sq-fx-ecb — findings & quirks

Living log. Update when you learn something new about ECB's feed or our handling.

ECB data model

• Daily reference rates published weekdays around 16:00 CET ("close of trading day in the Eurosystem").
• Format: 1 EUR = rate units of CCY. Every rate is EUR-cross.
• Daily file: https://www.ecb.europa.eu/stats/eurofxref/eurofxref-daily.xml
• 90-day rolling: …/eurofxref-hist-90d.xml
• Full history (since 1999): …/eurofxref-hist.xml (~1 MB) — wired: asof older than ~80 days transparently loads this file instead of the 90-day one. Live-verified 2026-06-11: USD→EUR at 2021-03-15, 2005-07-01 and 1999-01-08 all return that day's fixing.

Cache policy

• Daily file: 12h TTL. Survives a re-run within the trading day; refreshes once the next publishing window passes.
• 90-day file: 24h TTL. Rolls daily anyway.
• Cache dir: ~/.cache/sciqnt/fx-ecb/ (XDG-style; user-owned, outside the repo, survives git clean -fdx).
• Writes are atomic (.tmp + replace) so an interrupted refresh leaves no corrupt XML.

Triangulation

Non-EUR pairs (e.g. USD → GBP) computed via EUR:

rate(USD, GBP) = rate(EUR, GBP) / rate(EUR, USD)

Introduces ~1e-10 rounding (well below money-display granularity). Round-trip (e.g. convert(amount, USD, EUR) then back) does not always equal amount exactly — quantize at display.

Currencies excluded from ECB basket

• RUB — excluded since the 2022 sanctions. Any query for RUB returns None.
• Historically other currencies have entered/left the basket; check the ECB site if a query unexpectedly returns None.

Open issues / TODO

• Consider intraday — ECB ships a daily fixing only. For live FX-sensitive use, sq-fx-yfinance (when built) is the better source.
• Conformance suite uses fixture XML (no network); add an opt-in integration test (SQ_FX_ECB_LIVE=1) that hits the real ECB endpoint to detect schema drift early.