Locations API uses a PostgreSQL database to cache results from OS Places API; the provider where we get our postcode results from.
Read the original architectural plan for further details.
When a request for a postcode is made to Locations API, Locations API checks its database. If it doesn't have it, it will make a request to OS Places API, serve the response to the user, and save the result in the database.
We performed a one-off import of all known postcodes to prime the cache before making Locations API available for Production use.
The results for a postcode can change over time, e.g. a building being renamed.
To minimise the risk of serving out of date responses, we keep postcode results up to date by spawning a PostcodesCollectionWorker sidekiq worker every second. This worker grabs a few of the oldest records, and spawns a ProcessPostcodeWorker to update each one by performing a new fetch from OS Places API and overwriting the record.
We currently update 3 postcodes per second, which would process 1.8 million postcodes in around 7 days. We have to take care to avoid straining the OS Places API, which has a rate limit of 600 requests per minute.