398 lines
21 KiB
Markdown
398 lines
21 KiB
Markdown
# Solution Design: msghandler
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**Version**: 1.3.0
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**Date**: 2026-05-22
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**Status**: Active
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**Ground Truth**: [`src/msghandler.jl`](../src/msghandler.jl)
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**ASG Framework Alignment**: v8 pillars - Requirements → Solution Design → Specification → Walkthrough → Implementation Plan → Validation → Runbook
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---
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## 1. Problem Decomposition
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msghandler addresses the challenge of cross-platform data exchange between **Julia**, **JavaScript**, **Python**, **Dart**, **Rust**, and **MicroPython** applications using message brokers as transport layers.
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### User Problems
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| Problem | Description | User Impact | Requirement ID |
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|---------|-------------|-------------|----------------|
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| **P-001**: Cross-platform data serialization | Different languages have incompatible data types and serialization formats | Developers must write platform-specific conversion code | FR-001, FR-002 |
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| **P-002**: Large payload handling | Message brokers have size limits, but large files need to be transferred | Large files either fail or require complex workarounds | FR-003 |
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| **P-003**: Transport abstraction | Each platform has different message broker libraries and APIs | No unified interface across platforms | FR-013, FR-014 |
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| **P-004**: Request-response patterns | Bi-directional communication requires complex correlation tracking | Developers must implement custom message routing | FR-011 |
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| **P-005**: File server reliability | File server may be temporarily unavailable during downloads | Failed downloads without retry mechanism | FR-010 |
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| **P-006**: Payload type preservation | Different platforms have different type systems | Data corruption or misinterpretation on receiving end | FR-006, FR-007 |
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### Solution Boundaries
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**In Scope**:
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- Unified API for `smartpack()` and `smartunpack()` across all platforms
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- Automatic transport selection based on payload size
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- File server integration using Claim-Check pattern
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- Multi-payload support with mixed types in single message
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- Exponential backoff for reliable file downloads
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- Correlation ID propagation for message tracing
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**Out of Scope**:
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- Message compression (adds complexity without clear benefit)
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- Message encryption (application-layer concern)
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- Advanced message routing (simple topic matching sufficient)
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- Persistent message queues (transport pattern sufficient)
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### Decision IDs
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| Decision ID | Decision | Description | Requirement IDs | NFR IDs |
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|-------------|----------|-------------|-----------------|---------|
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| SD-001 | Claim-Check Pattern | Large payloads uploaded to HTTP server, small payloads sent directly | FR-003, FR-004 | NFR-104, NFR-105 |
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| SD-002 | Automatic Transport Selection | <0.5MB = direct, ≥0.5MB = link based on size threshold | FR-003, FR-004 | NFR-104, NFR-105 |
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| SD-003 | Handler Function Abstraction | Pluggable file server implementations via handler functions | FR-008, FR-009 | NFR-202 |
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| SD-004 | Unified Tuple Format | Same `(dataname, data, type)` format across all platforms | FR-006, FR-007 | - |
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| SD-005 | Base64 Encoding | JSON-compatible binary data transport | FR-012 | - |
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| SD-006 | Transport Abstraction | Support multiple broker protocols (NATS/MQTT/WebSocket) transparently | FR-013, FR-014 | NFR-201 |
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| SD-007 | Exponential Backoff | Retry failed file downloads with exponential backoff | FR-010 | NFR-202 |
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| SD-008 | Correlation ID Propagation | Propagate correlation IDs through all message processing steps | FR-011 | NFR-401, NFR-403 |
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---
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## 2. Solution Approach
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msghandler implements a **Claim-Check pattern** with intelligent transport selection:
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```
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Sender (smartpack) Transport Layer Receiver (smartunpack)
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┌─────────────────┐ ┌───────────────┐ ┌───────────────────┐
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│ │ │ │ │ │
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│ 1. Data tuples │────────────>│ │───────────>│ 1. Parse envelope │
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│ [(name, │ JSON │ Message │ JSON │ 2. Check transport│
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│ data, type)]│ format │ Broker │ format │ 3. Fetch/Decode │
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│ │ │ (NATS/MQTT/ │ │ 4. Return tuples │
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└─────────────────┘ │ WebSocket) │ │ │
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│ │ └───────────────────┘
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└───────────────┘
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```
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### Key Design Decisions
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| Decision ID | Decision | Rationale | Alternatives Rejected |
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|-------------|----------|-----------|----------------------|
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| **SD-001** | Claim-Check Pattern | Large payloads (>0.5MB) uploaded to HTTP server, small payloads sent directly via transport | Client-side compression - adds complexity; Server-side compression - not universally supported |
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| **SD-002** | Automatic Transport Selection | <0.5MB = direct (fast), ≥0.5MB = link (avoid transport limits) | Manual selection - error-prone; Fixed threshold - not adaptive |
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| **SD-003** | Handler Function Abstraction | Allows pluggable file server implementations (Plik, AWS S3, custom) | Hardcoded Plik - not flexible; Interface-based - too complex for this use case |
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| **SD-004** | Unified Tuple Format | Same input/output format across all platforms | Platform-native formats - no interoperability; Protocol buffers - too heavy |
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| **SD-005** | Base64 Encoding | JSON-compatible binary data transport | Raw bytes - not JSON-compatible; Hex encoding - 2x size overhead |
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| **SD-006** | Transport Abstraction | Support multiple broker protocols (NATS/MQTT/WebSocket) transparently | Platform-specific libraries - no interoperability |
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| **SD-007** | Exponential Backoff | Retry failed file downloads with exponential backoff | Simple retry - too aggressive; No retry - poor reliability |
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| **SD-008** | Correlation ID Propagation | Propagate correlation IDs through all message processing steps | Manual correlation - error-prone; No tracing - debug impossible |
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### Architecture Components
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```mermaid
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flowchart TB
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subgraph Client["Client Application"]
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direction TB
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APP["Application Code"]
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API["msghandler API"]
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APP -->|Data tuples| API
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API -->|JSON envelope| TRANSPORT
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end
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subgraph Transport["Transport Layer"]
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direction TB
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BROKER["Message Broker<br/>NATS/MQTT/WebSocket"]
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TOPICS["Topic Subscription"]
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API -->|Publish| BROKER
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BROKER -->|Deliver| TOPICS
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TOPICS -->|Subscribe| API
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end
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subgraph FileServer["File Server"]
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direction TB
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UPLOAD["Upload Handler"]
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DOWNLOAD["Download Handler"]
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API -.->|Upload URL| UPLOAD
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DOWNLOAD -.->|Fetch URL| API
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end
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style Client fill:#e1f5fe,stroke:#0288d1,stroke-width:2px
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style Transport fill:#ffe0b2,stroke:#f57c00,stroke-width:2px
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style FileServer fill:#c8e6c9,stroke:#43a047,stroke-width:2px
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```
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---
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## 3. Alternatives Considered
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| Alternative | Pros | Cons | Decision |
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|-------------|------|------|----------|
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| **gRPC/Protobuf** | Strong typing, efficient binary format | No native MicroPython support; Complex schema management | Rejected - not cross-platform enough |
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| **MessagePack** | Compact binary, good performance | Browser support limited; No standard for tabular data | Rejected - missing Arrow IPC alternative |
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| **Protocol Buffers** | Type-safe, efficient | No native support for tabular data exchange | Rejected - cannot represent DataFrames natively |
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| **REST HTTP Upload** | Simple, universal | High latency; No real-time capability | Rejected - not suitable for message broker pattern |
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| **Hybrid (direct/link)** | Optimal for both small and large payloads | More complex implementation | Accepted - matches user requirements (FR-003, FR-004) |
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| **Single transport type** | Simpler implementation | Cannot handle large payloads efficiently | Rejected - violates FR-003 requirement |
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| **Platform-specific APIs** | Native performance | No interoperability; Maintenance burden | Rejected - violates cross-platform goal |
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---
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## 4. High-Level Component Diagram
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```mermaid
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flowchart TD
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subgraph msghandler["msghandler Core Module"]
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direction TB
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subgraph Serialization["Serialization Layer"]
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DIR["Direct Transport"]
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LNK["Link Transport"]
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DIR -->|Base64| JSON_MSG
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LNK -->|HTTP URL| JSON_MSG
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end
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subgraph Envelope["Envelope Builder"]
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HDR["Message Header"]
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PAY["Payload Manager"]
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HDR --> PAY
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end
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subgraph Handlers["Handler Functions"]
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UPD["Upload Handler"]
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DWN["Download Handler"]
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UPD --> LNK
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DWN --> LNK
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end
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API["smartpack() / smartunpack()"]
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API -->|Input| Serialization
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API -->|Output| Serialization
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API -->|Configure| Handlers
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end
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subgraph Transport["Transport Layer"]
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BROKER["NATS / MQTT / WebSocket"]
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API -->|JSON| BROKER
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BROKER -->|JSON| API
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end
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subgraph FileServer["File Server"]
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Plik["HTTP Server"]
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UPD -.->|POST| Plik
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Plik -.->|URL| DWN
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end
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style msghandler fill:#b3e5fc,stroke:#0288d1,stroke-width:2px
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style Transport fill:#ffe0b2,stroke:#f57c00,stroke-width:2px
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style FileServer fill:#c8e6c9,stroke:#43a047,stroke-width:2px
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```
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### Component Responsibilities
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| Component | Responsibilities | Decision IDs | Requirements Addressed |
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|-----------|-----------------|--------------|----------------------|
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| **Serialization Layer** | Convert data types to transport format (Base64/URL) | SD-005 | FR-001, FR-002, FR-012 |
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| **Envelope Builder** | Create standardized message envelope with metadata | SD-001, SD-008 | FR-011, FR-013, FR-014 |
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| **Handler Functions** | Abstract file server operations for pluggability | SD-003, SD-007 | FR-008, FR-009, FR-010 |
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| **Transport Adapter** | Support multiple broker protocols transparently | SD-006 | FR-013, FR-014 |
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| **Payload Manager** | Track payload types, sizes, and encoding | SD-004 | FR-006, FR-007 |
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---
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## 5. Decision Rationale
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### SD-001: Why Claim-Check Pattern?
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**Requirement**: FR-003 (Large file handling), FR-004 (Direct transport for small payloads)
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**NFRs**: NFR-104 (File upload latency <1s), NFR-105 (File download latency <1s)
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**Rationale**:
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- Transport layers (NATS, MQTT) have message size limits (typically 1MB)
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- Direct transport is faster for small payloads (no file server round-trip)
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- Link transport avoids transport limits for large payloads
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- User doesn't need to manually choose - automatic selection based on threshold
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### SD-002: Why Handler Functions for File Server?
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**Requirement**: FR-008 (Plik integration), FR-009 (Custom file server support)
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**NFR**: NFR-202 (File server availability <5% failure rate)
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**Rationale**:
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- Plik is common open-source solution for file server
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- Some users need AWS S3 or custom implementation
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- Handler functions provide clean abstraction without vendor lock-in
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- Same signature across all platforms (unified API)
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### SD-003: Why Tuple Format for Payloads?
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**Requirement**: FR-006 (Multi-payload messages), FR-007 (Payload type preservation)
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**Rationale**:
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- `(dataname, data, type)` tuple is language-agnostic
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- Simple to understand: name, content, type
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- Supports mixed payload types in single message
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- Easy to serialize/deserialize across platforms
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### SD-004: Why Base64 Encoding?
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**Requirement**: FR-012 (Message serialization), FR-001 (Cross-platform text messaging)
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**Rationale**:
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- JSON is universal - works on all platforms
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- Base64 converts binary to ASCII for JSON compatibility
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- Standard format with native support in all languages
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- No additional dependencies needed
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### SD-005: Why Automatic Transport Selection?
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**Requirement**: FR-003 (Large file handling), FR-004 (Direct transport for small payloads)
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**NFRs**: NFR-104 (File upload latency <1s), NFR-105 (File download latency <1s)
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**Rationale**:
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- <0.5MB payloads use direct transport (<1s latency, FR-004 KPI)
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- ≥0.5MB payloads use link transport to avoid transport limits (FR-003 KPI: 99% successful uploads)
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- User doesn't need to manually choose - automatic selection based on threshold
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### SD-006: Why Transport Abstraction?
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**Requirement**: FR-013 (Transport publishing), FR-014 (Transport subscription)
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**NFR**: NFR-201 (Message delivery at-least-once)
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**Rationale**:
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- Support multiple broker protocols (NATS, MQTT, WebSocket) transparently
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- Caller handles actual transport publishing/subscription
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- Unified API across all platforms
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- At-least-once delivery semantics via transport layer
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### SD-007: Why Exponential Backoff?
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**Requirement**: FR-010 (Exponential backoff retry)
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**NFR**: NFR-202 (File server availability <5% failure rate)
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**Rationale**:
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- File server may be temporarily unavailable
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- Exponential backoff prevents overwhelming server during outages
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- Default: 5 retries, 100ms base delay, 5000ms max delay
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- 95% successful downloads within retry limit (FR-010 KPI)
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### SD-008: Why Correlation ID Propagation?
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**Requirement**: FR-011 (Correlation ID propagation)
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**NFRs**: NFR-401 (Required logs), NFR-403 (Tracing)
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**Rationale**:
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- Trace messages across distributed systems
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- Correlation ID logged with every message (NFR-401)
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- Propagated through all message processing steps (NFR-403)
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- Enables debugging and performance analysis in production
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---
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## 6. Risk Assessment
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| Risk | Impact | Probability | Mitigation | Requirement IDs | NFR IDs |
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|------|--------|-------------|------------|-----------------|---------|
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| **Performance degradation with >500KB payloads** | High | Medium | Size threshold detection; Link transport fallback | FR-003, FR-004 | NFR-104, NFR-105 |
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| **File server availability issues** | Medium | Low | Exponential backoff retry; Graceful degradation | FR-010 | NFR-202 |
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| **Platform-specific bugs** | Medium | Low | Comprehensive test suite per platform; CI validation | FR-001, FR-002, FR-006, FR-007 | - |
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| **Encoding mismatches between platforms** | High | Low | Strict specification; Test contracts; Validation rules | FR-012 | NFR-301 |
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| **Transport layer incompatibility** | Medium | Low | Transport-agnostic design; Handler abstraction | FR-013, FR-014 | NFR-201 |
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| **Correlation ID loss in processing** | Medium | Low | Centralized trace context management | FR-011 | NFR-401, NFR-403 |
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---
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## 7. Requirements Traceability
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| Solution Component | Decision ID | Requirement ID | Description |
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|-------------------|-------------|----------------|-------------|
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| **smartpack() function** | SD-001, SD-002, SD-004, SD-005, SD-006, SD-008 | FR-001, FR-002, FR-003, FR-004, FR-005, FR-006, FR-007, FR-008, FR-009, FR-010, FR-011, FR-012, FR-013, FR-014 | Unified API for sending messages across all platforms |
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| **smartunpack() function** | SD-001, SD-002, SD-004, SD-005, SD-006, SD-007, SD-008 | FR-001, FR-002, FR-003, FR-004, FR-005, FR-006, FR-007, FR-008, FR-009, FR-010, FR-011, FR-012, FR-013, FR-014 | Unified API for receiving messages across all platforms |
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| **Direct transport** | SD-002 | FR-004 | Send payloads < threshold directly via transport |
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| **Link transport** | SD-001, SD-002 | FR-003 | Upload payloads ≥ threshold to file server |
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| **File server handler** | SD-003, SD-007 | FR-008, FR-009, FR-010 | Pluggable upload/download handlers with retry logic |
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| **Payload type preservation** | SD-004 | FR-006, FR-007 | Support text, dictionary, arrowtable, jsontable, image, audio, video, binary |
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| **Correlation ID propagation** | SD-008 | FR-011 | Message tracing across distributed systems |
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| **Multi-payload support** | SD-004 | FR-006, FR-007 | List of (dataname, data, type) tuples |
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### Non-Functional Requirements Traceability
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| Solution Component | Decision ID | NFR ID | Description |
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|-------------------|-------------|--------|-------------|
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| **Serialization optimization** | SD-005 | NFR-101, NFR-102 | <50ms overhead for 10KB payloads |
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| **Transport efficiency** | SD-006 | NFR-103 | <100ms connection establishment |
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| **File server latency** | SD-001, SD-002 | NFR-104, NFR-105 | <1s upload/download for 0.5MB files |
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| **Concurrent connections** | SD-006 | NFR-106 | Support 100+ simultaneous connections |
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| **Message throughput** | SD-005, SD-006 | NFR-107 | Handle 1000+ messages/second per instance |
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| **At-least-once delivery** | SD-006 | NFR-201 | Transport layer semantics |
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| **Graceful degradation** | SD-003, SD-007 | NFR-202 | File server unavailability handling |
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| **Auto-reconnect** | SD-006 | NFR-203 | Transport connection failure recovery |
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| **Payload integrity** | SD-005 | NFR-301 | 100% SHA-256 checksum validation |
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| **Transport security** | SD-006 | NFR-302 | 100% TLS connections in production |
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| **File server security** | SD-003 | NFR-303 | 100% authenticated file uploads |
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| **Required logs** | SD-001, SD-008 | NFR-401 | Correlation ID, msg_id, timestamp, etc. |
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| **Critical metrics** | SD-001, SD-005 | NFR-402 | messages_sent_total, file upload/download duration |
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| **Tracing** | SD-001, SD-008 | NFR-403 | Correlation ID propagation |
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| **Alerting** | SD-007 | NFR-404 | <5min alert latency for `download_retry_exceeded` |
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---
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## 8. Gap-Check Validation
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| Stage Transition | Gap-Check Question | Status |
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|------------------|-------------------|--------|
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| **Requirements → Solution Design** | Does the Solution Design clearly explain how the system solves the user problem, not just what it does? | ✅ Verified - All user problems mapped to solution components with requirement ID and decision ID references |
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| **Solution Design → Specification** | Does the Specification define all technical details that the solution approach requires? | ⏳ Pending - Specification needs review for completeness |
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| **Solution Design → Walkthrough** | Does the Walkthrough reflect the complete flow including error states and timing? | ⏳ Pending - Walkthrough needs validation against design |
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### Solution Design Validation
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**User Problems** (from requirements.md):
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- **P-001**: Cross-platform data serialization (FR-001, FR-002)
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- **P-002**: Large payload handling (FR-003)
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- **P-003**: Transport abstraction (FR-013, FR-014)
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- **P-004**: Request-response patterns (FR-011)
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- **P-005**: File server reliability (FR-010)
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- **P-006**: Payload type preservation (FR-006, FR-007)
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**Solution Components**:
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1. **SD-001** - `smartpack()` / `smartunpack()` - Unified API for all platforms
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2. **SD-002** - Claim-Check pattern - Automatic transport selection based on size threshold
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3. **SD-003** - Handler function abstraction - Plik/AWS S3/custom file server support
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4. **SD-004** - Tuple format - `(dataname, data, type)` - platform-agnostic
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5. **SD-005** - Base64 encoding - JSON-compatible binary data transport
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6. **SD-006** - Transport abstraction - Support multiple broker protocols transparently
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7. **SD-007** - Exponential backoff - Reliable file downloads with retry logic
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8. **SD-008** - Correlation ID propagation - Message tracing across distributed systems
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**Requirement Mapping**:
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- **Functional Requirements**: FR-001 through FR-014 ✅
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- **Non-Functional Requirements**: NFR-101 through NFR-405 ✅
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**Gap Check**: Does this solution explain *how* users will actually use the system?
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**Answer**: Yes - the walkthrough provides concrete examples:
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1. JavaScript sends `[(msg, "Hello", "text"), (avatar, binary_data, "image")]`
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2. `smartpack()` automatically selects transport based on size (SD-002)
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3. Large file (≥0.5MB) → link transport → file server upload (SD-001)
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4. Small payload (<0.5MB) → direct transport → base64 encoding (SD-005)
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5. Receiver calls `smartunpack()` → receives same tuple format with preserved types
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**NFR Traceability**:
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- **Performance**: NFR-101 (serialization <50ms), NFR-102 (deserialization <50ms), NFR-103 (connection <100ms) ✅
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- **Reliability**: NFR-201 (at-least-once delivery), NFR-202 (file server <5% failure), NFR-203 (auto-reconnect <30s) ✅
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- **Security**: NFR-301 (SHA-256 checksum), NFR-302 (TLS 100%), NFR-303 (authenticated uploads) ✅
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- **Observability**: NFR-401 (required logs), NFR-402 (metrics), NFR-403 (tracing), NFR-404 (alerting <5min) ✅
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---
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*This solution design document is versioned and maintained in git alongside the codebase. All implementations must adhere to this design.*
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**Traceability Summary**:
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- All requirements traced to solution components with SD-XXX decision IDs
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- Each decision ID references the corresponding requirement IDs (FR-XXX, NFR-XXX)
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- Specification must cite SD-XXX references for each technical detail
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