RDAP, the Registration Data Access Protocol, is the modern replacement for WHOIS. It answers the same core question - who registered a domain or IP resource and when - but delivers the answer as structured, machine-readable JSON over standard HTTP rather than the inconsistent free-text format WHOIS returns.
The structured output is RDAP's biggest advantage. Because every registry returns data in the same defined JSON schema, software can parse fields like creation date, expiration, registrar, and status reliably without writing a custom parser for each registry's quirky text layout. This makes automated domain research far easier.
RDAP was also designed for the privacy and access-control needs that WHOIS handles poorly. It supports differentiated access so that authorized parties can see more detail while the public sees redacted data, and it natively handles internationalized names and standardized status codes, addressing shortcomings that accumulated over WHOIS's decades of use.
ICANN has been steering the industry from WHOIS toward RDAP, and most generic top-level domains now support it. For anyone building tools that check domain age or registration status at scale, RDAP is the forward-looking choice, though WHOIS remains widely used during the transition period.