Replica imaging and bootstrap

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 Etcd 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 [, ...]]
        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

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. Typically, such recovery.conf should contain at least one of the recovery_target_* parameters, together with the recovery_target_timeline set to promote.

Note

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

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 master 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_method:
        - wal_e
        - basebackup
    wal_e:
        command: patroni_wale_restore
        no_master: 1
        envdir: {{WALE_ENV_DIR}}
        use_iam: 1

The create_replica_method defines available replica creation methods and the order of executing them. Patroni will stop on the first one that returns 0. The basebackup is the built-in method and doesn’t require any configuration. The rest of the methods 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 (master or other replica). The user in the connection string can execute SQL and replication protocol commands.

A special no_master parameter, if defined, allows Patroni to call the replica creation method even if there is no running master 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.

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