API cannot connect to postgres database using docker compose - docker

Question

I'm trying to use docker compose to connect my ASP.Net Core web api to my postgres database.
Here is my docker-compose file
version: "3.9"
services:
db:
image: postgres:15.1-alpine
container_name: db
restart: always
environment:
POSTGRES_USER: postgres
POSTGRES_PASSWORD: "MyPassword123"
ports:
- "5432:5432"
volumes:
- db_data:/var/lib/postgresql/data
api:
image: ${DOCKER_REGISTRY-}api
container_name: api
environment:
ASPNETCORE_ENVIRONMENT: Development
ConnectionStrings_Default: Server=db;Port=5432;Database=DB;User Id=postgres;Password=MyPassword123;
ports:
- "7001:80"
depends_on:
- db
build:
context: .
dockerfile: src/Services/Api/Dockerfile
volumes:
db_data:
Here are the logs of my postgres DB
db | 2023-01-19 09:46:17.887 UTC [1] LOG: listening on IPv4 address "0.0.0.0", port 5432
db | 2023-01-19 09:46:17.887 UTC [1] LOG: listening on IPv6 address "::", port 5432
db | 2023-01-19 09:46:17.906 UTC [1] LOG: listening on Unix socket "/var/run/postgresql/.s.PGSQL.5432"
db | 2023-01-19 09:46:17.921 UTC [50] LOG: database system was shut down at 2023-01-19 09:46:17 UTC
db | 2023-01-19 09:46:17.929 UTC [1] LOG: database system is ready to accept connections
But on the API side I get the messsage:
Unhandled exception. Npgsql.NpgsqlException (0x80004005): Failed to connect to [::1]:5432
The db is correctly created and I can access it from my machine using PGAdmin using localhost and port 5432.
Should I create a user defined bridge network? I'm pretty sure everything should work like this.
Any help would be greatly appreciated.

Answer 1

The [::1] in the error message from the API is the IPv6 address for localhost. So it looks like it's not picking up the server name correctly from your connection string.
Looking at the docs, it seems like the keyword for the server is 'Host' and not 'Server', so try
ConnectionStrings_Default: Host=db;Port=5432;Database=DB;User Id=postgres;Password=MyPassword123;
If that doesn't help, then please provide more information on how you take the connection string and establish the connection to the database in C#.