Transcoders and Non-JSON Documents

The Go SDK supports common JSON document requirements out-of-the-box. Custom transcoders and serializers provide support for applications needing to perform advanced operations, including supporting non-JSON data.

The Go SDK uses the concept of transcoders, which are used whenever key-value data is sent to or retrieved from Couchbase Server.

Transcoders are only available to key value operations. Operations using search, analytics, query, or views will use the json package for serializing and deserializing data.

When sending data to Couchbase, the SDK passes the data being sent to a transcoder. The transcoder can either reject the data as being unsupported, or convert it into a []byte and a Common Flag. The Common Flag specifies whether the data is JSON, a non-JSON string, or raw binary data.

On retrieving key-value data from Couchbase, the fetched []byte and Common Flag are passed to a transcoder. The transcoder converts the bytes into a type (the application provides a pointer to a variable) if possible.

Many applications will not need to be aware of transcoders, as the defaults support most standard JSON use cases. The information in this page is only needed if the application has an advanced use-case, likely involving either non-JSON data, or a requirement for a particular JSON serialization library.

Default Behaviour

The ClusterOptions specify a global transcoder, which by default is a JSONTranscoder.

JSONTranscoder uses the json package for serializing and deserializing data.

On sending data to Couchbase, JSONTranscoder will send the data to its serializer to convert into a []byte. The serialized bytes are then sent to the Couchbase Server, along with a Common Flag of JSON.

JSONTranscoder will pass any data type to its serializer, apart from a []byte. It will reject this with an error, as it is ambiguous how it should be handled.

On retrieving data from Couchbase, JSONTranscoder passes the fetched []byte and Common Flag to its serializer to convert into a concrete value.

This table summarizes that information, and this more concise form will be used to describe the other transcoders included in the SDK.

Item	Result	Common Flag
string	Results of serializer	JSON
[]byte	error	-
Other	Results of serializer	JSON

Item

Result

Common Flag

string

Results of serializer

JSON

[]byte

error

Other

Results of serializer

JSON

RawJSONTranscoder

The RawJSONTranscoder provides the ability for the application to explicitly specify that the data they are storing or retrieving is JSON. This transcoder does not use a serializer, and always performs straight pass through of the data to the server. This enables the application to avoid unnecessary parsing costs when they are certain they are using JSON data.

It only accepts Strings and []byte.

Item	Result	Common Flag
string	Passthrough	JSON
[]byte	Passthrough	JSON
Other	InvalidArgumentException	-

Item

Result

Common Flag

string

Passthrough

JSON

[]byte

Passthrough

JSON

Other

InvalidArgumentException

This transcoder is particularly useful when working with third-party JSON libraries. Here we want to use ffjson for serialization, instead of the default json package:

	user := User{Name: "John Smith", Age: 27}
	transcoder := gocb.NewRawJSONTranscoder()

	b, err := ffjson.Marshal(user)
	if err != nil {
		panic(err)
	}

	_, err = collection.Upsert("john-smith", b, &gocb.UpsertOptions{
		Transcoder: transcoder,
	})
	if err != nil {
		panic(err)
	}

Since ffjson has already done the serialization work, we don’t want to use the default JSONTranscoder, as this will run the provided string needlessly through json.Marshal. Instead, RawJSONTranscoder is used, which just passes through the serialized bytes, and stores them in Couchbase with the JSON Common Flag set.

Similarly, the same transcoder is used on reading the document, so the raw bytes can be retrieved in a string without going through json.Unmarshal. ffjson can then be used for the deserialization.

	getRes, err := collection.Get("john-smith", &gocb.GetOptions{
		Transcoder: transcoder,
	})
	if err != nil {
		panic(err)
	}

	var returned []byte
	err = getRes.Content(&returned)
	if err != nil {
		panic(err)
	}

	err = ffjson.Unmarshal(returned, &user)
	if err != nil {
		panic(err)
	}

Non-JSON Transcoders

It is most common to store JSON with Couchbase. However, it is possible to store non-JSON documents, such as raw binary data, perhaps using an concise binary encoding like MessagePack or CBOR, in the Key-Value store.

It’s important to note that the Couchbase Data Platform includes multiple components other than the Key-Value store — including SQL++ (formerly N1QL) and its indexes, FTS (Search), analytics, and eventing — and these are optimized for JSON and will either ignore or provide limited functionality with non-JSON documents.

Also note that some simple data types can be stored directly as JSON, without recourse to non-JSON transcoding. A valid JSON document can be a simple integer (42), string ("hello"), array ([1,2,3]), boolean (true, false) and the JSON null value.

RawStringTranscoder

The RawStringTranscoder provides the ability for the user to explicitly store and retrieve raw string data with Couchbase. It can be used to avoid the overhead of storing the string as JSON, which requires two bytes for double quotes, plus potentially more for escaping characters.

Note that this transcoder does not accept a serializer, and always performs straight passthrough of the data to the server. It only accepts Strings.

Item	Result	Common Flag
string	Passthrough	String
[]byte	error	-
Other	error	-

Item

Result

Common Flag

string

Passthrough

String

[]byte

error

Other

error

Here’s an example of using the RawStringTranscoder:

	input := "hello world"
	transcoder := gocb.NewRawStringTranscoder()

	_, err = collection.Upsert("key", input, &gocb.UpsertOptions{
		Transcoder: transcoder,
	})
	if err != nil {
		panic(err)
	}

	getRes, err := collection.Get("key", &gocb.GetOptions{
		Transcoder: transcoder,
	})
	if err != nil {
		panic(err)
	}

	var returned string
	err = getRes.Content(&returned)
	if err != nil {
		panic(err)
	}

RawBinaryTranscoder

The RawBinaryTranscoder provides the ability for the user to explicitly store and retrieve raw byte data to Couchbase. The transcoder does not perform any form of real transcoding, and does not take a serializer, but rather passes the data through and assigns the appropriate binary Common Flag.

Item	Result	Common Flag
string	error	-
[]byte	Passthrough	Binary
Other	error	-

Item

Result

Common Flag

string

error

[]byte

Passthrough

Binary

Other

error

Here’s an example of using the RawBinaryTranscoder:

	input := []byte("hello world")
	transcoder := gocb.NewRawBinaryTranscoder()

	_, err = collection.Upsert("key", input, &gocb.UpsertOptions{
		Transcoder: transcoder,
	})
	if err != nil {
		panic(err)
	}

	getRes, err := collection.Get("key", &gocb.GetOptions{
		Transcoder: transcoder,
	})
	if err != nil {
		panic(err)
	}

	var returned []byte
	err = getRes.Content(&returned)
	if err != nil {
		panic(err)
	}

Custom Transcoders

More advanced transcoding needs can be accomplished if the application implements their own transcoders and serializers.

Creating a Custom Transcoder

Let’s look at a more complex example: encoding the JSON alternative, MessagePack. MessagePack is a compact binary data representation which is custom to our needs, so it should be stored with our with own Common Flag. The Common Flag is chosen by the transcoder, and none of the existing transcoders matches our needs (RawBinaryTranscoder does set the binary flag, but it passes data through directly rather than using a serializer, which could also cause issues if you access data through different SDKs). So we need to write one.

We create a transcoder that uses the msgp.Marshaler/msgp.Unmarshaler interfaces, and sets the our own Common Flag when storing the data:

const customFlags = (0x02 << 24) | ('M' << 16) | ('P' << 8) | ('K' << 0)

type MsgPackTranscoder struct {
}

func (t *MsgPackTranscoder) Encode(value interface{}) ([]byte, uint32, error) {
	msgPckVal, ok := value.(msgp.Marshaler)
	if !ok {
		return nil, 0, errors.New("MsgPackTranscoder only supports types that satisfy msgp.Marshaler")
	}

	data, err := msgPckVal.MarshalMsg(nil)
	if err != nil {
		return nil, 0, err
	}

	return data, customFlags, nil
}

func (t *MsgPackTranscoder) Decode(bytes []byte, flags uint32, out interface{}) error {
	if flags != customFlags {
		return errors.New("unexpected expectedFlags value")
	}

	msgPckVal, ok := out.(msgp.Unmarshaler)
	if !ok {
		return errors.New("MsgPackTranscoder only supports types that satisfy msgp.Unmarshaler")
	}

	var err error
	out, err = msgPckVal.UnmarshalMsg(bytes)
	if err != nil {
		return err
	}

	return nil
}

Note the use of customFlags. We are setting the flags to our own value so that our data cannot be misread by any other SDK accessing the data. We’d have to implement our transcoder in those SDKs too. The 0x02 << 24 value actually corresponds to an internal sdk flag signifying that the datatype is private, we then encode our own MsgPacK flag into it.

Now we can use the new transcoder to seamlessly store MessagePack data in Couchbase Server:

	transcoder := &MsgPackTranscoder{}

	_, err := collection.Upsert("key", input, &gocb.UpsertOptions{
		Transcoder: transcoder,
	})
	if err != nil {
		panic(err)
	}

	getRes, err := collection.Get("key", &gocb.GetOptions{
		Transcoder: transcoder,
	})
	if err != nil {
		panic(err)
	}

	var returned []byte
	err = getRes.Content(&returned)
	if err != nil {
		panic(err)
	}

Note that input in this example must be a type that has been enhanced using the msgpack tooling.

Transcoders and Non-JSON Documents

Default Behaviour

RawJSONTranscoder

Non-JSON Transcoders

RawStringTranscoder

RawBinaryTranscoder

Custom Transcoders

Creating a Custom Transcoder

Further reading