Stream data from a request response into a CSV in go - go

Question

I have something like a data pipeline.
API response (10k) rows as JSON.
=> Sanitize some of the data into a new structure
=> Create a CSV File
I can currently do that by getting the full response and doing that step by step.
I was wondering if there's a simpler way to stream the response reading into CSV right away and also writing in the file as it goes over the request-response.
Current code:
I will have a JSON like { "name": "Full Name", ...( 20 columns)} and that data repeats about 10-20k times with different values.
For request
var res *http.Response
if res, err = client.Do(request); err != nil {
return errors.Wrap(err, "failed to perform request")
}
For Unmarshal
var record []RecordStruct
if err = json.NewDecoder(res.Body).Decode(&record); err != nil {
return err
}
For CSV
var row []byte
if row, err = csvutil.Marshal(record); err != nil {
return err
}

Answer 1

To stream an array of JSON objects you have to decode nested objects instead of root object. To do this you need read data using tokens (check out Token method). According to the documentation:
Token returns the next JSON token in the input stream. At the end of the input stream, Token returns nil, io.EOF.
Token guarantees that the delimiters [ ] { } it returns are properly nested and matched: if Token encounters an unexpected delimiter in the input, it will return an error.
The input stream consists of basic JSON values—bool, string, number, and null—along with delimiters [ ] { } of type Delim to mark the start and end of arrays and objects. Commas and colons are elided.
That mean you can decode document part by part. Find an official example how to do it here
I will post a code snippet that shows how you can combine json stream technic with writing result to the CSV:
package main
import (
"encoding/csv"
"encoding/json"
"log"
"os"
"strings"
)
type RecordStruct struct {
Name string `json:"name"`
Info string `json:"info"`
// ... any field you want
}
func (rs *RecordStruct) CSVRecord() []string {
// Here we form data for CSV writer
return []string{rs.Name, rs.Info}
}
const jsonData =
`[
{ "name": "Full Name", "info": "..."},
{ "name": "Full Name", "info": "..."},
{ "name": "Full Name", "info": "..."},
{ "name": "Full Name", "info": "..."},
{ "name": "Full Name", "info": "..."}
]`
func main() {
// Create file for storing our result
file, err := os.Create("result.csv")
if err != nil {
log.Fatalln(err)
}
defer file.Close()
// Create CSV writer using standard "encoding/csv" package
var w = csv.NewWriter(file)
// Put your reader here. In this case I use strings.Reader
// If you are getting data through http it will be resp.Body
var jsonReader = strings.NewReader(jsonData)
// Create JSON decoder using "encoding/json" package
decoder := json.NewDecoder(jsonReader)
// Token returns the next JSON token in the input stream.
// At the end of the input stream, Token returns nil, io.EOF.
// In this case our first token is '[', i.e. array start
_, err = decoder.Token()
if err != nil {
log.Fatalln(err)
}
// More reports whether there is another element in the
// current array or object being parsed.
for decoder.More() {
var record RecordStruct
// Decode only the one item from our array
if err := decoder.Decode(&record); err != nil {
log.Fatalln(err)
}
// Convert and put out record to the csv file
if err := writeToCSV(w, record.CSVRecord()); err != nil {
log.Fatalln(err)
}
}
// Our last token is ']', i.e. array end
_, err = decoder.Token()
if err != nil {
log.Fatalln(err)
}
}
func writeToCSV(w *csv.Writer, record []string) error {
if err := w.Write(record); err != nil {
return err
}
w.Flush()
return nil
}
You can also use 3d party packages like github.com/bcicen/jstream