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From Andor Molnar <an...@cloudera.com>
Subject Re: Name resolution in StaticHostProvider
Date Thu, 22 Feb 2018 17:06:54 GMT
Did anybody happen to take a quick look by any chance?

I don't want to push this too hard, because I know it's a time consuming
topic to think about, but this is a blocker in 3.5 which has been hanging
around for a while and any feedback would be extremely helpful to close it


On Mon, Feb 19, 2018 at 12:18 PM, Andor Molnar <andor@cloudera.com> wrote:

> Hi all,
> We need more eyes and brains on the following PR:
> https://github.com/apache/zookeeper/pull/451
> I added a comment few days ago about the way we currently do DNS name
> resolution in this class and a suggestion on how we could simplify things a
> little bit. We talked about it with Abe Fine, but we're a little bit unsure
> and cannot get a conclusion. It would be extremely handy to get more
> feedback from you.
> To add some colour to it, let me elaborate on the situation here:
> In general, the task that StaticHostProvider does is to get a list of
> potentially unresolved InetSocketAddress objects, resolve them and iterate
> over the resolved objects by calling next() method.
> *Option #1 (current logic)*
> - Resolve addresses with getAllByName() which returns a list of IP
> addresses associated with the address.
> - Cache all these IP's, shuffle them and iterate over.
> - If client is unable to connect to an IP, remove all IPs from the list
> which the original servername was resolved to and re-resolve it.
> *Option #2 (getByName())*
> - Resolve address with getByName() instead which returns only the first IP
> address of the name,
> - Do not cache IPs,
> - Shuffle the *names* and resolve with getByName() *every time* when
> next() is called,
> - JDK's built-in caching will prevent name servers from being flooded and
> will do the re-resolution automatically when cache expires,
> - Names with multiple IPs will be handled by DNS servers which (if
> configured properly) return IPs in different order - this is called DNS
> Round Robin -, so getByName() will return different IP on each call.
> *Options #3*
> - There's a small problem with option#2: if DNS server is not configured
> properly and handles the round-robin case in a way that it always return
> the IP list in the same order, getByName() will never return the next ip,
> - In order to overcome that, use getAllByName() instead, shuffle the list
> and return the first IP.
> All feedback if much appreciated.
> Thanks,
> Andor

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