Long Polling vs WebSockets

Long Polling vs WebSockets

Both enable near-real-time communication between client and server. Long polling keeps an HTTP connection open until new data is available. WebSockets provide a persistent bidirectional connection.

The Case for Long Polling

Client sends an HTTP request. Server holds it open until new data or timeout. Client immediately re-requests.

What you gain: Works everywhere (standard HTTP). Simpler to implement. Works through all firewalls/proxies. Easy to load balance.

What you lose: Higher latency (new request after each response). More overhead (HTTP headers per request). Server holds connections idle. Not truly real-time.

Best when: Updates are infrequent; Infrastructure does not support WebSockets; Simple implementation is preferred; One-directional updates (server to client).

The Case for WebSockets

Persistent TCP connection after HTTP upgrade handshake. Full-duplex bidirectional communication.

What you gain: True real-time (sub-100ms latency). Low overhead (2-byte frame headers). Bidirectional communication. Efficient for high-frequency updates.

What you lose: Stateful connections (harder to scale). Not all proxies support WebSocket upgrade. Requires reconnection logic. More complex server infrastructure.

Best when: Real-time is critical (chat, gaming, trading); High-frequency updates (live dashboards, feeds); Bidirectional communication needed; Low latency is essential.

Side-by-Side

In the Real World

Slack uses WebSockets for real-time messaging — every keystroke in typing indicators is a WebSocket message.

Facebook historically used long polling for chat before migrating to WebSockets for better performance.

Stock trading platforms use WebSockets for real-time price feeds where every millisecond matters.

The Interview Take

Default to WebSockets for real-time features in your system design. Mention long polling as a fallback for clients that cannot support WebSockets.

Source | System-Design-Overview

Practical Implementation for .NET Developers

In a .NET application, you would typically implement this pattern using the following approach:

ASP.NET Core setup: Create a service class that encapsulates the logic, register it with dependency injection, and inject it into your controllers or minimal API endpoints. The built-in DI container handles lifecycle management.

Entity Framework Core: For database interactions, EF Core provides the ORM layer. Use migrations for schema management and raw SQL for performance-critical queries. Consider Dapper for read-heavy paths where EF Core's overhead matters.

Azure integration: If deploying to Azure, leverage managed services — Azure Cache for Redis, Azure SQL, Azure Service Bus, Azure Cosmos DB. These eliminate operational overhead and provide built-in monitoring through Application Insights.

Testing: Use xUnit with Testcontainers for integration tests that spin up real databases in Docker. Mock external dependencies with NSubstitute. The WebApplicationFactory class lets you test your entire HTTP pipeline in-process.

Monitoring: Add Application Insights telemetry to track request latency, dependency calls, and custom metrics. Use structured logging with Serilog to make production debugging possible:

Log.Information("Processing order {OrderId} for {CustomerId}", orderId, customerId);

This gives you searchable, structured logs in Azure Monitor or Seq.

Real-World Decision Framework

Long polling and WebSockets both enable real-time communication, but they work fundamentally differently. Long polling uses standard HTTP — the client sends a request, and the server holds it open until new data is available. WebSockets upgrade an HTTP connection to a persistent, bidirectional channel.

When Long Polling Wins

Long polling is simpler and works everywhere HTTP works:

• Notification systems: Check for new notifications by holding a request open. When a notification arrives, respond and the client immediately sends a new request.
• Simple chat: Early versions of Facebook Messenger used long polling because it worked through corporate firewalls and proxies that blocked WebSockets.
• Legacy environments: Systems behind strict firewalls or proxies that only allow HTTP traffic.
• Low-frequency updates: If updates come every 10-30 seconds, long polling is efficient enough.

Advantages: Works through all proxies and firewalls, uses standard HTTP, trivial to implement, stateless on the server side.

When WebSockets Win

WebSockets provide true real-time bidirectional communication:

• Live chat: Slack, Discord, and WhatsApp use WebSockets for instant message delivery with typing indicators.
• Collaborative editing: Google Docs and Figma push character-by-character changes over WebSockets.
• Live gaming: Online multiplayer games need sub-50ms bidirectional communication.
• Financial dashboards: Stock trading platforms push price updates multiple times per second.
• IoT: Connected devices stream sensor data continuously.

Advantages: Lower latency, lower overhead (no repeated HTTP headers), bidirectional communication, server can push without client requesting.

Performance Comparison

The Decision in Interviews

For most real-time features, mention WebSockets as the preferred approach, but acknowledge that you would fall back to long polling for compatibility. Mention that ASP.NET Core SignalR does exactly this — it negotiates WebSocket first, then falls back to Server-Sent Events, then long polling.

.NET Implementation

Long polling: Simple HTTP endpoint that uses TaskCompletionSource<T> to hold the response until data arrives. WebSocket: Use ASP.NET Core SignalR — it abstracts the transport, provides automatic reconnection, and scales with Azure SignalR Service for thousands of connections. For raw WebSockets, use WebSocketManager middleware.