Integration Principles

This page describes the common integration behaviour for the FAST Transaction Interface, including supported message modes, expected HTTP responses, response body requirements, Store and Forward (SAF), batch processing, and retry behaviour.

These principles apply to all FAST message types unless otherwise stated.

FAST Modes

The FAST Transaction Interface supports two operating modes:

Mode	Description
Active	Paymentology expects the client to return a business response in the HTTP response body.
Passive	Paymentology expects only an HTTP acknowledgement. No response body is required.

The supported mode for each FAST message type is shown below.

Message Type	Supported Mode
FAST Auth	Active or Passive
FAST Settlement	Passive
FAST Advice	Active

Message Types

Message Type	Mode	Description	Expected Client Response
FAST Auth	Active	Real-time authorisation request sent by Paymentology.	Return HTTP 200 OK and a response body containing the authorisation decision.
FAST Auth	Passive	Authorisation notification sent by Paymentology.	Return HTTP 200 OK. No response body is expected.
FAST Settlement	Passive	Settlement notification sent by Paymentology. Delivered using the Store and Forward (SAF) mechanism.	Return HTTP 200 OK only when all settlement messages in the request have been successfully processed. No response body is expected.
FAST Advice	Active	Advice-related messages, including advice, reversal, and repeat advice/reversal messages. Delivered using the Store and Forward (SAF) mechanism.	Return HTTP 200 OK or 201 Created together with a response body containing the advice response.

HTTP Response Requirements

Paymentology expects the client endpoint to return an HTTP response for every FAST request.

Message Type	Successful HTTP Response(s)	Response Body	Delivered via SAF
FAST Auth (Active)	200 OK	Required	No
FAST Auth (Passive)	200 OK	Not expected	No
FAST Settlement (Passive)	200 OK	Not expected	Yes
FAST Advice (Active)	200 OK, 201 Created	Required	Yes

If a response other than the expected HTTP status code(s) is returned, or no response is received, Paymentology considers the request delivery unsuccessful.

For FAST Settlement and FAST Advice, unsuccessful deliveries are automatically retried using the Store and Forward (SAF) mechanism.

Response Body Requirements

Message Type	Response Body
FAST Auth (Active)	A response body is mandatory and must conform to the published FAST Auth response schema.
FAST Auth (Passive)	No response body is expected. Any response body returned is ignored by Paymentology.
FAST Settlement (Passive)	No response body is expected. Any response body returned is ignored by Paymentology.
FAST Advice (Active)	A response body is mandatory and must conform to the published FAST Advice response schema.

Store and Forward (SAF)

FAST Settlement and FAST Advice messages are delivered using Paymentology's Store and Forward (SAF) mechanism to provide reliable message delivery.

Paymentology determines whether a message has been successfully delivered based on the HTTP response returned by the client.

Message Type	Successful HTTP Response(s)
FAST Settlement	200 OK
FAST Advice	200 OK, 201 Created

If any other HTTP status code is returned, or no response is received, Paymentology considers the delivery unsuccessful and retries the request according to the configured retry policy.

Batch Processing (FAST Settlement)

Paymentology may send multiple settlement messages within a single FAST Settlement request.

The client should process all settlement messages contained within the request before returning an HTTP response.

The HTTP response represents the processing outcome of the entire request, not individual settlement messages.

Example

FAST Settlement Request

Settlement A
Settlement B
Settlement C

Partial Processing

Return HTTP 200 OK only when every settlement message within the request has been successfully processed.

For example:

FAST Settlement Request

Settlement A
Settlement B
Settlement C

Possible outcome:

✅ Settlement A processed successfully
❌ Settlement B failed
✅ Settlement C processed successfully

Since one settlement message failed, the client should return an HTTP status code other than 200 OK.

Paymentology will retry the entire FAST Settlement request through the Store and Forward (SAF) mechanism, including all settlement messages contained within the original request.

Important

Clients should implement idempotent processing, as settlement messages that have already been processed successfully may be delivered again following a retry.

Best Practices

Return the expected HTTP success response for each FAST message type.
Return HTTP 200 OK for FAST Auth requests and include a valid response body when operating in Active mode.
Return HTTP 200 OK for FAST Settlement only when every settlement message within the request has been successfully processed.
Return HTTP 200 OK or 201 Created for FAST Advice together with a valid FAST Advice response body.
Do not return a response body for FAST Auth (Passive) or FAST Settlement requests.
Implement idempotent processing for FAST Settlement and FAST Advice messages, as the Store and Forward (SAF) mechanism may redeliver messages following a retry.