Customizable DNS resolution

[13h3r]

The current implementation of name resolution relies on the JVM name resolution implementation and in case of temporary DNS unavailability can't establish a connection to a remote host when the cached DNS record expires. Returning the latest know IP address might not be the right solution for all cases, but for some cases, it could be a reasonable fallback in case of DNS unavailability.

The primary use-case for this is API clients that are connecting to a limited number of endpoints and would like to have these connections reliable.

This issue is to discuss the need for such functionality in zio-nio. The new layer will be a simple String => IO[..., SocketAddress] and provide a few built-in building blocks like a fallback to the latest know address, background refresh (to avoid blocking calls when cache TTL expires), and others.

The API could be like (PoC)

package zio.nio

import zio.{Ref, UIO, ZIO}

import java.net.UnknownHostException

trait ZNameResolver[R, E] {
  def addr(): ZIO[R, E, InetAddress]
}

object ZNameResolver {
  type UNameResolver = ZNameResolver[Any, Nothing]
  type URNameResolver[R] = ZNameResolver[R, Nothing]
  type NameResolver[E] = ZNameResolver[Any, E]

  def regular(name: String): NameResolver[UnknownHostException] = () => InetAddress.byName(name)

  def onceAndForever[R, E](nameResolver: ZNameResolver[R, E]): ZIO[R, E, UNameResolver] = {
    nameResolver.addr().map(addr => () => UIO.succeed(addr))
  }

  def cacheLatestSuccessful[R, E](resolver: ZNameResolver[R, E]): ZIO[R, E, URNameResolver[R]] = {
    for {
      intialAddr <- resolver.addr()
      ref <- Ref.make(intialAddr)
    } yield () => resolver
      .addr()
      .tap(addr => ref.set(addr))
      .catchAll(_ => ref.get)
  }
}

This layer can also provide additional aspects:

• rotation among multiple addresses that corresponds to a single hostname
• periodical checking of network reachability and eviction of unreachable addresses from addresses pool
• support for SRV type of DNS records