Sse Vs Websockets

Server-Sent Events vs WebSockets - Streaming Trade-offs

SSE is one-way (server to client) over HTTP; WebSockets are two-way for low-latency interaction.

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Quick Overview

• SSE: Unidirectional (server to client), text-only, runs over HTTP.
• WebSockets: Bi-directional, full-duplex, supports binary and text.
• SSE is simpler: Works through proxies and load balancers with minimal setup.
• WebSockets are powerful: Best for interactive, low-latency apps.
• Trading example: Use SSE for price tick streams; use WebSockets for trading terminals with bidirectional commands.

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Detailed Explanation

Server-Sent Events (SSE)

What it is: A long-lived HTTP response that streams events to the client.

Strengths:

• Simple HTTP-based streaming
• Automatic reconnect support in browsers
• Works well with HTTP/2 and standard infrastructure

Constraints:

• Server to client only
• Text-only payloads (JSON is common)

C# example (ASP.NET Core):

app.MapGet("/prices/stream", async context =>
{
    context.Response.Headers.Append("Content-Type", "text/event-stream");
    context.Response.Headers.Append("Cache-Control", "no-cache");

    var ct = context.RequestAborted;
    while (!ct.IsCancellationRequested)
    {
        var tick = new { Symbol = "EURUSD", Bid = 1.0912m, Ask = 1.0914m };
        await context.Response.WriteAsync($"data: {JsonSerializer.Serialize(tick)}\n\n", ct);
        await context.Response.Body.FlushAsync(ct);
        await Task.Delay(TimeSpan.FromSeconds(1), ct);
    }
});

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WebSockets

What it is: A persistent TCP connection that allows full-duplex messaging.

Strengths:

• Bi-directional communication
• Low latency for interactive workflows
• Supports binary payloads

Constraints:

• More complex to scale and debug
• Requires explicit reconnection logic

C# example (ASP.NET Core):

app.UseWebSockets();

app.Map("/ws", async context =>
{
    if (!context.WebSockets.IsWebSocketRequest)
    {
        context.Response.StatusCode = StatusCodes.Status400BadRequest;
        return;
    }

    using var socket = await context.WebSockets.AcceptWebSocketAsync();
    var buffer = new byte[4096];

    while (true)
    {
        var result = await socket.ReceiveAsync(buffer, CancellationToken.None);
        if (result.MessageType == WebSocketMessageType.Close)
            break;

        await socket.SendAsync(buffer.AsMemory(0, result.Count), result.MessageType, result.EndOfMessage, CancellationToken.None);
    }

    await socket.CloseAsync(WebSocketCloseStatus.NormalClosure, "Done", CancellationToken.None);
});

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Comparison Snapshot

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Why It Matters for Interviews

• You can justify when a simpler transport is enough (SSE) vs when you need full-duplex.
• You can discuss scaling constraints (sticky sessions, backpressure, fan-out).
• You can tie decisions to latency and operational complexity.

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Common Pitfalls

• Using WebSockets for unidirectional updates where SSE is simpler.
• Forgetting heartbeats/pings and idle timeouts.
• Not handling reconnection or replay (event IDs) for SSE.
• Sending large payloads without backpressure control.

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Quick Reference

• SSE: Unidirectional, HTTP, easy to deploy, great for streaming data.
• WebSockets: Bi-directional, low latency, best for interactive workflows.

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Sample Interview Q&A

• Q: When would you choose SSE over WebSockets?
• A: For server-to-client streams like price ticks or notifications where clients do not need to send frequent messages back.

• Q: How do you handle reconnects with SSE?
• A: Use event IDs and allow clients to send Last-Event-ID so the server can replay missed data.

• Q: What is the biggest operational challenge with WebSockets?
• A: Scaling long-lived connections (load balancers, sticky sessions) and handling reconnects on network churn.