docs: rename REPORT.md -> Messaging_System_REPORT.md (FW-14)

Regularize the uncommitted rename via git mv so the working tree
is clean and the authoritative messaging-system spec is unambiguous.
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# Messaging System Technical Analysis & Architecture Report
This report provides a comprehensive, deep-dive analysis of the messaging system implemented in the `tmux-agent-orchestrate-delegate-job` skill. It covers the MQTT broker architecture, event protocols, job lifecycles, codebase internals, cross-system integration, and a list of known limitations along with production recommendations.
---
## 1. MQTT Broker Architecture: PoC vs. TLS Production
The messaging system is designed with a clear, decoupled transition pathway from a Proof of Concept (PoC) public broker setup to a secured, authenticated, and encrypted private production cluster. All configurations are resolved dynamically from the environment or overridden at the job level, requiring zero code modifications during deployment cut-over.
### 1.1 PoC Architecture (Public Sandbox)
In the initial development/testing phase, the system defaults to the public broker hosted by HiveMQ:
* **Host/IP**: `broker.hivemq.com`
* **Protocol/Port**: Plaintext MQTT over TCP on port `1883`.
* **Security & Auth**: None. No username, password, TLS encryption, or access control list (ACL) constraints are applied.
* **QoS Level**: QoS 1 (At Least Once) is requested for publishes and subscriptions, ensuring acknowledgement at the network layer.
#### Risks and Limitations of the PoC Setup:
1. **Zero Eavesdropping Protection**: Because the broker is public and unencrypted, any internet user can subscribe to the root topic (`python/mqtt/jobs/#`) and read the exact prompt, agent sessions, and intermediate progress events.
2. **Event Spoofing & Injection**: Anyone can publish messages to any job topic. An attacker could publish a malicious `completed` or `error` event, prematurely terminating a running subscriber or causing the delegator to execute unauthorized post-validation hooks.
3. **No Message Persistence**: Public brokers do not guarantee queue persistence or durable sessions for disconnected clients. If a subscriber briefly drops offline, QoS 1 messages published during the disconnect window may be discarded.
4. **Rate Limiting & Reliability**: Public sandboxes are subject to arbitrary rate limits, traffic spikes, and transient connection resets, leading to network-level timeouts.
---
### 1.2 Production Architecture (Secure Private Broker)
For production deployments, the system is designed to run on a private, self-hosted MQTT 5.0 broker such as **Mosquitto** or **EMQX**.
```mermaid
graph TD
subgraph "Secure Corporate Network"
Broker["Private MQTT Broker (Mosquitto/EMQX) <br> Ports: 8883 (TLS)"]
subgraph "Hermes (Delegator/Orchestrator)"
SubClient["job_subscriber.py <br> (Role: subscriber)"]
end
subgraph "Tmux Workspace (Agent Host)"
PubClient["publish_event.py <br> (Role: publisher)"]
end
SubClient -- "Subscribe (QoS 1) <br> Auth: hermes <br> ACL: Read jobs/+/events" --> Broker
PubClient -- "Publish (QoS 1 + Retain Terminal) <br> Auth: claude-worker <br> ACL: Write jobs/+/events" --> Broker
end
```
#### Production Security & Hardening Controls:
1. **Transport Layer Security (TLS v1.3)**: Traffic is encrypted over port `8883` using a private Certification Authority (CA). The orchestrator validates the broker using `MQTT_CA_CERTS` (CA bundle path). Optionally, Mutual TLS (mTLS) is supported via client-side certificate keys (`MQTT_CERTFILE`/`MQTT_KEYFILE`) for cryptographic device identities.
2. **Strict Client Authentication**: All clients must supply credentials (`MQTT_USERNAME` / `MQTT_PASSWORD`) to establish a connection. Anonymous logins are explicitly disabled (`allow_anonymous false`).
3. **Role-Based Topic Access Control Lists (ACLs)**:
* **Orchestrator/Hermes (Subscriber)**: Authenticates as user `hermes` with read-only access to all event streams:
```conf
user hermes
topic read python/mqtt/jobs/+/events
```
* **Agent/Worker (Publisher)**: Authenticates as user `claude-worker` with write-only access restricted to the job event sub-topics:
```conf
user claude-worker
topic write python/mqtt/jobs/+/events
```
This prevents workers from eavesdropping on sister agents or intercepting commands on other jobs.
4. **Durable Message Queues & Session State**:
* The broker is configured with `persistence true` and a dedicated disk storage path.
* Subscribers connect with persistent session flags to ensure the broker buffers QoS 1 messages during temporary network drops.
5. **Retained Terminal Events**: Terminal events (`completed`/`error`) are published with the `retain=True` flag. This allows a late-joining or recovering subscriber to instantly retrieve the final job status without waiting for active transmissions.
---
### 1.3 Production Mosquitto Configuration Reference
A hardened `/etc/mosquitto/mosquitto.conf` production configuration includes:
```conf
# Persistence settings
persistence true
persistence_location /var/lib/mosquitto/
# Authentication and Authorization
password_file /etc/mosquitto/auth/passwd
acl_file /etc/mosquitto/auth/acl
allow_anonymous false
# Listener and TLS Configuration
listener 8883
cafile /etc/mosquitto/certs/ca.crt
certfile /etc/mosquitto/certs/server.crt
keyfile /etc/mosquitto/certs/server.key
tls_version tlsv1.3
```
---
## 2. Event Protocol Specification
The event protocol defines a strict, single-direction JSON wire schema. It acts as the contract between the worker agent (the publisher) and the delegator/orchestrator (the subscriber).
### 2.1 Wire Schema (JSON UTF-8, `schema_version = 1`)
Every event payload must adhere to the following schema structure:
```json
{
"schema_version": 1,
"seq": 2,
"job_id": "918b0612",
"event": "progress",
"timestamp": "2026-06-20T14:48:58Z",
"detail": "Section 1: MQTT Broker Architecture completed",
"data": {
"auth_token": "URL-safe-base64-random-token-here",
"custom_metric": 42
}
}
```
### 2.2 Wire Schema Field Dictionary
| Field | Type | Required | Description & Validation Rules |
| :--- | :--- | :--- | :--- |
| `schema_version` | Integer | **Yes** | Must be exactly `1`. Subscribers discard payloads with mismatched version numbers to prevent parser crashes on schema drift. |
| `seq` | Integer | **Yes** | Monotonic sequence counter starting at `1` for the first publish. Incremented and stored in the job's registry file (`last_seq`) to survive agent pane crashes. |
| `job_id` | String | **Yes** | The 8-character hex string identifying the target job. Subscribers discard any messages whose `job_id` is unexpected or unrequested. |
| `event` | String | **Yes** | The event classification: `started`, `progress`, `permission_required`, `completed`, or `error`. |
| `timestamp` | String | **Yes** | ISO-8601 UTC timestamp with a trailing `Z` suffix. (Advisory only; never trusted for timeouts). |
| `detail` | String | **Yes** | Generalized, safe text description. Strict rules prohibit absolute paths, workspace paths, passwords, or raw environment variables. |
| `data` | Object | **Yes** | Metadata dictionary. Used in production to pass `auth_token` or structured execution metrics. |
---
### 2.3 Event Type Dictionary and Schemas
#### 1. `started`
* **Emit Trigger**: Emitted by the worker agent immediately upon boot inside the tmux session, indicating it has parsed the instructions and started execution.
* **Payload Constraints**: `seq` must be `1`. Status in registry is transitioned to `running`.
* **Example Detail**: `"Job 918b0612 started"`
#### 2. `progress`
* **Emit Trigger**: Optional. Emitted at major check-points, long loops, or sub-task boundaries.
* **Payload Constraints**: None.
* **Example Detail**: `"Section 1: MQTT Broker Architecture completed"`
#### 3. `permission_required`
* **Emit Trigger**: Emitted when the agent needs human confirmation (e.g. to run a destructive command or read/write critical system files).
* **Payload Constraints**: `detail` contains the resource/action requested.
* **Example Detail**: `"needs write permission to REPORT.md"`
#### 4. `completed` (Terminal)
* **Emit Trigger**: Successful job completion. The agent has generated all expected artifacts and verified correctness.
* **Payload Constraints**: Must be the final event. Published with `retain=True`.
* **Example Detail**: `"deep report written and committed to git"`
#### 5. `error` (Terminal)
* **Emit Trigger**: Terminal failure. Agent encountered an unhandled exception, syntax error, or validation script fail.
* **Payload Constraints**: Must be the final event. Published with `retain=True`.
* **Example Detail**: `"validation fail: missing files"`
---
### 2.4 Integrity and Authentication Verification (Bearer Auth)
To prevent unauthorized users from hijacking or spoofing events on public brokers:
1. When a job is registered, a cryptographic token (`auth_token`) is generated (`secrets.token_urlsafe(32)`).
2. The publisher reads this token from the local job file and injects it into `data.auth_token` for all outgoing messages.
3. The subscriber (`job_subscriber.py`) reads the expected `auth_token` from the local registry and performs a plaintext bearer-token check on all received messages. Mismatched or missing tokens are discarded immediately.
---
## 3. Job Lifecycle & State Transitions
The lifecycle of a delegated job progresses through a highly coordinated state machine, involving file-based registry claiming, asynchronous message subscription, and multi-faceted event publishing.
```mermaid
stateDiagram-v2
[*] --> pending : register_job()
pending --> running : pick_pending()
running --> completed : publish_event(--event completed)
running --> error : publish_event(--event error)
running --> cancelled : update_status(..., cancelled)
pending --> cancelled : update_status(..., cancelled)
completed --> [*]
error --> [*]
cancelled --> [*]
```
### 3.1 Step-by-Step Lifecycle Phase Details
#### Phase 1: Registration (`register`)
* **Trigger**: A delegator triggers `registry.py register` (or the `tmux-agent-orchestrate-delegate-job submit` command).
* **Registry State**: Flips from non-existent to `pending` inside `.hermes/jobs/<job_id>.json`. A `last_seq` counter is initialized to `0`.
* **Locking**: Exclusive fcntl file lock acquired over `.lock` during write.
* **Durable Audit Log**: Writes `<logs>/<job_id>/meta.json`, sets status to `pending` in `status.json`, and appends a `registered` event line to `events.ndjson`.
#### Phase 2: Claiming (`pick_pending`)
* **Trigger**: An agent session starts up and calls `registry.py pick --agent-session <session_label>`.
* **Registry State**: Oldest matching `pending` record is scanned. Status is atomically updated to `running`. `updated_at` is stamped.
* **Locking**: Reads and writes occur inside the exclusive fcntl lock block.
* **Durable Audit Log**: Status is synced to `running` in `status.json` and a `status_changed` event is appended to `events.ndjson`.
#### Phase 3: Listening (`subscribe`)
* **Trigger**: The wrapper command launches `job_subscriber.py --job <job_id>` in the background **before** launching the agent.
* **Broker Connection**: Connects to the resolved host, issues a QoS 1 subscription to `python/mqtt/jobs/<job_id>/events`, and blocks on an event queue.
* **Timeout Initialization**: Dual timeouts (wall-clock budget and activity idle timer) are calculated and start ticking.
#### Phase 4: Execution & Progress Events (`publish`)
* **Trigger**: The agent executes prompts within tmux and runs `publish_event.py` at boot and checkpoint stages.
* **Network Handshake**: Publisher opens a fresh TCP/TLS socket to the broker, awaits CONNACK, publishes a single QoS 1 message, waits for PUBACK, and gracefully disconnects to avoid socket resource leaks.
* **State Updates**: Updates `last_seq` monotonically, updates `status` to `running` (if not already), and mirrors the published payload into the local audit logs (`events.ndjson`).
* **Subscriber Capture**: The subscriber captures the payload, performs bearer token checks, prints the formatted line to stdout, and resets its idle timer.
#### Phase 5: Terminal Finalization
* **Trigger**: Agent publishes `--event completed` or `--event error`.
* **Registry Transition**: State becomes `completed` or `error`.
* **Retained Messaging**: The terminal event is published with `retain=True` on the broker.
* **Subscriber Exit**: The subscriber processes the terminal event exactly once, terminates its background loops, and exits (code `0` for completed, `1` for error).
---
## 4. Code Internals Analysis
### 4.1 `registry.py` & `lib.sh` (Locking & Atomicity)
Two concurrency control schemes co-exist in this workspace to coordinate state modification:
1. **`lib.sh::atomic_dump_yaml()`**: Used for workspace-wide tmux session inventory (`agent-sessions.yaml`).
* **Locking**: Uses POSIX advisory locking via python's `fcntl.flock(lock_fh, fcntl.LOCK_EX)` over a sidecar lock file `<yaml_path>.lock`.
* **Safe Mutation**: The mutation source code is passed in an environment variable `AGENT_SESSIONS_MUTATION` and executed dynamically using `exec(compile(..., 'exec'), globals())`. This isolates the execution and avoids command-injection vectors.
* **Atomicity**: Writes to a temp file in the same directory using `tempfile.mkstemp()`, then performs an `os.replace()` rename. POSIX guarantees the replacement is atomic, preventing half-written YAML reads. A `.bak` backup copy is also preserved.
2. **`registry.py::register_job() / pick_pending() / _atomic_write_record()`**: Used for job-level metadata JSON files (`<job_id>.json`).
* **Locking**: Wraps operations in a `registry_lock(registry_dir)` context manager, implementing an advisory exclusive lock on `.lock` via `fcntl.flock`.
* **Atomicity**: In `_atomic_write_record()`, it uses `tempfile.mkstemp` inside the parent registry folder, serializes the updated job record to the temp file, flushes it, triggers a physical disk sync via `os.fsync(fh.fileno())`, and executes `os.replace` to replace the main JSON record file. The file permission is restricted to `0o600` immediately.
---
### 4.2 `publish_event.py` (Retries and Handshakes)
The publisher script enforces robust error handling when sending status updates:
* **Fresh Connection Pattern**: Instead of maintaining a persistent socket connection (which is susceptible to socket timeouts or channel leaks), `publish_event.py` opens a fresh socket, completes the authentication/TLS handshake, publishes a single QoS 1 event, waits for `PUBACK`, and closes the connection.
* **Exponential Backoff**: Wrapped in the `with_retry()` decorator from `mqtt_common.py`. In case of socket errors (`OSError`, `TimeoutError`, `ConnectionError`), it retries up to 3 times (configurable via `--attempts`) with backoff:
$$\text{delay} = \min(\text{base\_delay} \times \text{factor}^{\text{attempt}-1}, \text{max\_delay})$$
Default parameters: `base_delay = 0.5s`, `factor = 2.0`, `max_delay = 8.0s`.
* **ACK Handshake Deadlines**:
* `CONNECT_ACK_TIMEOUT = 10s` (stops hangs during broker downtime).
* `PUBLISH_ACK_TIMEOUT = 5s` (guarantees QoS 1 message acknowledgment before marking as published).
---
### 4.3 `job_subscriber.py` (Timers and Queue Semantics)
The subscriber acts as the central execution watchdog:
* **Queue Serialization**: Uses a thread-safe `queue.Queue` internally. The Paho MQTT callback thread adds messages to the queue, and the main thread processes them sequentially. This separates network I/O from state machine validation.
* **State Machine Protection**: To safeguard against QoS 1 duplicate delivery or out-of-order broker retries, the subscriber runs a terminal state machine. It records job completion in an internal `terminal` dictionary. Once a job is marked `completed` or `error`, any subsequent events for that `job_id` are ignored:
```python
if event in TERMINAL_EVENTS:
if jid in terminal:
logger.info("ignoring duplicate terminal %s for %s", event, jid)
continue
terminal[jid] = event
pending.discard(jid)
```
* **Dual Timeout Semantics**:
1. **Wall-Clock Timeout**: Calculated relative to absolute startup time (`wall_deadline = start + wall_timeout`). It acts as a hard budget limit, guarding against an agent hanging indefinitely.
2. **Activity Idle Timeout**: Measured as the difference between the current monotonic time and the last packet arrival time (`idle_left = idle_timeout - (now - last_event)`). If the agent fails to print logs or publish progress updates for the duration of the idle window, the subscriber aborts and exits with code 2.
---
### 4.4 `mqtt_common.py` (Logging & Config Resolution)
* **Log Routing isolation**: Configured via `setup_logging()`. The root logger is bound to `sys.stderr`. This preserves the standard output stream (`stdout`) exclusively for clean JSON-lines payloads, enabling downstream bash tools to pipeline event feeds cleanly (e.g., `job_subscriber.py ... | jq`).
* **Broker Config Resolution**: Configured in `broker_config_from_job()`. Resolves credentials hierarchically:
1. Defaults to environment configurations (e.g. `MQTT_BROKER`, `MQTT_PORT`, `MQTT_TLS`, `MQTT_CA_CERTS`).
2. Overlays credentials specified inside the job record JSON block (`broker.*`). This allows the agent to fetch its dedicated target broker credentials on a per-job basis.
---
## 5. Cross-System Integration
The delegated messaging system functions as a critical control backplane, binding shell wrappers and monitoring loops across the orchestration stack.
```mermaid
graph LR
User["User/Cron Client"] -->|submit| Wrap["tmux-agent-orchestrate-delegate-job (Bash)"]
Wrap -->|registers| Reg["registry.py (Live Registry)"]
Wrap -->|spawns background| Sub["job_subscriber.py"]
Wrap -->|spawns tmux pane| Tmux["tmux Session (Agent Pane)"]
Tmux -->|executes agent| Agent["Claude / Codex Agent"]
Agent -->|publish_event.py| Broker["MQTT Broker"]
Broker -->|delivers events| Sub
Broker -->|delivers events| Mon["reconcile.sh (Monitor Loop)"]
Mon -->|updates| Inv["agent-sessions.yaml <br> (lib.sh::atomic_dump_yaml)"]
```
### 5.1 Orchestration Wrappers (`tmux-agent-orchestrate-*`)
1. **`tmux-agent-orchestrate-delegate-job (submit)`**:
* Registers a job, spawns `job_subscriber.py` to capture standard output streams to `.hermes/jobs/<job_id>.subscriber.out`, and sleeps for `1` second.
* Boots the agent pane in tmux:
```bash
tmux new-session -d -s "$sess" -c "$WORKDIR" \
"printf '%s' \"$instructions\" | $bin --dangerously-skip-permissions; echo; read"
```
* Pre-seeds agent instruction headers via stdin to enforce that the agent runs `publish_event.py` for its transitions.
* Blocks on `wait $sub_pid`, and finally prints the audit log directory.
2. **`tmux-agent-orchestrate-monitor` (`reconcile.sh` & `watchdog.sh`)**:
* **Watchdog Integration**: Starts a subscriber monitoring loop (`watchdog.sh`) to detect orphaned agent panes or locked workspaces.
* **Reconciliation loop**: Subscribes to the global job topic. On terminal events, it invokes `lib.sh::atomic_dump_yaml` to sync status drifts (e.g. setting tmux sessions to `terminated` in `agent-sessions.yaml` once the agent exits).
3. **`tmux-agent-orchestrate-create / delete / resume`**:
* Integrates the job life status into session metadata updates, ensuring standard tmux cleanup triggers state updates in the registry and audit logs.
---
## 6. Known Limitations & Recommendations
### 6.1 Limitations
1. **Single-Host File Locking Vulnerability**:
The advisory locking system relies on `fcntl.flock` on `.hermes/jobs/.lock`. This works perfectly for local processes but is **broken on network filesystems (NFS)** or across multi-host environments where locks may fail or behave non-atomically.
2. **Bearer Token Leakage over Plaintext (Public Broker)**:
The `auth_token` mechanism is a simple plaintext bearer comparison. If the transport layer is unencrypted (e.g., using `broker.hivemq.com` on port `1883`), any eavesdropper on the network can steal the token and spoof legitimate events.
3. **Subscriber Network Drop Orphanage**:
`job_subscriber.py` does not implement automatic reconnection loops. If the subscriber loses connection to the broker, it exits, leaving the running tmux agent orphaned and without a validation/collection hook.
4. **Lack of Ordering Guarantees in QoS 1**:
QoS 1 guarantees delivery but not strict ordering. Under heavy backoff retries, a late-delivered progress event could land after a terminal event, causing state inconsistencies.
---
### 6.2 Recommendations
1. **Migrate to SQLite WAL Backend**:
Replace the raw directory lock in `registry.py` and `mqtt_common.py` with a SQLite database configured with Write-Ahead Logging (`PRAGMA journal_mode=WAL`). SQLite handles concurrent reads and serializes writes safely across multi-process applications without blocking.
2. **Implement Signature-Based Payload Verification**:
Rather than sending a plaintext token, utilize HMAC signatures. The delegator and worker share a secret key; the worker publishes a signature of the payload (e.g. `HMAC-SHA256(secret_key, payload_bytes)`). The subscriber validates the signature, preventing token interception.
3. **Enforce Mandatory Broker-Side TLS and ACLs**:
De-prioritize plaintext support. Enforce connection over port `8883` with verified TLS certificates. Implement client certificates (mTLS) for agent authentication.
4. **Build Auto-Reconnecting Subscriber Loops**:
Upgrade `job_subscriber.py` to handle disconnect callbacks. Maintain a persistent queue in memory and allow the client to reconnect with exponential backoff, preventing socket dropout from terminating the orchestration flow.