Watchdog is a sub process of Pgpool-II, which adds the high availability and resolves the single point of failure by coordinating multiple Pgpool-II. The watchdog process automatically starts (if enabled) when the Pgpool-II starts up and consists of two main components, Watchdog core and the lifecheck system.
Watchdog core referred as a "watchdog" is a Pgpool-II child process that manages all the watchdog related communications with the Pgpool-II nodes present in the cluster and also communicates with the Pgpool-II parent and lifecheck processes.
The heart of a watchdog process is a state machine that starts from its initial state (WD_LOADING) and transit towards either standby (WD_STANDBY) or leader/coordinator (WD_COORDINATOR) state. Both standby and leader/coordinator states are stable states of the watchdog state machine and the node stays in standby or leader/coordinator state until some problem in local Pgpool-II node is detected or a remote Pgpool-II disconnects from the cluster.
The watchdog process performs the following tasks:
Manages and coordinates the local node watchdog state.
Interacts with built-in or external lifecheck system for the of local and remote Pgpool-II node health checking.
Interacts with Pgpool-II main process and provides the mechanism to Pgpool-II parent process for executing the cluster commands over the watchdog channel.
Communicates with all the participating Pgpool-II nodes to coordinate the selection of leader/coordinator node and to ensure the quorum in the cluster.
Manages the Virtual-IP on the active/coordinator node and allow the users to provide custom scripts for escalation and de-escalation.
Verifies the consistency of Pgpool-II configurations across the participating Pgpool-II nodes in the watchdog cluster.
Synchronize the status of all PostgreSQL backends at startup.
Provides the distributed locking facility to Pgpool-II main process for synchronizing the different failover commands.
Watchdog uses TCP/IP sockets for all the communication with other nodes. Each watchdog node can have two sockets opened with each node. One is the outgoing (client) socket which this node creates and initiate the connection to the remote node and the second socket is the one which is listening socket for inbound connection initiated by remote watchdog node. As soon as the socket connection to remote node succeeds watchdog sends the ADD NODE (WD_ADD_NODE_MESSAGE) message on that socket. And upon receiving the ADD NODE message the watchdog node verifies the node information encapsulated in the message with the Pgpool-II configurations for that node, and if the node passes the verification test it is added to the cluster otherwise the connection is dropped.
Watchdog process exposes a UNIX domain socket for IPC communications, which accepts and provides the data in JSON format. All the internal Pgpool-II processes, including Pgpool-II's built-in lifecheck and Pgpool-II main process uses this IPC socket interface to interact with the watchdog. This IPC socket can also be used by any external/3rd party system to interact with watchdog.
See Section 4.2 for details on how to use watchdog IPC interface for integrating external/3rd party systems.
Watchdog lifecheck is the sub-component of watchdog that monitors the health of Pgpool-II nodes participating in the watchdog cluster. Pgpool-II watchdog provides three built-in methods of remote node health checking, "heartbeat", "query" and "external" mode.
In "heartbeat" mode, The lifecheck process sends and receives the data over UDP socket to check the availability of remote nodes and for each node the parent lifecheck process spawns two child process one for sending the heartbeat signal and another for receiving the heartbeat. While in "query" mode, The lifecheck process uses the PostgreSQL libpq interface for querying the remote Pgpool-II. And in this mode the lifecheck process creates a new thread for each health check query which gets destroyed as soon as the query finishes. While in "external" mode, this mode disables the built in lifecheck of Pgpool-II, and expects that the external system will monitor local and remote node instead.
Apart from remote node health checking watchdog lifecheck can also check the health of node it is installed on by monitoring the connection to upstream servers. For monitoring the connectivity to the upstream server Pgpool-II lifecheck uses execv() function to executes 'ping -q -c3 hostname' command. So a new child process gets spawned for executing each ping command. This means for each health check cycle a child process gets created and destroyed for each configured upstream server. For example, if two upstream servers are configured in the lifecheck and it is asked to health check at ten second intervals, then after each ten second lifecheck will spawn two child processes, one for each upstream server, and each process will live until the ping command is finished.