MailboxRPC 3/7: contract notes (non-normative)

[bhandras]

Summary

Adds RPC_MAILBOX_CONTRACT.md: non-normative contract notes that complement the
implementation-ready spec in lightninglabs/darepo#71.

Motivation

Review churn on the core contract is high right now. Keeping these notes in the
public client repo makes it easier to iterate without referencing private repos
or local scratchpads.

References

• Spec issue: https://github.com/lightninglabs/darepo/issues/71
• Durability reference: MailboxRPC 1/7: durability foundation #48

[gemini-code-assist]

Summary of Changes

Hello @bhandras, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a new documentation file, RPC_MAILBOX_CONTRACT.md, which outlines non-normative contract notes for an RPC-over-mailbox transport. The primary goal is to provide a public and accessible reference for design considerations and constraints, thereby streamlining the development and review process by reducing reliance on internal documentation and accelerating iteration on the core contract.

Highlights

• New Documentation Added: A new document, RPC_MAILBOX_CONTRACT.md, has been added to the repository.
• RPC-over-Mailbox Contract Notes: The new document provides non-normative contract notes and design constraints for an RPC-over-mailbox transport, serving as a 'scratchpad' companion to an existing implementation-ready specification.
• Reduced Review Churn: The motivation for adding these notes to the public client repository is to make it easier to iterate on the core contract and reduce review churn by avoiding references to private repositories or local artifacts.
• Detailed Contract Aspects: The document thoroughly covers various aspects of the RPC-over-mailbox contract, including its scope, high-level model, addressing and routing, idempotency and retries, correlation, ordering, ack watermark behavior, long-poll pull, payload encoding, error handling, and versioning/extensibility.

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[gemini-code-assist]

Code Review

This pull request introduces RPC_MAILBOX_CONTRACT.md, a non-normative document outlining the design and constraints for an RPC-over-mailbox transport. The document is well-structured and clearly explains the high-level model, idempotency, correlation, ordering, and other important aspects of the contract. My review includes a couple of suggestions to enhance clarity regarding the relationship between idempotency keys and correlation IDs, and the location of the idempotency key within the envelope. Overall, this is a valuable addition that will help developers understand the system.

[bhandras]

@codex review pls

[chatgpt-codex-connector]

To use Codex here, create a Codex account and connect to github.

[litbot-9000]

@bhandras, remember to re-request review from reviewers when ready

[Roasbeef]

LGTM 🐸