咨询：搭建支持SSE订阅与MQTT式通配符的HTTP Pub/Sub服务器

Great question! Building an HTTP-based Pub/Sub system with SSE subscriptions and MQTT-style wildcards is a smart pick for real-time web apps—let’s walk through how to pull this off, plus some existing tools you can use instead of building from scratch.

Core Architecture Design #

Start with a straightforward three-part flow to keep things maintainable:

• Publisher Flow: Accept HTTP POST requests (either to /publish/{channel} or with the channel specified in the request body). Validate the request, extract the target channel and message content, then pass it to your broker for routing.
• Subscriber Flow: Use EventSource (SSE) for long-lived, low-overhead connections. Subscribers connect to /subscribe/{topic-filter} where the filter can include MQTT-style wildcards. Your broker needs to track which filters map to active SSE connections.
• Broker Core: The central brain that manages:
  • A registry of active subscribers (mapping filter strings to lists of open SSE connections)
  • A message routing engine that matches incoming message channels to subscriber filters

MQTT Wildcard Matching Logic #

You’ll need to implement the two standard MQTT wildcard rules—here’s how to translate them into code:

• Single-level wildcard (+): Matches exactly one segment in a channel hierarchy. For example, home/+/temp matches home/kitchen/temp but not home/upstairs/bedroom/temp.
• Multi-level wildcard (#): Matches all remaining segments from its position onward, and must be the last character in the filter. For example, home/# matches every channel under home, whether that’s home/livingroom/light or home/garage/door/status.

Matching Implementation Tips #

1. Pre-process both incoming channels and subscriber filters by splitting them into segments using / as the delimiter (do this once when a subscriber connects to save time during message routing).
2. Iterate through each segment pair to check for matches:
   • If the filter segment is #, immediately return a match (it covers all remaining segments).
   • If the filter segment is +, skip to the next pair (it matches any single segment).
   • If the segments don’t match, reject the filter.
3. Ensure the segment counts match unless the filter ends with # (handled in the first bullet above).

Key Components to Build #

• Connection Manager: Track active SSE connections, detect disconnections (use SSE’s built-in ping frames or implement a custom heartbeat), and clean up stale connections to avoid memory leaks.
• Message Router: When a publish request hits, split the target channel into segments, then loop through all subscriber filters to find matches. For each match, send the message via the SSE connection using the required format: data: {your-message-content}\n\n.
• API Endpoints:
  • POST /publish: Accepts JSON with channel and message fields (e.g., {"channel": "home/kitchen/temp", "message": "22.5"}). Return clear HTTP status codes (200 OK for success, 400 Bad Request for invalid input, 401 Unauthorized if auth is required).
  • GET /subscribe/{filter}: Establishes the SSE connection, sends a welcome message to confirm the subscription, then waits to forward matching messages.

Critical Considerations #

• Scalability: If you need to handle thousands of subscribers, use a distributed backend like Redis Pub/Sub to sync messages across multiple server instances. Each instance manages its own SSE connections, while Redis handles cross-instance message routing.
• Security: Add authentication (API keys, OAuth2) for publish endpoints to block unauthorized message injection. For subscribers, restrict which filters they can subscribe to if needed.
• Message Persistence: If subscribers need to catch up on missed messages, add a message store (like Redis Streams or a lightweight database) that retains messages for a configurable time. Send recent matching messages to new subscribers when they connect.
• Error Handling: For SSE connections, handle unexpected drops gracefully and guide clients to reconnect automatically (most browsers will retry SSE connections by default, but you can add explicit retry logic in your client code).

If you don’t want to build from scratch, here are some tools that fit your requirements:

• EventSource.io: A managed service that provides SSE-based Pub/Sub with MQTT-style wildcard support out of the box.
• Pusher Channels: While known for WebSockets, it also supports SSE for subscribers and allows pattern-based channel subscriptions that mirror MQTT wildcards.
• Redis + SSE Proxy: Use Redis Pub/Sub as the backend, then leverage open-source proxies like redis-sse to handle SSE connections, wildcard matching, and message forwarding to clients.
• HiveMQ SSE Bridge: The popular MQTT broker HiveMQ offers a bridge that lets clients connect via SSE, giving you access to MQTT’s robust wildcard subscription features over HTTP.

内容的提问来源于stack exchange，提问作者user698494