How to Efficiently Marshal Large Data Streams in JSON without Loading into Memory?

Marshaling Large Data Streams in JSON without Loading into Memory

The need to encode large data streams into JSON often arises, but loading the entire stream into memory at once can be impractical. This article explores ways to overcome this challenge without the built-in json.Marshaler interface.

Encoding with json.Encoder: A Limitation

Attempts to encode a large stream of data with json.Encoder will fail due to its inability to handle channels (chan string) in memory.

Custom JSON Encoding

In the absence of a suitable built-in solution, custom JSON encoding becomes necessary. This involves manually building the JSON string, as demonstrated in the snippet below:

w := os.Stdout
w.WriteString(`{ "Foo": "` + t.Foo + `", "Bar": [`)

for x := range t.Bar {
    _ = json.NewEncoder(w).Encode(x)
    w.WriteString(`,`)
}

w.WriteString(`]}`)

Extending encoding/json for Channel Support

To enhance the encoding/json package with channel support, you can modify the reflectValueQuoted function in encoding/json/encode.go. Specifically, add a case for channels similar to the following:

case reflect.Chan:
    e.WriteByte('[')
    i := 0
    for {
        x, ok := v.Recv()
        if !ok {
            break
        }
        if i > 0 {
            e.WriteByte(',')
        }
        e.reflectValue(x)
        i++
    }
    e.WriteByte(']')

Conclusion

While the encoding/json package does not currently support channel encoding, this article provides alternative approaches for marshaling large data streams in JSON efficiently. Custom encoding allows streaming data directly into the JSON output, while extending encoding/json offers a more robust solution.

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