PostgreSQL Architecture

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How Connections are established:


PostgreSQL is implemented using a simple “process per user” client/server model. In this model there is one client process connected to exactly one server process. As we do not know ahead of time how many connections will be made, we have to use a master process that spawns a new server process every time a connection is requested. This master process is called postgres and listens at a specified TCP/IP port for incoming connections. Whenever a request for a connection is detected the postgres process spawns a new server process. The server tasks communicate with each other using semaphores and shared memory to ensure data integrity throughout concurrent data access.

Once a connection is established the client process can send a query to the backend (server). The query is transmitted using plain text, i.e., there is no parsing done in the frontend (client). The server parses the query, creates an execution plan, executes the plan and returns the retrieved rows to the client by transmitting them over the established connection.

PostgreSQL Architecture:

Libpq:

Libpq is the C application programmer's interface to PostgreSQL. Libpq is a set of library functions that allow client programs to pass queries to the PostgreSQL backend server and to receive the results of these queries.

Libpq is also the underlying engine for several other PostgreSQL application interfaces, including those written for C++, Perl, Python, Tcl and ECPG

Postmaster:
When a client request for connection to the database, firstly request is hit to Postmaster daemon process. After performing authentication and authorization it forks one new backend server process (postgres). Henceforth, the frontend process and the backend server communicate directly without intervention by the postmaster.

The postmaster is always running, waiting for connection requests, whereas frontend and backend processes come and go. The libpq library allows a single frontend to make multiple connections to backend processes.

However, each backend process is a single-threaded process that can only execute one query at a time; so the communication over any one frontend-to-backend connection is single-threaded.

One postgres process exists for every open database session. Once authenticated with user connection, it directly connects with shared memory.

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