How Aardling facilitated Billie’s strategic pivot using Domain-Driven Design


  • Strategy
  • Domain modelling

Aardling helped Billie during their pivot to Buy Now Pay Later (BNPL) by doing both strategic software mapping and detailed domain modelling.


  • Aardling came in during Billie's pivot to Buy Now Pay Later (BNPL) to do strategic software mapping and detailed domain modelling.

  • Better, more refined models emerged; representing the business process more accurately.

  • Better communication between teams emerged and as a consequence of the improved separation of Bounded Contexts, a better team organisation was established too.

About Billie

Founded in Berlin in 2016, Billie started by providing easy access to short-term liquidity with one of the first fully digital factoring services. Many businesses pass on their invoices to a third party, which then collects the money from the customer.


Third party invoicing is traditionally a highly manual process, but Billie has managed to turn it completely digital. Now, they are expanding by offering a Buy Now Pay Later (BNPL) payment method for B2B e-commerce. With a $100 million Series C investment and a partnership with Klarna, they continue to succesfully simplify the business purchasing experience.

"After the first years of our market fit journey, we pivoted to BNPL. As is common in fast changing startups,

our early software architecture and anaemic domain models weren’t fit for purpose anymore.This is why we decided to bring in Aardling."

Ahmed Gaber

VP of Engineering

The process

Mathias Verraes and Stijn Vannieuwenhuyse, Aardling consultants, helped Billie look holistically across their whole business: the checkout journey, the domain models, the boundaries and their teams.

In a series of strategy sessions, they created various maps and big-picture models. This showed that the way the teams and the codebase were organised around features wasn’t cutting it. To support fast, sustainable growth, they needed to organise around domains instead.

This business model pivot from automated factoring to BNPL caused a lot of complexity in the Payments and Ledger systems. Aardling made sure to include financial experts from across Billie in the Domain Discovery sessions.

By bridging the gap between domain experts and engineering, we uncovered that some wrong assumptions were made about the journal, payments execution, and reconciliation processes.

After the initial strategy sessions, Stijn worked with teams in Billie’s core domain to refine the models of Financing Authorisation, Payments Execution, and Ledger.

The Solution

Stijn Vannieuwenhuyse started hammering down on the names in the model. Everything was modeled as Tickets in a Customer Support-style system. Stijn remodelled these as Factoring Cases, and broke these further down into separate processes for selling an invoice to an investor, reselling it, and buying it back.

This new model was much more representative of how the business operated. There was a Payments Ledger model, but that was exposing a broad set of data that needed to be re-interpreted by the downstream system.

By extracting a Financing History model from Ledger, focused on the specific needs of Factoring Cases, communication with downstream components became a lot easier. The teams were changed as well, separating the Case Management team from the Ledger team.

"Working with Aardling to set up better boundaries in our architecture and teams paid off in terms of collaboration and simplicity.

Their facilitation techniques reduced the gap in knowledge, strategically and down at the level of code. Domain-Driven Design is somewhat radical, but they brought everyone on board from engineering to business to financial experts."

Ahmed Gaber

VP of Engineering

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