Server-Sent Events (SSE) Cheat Sheet

Updated 2026-03-18

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Server-Sent Events (SSE) is a standard web technology that enables servers to push real-time updates to browser clients over a persistent HTTP connection using the text/event-stream content type. Unlike WebSockets, SSE provides unidirectional server-to-client streaming using familiar HTTP infrastructure, making it simpler to implement and deploy for scenarios like live notifications, dashboards, and progress updates. The key advantage is automatic reconnection: when a connection drops, the browser's EventSource API automatically attempts to reconnect, optionally resuming from the last event using the Last-Event-ID header. SSE works seamlessly with HTTP/2 multiplexing, eliminating the historical 6-connection-per-domain limitation that affected HTTP/1.1, and requires no special proxy configuration beyond disabling response buffering.

What This Cheat Sheet Covers

This topic spans 14 focused tables and 100 indexed concepts. Below is a complete table-by-table outline of this topic, spanning foundational concepts through advanced details.

Table 1: Core Concepts & EventSource APITable 2: Event Stream Format & SyntaxTable 3: Connection Management & LifecycleTable 4: Event Handlers & State ManagementTable 5: Named Events & Event IDsTable 6: Reconnection & Stream ResumptionTable 7: CORS & AuthenticationTable 8: HTTP Protocol & TransportTable 9: Proxy & Server ConfigurationTable 10: Performance & ScalabilityTable 11: Browser Support & PolyfillsTable 12: Use Cases & PatternsTable 13: SSE vs WebSockets ComparisonTable 14: Best Practices & Optimization

Table 1: Core Concepts & EventSource API

Concept	Example	Description
EventSource interface	`const es = new EventSource('/events')`	• Browser API that opens a persistent HTTP connection and receives server-pushed events • handles parsing, reconnection, and event dispatching automatically.
text/event-stream content type	`Content-Type: text/event-stream`	• Required HTTP response header that signals SSE protocol • charset UTF-8 is implicit and should not be explicitly added as it can break some implementations.
UTF-8 encoding	All messages must be UTF-8 text	• SSE exclusively supports text data encoded as UTF-8 • binary data requires Base64 encoding or similar text-safe transformation before transmission.
Unidirectional streaming	Server → Client only	• SSE is server-to-client only • client sends regular HTTP requests if bidirectional communication is needed (contrast with WebSocket's full-duplex design).

Table 1: Core Concepts & EventSource API

Concept	Example	Description
EventSource interface	`const es = new EventSource('/events')`	• Browser API that opens a persistent HTTP connection and receives server-pushed events • handles parsing, reconnection, and event dispatching automatically.
text/event-stream content type	`Content-Type: text/event-stream`	• Required HTTP response header that signals SSE protocol • charset UTF-8 is implicit and should not be explicitly added as it can break some implementations.
UTF-8 encoding	All messages must be UTF-8 text	• SSE exclusively supports text data encoded as UTF-8 • binary data requires Base64 encoding or similar text-safe transformation before transmission.
Unidirectional streaming	Server → Client only	• SSE is server-to-client only • client sends regular HTTP requests if bidirectional communication is needed (contrast with WebSocket's full-duplex design).