Content types #

Content type is MIME type, that specifies how the data is encoded in the message payload.

go-asyncapi fully supports the content type property, both a message-specific and default document-wide.

Content types supported by go-asyncapi are listed in features page.

Default behavior #

go-asyncapi generates the encoder/decoder code depending on the content type set in AsyncAPI message. If it’s missed, the defaultContentType document field is used. If it’s missed as well, the default content type is application/json.

To render the marshalling/unmarshalling code, go-asyncapi looks for templates code/proto/mime/messageDecoder/<mime> and code/proto/mime/messageEncoder/<mime>. mime is the content type name (MIME type), for example, application/json, text/plain, etc.

If not found, the default templates are code/proto/mime/messageDecoder/default and code/proto/mime/messageEncoder/default (which are also may be customized).

This default behavior can be customized in templates code/proto/message/encoder and code/proto/message/decoder.

More about templates customization.

Adding a new content type #

It is very easy, just define templates code/proto/mime/messageDecoder/<mime> and code/proto/mime/messageEncoder/<mime>. The result will be substituted to Marshal and Unmarshal message methods instead of default encoder/decoder.

For example, to add support for text/plain content type, define the templates code/proto/mime/messageDecoder/text/plain and code/proto/mime/messageEncoder/text/plain with encoding/decoding code and put them to a file with any name, say content_types.tmpl.

For example:

{{ define "code/proto/mime/messageDecoder/text/plain" }}
var b {{ goQual "strings.Builder" }}
if err := {{ goQual "io.Copy" }}(&b, r); err != nil {
    return nil, fmt.Errorf("read message: %w", err)
}
m.Payload = b.String()
{{ end }}

{{ define "code/proto/mime/messageEncoder/text/plain" }}
r := {{ goQual "strings.NewReader" }}(m.Payload)
if _, err := {{ goQual "io.Copy" }}(w, r); err != nil {
    return fmt.Errorf("write message: %w", err)
}
{{ end }}

Then run the code generation:

go-asyncapi code -T my_templates my_asyncapi.yaml

Replacing the default encoder/decoder #

Everything the same as for adding a new content type, i.e. by defining the templates code/proto/mime/messageDecoder/<mime> and code/proto/mime/messageEncoder/<mime>. See Adding a new content type section above.