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Golang / GoLang Production Patterns and Web Standards Interview Questions

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()
    }
}

What interface must an http.ResponseWriter implement to support Flush()?
Why is io.Copy preferred over reading an entire file into memory and then writing it?

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