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+# SB-2 (Pluggable Transport)
+
+**Status**: Under Review
+
+This proposal aims to introduce a pluggable transport to seabird-core and
+introduce an alternative to gRPC.
+
+## Context
+
+All of the seabird APIs have been built around gRPC and protobufs. This is a
+format which is backed by Google for RPC servers and includes code for client
+and server autogeneration. Protobufs have a number of rough edges because of the
+various limitations. However, even though this works fairly well for
+server-to-server communication, it still falls short for web applications.
+
+## Dependencies
+
+- [SB-1 (AMQP Backed Implementation)](./SB-1.md) - SB-2 can be completed
+  without [SB-1](./SB-1.md), but it would be much easier to do this after
+  there's a simpler implementation.
+- [SB-3 (API Refactor)](./SB-3.md) - this refactor would allow us to only
+  support server streams and would simplify this implementation more.
+
+## Goals
+
+- Introduce a new transport
+- Allow multiple transports to be running at once, selectable by the backend or
+  plugin.
+- Continue to be backwards compatible
+
+## Potential Problems
+
+- How to best handle bidirectional streaming/chat ingest
+  - The current plan is to use HTTP/1.1 Chunked Encoding (or another similar
+    technology) once the API only requires server streaming and not ingestion
+    streaming.
+
+## Details
+
+The main complication with other transports relates to bidirectional streaming.
+Currently, the only APIs which use bidirectional streaming are the chat
+backends. Because most users only implement plugins, it may be acceptable to add
+additional complexity there and drop bidirectional streaming.
+
+### Data Format
+
+In general, either JSON (with `application/json`) or msgpack (with
+`application/msgpack`) should be used for the serialization/deserialization.
+
+#### Replacing anyof
+
+Essentially, anyof is a tagged union. The simplest way to decode this in Go is
+to have a wrapper struct with `type` and `payload` with the `payload` being a
+`json.RawMessage`. Some msgpack libraries have an alternative to this. With
+minimal glue, it should be possible to create a `RawJsonOrMsgpackMessage` which
+would let us deserialize to either type trivially. Unfortunately, this also
+means a two-step encoding process, so additional wrappers would need to be
+written.
+
+For rust, serde supports tagged unions out of the box with `#[serde(tag =
+"type", payload = "payload")]`.
+
+### Endpoints
+
+#### Request/Response
+
+Most endpoints are fairly straight-forward. Request-response over HTTP will
+react fairly similarly no matter if you're using the data format in this
+proposal or gRPC/protobufs.
+
+#### Chat Ingest
+
+Because Chat Ingest is bidirectional, it doesn't cleanly map onto HTTP.
+
+Thankfully there are other options:
+- websockets (which require server support and
+additional wrappers for reading auth from query params but support web cleanly
+and matches the current paradigm)
+- HTTP Multipart (which also requires some
+additional wrappers to properly support streaming the form data)
+- HTTP/2 (which
+requires server support and has relatively limited library support, especially
+on the frontend)
+- HTTP/1.1 chunked encoding (though this would require having
+additional ingestion endpoints as it would only be usable for events streaming
+from the server).
+- HTTP SSE (with newline delimited json events)
+- HTTP body streaming (with newline delimited json events)
+
+See https://github.com/twitchtv/twirp/issues/3
+
+Websockets and HTTP/2 are the cleanest way to get messages, though newline
+delimited JSON would also work. Additionally websockets have fairly good browser
+support. It gets more complicated for binary formats because you need to invent
+a new container format.
+
+#### Event Stream
+
+Because the event stream is only reading events (and not bidirectional), it will
+also map onto the same pattern as chat ingest, but without the additional work
+to support sending events.