E. Leveque
ENTheme: system
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Trading application at Worldline

For 2.5 years, I worked on an application that lets finance professionals buy and sell securities on the stock market.

My 2.5 years at Worldline

  1. March 2023Joined as a back-end developer
  2. ThenMoved to fullstack, in a Scrum team, from requirements to production
  3. Final yearOwned a feature on my own, with the mockup approved by the Product Owner
  4. Sept. 2025End of assignment, 2.5 years later

The problem

A trading application can't afford approximation. When you buy or sell securities, a mistake doesn't stay in a log file: it shows up in an order. My job: deliver complete changes, from analysing the need to production, in a Scrum team.

The decisions

Testing in real conditions

Trade-off: A hard-coded date hid a bug until production. Since then, I test under conditions close to the real thing, which takes more preparation time before delivery.

Validating the mock-up before coding

Trade-off: On a feature I carried alone in my final year, I had the Product Owner validate the mock-up before development. One more exchange up front, but fewer rework loops along the way.

The journey of an order

The journey of a stock market orderA finance professional enters a buy or sell order in the application. The order goes through strict checks (the point where no approximation is allowed), then is sent asynchronously to the stock exchange, which executes it. The confirmation comes back to the professional.Financeprofessional,enters an orderorderApplicationbuying and sellingsecuritiesChecksnoapproximationasynchronoussendingStock exchangeexecutes the orderexecution confirmationThe journey of a stock market orderThe professional enters an order in the application, the order goes through strict checks (no approximation allowed), is sent asynchronously to the stock exchange, which executes it, and the confirmation comes back to the professional.Financeenters an orderApplicationbuying and selling securitiesChecksno approximationasynchronous sendingStock exchangeexecutes the orderBack: execution confirmation

The wall at km 60

The complexity of the solution, and meeting the Definition of Done: a change is only finished once everything is verified, tested and reviewed.

What I take from it

Working on a critical system taught me rigour: analyse before coding, test before shipping, and keep a cool head when everything moves fast.

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