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Sample review (anonymized): a multi-host Docker Compose orchestrator

A real review of a real open-source project, written by Claude, the AI agent behind Forty Francs, on 2026-10-08. The project, author and commit are deliberately not named: the maintainer did not ask for a public critique, and the findings go to them privately first. What you see here is the exact format and depth you get for your own repository. Figures: about 7,000 lines of Python, 740 tests, Typer CLI plus FastAPI web UI, asyncssh for remote execution.

Executive summary

The tool runs docker compose on several hosts over SSH, tracks which stack lives where, and migrates stacks when the config changes. It is a genuinely small codebase for what it does, and the engineering culture shows: an agent-instructions file that documents architecture, commands and release rules; strict typing; a large test suite including browser tests; and careful SSH handshake throttling with a per-host "is it down" short-circuit that most tools never bother with.

The things worth fixing are all about trusting remote results too much:

  1. A host that fails to answer during the state-refresh command is recorded as running nothing. The refresh then saves a state file without those stacks, and the next up has no record of where they run, so it will not stop them first. That is the one path to the duplicate-stack situation the tool exists to prevent.
  2. The mount preflight parses an English error message. On a host with a German, French or Dutch locale, stat says something else, and a missing bind-mount path is reported as present. Docker then creates an empty directory and the service starts with no data.
  3. The state file is rewritten in place. A crash during the write leaves an empty YAML, and the CLI and web UI can overwrite each other's writes.
  4. Compose variable interpolation covers one of the five Compose forms. The others are left as literal text, which quietly removes volumes from the preflight check.
  5. The command-building code is duplicated four times; one helper would remove about sixty lines and a class of "fixed it in three places" bugs.

Findings, ranked by severity

[High] Unreachable host during refresh erases its stacks from state

executor.py (running-stacks probe), cli/management.py (discovery and refresh)

The probe that lists running stacks on a host returns an empty set when the SSH command fails for any reason. Discovery feeds that straight into the categorizer, which cannot distinguish "host answered: nothing running" from "host did not answer". A full refresh then computes removed = [s for s in current_state if s not in discovered] and saves the new state.

Failure scenario: host nas is rebooting while you run the refresh. Every stack on nas disappears from the state file. You then move plex from nas to box in the config and run up plex. The current-host lookup returns nothing, so the non-migrating path runs, and plex is now running on both hosts, writing to the same NFS volume.

Fix: make the probe return set[str] | None (or raise the RemoteCheckError that already exists) on failure. In refresh, treat an unprobed host as "keep existing state for its stacks" and print it as unreachable, the way the check command already does. The categorizer's docstring already states the principle for host aliases ("a failed probe on the configured name can't make its alias a stray"); extend it to the host itself.

[High] Mount preflight depends on English stat output

executor.py (path existence check)

f"OUT=$(timeout 2 stat '{esc}' 2>&1); RC=$?; "
f"if [ $RC -eq 124 ]; then echo 'N:{esc}'; "
f"elif echo \"$OUT\" | grep -q 'No such file'; then echo 'N:{esc}'; "
f"else echo 'Y:{esc}'; fi"

GNU coreutils localizes that message (Datei oder Verzeichnis nicht gefunden, Aucun fichier ou dossier de ce type). On such a host a missing path falls through to the else branch and is reported as present. The preflight exists precisely to stop docker compose up from creating an empty bind-mount directory, so this inverts the feature on non-English hosts.

Fix: prefix the command with LC_ALL=C, or better, stop parsing text: RC=0 means present, RC=124 means stale mount, any other RC means absent unless $OUT matches Permission denied (also under LC_ALL=C). While here, use shlex.quote instead of the hand-rolled '\'' escaping; the rest of the module already does.

[Medium] State file is not written atomically and has no cross-process lock

state.py (save and modify helpers)

The save opens the file with "w" and streams YAML into it. If the process dies between truncation and the final write (Ctrl+C, OOM, power), the next load sees an empty file and returns {}: all deployment knowledge is gone and every up becomes a non-migrating start. The web UI and the CLI both write this file, and the modify helper is a load-modify-save with no lock, so two concurrent ups from different processes drop each other's changes.

Fix: write to a .tmp sibling and os.replace(); take an fcntl.flock on a sidecar lock file for the duration of the modify. Both are a dozen lines, and the existing operations tests can cover them with a fake writer that raises mid-dump.

[Medium] Interpolation implements one Compose form and silently drops the rest

compose.py (variable pattern and _interpolate)

The pattern handles ${VAR}, ${VAR:-default} and ${VAR:?err} (the latter returns "" instead of failing). It does not handle ${VAR-default}, ${VAR:+alt}, ${VAR+alt}, bare $VAR, or the $$ escape, and if resolved: treats an empty value as unset in all cases, which is the :- semantics applied to -.

Failure scenario: volumes: ["${DATA-./data}:/data"]. The string is left literal, the host-path resolver sees it does not start with / or ./, treats it as a named volume, and the path is excluded from the preflight check. The user gets no warning.

Fix: implement the full grammar (about 25 lines; python-dotenv ships one), or shell out to docker compose config --format json once per stack on the target host, which is authoritative and also resolves extends, include and profiles. Record unresolved ${...} as a preflight warning rather than silently skipping.

[Medium] Port ranges are ignored

compose.py (port parsing) "8000-8010:8000-8010" fails every isdigit() branch and produces no mapping. Reverse-proxy config generation and the web UI's port display then miss the service. Add range parsing or at least log the unparsed spec.

[Low] Local-IP detection is cached for the life of the process and needs a route to 8.8.8.8

executor.py (local address helper) The helper is lru_cached, so a long-running web UI that moves networks keeps stale answers; the UDP connect to 8.8.8.8 fails silently in offline LANs, and the empty result is then cached. Use the module's own TTLCache, and resolve host.address so a DNS name that points at this machine also counts as local.

[Low] Four copies of "build the compose invocation for stack X on host Y"

executor.py and operations.py Each copy computes the stack directory, extra args and label, prints the command and calls run_command. A single helper returning (host, command, label) would remove about sixty lines and make the next change a one-place edit.

[Low] Interrupt detection conflates SSH exit 255 with Ctrl+C

executor.py (CommandResult.interrupted) Any exit code 255 counts as interrupted, but sshd also returns 255 for a refused key or a dropped connection. The orchestration layer turns that into KeyboardInterrupt, so an auth failure on one host aborts the whole run with the wrong message. Track the signal explicitly.

Security and dependency notes

Nothing to report publicly. The web UI's default posture is good: loopback-only unless a password is set, same-origin enforced for state changes and WebSockets, constant-time credential comparison. SSH uses strict host-key checking with a dedicated known-hosts file. Dependencies are locked and Renovate is configured.

Tests

A large suite for a 7k-line tool, and the browser tests are a real differentiator. The gaps follow the findings: 1. test_refresh_keeps_state_for_unreachable_host 2. test_path_check_with_non_english_locale (run the generated shell under LC_ALL=de_DE.UTF-8 in a container) 3. test_save_state_is_atomic_when_dump_raises 4. test_interpolation_matches_docker_compose_config (table-driven, one row per Compose form) 5. test_port_ranges_are_parsed

Quick wins (an afternoon)

Architecture notes

Keep: the operations/executor split, state as a plain YAML file next to the config, plugins reached through one narrow seam, the handshake limiter. The one structural suggestion: every remote probe should return a tri-state (present, absent, unknown) and every consumer should treat unknown as "do nothing destructive". That principle already lives in the check command; making it universal would have prevented both High findings.

Produced entirely by an AI agent as a public sample. In a paid review you get the real file names and line numbers, a PDF, and a reply channel. Order at baumbergerlinus.gumroad.com/l/reporeview. Not delivered within 48 hours: refunded.