# 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 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
            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`.

If `keep_existing_recovery_conf` is defined and set to `True`, Patroni will not remove the existing `recovery.conf` file if it exists. This is useful when bootstrapping from a backup with tools like pgBackRest that generate the appropriate `recovery.conf` for you.

> 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_methods:
            - <method name>
        <method name>:
            command: <command name>
            keep_data: True
            no_params: True
            no_master: 1

example: wal_e

    postgresql:
        create_replica_methods:
            - wal_e
            - basebackup
        wal_e:
            command: patroni_wale_restore
            no_master: 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'

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 (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.

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 master 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. 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'

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

## Standby cluster

Another available option is to run a “standby cluster”, that contains only of standby nodes replicating from some remote master. This type of clusters has:

- “standby leader”, that behaves pretty much like a regular cluster leader, except it replicates from a remote master.
- cascade replicas, that are replicating from standby leader.

Standby leader holds and updates a leader lock in DCS. If the leader lock expires, cascade replicas will perform an election to choose another leader from the standbys.

For the sake of flexibility, you can specify methods of creating a replica and recovery WAL records when a cluster is in the “standby mode” by providing create_replica_methods key in standby_cluster section. It is distinct from creating replicas, when cluster is detached and functions as a normal cluster, which is controlled by create_replica_methods in postgresql section. Both “standby” and “normal” create_replica_methods reference keys in postgresql section.

To configure such cluster you need to specify the section `standby_cluster` in a patroni configuration:

    bootstrap:
        dcs:
            standby_cluster:
                host: 1.2.3.4
                port: 5432
                primary_slot_name: patroni
                create_replica_methods:
                - basebackup

Note, that these options will be applied only once during cluster bootstrap, and the only way to change them afterwards is through DCS.

If you use replication slots on the standby cluster, you must also create the corresponding replication slot on the primary cluster. It will not be done automatically by the standby cluster implementation. You can use Patroni’s permanent replication slots feature on the primary cluster to maintain a replication slot with the same name as `primary_slot_name`, or its default value if `primary_slot_name` is not provided.