The WHOIS query and response protocol is widely used for databases holding various Internet resources like IPv4 assignments, IPv6 assignments, AS numbers and RIR (for example ARIN in USA or RIPE in Europe).  Members need (details depend on RIR) to keep their records in those databases. In addition, lots of automation in ISP world happens based on those objects in those RIR WHOIS databases. For example whois.ripe.net for RIPE region or whois.apnic.net for APNIC region. In which WHOIS databases are the domain names kept? Is it mandatory for domain registrar to make a record to this/those WHOIS database? I would guess yes, because otherwise the other domain registers would not know that the domain is in use. Is there a RFC which defines what information besides the domain name that will go to this database?
How do domain names get into WHOIS database and which databases get updated for each domain?
domains;dns;domain registration;database;whois
I'll be quoting wikipedia on this, but I believe in this case the information is reliable:WHOIS servers operated by Regional Internet Registries (RIR) can be  queried directly to determine the Internet Service Provider  responsible for a particular resource.The records of each of these registries are cross-referenced, so that  a query to ARIN for a record which belongs to RIPE will return a  place-holder pointing to the RIPE WHOIS server. This lets the WHOIS  user making the query know that the detailed information resides on  the RIPE server. In addition to the RIRs servers, commercial services  exist, such as the Routing Assets Database used by some large networks  (e.g., large Internet providers that acquired other ISPs in several  RIR areas).About Server Discovery, they say this:There is currently no standard for determining the responsible WHOIS  server for a DNS domain, though a number of methods are in common use  for top-level domains (TLDs). Some WHOIS lookups require searching the  procuring domain registrar to display domain owner details.Source: Whois