# Glossary

### Topic

`topic`: is like a category for the messages (grouping). Basically, you can:

* Use a single topic (using the 3rd service name) for all requests.
* Or use multiple topics for the same service as well if you are confident that there won't be any conflicts.

Example:

* `inbox/stripe`
* `inbox/currency-cloud`
* `inbox/stripe-invoice`
* `inbox/stripe-payment`
* ...

### Unique External Id

In order to ensure the msgs are unique, you must determine the unique external id **for each topic**.

Something like: the unique ID of the 3rd-party entity.

If the entity changes the stage frequently, you can add a prefix.

I had a real use case a while back. The Payment entity status would be updated frequently so I introduced:

* `payment-received-{paymentUuid}`
* `payment-handling-{paymentUuid}`
* `payment-sending-{paymentUuid}`
* `payment-sent-{paymentUuid}`

DB Constraint: `UNIQUE(topic, external_id)`

### Ordering

To keep the ordering, we are using the **UNIX timestamp in milliseconds** instead of using timestamp or datetime from DB.

**PROs**:

* Faster sorting & pulling the unresolved msgs.
* Keeping the correct order at the millisecond level.

We also add a **created\_at** timestamp column to increase your visibility too (readable).