How Can We Encode Large Data Streams in JSON Without Loading Objects in Memory?

Encoding Large Data Streams in JSON without In-Memory Object Loading

In scenarios involving extensive data streams, the json package may encounter limitations when attempting to encode all objects simultaneously into memory. Consider this scenario:

<code class="go">type T struct {
    Foo string

    // Bar represents a large stream of objects
    // that we don't want to load entirely into memory.
    Bar chan string
}</code>

While attempting to marshal t using json.Encoder, an error occurs due to the unsupported type chan string.

<code class="go">if err := json.NewEncoder(os.Stdout).Encode(&t); err != nil {
    log.Fatal(err)
}</code>

In situations like these, manually constructing the JSON string becomes necessary. However, a more efficient mechanism is desirable.

If the json.Marshaler interface resembled this structure, the encoding process would be more straightforward:

<code class="go">type Marshaler interface {
    MarshalJSON(io.Writer) error
}</code>

Unfortunately, the encoding/json package currently lacks such a mechanism. Therefore, customizing the built-in package may be necessary. To achieve this, modifications to the reflectValueQuoted function in encoding/json/encode.go are required. Specifically, focus should be placed on the Array case (with fallthrough to Slice) and the addition of a case for channels.

<code class="go">// Case Array:
e.WriteByte('[')
n := v.Len()
for i := 0; i < n; i++ {
    if i > 0 {
        e.WriteByte(',')
    }
    e.reflectValue(v.Index(i))
}
e.WriteByte(']')

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

Note: The channel case may require additional checks that are not evident in the above code.

By implementing these modifications, channels would be treated similar to arrays within the encoding process. This modification could be submitted as a patch to the encoding/json package for potential inclusion.

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