Field Notes
How we build systems that move money
Practical engineering writing on continuous integration, deployment, determinism and modernisation for institutional trading systems.
What continuous integration actually means for a trading system
CI in a trading business is not "the tests go green". It is the machinery that lets you change a system that moves real money, on an ordinary Tuesday, without holding your breath.
Deploying a trading system without stopping the market
Zero-downtime deployment is a solved problem for web services and an unsolved one for anything holding a position. Here is how the patterns change when state and orders are on the line.
Determinism in market-data pipelines: testing a run you cannot repeat
If replaying yesterday’s data does not reproduce yesterday’s decisions, you cannot test anything meaningful. Determinism is the property that makes every other guarantee possible.
Latency is a budget, not a number
A single average latency figure hides everything that matters. Treating latency as an allocated budget across a path, measured at the tail, is what turns optimisation from guesswork into engineering.
Migrating a legacy trading stack without a big-bang cutover
The weekend cutover is the most expensive migration strategy available and the most commonly chosen. The alternative is slower to start, far cheaper to finish, and reversible at every step.
The build pipeline as a risk control
Regulators and risk committees ask who can change a production trading system and how that change is evidenced. A well-designed pipeline answers both automatically, as a by-product of working normally.
Illustrative engineering writing. Examples are generalised from common patterns, not from any single client system.