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. .. _custom_bootstrap: 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: .. code:: YAML bootstrap: method: : command: [param1 [, ...]] keep_existing_recovery_conf: False no_params: False recovery_conf: recovery_target_action: promote recovery_target_timeline: latest 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_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 (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: .. code:: YAML bootstrap: method: : command: 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: .. code:: YAML bootstrap: method: barman barman: keep_existing_recovery_conf: true command: patroni_barman_recover api-url: https://barman-host:7480 barman-server: my_server ssh-command: ssh postgres@patroni-host .. note:: ``patroni_barman_recover`` requires that you have both Barman and ``pg-backup-api`` configured in the Barman host, so it can execute a remote ``barman recover`` through the backup API. The above example uses a subset of the available parameters. You can get more information running ``patroni_barman_recover --help``. .. _custom_replica_creation: 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: .. code:: YAML postgresql: create_replica_methods: - : command: keep_data: True no_params: True no_leader: 1 example: wal_e .. code:: YAML 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 .. code:: YAML postgresql: create_replica_methods: - pgbackrest - basebackup pgbackrest: command: /usr/bin/pgbackrest --stanza= --delta restore keep_data: True no_params: True basebackup: max-rate: '100M' example: Barman .. code:: YAML postgresql: create_replica_methods: - barman - basebackup barman: command: patroni_barman_recover api-url: https://barman-host:7480 barman-server: my_server ssh-command: ssh postgres@patroni-host basebackup: max-rate: '100M' .. note:: ``patroni_barman_recover`` requires that you have both Barman and ``pg-backup-api`` configured in the Barman host, so it can execute a remote ``barman recover`` through the backup API. The above example uses a subset of the available parameters. You can get more information running ``patroni_barman_recover --help``. 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: .. code:: YAML postgresql: basebackup: max-rate: '100M' checkpoint: 'fast' and .. code:: YAML 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. .. _standby_cluster: Standby cluster --------------- Another available option is to run a "standby cluster", that contains only of standby nodes replicating from some remote node. This type of clusters has: * "standby leader", that behaves pretty much like a regular cluster leader, except it replicates from a remote node. * 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. There is no further relationship between the standby cluster and the primary cluster it replicates from, in particular, they must not share the same DCS scope if they use the same DCS. They do not know anything else from each other apart from replication information. Also, the standby cluster is not being displayed in :ref:`patronictl_list` or :ref:`patronictl_topology` output on the primary cluster. 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: .. code:: YAML 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. Patroni expects to find `postgresql.conf` or `postgresql.conf.backup` in PGDATA of the remote primary and will not start if it does not find it after a basebackup. If the remote primary keeps its `postgresql.conf` elsewhere, it is your responsibility to copy it to PGDATA. 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.