Add Pluggable Codec System for Request/Response Serialization

[om26er]

Problem

Right now, the library handles message serialization in a fixed format (e.g., JSON or MessagePack).
We’d like to make it possible to pass structured objects (like Protobuf or Cap’n Proto messages) into WAMP calls and get objects back — with minimal boilerplate.

Example desired API:

result = await session.call(
    "com.example.process",
    MyInputProto(id=123),
    out_type=MyOutputProto,
)

This should:

• Serialize MyInputProto to bytes using the selected codec.
• Send the bytes over WAMP.
• Decode the response into MyOutputProto automatically.

---

🧩 Codec Interface

from typing import Any, Generic, Type, TypeVar

T = TypeVar("T")

class Codec(Generic[T]):
    name: str

    def encode(self, obj: Any) -> bytes:
        """Serialize a Python object to bytes."""
        raise NotImplementedError

    def decode(self, data: bytes, out_type: Type[T]) -> T:
        """Deserialize bytes into an instance of out_type."""
        raise NotImplementedError

---

✅ Example Implementations

Protobuf

from google.protobuf.message import Message

class ProtobufCodec(Codec):
    name = "protobuf"

    def encode(self, obj: Message) -> bytes:
        return obj.SerializeToString()

    def decode(self, data: bytes, out_type: Type[Message]) -> Message:
        msg = out_type()
        msg.ParseFromString(data)
        return msg

Cap’n Proto

import capnp

class CapnpCodec(Codec):
    name = "capnp"

    def encode(self, obj) -> bytes:
        return obj.to_bytes_packed()

    def decode(self, data: bytes, out_type):
        return out_type.from_bytes_packed(data)

---

⚙️ Integrating with the WAMP Session

class Session:
    def __init__(self,):
        self.codec = None

    def set_payload_codec(codec: Codec):
        self.codec = codec

    async def call(self, uri: str, in_obj: Any, out_type: Type[Any]) -> Any:
        payload = self.codec.encode(in_obj)
        raw = await self._send_and_receive(uri, payload)
        return self.codec.decode(raw, out_type)

Usage:

session = Session(codec=ProtobufCodec())
result = await session.call("com.example.add", MyRequest(a=2, b=3), MyResponse)

Switching codecs is trivial:

session.codec = CapnpCodec()

---

✅ Benefits

Benefit	Description
🔌 Extensible	Users can plug in new codecs (e.g. JSON, FlatBuffers)
🧠 Clean	Transport code remains format-agnostic
⚙️ Flexible	Codec can be changed dynamically or negotiated at runtime
🧾 Reusable	Can also be reused for publish/subscribe and registration calls

---

Summary

Add a codec abstraction to the WAMP session layer to allow encode()/decode() handling to be pluggable.
This enables passing and returning Protobuf, Cap’n Proto, or other structured data seamlessly.