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Nothing runs on an empty machine.

Rule

Before you read a line of application code, prove the environment can actually run it. Six checks, in order, each one either pass or fail with the observation that decided it. You may not skip a check because the previous one passed, and you may not infer a pass from configuration that merely looks correct. Secrets are confirmed by presence and shape, never by value.

Procedure

Run all six, in this order. Stop and report at the first failure — a failed check is the finding, not a detour.

1. Is it running, and is it listening? A live process is not a bound socket. Confirm the process exists (ps, docker ps, systemctl status, container health) and that something is listening on the expected address and port (ss -ltnp, lsof -i :PORT). A process stuck in startup passes the first half and fails the second.
2. Is the dependency installed, at the version the code imports? Presence is not enough. Compare the installed version (node_modules/<pkg>/package.json, pip show, <bin> --version) against what the manifest declares and what the code imports. Also confirm you are in the interpreter or virtualenv that owns that install — the right package under the wrong python is a fail.
3. Are the required env vars and credentials set, non-empty, and unexpired? Verify without printing values. Check that the name is defined, that the length is non-zero and plausible, that the prefix or format matches the expected shape, and that any embedded expiry (token exp, cert notAfter) is in the future. Check the process's own environment, not just the .env file on disk — the file may never have been loaded.
4. Are local resources available? The port free (or bound by the intended process, not a leftover), disk space on the volumes that matter (df -h, including /tmp and the data directory), memory headroom, and file-descriptor limits (ulimit -n) above what the workload opens. "No space left" and "too many open files" surface as unrelated application errors.
5. Is the host reachable, and does the name resolve? Resolve the hostname first (dig, getent hosts), then reach the port (nc -z, curl -v). DNS failure and connection refusal are different diagnoses, and inside containers or VPNs the name may resolve differently than it does in your shell.
6. Does the connection actually open, end to end? One real round trip from the same context the application runs in — a query, an authenticated ping, a health endpoint — returning a real response. Reading the config file is not this check. Credentials that are present and well-formed can still be revoked.