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feat(*): Updates messaging to support request-reply #21

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feat(*): Updates messaging to support request-reply #21

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2 changes: 1 addition & 1 deletion README.md
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Expand Up @@ -45,7 +45,7 @@ Overall, the messaging interfaces aim to make it easier to build complex and sca
### Non-goals

- The messaging service interfaces do not aim to provide advanced features of message brokers, such as broker clustering, message persistence, or guaranteed message delivery. These are implementation-specific details that are not addressed by the interfaces.
- The messaging service interfaces do not aim to provide support for every possible messaging pattern or use case. Instead, they focus on the common use cases of pub-sub and push-based message delivery. Other messaging patterns, such as request-reply or publish-confirm-subscribe, are outside the scope of the interfaces.
- The messaging service interfaces do not aim to provide support for every possible messaging pattern or use case. Instead, they focus on the common use cases of pub-sub, push-based message delivery, and request-reply. Other messaging patterns, such as publish-confirm-subscribe, are outside the scope of the interfaces.
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Adding support for request-reply was rejected during our stakeholder review - here's a relevant comment explaining why: #8 (comment)

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@thomastaylor312 thomastaylor312 Mar 4, 2024 •
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So I missed that when I was digging around. However, I think I need to do some more thinking on the reasoning there. I understand the motivation as described there for not adding it, but based on everything I've seen in real usage of the wasmcloud messaging interface and in my review of most of the common messaging systems, request/reply showed up everywhere. Let me do some more reading and come back to this, because, like you mentioned, this should reflect real world usage

- The messaging service interfaces do not aim to provide a specific implementation of a message broker. Instead, they provide a standard way to interact with any message broker that supports the interfaces.

### API walk-through
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2 changes: 2 additions & 0 deletions wit/producer.wit
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@@ -1,5 +1,7 @@
interface producer {
use messaging-types.{client, channel, message, error};

/// Sends a message to the given channel/topic. This topic can be overridden if a message has a
/// non-empty topic field
send: func(c: client, ch: channel, m: list<message>) -> result<_, error>;
}
21 changes: 7 additions & 14 deletions wit/types.wit
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Expand Up @@ -23,22 +23,15 @@ interface messaging-types {
extensions: option<list<tuple<string, string>>>
}

/// Format specification for messages
/// - more info: https://github.com/clemensv/spec/blob/registry-extensions/registry/spec.md#message-formats
/// - message metadata can further decorate w/ things like format version, and so on.
enum format-spec {
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format-spec wasn't part of the original interface, but was added during our stakeholder review. Here's some relevant comments you may want to take a look at:
(1) #9 (comment)
(2) #9 (comment)

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Hmmm...I think agree with that second comment that this should be a either a bare string or at least called content-type as the link in the comment was about registry stuff and nothing to do with content-type. The thread in (2) suggests this but I didn't seem to understand the argument that format-spec should be the thing we use

cloudevents,
http,
amqp,
mqtt,
kafka,
raw
}

/// A message with a binary payload, a format specification, and decorative metadata.
/// A message with a binary payload and additional information
record message {
/// The topic or subject this message was received or should be sent on
topic: string,
/// An optional topic for use in request/response scenarios
reply-to: option<string>,
/// An opaque blob of data
data: list<u8>,
format: format-spec,
/// Optional metadata (also called headers or attributes in some systems) attached to the message
metadata: option<list<tuple<string, string>>>
}
}