configured in the postgresql configuration block:

postgresql:
    create_replica_methods:
        - <method name>
    <method name>:
        command: <command name>
        keep_data: True
        no_params: True
        no_leader: 1

example: wal_e

postgresql:
    create_replica_methods:
        - wal_e
        - basebackup
    wal_e:
        command: patroni_wale_restore
        no_leader: 1
        envdir: {{WALE_ENV_DIR}}
        use_iam: 1
    basebackup:
        max-rate: '100M'

example: pgbackrest

postgresql:
    create_replica_methods:
        - pgbackrest
        - basebackup
    pgbackrest:
        command: /usr/bin/pgbackrest --stanza=<scope> --delta restore
        keep_data: True
        no_params: True
    basebackup:
        max-rate: '100M'

example: Barman

postgresql:
    create_replica_methods:
        - barman
        - basebackup
    barman:
        command: patroni_barman --api-url https://barman-host:7480 recover
        barman-server: my_server
        ssh-command: ssh postgres@patroni-host
    basebackup:
        max-rate: '100M'

The create_replica_methods defines available replica creation methods and the order of executing them. Patroni will stop on the first one that returns 0. Each method should define a separate section in the configuration file, listing the command to execute and any custom parameters that should be passed to that command. All parameters will be passed in a --name=value format. Besides user-defined parameters, Patroni supplies a couple of cluster-specific ones:

--scope
Which cluster this replica belongs to

--datadir
Path to the data directory of the replica

--role
Always ‘replica’

--connstring
Connection string to connect to the cluster member to clone from (primary or other replica). The user in the connection string can execute SQL and replication protocol commands.

A special no_leader parameter, if defined, allows Patroni to call the replica creation method even if there is no running leader or replicas. In that case, an empty string will be passed in a connection string. This is useful for restoring the formerly running cluster from the binary backup.

A special keep_data parameter, if defined, will instruct Patroni to not clean PGDATA folder before calling restore.

A special no_params parameter, if defined, restricts passing parameters to custom command.

A basebackup method is a special case: it will be used if create_replica_methods is empty, although it is possible to list it explicitly among the create_replica_methods methods. This method initializes a new replica with the pg_basebackup, the base backup is taken from the leader unless there are replicas with clonefrom tag, in which case one of such replicas will be used as the origin for pg_basebackup. It works without any configuration; however, it is possible to specify a basebackup configuration section. Same rules as with the other method configuration apply, namely, only long (with –) options should be specified there. Not all parameters make sense, if you override a connection string or provide an option to created tar-ed or compressed base backups, patroni won’t be able to make a replica out of it. There is no validation performed on the names or values of the parameters passed to the basebackup section. Also note that in case symlinks are used for the WAL folder it is up to the user to specify the correct --waldir path as an option, so that after replica buildup or re-initialization the symlink would persist. This option is supported only since v10 though.

You can specify basebackup parameters as either a map (key-value pairs) or a list of elements, where each element could be either a key-value pair or a single key (for options that does not receive any values, for instance, --verbose). Consider those 2 examples:

postgresql:
    basebackup:
        max-rate: '100M'
        checkpoint: 'fast'

and

postgresql:
    basebackup:
        - verbose
        - max-rate: '100M'
        - waldir: /pg-wal-mount/external-waldir

If all replica creation methods fail, Patroni will try again all methods in order during the next event loop cycle.