Migrating Marginalen Bank to Mambu

Client’s context

Marginalen Bank, based in Sweden, offers a range of financial services, including savings, loans, credit cards, and business services. Working for around 300,000 customers with 300 employees, the bank focuses on key areas like Collections, Lending, and Saving Deposits. However, Marginalen’s legacy Tieto EntraBank core system prevented the bank from improving the delivery of their digital services, particularly through the mobile app, which is key to success in today’s client experience-driven banking industry.

Business challenge

Marginalen Bank needed a flexible, scalable core banking solution to quickly meet business demands while reducing costs. Their existing core system lacked the adaptability required to address these needs. The transition to Mambu was a crucial step for achieving greater efficiency and compliance within a short timeline.

With Marginalen’s tech organization driving the bank’s digital transformation, supporting online services and daily operations, the client was looking for a partner who would adapt to their focus on in-house development and microservice architecture. Furthermore, the existing technical environment was complex, and it required not only technical but also deep domain knowledge. New engineers stepping into the project have had to undergo knowledge transfer sessions to navigate both the technical and the legal sides of the Swedish banking systems.

Solution

Process

 Discover

We ran joint workshops to capture objectives, compliance boundaries and timelines, then audited EntraBank tables, interfaces and reports while mapping must-keep features and fourteen external integrations.

The discovery sprint produced a clear migration scope, prioritised backlog and evidence-based 16-month schedule, trimming the usual planning effort from three months to six weeks.

Define

Architects framed the target state that combines Mambu, Azure services and a new ODS. We agreed on nightly delta migrations with a single-weekend cut-over and staffed three Scrum squads, Migration, Integration and ODS, around a shared CI/CD pipeline.

Early alignment locked requirements, let security and procurement run in parallel and removed four weeks from the critical path while eliminating scope drift.

Develop

The Migration squad built a .NET engine for cleansing, validation and incremental loads; the Integration squad delivered reusable Azure adapters for fourteen systems; and the ODS squad stood up Data Factory pipelines and a relational model that refreshes after every EOD cycle.

Eight full dress-rehearsal migrations plus continuous testing meant the Mambu environment was fully synced four months before go-live, allowing business users to complete acceptance ahead of schedule.

Deliver

Following the rehearsals, we executed the final switch during a planned weekend window with zero customer downtime, then entered a hyper-care period with 24/7 monitoring, joint war-room support and rapid knowledge transfer to Marginalen’s ops team.

Marginalen went live on Mambu inside the 16-month window, launched two new savings products in the first quarter and cut core-platform change lead-time.

Results

• Cloud-native core live in 16 months. A phased, no-downtime migration replaced Tieto EntraBank with Mambu while all customer channels stayed fully operational.
• New, Avenga-built Operational Data Store (ODS). Our team developed a cloud-native platform that replaces the legacy system’s reporting functions. We delivered T-1 data automatically to finance, treasury, tax, and audit teams. No more manual extracts.
• Integration with fourteeninternal and external systems. Thesekeep payments, compliance checks and back-office workflows running without interruption.

The new Mambu-powered core gives Marginalen Bank a modern, API-first foundation that scales with demand, refreshes its data warehouse every night for next-day reporting, all while preserving real-time payments and back-office workflows through fourteen seamless integrations.