Payments reconciliation rebuild
2025Fintech · 6 months
Replaced a nightly batch that silently dropped rows with an event-sourced ledger. Discrepancies went from hundreds a week to zero.
ida-renner / about
$ available from March
I fix the systems that page you at 3am
Freelance backend engineer. I take on the parts of a system that are expensive to get wrong: payments, queues and the migration nobody wants to own.
Four weeks notice. Outside IR35. References on request.
Selected work
Things I shipped and what changed
Client names on request. Every one of these is something I would be happy to walk you through in detail.
Postgres to Aurora migration
2024Marketplace · 3 months
4TB moved with dual-write and backfill. Eleven seconds of write downtime, planned and announced.
Queue redesign
2024Logistics · 4 months
At-least-once with idempotent consumers, replacing an at-most-once system that lost orders under load.
API versioning scheme
2023SaaS · 2 months
Contract diffing in CI, so a breaking change fails the build rather than a customer's integration.
What I take on
The expensive parts, not the whole backlog
Payments & billing
Idempotency, reconciliation and the failure cases nobody writes tests for until after the incident.
Queues & event systems
Delivery guarantees, ordering, dead letters and a replay path you can actually run under pressure.
Migrations
Expand and contract, backfill, dual-write and the rollback plan written before the first deploy.
API design
Contracts your consumers can depend on, versioned properly, with breaking changes graded not guessed.
Performance work
Profile first. Most of what people want rewritten turns out to be one query and one missing index.
Handover
Runbooks, diagrams and a fortnight of pairing. You should not need me after I leave.
Why people pick us
The bits that go wrong on a build, written down before we start
Two clients at a time
You get someone who has actually loaded your system into their head, not a slice of someone's week.
I will talk you out of the rewrite
Usually it is one query, one index and a queue. I would rather tell you that than bill you for a year.
Written down, not in my head
Design docs before, runbooks after. The work has to survive me leaving or it was not finished.
Rollback plan before the first deploy
Every migration ships with the way back, tested. That is what makes the way forward safe.
12
Years engineering
31
Projects shipped
2
Clients at a time
4
Weeks notice
Reviews
What customers said afterwards
She spent the first week reading and asking questions rather than writing code, which made me nervous. The design doc at the end of it was the clearest description of our own system anyone here had produced.
The handover was the good bit. Runbooks, diagrams, and two weeks of pairing. Nobody has needed to call her since, which I think was the point.
Told us the rewrite we asked for was not necessary and fixed it with an index and a queue change instead. Cost us a fifth of the budget we had set aside.
Get in touch
Tell me what is breaking
A paragraph is plenty. If it is not something I should take on I will say so, and usually suggest who should.