Glossary
What is Rate limit?
A rate limit caps how many requests a client can make to an API in a given time; a request over the limit is refused, usually with HTTP status 429 Too Many Requests.
Status 429 comes from RFC 6585, published in April 2012, which says it "indicates that the user has sent too many requests in a given amount of time" and lets the server add a Retry-After header saying how long to wait. A client that reads Retry-After and waits that long, instead of retrying at once, gets through a busy period with the fewest refusals.
Rate limits are why a big send works better as one request carrying many numbers than as a long run of single sends. GOV.UK Notify, for example, limits a service to 3,000 messages a minute for each type of API key.
CaptiFi Connect allows each API key 20 requests a second, with bursts up to 40, and answers anything over that with a 429 and a Retry-After in whole seconds. One send request can carry up to 1,000 typed numbers or a whole contact list, so a campaign rarely comes near the limit.
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