WebSocket vs SSE: Architecting Real-Time HMI Telemetry

WebSocket vs Server-Sent Events (SSE) for Real-Time HMI Telemetry

Modern industrial control systems require sub-second data streaming to provide operators with accurate situational awareness. When architecting web-based HMI telemetry dashboards, automation engineers face a critical decision: should you implement full-duplex WebSockets or rely on unidirectional Server-Sent Events (SSE)? Both protocols eliminate legacy polling techniques, but choosing the wrong architecture can introduce unnecessary overhead or limit command capabilities on the plant floor.

Key Takeaways

• Server-Sent Events (SSE) excel in unidirectional telemetry, offering built-in reconnection and simpler load-balancer integration for read-only dashboards.
• WebSockets provide low-latency, full-duplex communication required for active operator control and binary payload transmission (e.g., raw PLC packets).
• Evaluating your specific SCADA topology—whether it’s pure monitoring or active supervisory control—dictates the correct protocol choice.

flowchart LR
    client["Web HMI Client"]
    server["SCADA Gateway"]

    client -->|"HTTP Upgrade (ws://)"| server
    server -->|"Full-Duplex (WebSocket)"| client
    
    client -.->|"Standard HTTP"| server
    server ==>|"Unidirectional Push (SSE)"| client

    classDef wsFill fill:#2563eb,color:#ffffff,stroke:#1e40af
    classDef sseFill fill:#16a34a,color:#ffffff,stroke:#166534
    
    client:::wsFill
    server:::wsFill

The Limitations of Traditional Polling in SCADA

Historically, web-based HMIs relied on AJAX polling, where the client repeatedly asks the server for new tag values every few seconds. In large-scale deployments with thousands of tags, polling generates massive HTTP overhead, saturating network bandwidth and increasing latency. Modern HMI telemetry demands push-based architectures where the server transmits data only when tag values change (report-by-exception).

Server-Sent Events (SSE): The Unidirectional Workhorse

Server-Sent Events utilize standard HTTP connections to push continuous text streams from the server to the client. Once the HMI client establishes the connection, the SCADA server can transmit telemetry updates indefinitely.

Advantages for HMI Telemetry:

• Built-In Reconnection: If an operator’s tablet drops network coverage in the plant, the browser automatically attempts to reconnect and resumes the event stream, reducing custom error-handling logic.
• Infrastructure Compatibility: Because SSE operates over standard HTTP/HTTPS, it traverses corporate firewalls, reverse proxies, and load balancers effortlessly without requiring protocol upgrades.
• Simplicity: It is exceptionally lightweight for read-only dashboards displaying real-time OEE metrics or sensor trends.

flowchart TD
    subgraph sse_architecture ["SSE Telemetry Flow"]
        client["HMI Dashboard"]
        proxy["Reverse Proxy"]
        scada["SCADA Server"]
        
        client -->|"1. HTTP GET /stream"| proxy
        proxy -->|"2. Forward Request"| scada
        scada ==>|"3. Push Tag Data (Continuous)"| proxy
        proxy ==>|"4. Push Tag Data"| client
    end

    classDef blueFill fill:#2563eb,color:#ffffff,stroke:#1e40af
    classDef greenFill fill:#16a34a,color:#ffffff,stroke:#166534
    classDef redFill fill:#dc2626,color:#ffffff,stroke:#991b1b

    client:::blueFill
    proxy:::greenFill
    scada:::redFill

WebSockets: The Full-Duplex Standard

WebSockets establish a persistent, bidirectional TCP connection between the client and server. After an initial HTTP handshake, the connection is upgraded to the ws:// or wss:// protocol, allowing simultaneous, low-latency communication in both directions.

Advantages for Active Control:

• Bidirectional Command & Control: If your HMI requires operators to frequently acknowledge alarms, start/stop motors, or adjust PID setpoints, WebSockets handle both the incoming telemetry and outgoing commands over a single socket.
• Binary Data Support: Unlike SSE (which is restricted to UTF-8 text), WebSockets can transmit binary payloads. This is crucial when streaming raw, serialized protocols or compressed waveform data for vibration analysis.
• Ultra-Low Latency: The minimal framing overhead makes WebSockets ideal for high-speed, closed-loop visualization where millisecond precision matters.

Comparing the Architectures

Conclusion

When engineering HMI telemetry solutions, the choice depends on operator interaction. Use Server-Sent Events for pure monitoring dashboards where robustness and simplicity are paramount. Opt for WebSockets when building comprehensive supervisory control interfaces that require low-latency, bidirectional interaction.

Frequently Asked Questions

Can I use both SSE and WebSockets in the same HMI?

Yes. A hybrid approach uses SSE for streaming high-volume tag updates and a standard REST API (or a dedicated WebSocket) strictly for dispatching control commands.

How do WebSockets handle network drops compared to SSE?

WebSockets do not automatically reconnect when a connection is lost. Developers must implement custom heartbeat (ping/pong) mechanisms and reconnection backoff algorithms, whereas SSE handles this natively in the browser.

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