# Requirements Document: msghandler

**Version**: 1.2.0  
**Date**: 2026-05-13  
**Status**: Active  
**Ground Truth**: [`src/msghandler.jl`](../src/msghandler.jl)

---

## 1. Business Context & Success Metrics

### 1.1 Business Goal

msghandler is a cross-platform, bi-directional data bridge that enables seamless communication between **Julia**, **JavaScript**, **Python**, **Dart**, **Rust**, and **MicroPython** applications using a message broker as the transport layer. The system implements the **Claim-Check pattern** for efficient handling of large payloads (>0.5MB) by uploading them to an HTTP file server instead of sending raw binary data over the transport layer.

### 1.2 User Stories (with acceptance criteria)

| Story | Priority | Acceptance Criteria |
|-------|----------|---------------------|
| **As a Julia developer**, I want to send text messages to JavaScript/Dart applications that lives on a server and also on a browser | P1 | Text messages are serialized, encoded, and received correctly across platforms |
| **As a Python developer**, I want to send tabular data to Julia/Dart applications | P1 | DataFrame exchange works with both Arrow IPC and JSON formats |
| **As a JavaScript developer**, I want to send large files (>0.5MB) from JavaScript applications that lives on a server and also on a browser to other applications | P1 | Large files are automatically uploaded to file server and URLs are sent via the transport layer |
| **As a Dart developer**, I want to send text messages to other platforms | P1 | Text messages are serialized, encoded, and received correctly across platforms |
| **As a Dart developer**, I want to send dictionary data to other platforms | P1 | JSON-serializable data is exchanged correctly |
| **As a Dart developer**, I want to send tabular data (List<Map>) to other platforms | P1 | JSON table format exchange works with Arrow IPC on desktop |
| **As a Dart developer**, I want to send large files (>0.5MB) | P1 | Large files are automatically uploaded to file server and URLs are sent via the transport layer |
| **As a MicroPython developer**, I want to send sensor data with minimal memory usage | P1 | Direct transport works for payloads <100KB on memory-constrained devices |
| **As a Rust developer**, I want to send and receive messages with type-safe APIs | P1 | Rust implementation uses serde for serialization, tokio for async, and transport-agnostic client for connectivity |
| **As a developer**, I want to send mixed-content messages (text + image + file) | P1 | msghandler accepts list of (dataname, data, type) tuples and handles each payload appropriately |
| **As a developer**, I want to receive multi-payload messages | P1 | msghandler returns payloads as list of tuples with correct types preserved |
| **As a developer**, I want to use Plik as the file server | P2 | Plik one-shot upload mode is supported with upload ID and token handling |
| **As a developer**, I want to use custom HTTP file servers | P2 | Handler function abstraction allows plugging in AWS S3 or custom implementations |
| **As a developer**, I want automatic retry on file server download failures | P1 | Exponential backoff with configurable retries (default: 5, base_delay: 100ms, max_delay: 5000ms) |
| **As a developer**, I want message tracing across distributed systems | P1 | Correlation ID is propagated through all message processing steps |

### 1.3 KPIs & Targets

| Metric | Target | Measurement Method |
|--------|--------|-------------------|
| 95% of messages complete within 200ms | 95% | Synthetic monitoring |
| <2 days from onboarding to first PR | 2 days | PR timeline tracking |
| 100% of messages validate against spec | 100% | CI block rate |
| >80% unit test coverage | 80% | Test coverage tools |
| <1% of PRs bypass validation gates | 1% | CI gate analysis |
| MTTR <15 minutes for P1 incidents | 15 minutes | Incident tracking |

---

## 2. Technical Boundaries

### 2.1 In Scope

| Feature | Description |
|---------|-------------|
| Cross-platform interoperability | Seamless data exchange between Julia, JavaScript, Python, Dart, Rust, and MicroPython |
| Intelligent transport selection | Direct transport (<0.5MB) vs Link transport (≥0.5MB) based on payload size |
| Unified API | Consistent `smartpack()` and `smartunpack()` functions across all platforms |
| Multi-payload support | List of (dataname, data, type) tuples with appropriate handling |
| File server integration | Plik one-shot upload and custom HTTP server support |
| Reliability features | Exponential backoff retry and correlation ID propagation |
| Message serialization | Converts data types to binary format (Base64, JSON, Arrow IPC) |
| Transport communication | Publishing and subscription via message broker (NATS, MQTT, WebSocket, etc.) |

### 2.2 Out of Scope

| Feature | Reason |
|---------|--------|
| Advanced transport features | Basic transport sufficient for current use cases |
| Message compression | Compression adds complexity without clear benefit |
| Message encryption | Payload encryption is application-layer concern |
| Persistent message queues | Transport request-reply pattern sufficient |
| Advanced routing rules | Simple topic matching sufficient |

### 2.3 Dependencies

| Platform | Package | Version |
|----------|---------|---------|
| Julia | JSON.jl | Latest stable |
| Julia | Arrow.jl | Latest stable |
| Julia | HTTP.jl | Latest stable |
| Julia | UUIDs.jl | Latest stable |
| Node.js | node-fetch | Latest stable |
| Python | aiohttp | Latest stable |
| Python | pyarrow | Latest stable |
| Browser | - | Transport-agnostic (caller provides) |
| Dart | http | Latest stable |
| Dart | uuid | Latest stable |
| Rust | serde | Latest stable |
| Rust | serde_json | Latest stable |
| Rust | tokio | Latest stable |
| Rust | uuid | Latest stable |

### 2.4 Platform Compatibility

| Platform | Minimum Version | Notes |
|----------|-----------------|-------|
| Julia | 1.7+ | Arrow.jl required for arrowtable support |
| Node.js | 16+ | Transport client required, Arrow IPC supported |
| Python | 3.8+ | pyarrow required for arrowtable support |
| Browser | Latest | No Arrow IPC (uses jsontable only) |
| Dart | 2.17+ | Supports Desktop (Dart SDK), Flutter (Dart SDK), and Web (Dart SDK) |
| Rust | 1.70+ | Full support with async/await, Arrow IPC on desktop |
| MicroPython | 1.19+ | Limited to direct transport |

---

## 3. Functional Requirements (FR)

| ID | Requirement | Description |
|----|-------------|-------------|
| **FR-001** | Cross-platform text messaging | System shall allow users to send text messages between Julia, JavaScript, Python, and MicroPython applications |
| **FR-002** | Cross-platform tabular data | System shall support DataFrame exchange between Julia and Python applications using Arrow IPC format |
| **FR-003** | Large file handling | System shall automatically detect payloads ≥0.5MB and upload them to HTTP file server instead of sending via transport |
| **FR-004** | Direct transport for small payloads | System shall send payloads <0.5MB directly via transport without file server upload |
| **FR-005** | MicroPython support | System shall support payloads <100KB on MicroPython devices using direct transport |
| **FR-006** | Multi-payload messages | System shall accept and process lists of (dataname, data, type) tuples |
| **FR-007** | Payload type preservation | System shall preserve payload types when returning multi-payload messages |
| **FR-008** | Plik file server integration | System shall support Plik one-shot upload mode with upload ID and token handling |
| **FR-009** | Custom file server support | System shall provide handler function abstraction for custom HTTP file server implementations |
| **FR-010** | Exponential backoff retry | System shall implement exponential backoff with configurable retries (default: 5, base_delay: 100ms, max_delay: 5000ms) for file server download failures |
| **FR-011** | Correlation ID propagation | System shall propagate correlation IDs through all message processing steps |
| **FR-012** | Message serialization | System shall serialize data types using Base64, JSON, or Arrow IPC encoding |
| **FR-013** | Transport publishing | System shall return JSON string representation for caller to publish via transport layer (caller is responsible for actual transport publish) |
| **FR-014** | Transport subscription | System shall receive and process messages by accepting JSON string from transport payload |

---

## 4. Non-Functional Requirements (NFRs)

### 4.1 Performance & Scalability

| ID | Requirement | Specification | Test Method |
|----|-------------|---------------|-------------|
| **NFR-101** | Message serialization overhead | <50ms for 10KB payload | Benchmark tests |
| **NFR-102** | Message deserialization overhead | <50ms for 10KB payload | Benchmark tests |
| **NFR-103** | Transport connection establishment | <100ms | Connection pool benchmarks |
| **NFR-104** | File upload latency | <1s for 0.5MB file | Integration tests |
| **NFR-105** | File download latency | <1s for 0.5MB file | Integration tests |
| **NFR-106** | Concurrent connections | Support 100+ simultaneous transport connections | Scale testing |
| **NFR-107** | Message throughput | Handle 1000+ messages/second per instance | Load testing |
| **NFR-108** | File server scalability | Support horizontal scaling of file server backend | Architecture review |

### 4.2 Availability & Reliability

| ID | Requirement | Specification |
|----|-------------|---------------|
| **NFR-201** | Message delivery | At-least-once delivery semantics via transport |
| **NFR-202** | File server availability | Graceful degradation when file server is unavailable |
| **NFR-203** | Connection recovery | Auto-reconnect on transport connection failure |

### 4.3 Privacy & Security

| ID | Requirement | Specification |
|----|-------------|---------------|
| **NFR-301** | Payload integrity | SHA-256 checksum support via metadata |
| **NFR-302** | Transport security | TLS support for transport connections |
| **NFR-303** | File server security | Authentication token for file uploads |

### 4.4 Observability & Telemetry

| ID | Requirement | Specification |
|----|-------------|---------------|
| **NFR-401** | Required logs | `correlation_id`, `msg_id`, `timestamp`, `sender_name`, `receiver_name`, `payload_type`, `transport` |
| **NFR-402** | Critical metrics | `messages_sent_total`, `messages_received_total`, `file_upload_duration_seconds`, `file_download_duration_seconds`, `retry_attempts_total` |
| **NFR-403** | Tracing | Correlation ID propagation for request tracing |
| **NFR-404** | Alerting | `download_retry_exceeded` triggers alert when max retries exceeded |
| **NFR-405** | Retention | Logs: 30 days, Metrics: 1 year |

---

## 5. Acceptance Conditions

| Condition | Description |
|-----------|-------------|
| **AC-001** | All functional requirements FR-001 through FR-014 are implemented and tested |
| **AC-002** | All non-functional requirements NFR-101 through NFR-405 meet specified targets |
| **AC-003** | Cross-platform text message test passes (Julia ↔ JavaScript ↔ Python) |
| **AC-004** | Cross-platform tabular data test passes with Arrow IPC round-trip (Desktop) |
| **AC-005** | Cross-platform tabular data test passes with JSON table round-trip (Browser) |
| **AC-006** | Large file transfer test passes with file server upload/download |
| **AC-007** | Multi-payload mixed content test passes with all payload types in one message |
| **AC-008** | CI validation gates block PRs on specification violations |
| **AC-009** | Unit test coverage exceeds 80% |
| **AC-010** | Documentation is complete and includes walkthroughs, architecture, and runbook |

---

## 6. Payload Type Requirements

### 6.1 Supported Payload Types

| Type | Julia | JavaScript | Python | Dart | MicroPython | Description |
|------|-------|------------|--------|------|-------------|-------------|
| `text` | `String` | `string` | `str` | `String` | `String` | `str` | Plain text strings |
| `dictionary` | `Dict`, `NamedTuple` | `Object`, `Array` | `dict`, `list` | `Map`, `serde_json::Value` | `String` | `dict` | JSON-serializable data |
| `arrowtable` | `DataFrame`, `Arrow.Table` | ❌ (Browser), ✅ (Node.js) | `pandas.DataFrame` | `List<Map>` (Desktop), `List<dynamic>` (Flutter) | `arrow2::Table` | ❌ | Tabular data (Arrow IPC) |
| `jsontable` | `Vector{NamedTuple}` | `Array<Object>` | `list[dict]` | `Vec<Map>` | ⚠️ | Tabular data (JSON) - **Only table type in Browser** |
| `image` | `Vector{UInt8}` | `Uint8Array`, `Buffer` | `bytes` | `Uint8List` | `Vec<u8>` | `bytearray` | Image binary data |
| `audio` | `Vector{UInt8}` | `Uint8Array`, `Buffer` | `bytes` | `Uint8List` | `Vec<u8>` | `bytearray` | Audio binary data |
| `video` | `Vector{UInt8}` | `Uint8Array`, `Buffer` | `bytes` | `Uint8List` | `Vec<u8>` | `bytearray` | Video binary data |
| `binary` | `Vector{UInt8}`, `IOBuffer` | `Uint8Array`, `Buffer` | `bytes`, `bytearray` | `Uint8List` | `Vec<u8>` | `bytearray` | Generic binary data |

### 6.2 Encoding Requirements

| Payload Type | Encoding Method | Notes |
|--------------|-----------------|-------|
| `text` | UTF-8 → Base64 | Text must be String type |
| `dictionary` | JSON → Base64 | JSON.jl for Julia |
| `arrowtable` | Arrow IPC → Base64 | Requires Arrow.jl/pyarrow (Desktop only) |
| `jsontable` | JSON → Base64 | Human-readable format - **Browser uses this only** |
| `image`/`audio`/`video`/`binary` | Direct → Base64 | Binary data preserved |

---

## 7. Size Threshold Requirements

### 7.1 Direct Transport Threshold

| Platform | Threshold | Notes |
|----------|-----------|-------|
| Desktop (Julia/JS/Python/Dart) | 0.5MB | Default size threshold |
| Dart Desktop | 0.5MB | Default size threshold |
| Dart Flutter | 0.5MB | Default size threshold |
| Dart Web | 0.5MB | Default size threshold |
| Rust | 0.5MB | Default size threshold |
| MicroPython | 100KB | Lower threshold for memory constraints |

### 7.2 Maximum Payload Size

| Platform | Maximum | Notes |
|----------|---------|-------|
| Desktop | Unlimited | Limited by transport server configuration |
| Dart Desktop | Unlimited | Limited by transport server configuration |
| Dart Flutter | Unlimited | Limited by transport server configuration |
| Dart Web | Unlimited | Limited by transport server configuration |
| Rust | Unlimited | Limited by transport server configuration |
| MicroPython | 50KB | Hard limit due to 256KB-1MB memory |

---

## 8. Message Envelope Requirements

### 8.1 Required Fields

| Field | Type | Purpose |
|-------|------|---------|
| `correlation_id` | String (UUID) | Track message flow across systems |
| `msg_id` | String (UUID) | Unique message identifier |
| `timestamp` | String (ISO 8601) | Message publication timestamp |
| `send_to` | String | Topic/subject to publish to |
| `msg_purpose` | String | ACK, NACK, updateStatus, shutdown, chat |
| `sender_name` | String | Sender application name |
| `sender_id` | String (UUID) | Sender unique identifier |
| `receiver_name` | String | Receiver application name (empty = broadcast) |
| `receiver_id` | String (UUID) | Receiver unique identifier (empty = broadcast) |
| `reply_to` | String | Topic for reply messages |
| `reply_to_msg_id` | String | Message ID being replied to |
| `broker_url` | String | Broker URL for the transport layer |
| `metadata` | Dict | Message-level metadata |
| `payloads` | Array | List of payload objects |

### 8.2 Payload Fields

| Field | Type | Purpose |
|-------|------|---------|
| `id` | String (UUID) | Unique payload identifier |
| `dataname` | String | Name of the payload |
| `payload_type` | String | Type: text, dictionary, arrowtable, etc. |
| `transport` | String | direct or link |
| `encoding` | String | none, json, base64, arrow-ipc |
| `size` | Integer | Payload size in bytes |
| `data` | Any | Base64 string or URL |
| `metadata` | Dict | Payload-level metadata |

---

## 9. Error Handling Requirements

### 9.1 Error Codes

| Error | Condition | Response |
|-------|-----------|----------|
| `Unknown payload_type` | Unsupported type | Throw error |
| `Failed to upload` | File server error | Throw error |
| `Failed to fetch` | File server unavailable | Retry with exponential backoff |
| `Unknown transport` | Invalid transport type | Throw error |
| `Transport connection failed` | Transport/broker unavailable | Throw error |

### 9.2 Exception Handling

| Scenario | Handler |
|----------|---------|
| File server unavailable | Retry up to 5 times with exponential backoff |
| Transport publish failure | Handled by caller |
| Deserialization error | Log correlation ID and throw error |
| Memory overflow (MicroPython) | Reject payloads >50KB |

---

## 10. Testing Requirements

### 10.1 Unit Tests

| Test Category | Coverage | Files |
|---------------|----------|-------|
| Serialization | All payload types | `test/test_*_sender.*` |
| Deserialization | All payload types | `test/test_*_receiver.*` |
| Transport selection | Direct vs link | `test/test_*_mix_payloads.*` |
| File server upload | Plik integration | Platform-specific |
| File server download | Exponential backoff | Platform-specific |

### 10.2 Integration Tests

| Test Scenario | Success Criteria |
|-------------|-----------------|
| Cross-platform text message | Julia ↔ JavaScript ↔ Python |
| Cross-platform tabular data (Desktop) | Arrow IPC round-trip |
| Cross-platform tabular data (Browser) | JSON table round-trip |
| Large file transfer | File server upload/download |
| Multi-payload mixed content | All payload types in one message |

---

## 11. API Contract

### 11.1 smartpack Signature

```julia
function smartpack(
    subject::String,
    data::AbstractArray{Tuple{String, T1, String}, 1};
    broker_url::String = DEFAULT_BROKER_URL,
    fileserver_url::String = DEFAULT_FILESERVER_URL,
    fileserver_upload_handler::Function = plik_oneshot_upload,
    size_threshold::Int = DEFAULT_SIZE_THRESHOLD,
    correlation_id::String = string(uuid4()),
    msg_purpose::String = "chat",
    sender_name::String = "msghandler",
    receiver_name::String = "",
    receiver_id::String = "",
    reply_to::String = "",
    reply_to_msg_id::String = "",
    msg_id::String = string(uuid4()),
    sender_id::String = string(uuid4())
)::Tuple{msg_envelope_v1, String} where {T1<:Any}
```

**Note**: Publishing via the transport layer is the caller's responsibility. `smartpack` returns `(env::msg_envelope_v1, env_json_str::String)`.

### 11.2 smartunpack Signature

```julia
function smartunpack(
    msg_json_str::String;
    fileserver_download_handler::Function = _fetch_with_backoff,
    max_retries::Int = 5,
    base_delay::Int = 100,
    max_delay::Int = 5000
)::JSON.Object{String, Any}
```

**Note**: Pass the payload string from the transport subscription to `smartunpack`. The input is the JSON string payload from the transport message, not the transport message object directly.

**Note**: Pass the payload from the transport subscription to `smartunpack`.

---

## 12. Deployment Requirements

### 12.1 Minimum Infrastructure

| Component | Minimum | Notes |
|-----------|---------|-------|
| Message Broker | 1 instance | Single node for development |
| File Server | 1 instance | HTTP server for large payloads |
| Client Memory | 50MB | Desktop platforms (Julia/JS/Python/Dart) |
| Client Memory | 256KB | MicroPython devices |

### 12.2 Environment Variables

| Variable | Default | Description |
|----------|---------|-------------|
| `BROKER_URL` | `ws://localhost:4222` | Message broker URL |
| `FILESERVER_URL` | `http://localhost:8080` | HTTP file server URL |
| `SIZE_THRESHOLD` | `500000` | Size threshold in bytes (0.5MB) |

---

## 13. Versioning

### 13.1 Current Version

- **Major**: 1 (Breaking changes require major version bump)
- **Minor**: 0 (Feature additions)
- **Patch**: 0 (Bug fixes)

### 13.2 Version Compatibility

| Version | Supported Platforms |
|---------|---------------------|
| v1.0.x | Julia 1.7+, Node.js 16+, Python 3.8+, Dart 2.17+, Rust 1.70+, Browser (latest), MicroPython 1.19+ |

---

## 14. Change Log

| Date | Version | Changes |
|------|---------|---------|
| 2026-05-15 | 1.3.0 | Made transport layer agnostic |
| - | - | Removed all NATS-specific dependencies and references |
| - | - | Updated all NATS references to generic "transport layer"/"message broker" |
| - | - | Removed NATS client packages from dependencies tables |
| 2026-05-13 | 1.2.0 | Aligned with ground truth implementation (src/msghandler.jl) |
| - | - | Fixed smartpack signature: removed is_publish, NATS_connection; added sender_name |
| - | - | Fixed smartunpack signature: takes msg_json_str::String instead of msg::NATS.Msg |
| - | - | Fixed size_threshold default from 1,000,000 to 500,000 |
| - | - | Updated FR-013/FR-014 to reflect caller responsibility for NATS publishing |
| - | - | Updated FR-008/FR-009 to include file path upload overload |
| - | - | Updated SIZE_THRESHOLD env var default to 500000 |
| 2026-03-23 | 1.0.0 | Updated to ASG Framework requirements structure |

---

## 15. References

- [`src/msghandler.jl`](../src/msghandler.jl) - Ground truth implementation (Julia)
- [`src/msghandler_ssr.js`](../src/msghandler_ssr.js) - Server-side JavaScript implementation
- [`src/msghandler_csr.js`](../src/msghandler_csr.js) - Client-side JavaScript implementation
- [`src/msghandler.py`](../src/msghandler.py) - Python implementation
- [`src/msghandler.dart`](../src/msghandler.dart) - Dart implementation
- [`src/msghandler_mpy.py`](../src/msghandler_mpy.py) - MicroPython implementation
- [`src/msghandler.rs`](../src/msghandler.rs) - Rust implementation
- [`README.md`](../README.md) - Project overview
- [`docs/specification.md`](./specification.md) - Technical specification
- [`docs/ui-specification.md`](./ui-specification.md) - UI specification
- [`docs/walkthrough.md`](./walkthrough.md) - End-to-end walkthrough
- [`docs/architecture.md`](./architecture.md) - Architecture documentation
- [`docs/validation.md`](./validation.md) - Validation and CI/CD
- [`docs/runbook.md`](./runbook.md) - Operational runbook