How do you implement streaming HTTP responses in Go?

Streaming is useful when the response is large, generated incrementally, or delivered in real-time (server-sent events, file downloads). Go's http.Flusher interface allows the handler to push buffered data to the client without waiting for the full response.

// Check if ResponseWriter supports flushing
func streamHandler(w http.ResponseWriter, r *http.Request) {
    flusher, ok := w.(http.Flusher)
    if !ok {
        http.Error(w, "streaming not supported", http.StatusInternalServerError)
        return
    }

    w.Header().Set("Content-Type", "text/event-stream")
    w.Header().Set("Cache-Control", "no-cache")
    w.Header().Set("Connection", "keep-alive")

    ctx := r.Context()
    ticker := time.NewTicker(1 * time.Second)
    defer ticker.Stop()

    for {
        select {
        case <-ctx.Done():
            return // client disconnected
        case t := <-ticker.C:
            fmt.Fprintf(w, "data: %s\n\n", t.Format(time.RFC3339))
            flusher.Flush() // push to client immediately
        }
    }
}

// Large file download — stream without loading into memory
func downloadHandler(w http.ResponseWriter, r *http.Request) {
    f, err := os.Open("/data/large-file.csv")
    if err != nil {
        http.Error(w, "file not found", http.StatusNotFound)
        return
    }
    defer f.Close()

    w.Header().Set("Content-Type", "text/csv")
    w.Header().Set("Content-Disposition", `attachment; filename="data.csv"`)

    // io.Copy streams from file to response in 32 KB chunks
    // without loading the entire file into memory
    if _, err := io.Copy(w, f); err != nil {
        log.Printf("download error: %v", err)
    }
}

// NDJSON streaming (newline-delimited JSON for large result sets)
func streamResults(w http.ResponseWriter, r *http.Request) {
    w.Header().Set("Content-Type", "application/x-ndjson")
    enc := json.NewEncoder(w)
    for _, item := range largeResultSet {
        enc.Encode(item)  // writes JSON + newline
        flusher.Flush()
    }
}

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