NewDNSResolver creates a DNS Resolver that can resolve DNS names, and create watchers that poll the DNS server using the default frequency defined by defaultFreq.

NewDNSResolverWithFreq creates a DNS Resolver that can resolve DNS names, and create watchers that poll the DNS server using the frequency set by freq.

Add indicates a new address is added.

Backend indicates the server is a backend server.

Delete indicates an exisiting address is deleted.

GRPCLB indicates the server is a grpclb load balancer.

AddrMetadataGRPCLB contains the information the name resolver for grpclb should provide.

Update defines a name resolution update.

Resolver creates a Watcher for a target to track its resolution changes.

Watcher watches for the updates on the specified target.

AddressType indicates the address type returned by name resolution.

Operation defines the corresponding operations for a name resolution change.