`dns.setServers` doesn't play well with async code

[silverwind]

nodejs/node-v0.x-archive#9243 (comment) led me to realizing that the way dns.setServers is implemented doesn't play too well with async code.

The current target server is stored in a per-app global state, which would probably be fine with sync code, but complicates things if you want to query multiple servers with our async resolve.

I'd propose adding an options object to resolve containing servers, while deprecating dns.setServers in a major version. I think having an option object on resolve methods could prove useful later.

Note: I haven't checked if c-ares enables per-query target servers easily, maybe @indutny can comment on that part.

[indutny]

Hm... this is not a bad suggestion at all, but I this is not the way c-ares works :) The thing that you propose will require creating and keeping alive separate channels in a hashmap.

[silverwind]

Ah, so there is a reason it's implemented in this way.

Maybe something that should be kept in mind for #1013.

[piscisaureus]

Another solution would be to allow multiple "resolver" instances, e.g. support the following pattern:

var resolver = dns.createResolver(server);
resolver.lookup(...);
resolver.close();

The default lookup method would just delegate to a default resolver.

[indutny]

Please don't close this, I didn't mean that it wasn't feasible :) Just wanted to stress out how it will be implemented.

[silverwind]

"resolver" instances

This is what node-dns basically does. It's an option, but I think I'm leaning more towards the option object for simplicity.

[ChALkeR]

Atm, dns.setServers asserts when there are running dns queries or callbacks of dns queries.

This crashes:

var dns = require('dns');

var servers = dns.getServers();
dns.setServers(['208.67.222.222', '208.67.220.220']);
dns.resolve4('myip.opendns.com', function(err, addresses, family) {
    dns.setServers(servers);
    console.log('addresses:', addresses);
});

This works:

var dns = require('dns');

var servers = dns.getServers();
dns.setServers(['208.67.222.222', '208.67.220.220']);
dns.resolve4('myip.opendns.com', function(err, addresses, family) {
    setTimeout(function() { dns.setServers(servers); }, 0);
    console.log('addresses:', addresses);
});

[silverwind]

@ChALkeR this is known, see #894.

[snsparrish]

I hope this idea hasn't died. I'd really like to see this functionality. The node-dns alternative doesn't appear to be functioning at all in 2.2.1.