CHANGELOG.md

@@ -5,6 +5,59 @@ All notable changes to SLMM (Sound Level Meter Manager) will be documented in th
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.3.0] - 2026-02-17
+
+### Added
+
+#### Persistent TCP Connection Pool
+- **Connection reuse** - TCP connections are cached per device and reused across commands, eliminating repeated TCP handshakes over cellular modems
+- **OS-level TCP keepalive** - Configurable keepalive probes keep cellular NAT tables alive and detect dead connections early (default: probe after 15s idle, every 10s, 3 failures = dead)
+- **Transparent retry** - If a cached connection goes stale, the system automatically retries with a fresh connection so failures are never visible to the caller
+- **Stale connection detection** - Multi-layer detection via idle TTL, max age, transport state, and reader EOF checks
+- **Background cleanup** - Periodic task (every 30s) evicts expired connections from the pool
+- **Master switch** - Set `TCP_PERSISTENT_ENABLED=false` to revert to per-request connection behavior
+
+#### Connection Pool Diagnostics
+- `GET /api/nl43/_connections/status` - View pool configuration, active connections, age/idle times, and keepalive settings
+- `POST /api/nl43/_connections/flush` - Force-close all cached connections (useful for debugging)
+- **Connections tab on roster page** - Live UI showing pool config, active connections with age/idle/alive status, auto-refreshes every 5s, and flush button
+
+#### Environment Variables
+- `TCP_PERSISTENT_ENABLED` (default: `true`) - Master switch for persistent connections
+- `TCP_IDLE_TTL` (default: `300`) - Close idle connections after N seconds
+- `TCP_MAX_AGE` (default: `1800`) - Force reconnect after N seconds
+- `TCP_KEEPALIVE_IDLE` (default: `15`) - Seconds idle before keepalive probes start
+- `TCP_KEEPALIVE_INTERVAL` (default: `10`) - Seconds between keepalive probes
+- `TCP_KEEPALIVE_COUNT` (default: `3`) - Failed probes before declaring connection dead
+
+### Changed
+- **Health check endpoint** (`/health/devices`) - Now uses connection pool instead of opening throwaway TCP connections; checks for existing live connections first (zero-cost), only opens new connection through pool if needed
+- **Diagnostics endpoint** - Removed separate port 443 modem check (extra handshake waste); TCP reachability test now uses connection pool
+- **DRD streaming** - Streaming connections now get TCP keepalive options set; cached connections are evicted before opening dedicated streaming socket
+- **Default timeouts tuned for cellular** - Idle TTL raised to 300s (5 min), max age raised to 1800s (30 min) to survive typical polling intervals over cellular links
+
+### Technical Details
+
+#### Architecture
+- `ConnectionPool` class in `services.py` manages a single cached connection per device key (NL-43 only supports one TCP connection at a time)
+- Uses existing per-device asyncio locks and rate limiting — no changes to concurrency model
+- Pool is a module-level singleton initialized from environment variables at import time
+- Lifecycle managed via FastAPI lifespan: cleanup task starts on startup, all connections closed on shutdown
+- `_send_command_unlocked()` refactored to use acquire/release/discard pattern with single-retry fallback
+- Command parsing extracted to `_execute_command()` method for reuse between primary and retry paths
+
+#### Cellular Modem Optimizations
+- Keepalive probes at 15s prevent cellular NAT tables from expiring (typically 30-60s timeout)
+- 300s idle TTL ensures connections survive between polling cycles (default 60s interval)
+- 1800s max age allows a single socket to serve ~30 minutes of polling before forced reconnect
+- Health checks and diagnostics produce zero additional TCP handshakes when a pooled connection exists
+- Stale `$` prompt bytes drained from idle connections before command reuse
+
+### Breaking Changes
+None. This release is fully backward-compatible with v0.2.x. Set `TCP_PERSISTENT_ENABLED=false` for identical behavior to previous versions.
+
+---
+
 ## [0.2.1] - 2026-01-23
 
 ### Added
@@ -146,6 +199,7 @@ None. This release is fully backward-compatible with v0.1.x. All existing endpoi
 
 ## Version History Summary
 
+- **v0.3.0** (2026-02-17) - Persistent TCP connections with keepalive for cellular modem reliability
 - **v0.2.1** (2026-01-23) - Roster management, scheduler hooks, FTP logging, doc cleanup
 - **v0.2.0** (2026-01-15) - Background Polling System
 - **v0.1.0** (2025-12-XX) - Initial Release

README.md

@@ -1,6 +1,6 @@
 # SLMM - Sound Level Meter Manager
 
-**Version 0.2.1**
+**Version 0.3.0**
 
 Backend API service for controlling and monitoring Rion NL-43/NL-53 Sound Level Meters via TCP and FTP protocols.
 
@@ -12,8 +12,9 @@ SLMM is a standalone backend module that provides REST API routing and command t
 
 ## Features
 
-- **Background Polling** ⭐ NEW: Continuous automatic polling of devices with configurable intervals
+- **Persistent TCP Connections**: Cached per-device connections with OS-level keepalive, tuned for cellular modem reliability
-- **Offline Detection** ⭐ NEW: Automatic device reachability tracking with failure counters
+- **Background Polling**: Continuous automatic polling of devices with configurable intervals
+- **Offline Detection**: Automatic device reachability tracking with failure counters
 - **Device Management**: Configure and manage multiple NL43/NL53 devices
 - **Real-time Monitoring**: Stream live measurement data via WebSocket
 - **Measurement Control**: Start, stop, pause, resume, and reset measurements
@@ -22,6 +23,7 @@ SLMM is a standalone backend module that provides REST API routing and command t
 - **Device Configuration**: Manage frequency/time weighting, clock sync, and more
 - **Rate Limiting**: Automatic 1-second delay enforcement between device commands
 - **Persistent Storage**: SQLite database for device configs and measurement cache
+- **Connection Diagnostics**: Live UI and API endpoints for monitoring TCP connection pool status
 
 ## Architecture
 
@@ -29,29 +31,39 @@ SLMM is a standalone backend module that provides REST API routing and command t
 ┌─────────────────┐         ┌──────────────────────────────┐         ┌─────────────────┐
 │                 │◄───────►│  SLMM API                    │◄───────►│  NL43/NL53      │
 │  (Frontend)     │  HTTP   │  • REST Endpoints            │  TCP    │  Sound Meters   │
-└─────────────────┘         │  • WebSocket Streaming       │         └─────────────────┘
+└─────────────────┘         │  • WebSocket Streaming       │  (kept  │  (via cellular  │
-                            │  • Background Poller ⭐ NEW  │                ▲
+                            │  • Background Poller         │  alive) │   modem)        │
-                            └──────────────────────────────┘                │
+                            │  • Connection Pool (v0.3)    │         └─────────────────┘
-                                          │                         Continuous
+                            └──────────────────────────────┘
-                                          ▼                          Polling
+                                          │
-                                  ┌──────────────┐                      │
+                                          ▼
-                                  │  SQLite DB   │◄─────────────────────┘
+                                  ┌──────────────┐
+                                  │  SQLite DB   │
                                   │  • Config    │
                                   │  • Status    │
                                   └──────────────┘
 ```
 
+### Persistent TCP Connection Pool (v0.3.0)
+
+SLMM maintains persistent TCP connections to devices with OS-level keepalive, designed for reliable operation over cellular modems:
+
+- **Connection Reuse**: One cached TCP socket per device, reused across all commands (no repeated handshakes)
+- **TCP Keepalive**: Probes keep cellular NAT tables alive and detect dead connections early
+- **Transparent Retry**: Stale cached connections automatically retry with a fresh socket
+- **Configurable**: Idle TTL (300s), max age (1800s), and keepalive timing via environment variables
+- **Diagnostics**: Live UI on the roster page and API endpoints for monitoring pool status
+
 ### Background Polling (v0.2.0)
 
-SLMM now includes a background polling service that continuously queries devices and updates the status cache:
+Background polling service continuously queries devices and updates the status cache:
 
 - **Automatic Updates**: Devices are polled at configurable intervals (10-3600 seconds)
 - **Offline Detection**: Devices marked unreachable after 3 consecutive failures
 - **Per-Device Configuration**: Each device can have a custom polling interval
 - **Resource Efficient**: Dynamic sleep intervals and smart scheduling
-- **Graceful Shutdown**: Background task stops cleanly on service shutdown
 
-This makes Terra-View significantly more responsive - status requests return cached data instantly (<100ms) instead of waiting for device queries (1-2 seconds).
+Status requests return cached data instantly (<100ms) instead of waiting for device queries (1-2 seconds).
 
 ## Quick Start
 
@@ -96,9 +108,18 @@ Once running, visit:
 
 ### Environment Variables
 
+**Server:**
 - `PORT`: Server port (default: 8100)
 - `CORS_ORIGINS`: Comma-separated list of allowed origins (default: "*")
 
+**TCP Connection Pool:**
+- `TCP_PERSISTENT_ENABLED`: Enable persistent connections (default: "true")
+- `TCP_IDLE_TTL`: Close idle connections after N seconds (default: 300)
+- `TCP_MAX_AGE`: Force reconnect after N seconds (default: 1800)
+- `TCP_KEEPALIVE_IDLE`: Seconds idle before keepalive probes (default: 15)
+- `TCP_KEEPALIVE_INTERVAL`: Seconds between keepalive probes (default: 10)
+- `TCP_KEEPALIVE_COUNT`: Failed probes before declaring dead (default: 3)
+
 ### Database
 
 The SQLite database is automatically created at [data/slmm.db](data/slmm.db) on first run.
@@ -126,7 +147,7 @@ Logs are written to:
 | GET | `/api/nl43/{unit_id}/live` | Request fresh DOD data from device (bypasses cache) |
 | WS | `/api/nl43/{unit_id}/stream` | WebSocket stream for real-time DRD data |
 
-### Background Polling Configuration ⭐ NEW
+### Background Polling
 
 | Method | Endpoint | Description |
 |--------|----------|-------------|
@@ -134,6 +155,13 @@ Logs are written to:
 | PUT | `/api/nl43/{unit_id}/polling/config` | Update polling interval and enable/disable polling |
 | GET | `/api/nl43/_polling/status` | Get global polling status for all devices |
 
+### Connection Pool
+
+| Method | Endpoint | Description |
+|--------|----------|-------------|
+| GET | `/api/nl43/_connections/status` | Get pool config, active connections, age/idle times |
+| POST | `/api/nl43/_connections/flush` | Force-close all cached TCP connections |
+
 ### Measurement Control
 
 | Method | Endpoint | Description |
@@ -255,6 +283,9 @@ Caches latest measurement snapshot:
 
 ### TCP Communication
 - Uses ASCII command protocol over TCP
+- Persistent connections with OS-level keepalive (tuned for cellular modems)
+- Connections cached per device and reused across commands
+- Transparent retry on stale connections
 - Enforces ≥1 second delay between commands to same device
 - Two-line response format:
   - Line 1: Result code (R+0000 for success)
@@ -320,6 +351,16 @@ curl http://localhost:8100/api/nl43/meter-001/polling/config
 curl http://localhost:8100/api/nl43/_polling/status
 ```
 
+### Check Connection Pool Status
+```bash
+curl http://localhost:8100/api/nl43/_connections/status | jq '.'
+```
+
+### Flush All Cached Connections
+```bash
+curl -X POST http://localhost:8100/api/nl43/_connections/flush
+```
+
 ### Verify Device Settings
 ```bash
 curl http://localhost:8100/api/nl43/meter-001/settings
@@ -388,11 +429,19 @@ See [API.md](API.md) for detailed integration examples.
 ## Troubleshooting
 
 ### Connection Issues
+- Check connection pool status: `curl http://localhost:8100/api/nl43/_connections/status`
+- Flush stale connections: `curl -X POST http://localhost:8100/api/nl43/_connections/flush`
 - Verify device IP address and port in configuration
 - Ensure device is on the same network
 - Check firewall rules allow TCP/FTP connections
 - Verify RX55 network adapter is properly configured on device
 
+### Cellular Modem Issues
+- If modem wedges from too many handshakes, ensure `TCP_PERSISTENT_ENABLED=true` (default)
+- Increase `TCP_IDLE_TTL` if connections expire between poll cycles
+- Keepalive probes (default: every 15s) keep NAT tables alive — adjust `TCP_KEEPALIVE_IDLE` if needed
+- Set `TCP_PERSISTENT_ENABLED=false` to disable pooling for debugging
+
 ### Rate Limiting
 - API automatically enforces 1-second delay between commands
 - If experiencing delays, this is normal device behavior

app/main.py

@@ -52,7 +52,7 @@ async def lifespan(app: FastAPI):
 app = FastAPI(
     title="SLMM NL43 Addon",
     description="Standalone module for NL43 configuration and status APIs with background polling",
-    version="0.2.0",
+    version="0.3.0",
     lifespan=lifespan,
 )