Add a DNS-based EndpointGroup

Many service discovery systems support returning a list of endpoints as records in a DNS response, and since DNS resolution is built into netty, it makes sense for armeria to add support out of the box. Some methods use A records with IP addresses and others use SRV records with CNAMEs - so armeria should ideally support at least both of these.

I dug through netty’s DNS resolver package and found a lot of useful logic, but am not sure if there is a good integration point for it.

The biggest problem is I don’t see a way to support SRV records - the API only exposes knobs for address family, e.g. IPV4 and IPV6, which correspond to A and AAAA only.

https://github.com/netty/netty/blob/1767814a466f9fc6a51c87a84bd73d93eb0ca1d7/resolver-dns/src/main/java/io/netty/resolver/dns/DnsNameResolver.java

I think we also need a way to always be able to know the most recent result for the DNS query for monitoring - it’s very important to be able to present the mapping from service name -> resolved endpoints, and it’s not clear this can be accomplished with the default DnsNameResolver (maybe by showing the state of the dns cache?).

That said, there’s much logic in DnsNameResolver, DnsNameResolverContext, and RoundRobinDnsAddressResolverGroup that I don’t think we want to duplicate into an endpoint group implementation.

[anuraaga]

Just to follow up, I’ve mostly completed an implementation that manually handles the query response. While a good amount of copy-paste, by removing some of the more esoteric features that probably don’t apply in service discovery, I could simplify it quite a bit.

https://github.com/anuraaga/armeria/blob/dev_rag/core/src/main/java/com/linecorp/armeria/client/endpoint/dns/DnsEndpointGroup.java

[anuraaga]

Yeah - I found query and am able to get DNS responses - but found there’s tons and tons of logic dealing with correctly handling the response. It would be quite sad to have to duplicate it

https://github.com/netty/netty/blob/4.1/resolver-dns/src/main/java/io/netty/resolver/dns/DnsNameResolverContext.java#L336

It’s why using resolve would be much nicer, but has the limitation of no SRV support. Any thoughts on a cleaner way to avoid duplicating what looks like pretty hairy (i.e. easily buggy) code?

If there doesn’t seem to be a good way, or clean netty integration point, will just copy-paste 😃