Accessing multiple PostgreSQL servers using the same host and port typically involves a technique called connection pooling or the use of a proxy that can direct traffic to different database instances based on the incoming request criteria. Here’s how you can set up and manage access to multiple PostgreSQL servers on the same host and port:

Using Connection Pooling

Connection pooling software like PgBouncer or pgpool-II can manage connections between PostgreSQL clients and multiple PostgreSQL server instances. These tools don't exactly allow multiple servers on the same port directly but manage connections to various servers from a single endpoint.

PgBouncer

PgBouncer is a lightweight connection pooler for PostgreSQL that can manage database connections and route them to one or more PostgreSQL servers.

1. Installation: Install PgBouncer on your system where it can access all PostgreSQL servers.

2. Configuration: Configure pgbouncer.ini to manage connections to different PostgreSQL servers. For example:

    [databases]
    db1 = host=server1 port=5432 dbname=mydb
    db2 = host=server2 port=5432 dbname=mydb
   
    [pgbouncer]
    listen_port = 6432
    listen_addr = *
    auth_type = md5
    auth_file = /etc/pgbouncer/userlist.txt
   

3. Running PgBouncer: Start PgBouncer, and it will listen on a specified port (e.g., 6432). All applications connect through PgBouncer, which then routes the connections to the appropriate PostgreSQL server.

pgpool-II

pgpool-II is another middleware that sits between PostgreSQL servers and a PostgreSQL client. It provides connection pooling, load balancing, and more.

1. Installation and Configuration: Similar to PgBouncer, you install and configure pgpool-II to manage connections to multiple PostgreSQL databases.

2. Features: pgpool-II can also split read and write operations, manage replication, and perform load balancing across multiple servers.

Using a TCP Proxy

A TCP proxy like HAProxy can be used to route database traffic to different PostgreSQL servers based on predefined rules. This setup is more about directing traffic than managing database connections.

1. Setup HAProxy: Install and configure HAProxy to listen on a common port and forward requests to different PostgreSQL servers based on the source, destination, or other criteria.

2. Configuration Example:

    frontend psql_front
      bind *:5432
      default_backend psql_back
   
    backend psql_back
      server server1 192.168.1.101:5432 check
      server server2 192.168.1.102:5432 check
   

This configuration directs all traffic coming to port 5432 on the HAProxy machine to one of the configured PostgreSQL servers.

Considerations

• Security: Ensure that connection pooling or proxy solutions do not become a single point of failure or a security risk.

• Performance: These tools can introduce latency, so it’s crucial to monitor performance and adjust configurations as needed.

• Maintenance: Managing a proxy or a connection pooler requires additional maintenance and monitoring for stability and performance.

Conclusion

While you cannot directly have multiple PostgreSQL servers listening on the exact same host and port without additional software, using connection pooling or a TCP proxy provides robust methods to manage and route connections effectively to multiple PostgreSQL instances from a single access point. These methods enhance scalability, performance, and manageability of accessing multiple databases within PostgreSQL environments.