# Contract Delta: 006 Messaging NATS Replacement

> For the complete documentation index, see [llms.txt](https://deploy-preview-472--lucky-concha-f3599f.netlify.app/llms.txt).

Parent state: `005-postgres-database-replacement`

## OpenAPI Changes

*   No REST OpenAPI contract changes required in this state.
*   Existing service REST contracts from prior state remain valid.

## Event Contract Changes

Socket.IO channels are replaced by NATS subjects.

Canonical subject families:

*   `trades.new` (trade submission events)
*   `trades.processed` (trade lifecycle updates)
*   `positions.account.<accountId>.updated`
*   `trades.account.<accountId>.updated`

Payload compatibility goal:

*   Keep semantic payload shape equivalent to prior state events where possible.
*   Any payload/schema drift must be documented explicitly before implementation.

## Compatibility Notes

*   Functional UX behavior should remain unchanged for baseline flows.
*   Transport-level compatibility is not required (Socket.IO clients are replaced).
*   Migration should proceed service-by-service with temporary dual-publish allowed only during cutover implementation.