“non-optional + non-trivial” is the key.
Context – trading systems.
I feel trade booking/capture is among the “least optional”. Similarly, settlement, cash mgmt, GL, position database, daily pnl (incl. unrealized). Even the smallest trading shops have these automated. Reasons – automation, reliability, volume. Relational Database is necessary and proven. These are Generally the very first wave of boring, low-pay IT systems. In contrast a lot of new, hot technologies look experimental, evolving and not undoubtedly necessary or proven —
* Sophisticated risk engine is less proven. I don’t know if traders really trust it.
* Pre-trade analysis is less proven.
* huge Market data often feed into research department, risk/analysis systems. I feel some small portion of market data is necessary.
* Algo trading, often based on market data and models
* object DB, dist cache, cloud aren’t always needed
* MOM? i guess many trading systems don’t use MOM but very rare.