This chapter provides a high-level overview of repmgr's components and functionality. This guide assumes that you are familiar with PostgreSQL administration and streaming replication concepts. For further details on streaming replication, see the PostgreSQL documentation section on streaming replication. The following terms are used throughout the `repmgr` documentation. replication cluster In the `repmgr` documentation, "replication cluster" refers to the network of PostgreSQL servers connected by streaming replication. node A node is a server within a replication cluster. upstream node The node a standby server connects to, in order to receive streaming replication. This is either the primary server or in the case of cascading replication, another standby. failover This is the action which occurs if a primary server fails and a suitable standby is promoted as the new primary. The `repmgrd` daemon supports automatic failover to minimise downtime. switchover In certain circumstances, such as hardware or operating system maintenance, it's necessary to take a primary server offline; in this case a controlled switchover is necessary, whereby a suitable standby is promoted and the existing primary removed from the replication cluster in a controlled manner. The `repmgr` command line client provides this functionality. fencing In a failover situation, following the promotion of a new standby, it's essential that the previous primary does not unexpectedly come back on line, which would result in a split-brain situation. To prevent this, the failed primary should be isolated from applications, i.e. "fenced off". `repmgr` is a suite of open-source tools to manage replication and failover within a cluster of PostgreSQL servers. It supports and enhances PostgreSQL's built-in streaming replication, which provides a single read/write primary server and one or more read-only standbys containing near-real time copies of the primary server's database. It provides two main tools: repmgr A command-line tool used to perform administrative tasks such as: setting up standby servers promoting a standby server to primary switching over primary and standby servers displaying the status of servers in the replication cluster repmgrd A daemon which actively monitors servers in a replication cluster and performs the following tasks: monitoring and recording replication performance performing failover by detecting failure of the primary and promoting the most suitable standby server provide notifications about events in the cluster to a user-defined script which can perform tasks such as sending alerts by email In order to effectively manage a replication cluster, `repmgr` needs to store information about the servers in the cluster in a dedicated database schema. This schema is automatically by the `repmgr` extension, which is installed during the first step in initialising a `repmgr`-administered cluster (`repmgr primary register`) and contains the following objects: Tables repmgr.events: records events of interest repmgr.nodes: connection and status information for each server in the replication cluster repmgr.monitoring_history: historical standby monitoring information written by `repmgrd` Views repmgr.show_nodes: based on the table `repl_nodes`, additionally showing the name of the server's upstream node repmgr.replication_status: when `repmgrd`'s monitoring is enabled, shows current monitoring status for each standby. The `repmgr` metadata schema can be stored in an existing database or in its own dedicated database. Note that the `repmgr` metadata schema cannot reside on a database server which is not part of the replication cluster managed by `repmgr`. A database user must be available for `repmgr` to access this database and perform necessary changes. This user does not need to be a superuser, however some operations such as initial installation of the `repmgr` extension will require a superuser connection (this can be specified where required with the command line option `--superuser`).