Great question! Let's break down the purpose and impact of setting hostname: db for your dbc MySQL container, especially since each container already gets its own unique IP address.

• It sets the internal system hostname of the container
  When you shell into the dbc container (run docker exec -it <your-dbc-container-id> bash), running the hostname command will output db. This value is written to the container's /etc/hostname and /etc/hosts files, so any process inside the container that relies on the system's hostname (like logging tools that tag entries with the host) will use this value instead of the default random hostname (which is usually the first 12 characters of the container ID).

• It provides an additional DNS alias for container-to-container communication
  In your setup, the agent service uses PROBE_HOST: "db" to reach the MySQL container. Since both services are on the same Docker network (the default bridge network, or a custom one if you've defined it), Docker's internal DNS resolver will map the hostname db to the dbc container's IP address. That said, using the service name (dbc) is generally more reliable for inter-container communication—Docker Compose automatically sets up DNS for service names, and this works even if you scale the service to multiple containers (where duplicate hostnames would break DNS resolution).

• It’s separate from the container’s network IP
  The container's unique IP address is its network identity within the Docker network, while the hostname is its system-level identity. These two aren't tied together: if the container restarts and gets a new IP, the hostname db will remain the same.

• It improves readability for logs and monitoring
  Tools like log collectors (e.g., Filebeat, Fluentd) or monitoring systems (e.g., Prometheus) often use the container's hostname to tag data. Having a human-readable hostname like db makes it much easier to identify which container generated a log entry or metric, instead of sorting through random container IDs.

To sum up: your hostname: db setting gives the container a meaningful internal system name, enables DNS resolution via that name for other containers on the same network, and makes debugging/monitoring easier. For inter-container communication though, sticking to the service name (dbc) is the more robust approach.

内容的提问来源于stack exchange，提问作者overexchange