Opaque Format
Applications can use the built-in opaque format to exchange messages that contain unstructured data. Opaque messages are very efficient, and can achieve very low-latency performance.
Opaque messages are an excellent choice for streaming data.
Application code can also embed structured data within opaque messages, however, application code is entirely responsible for the structure: the application must pack the data at the sender, and unpack the data at the receiver.
Content matchers cannot select opaque messages. (Recall that content matchers require an exact match on all specified field names and field values. Opaque values vary too much for content matchers.)
To optimize performance, built-in opaque formats limit the size of the data payload. If you need to send larger payloads, define a static format with an opaque field.