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Every Transactional asks one question besides the event and the messages: what identifies each send? The answer is a field of your event, and it decides how many messages a customer gets.

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The short version

Pick the field that is different on every event, usually an id: paymentLinkId, orderId, appointmentId.
  • Each distinct value sends its messages once, ever, per customer.
  • The same value arriving again for that customer sends nothing.
  • Two different values for the same customer send two messages, even at the same time.

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An example

Your system records a payment_link_created event each time a payment link is made, and the Transactional identifies each send by paymentLinkId. Without that field, Boom could not tell a new payment link from a retry of an old one, and would either message the customer every time your system retries or only once for all of their links.

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Choosing the field

  • Use an id that your system creates once per thing, the same one you would use to look it up later.
  • Do not use a field shared by many events, like the customer id, a status or an amount. A customer id would send only the first message the customer ever gets, and never again.
  • Do not use a field that changes on a retry, like a timestamp or a request id. Every retry would send again.
When the event has exactly one field ending in Id, Boom picks it for you. If your event has not arrived yet, type the field’s name; it only has to match what your system will send.

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Sending the field

Put the field in the event’s properties:
An event that arrives without the field, or with an empty one, sends nothing, because Boom cannot tell which send it belongs to. So does a value longer than 255 characters.
externalId is a different thing: it identifies the event record itself, so recording the same externalId twice stores it once. The field that identifies each send is what stops a customer from getting the same message twice.
See Send transactional messages for the full set-up, and events for the event API.