Speaking the Bank's Language

National financial switches — the infrastructure that lets a card or wallet transaction clear between institutions — overwhelmingly still run on ISO 8583, a message format built around fixed and bitmap-indicated fields rather than JSON. Zambia's National Financial Switch was no exception. Airtel's payment platform, like everything built in the last decade, is REST-native end to end.

To settle transactions through the switch, every outbound request needed to become a correctly formatted ISO 8583 message, and every response — approval, decline, reversal — needed to come back out as something the rest of the platform could actually consume.

I built a new microservice on jPOS, the standard Java toolkit for ISO 8583 messaging, to own the switch-facing side of the connection: framing, bitmaps, MTI (Message Type Indicator) handling, and the raw socket-level conversation with the switch.

The transformation layer itself — REST in, ISO 8583 out, and back — lived in Archway, implemented with Apache FreeMarker templates mapping REST payload fields to ISO 8583 field positions and back. Using a templating engine rather than hardcoded mapping code meant field mappings could be adjusted per switch or per message type without a redeploy, which mattered because national switch specs are rarely stable documents in practice.

The integration processed live transaction volume against Zambia's National Financial Switch with materially fewer failures than the prior approach, to the point that the Bankers Association of Zambia formally recognized the work — an unusual thing to receive as an engineer several layers removed from any banking relationship, and a signal that the reliability of the message-level bridge was visible all the way up at the institutional level.