Process and Memory architecture in PostgreSQL

The chapter provides an overview of PostgreSQL's process architecture, which follows a client/server model and multi-process architecture, running on a single host. A 'PostgreSQL server' comprises multiple processes managing a database cluster, including a postgres server process, backend processes, various background processes, replication-associated processes, and background worker processes from version 9.3.
The postgres server process is the parent of all processes related to database cluster management. It starts by executing the pg_ctl utility, allocating shared memory, and waiting for client connection requests. PostgreSQL listens to one network port (default 5432) and can run multiple servers on different port numbers.
Backend processes handle queries and statements from connected clients. PostgreSQL allows multiple simultaneous client connections, with the max_connections parameter controlling the maximum number (default is 100). To address connection and disconnection costs, pooling middleware like pgbouncer or pgpool-II is often used.
Background processes are responsible for different functions like writing dirty pages to persistent storage, performing checkpoints, invoking autovacuum workers, writing and flushing WAL data, collecting statistics, writing error messages into log files, and executing archiving logging.
PostgreSQL's memory architecture is divided into two main categories: local memory area and shared memory area. The local memory area is allocated by each backend process and contains sub-areas for query processing, maintenance operations, and temporary tables. The shared memory area, allocated during server startup, includes sub-areas for shared buffer pool, WAL buffer, commit log, access control mechanisms, background processes, and transaction processing.
In summary, the chapter explains PostgreSQL's process architecture, which includes various types of processes and memory areas for managing database clusters and handling client connections, queries, and statements.