go 46 lines · 8 steps

Streaming a CSV export in Gin

Stream database rows straight to an HTTP response as CSV without buffering the whole result set in memory.

Explained by highlit
Gin Intermediate streaming csv-export database-cursor http-handlers memory-efficiency
1func (h *TransactionHandler) ExportCSV(c *gin.Context) {
2 ctx := c.Request.Context()
3 filters := parseTransactionFilters(c)
4 
5 rows, err := h.repo.StreamTransactions(ctx, filters)
6 if err != nil {
7 c.AbortWithStatusJSON(http.StatusInternalServerError, gin.H{"error": "failed to query transactions"})
8 return
9 }
10 defer rows.Close()
11 
12 filename := fmt.Sprintf("transactions-%s.csv", time.Now().Format("2006-01-02"))
13 c.Header("Content-Type", "text/csv")
14 c.Header("Content-Disposition", fmt.Sprintf(`attachment; filename="%s"`, filename))
15 
16 w := csv.NewWriter(c.Writer)
17 _ = w.Write([]string{"id", "account", "amount", "currency", "status", "created_at"})
18 w.Flush()
19 
20 c.Stream(func(out io.Writer) bool {
21 if !rows.Next() {
22 return false
23 }
24 
25 var t Transaction
26 if err := rows.Scan(&t.ID, &t.Account, &t.Amount, &t.Currency, &t.Status, &t.CreatedAt); err != nil {
27 c.Error(err)
28 return false
29 }
30 
31 record := []string{
32 t.ID,
33 t.Account,
34 strconv.FormatInt(t.Amount, 10),
35 t.Currency,
36 t.Status,
37 t.CreatedAt.UTC().Format(time.RFC3339),
38 }
39 if err := w.Write(record); err != nil {
40 c.Error(err)
41 return false
42 }
43 w.Flush()
44 return w.Error() == nil
45 })
46}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Streaming rows through a cursor keeps memory flat regardless of how many records you export.
  2. 2Setting Content-Type and Content-Disposition headers before writing the body makes the browser treat the response as a file download.
  3. 3Gin's Stream callback returning false ends the response, so it doubles as both the loop condition and the error exit.

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