Cache only queries

[sipsorcery]

I've recently replaced some decades old DNS logic with DnsClient.NET (went from 10k lines down to 1k). One feature I'm now missing is the ability to get results directly from the in-memory cache. I can implement a custom feature but I thought I'd first check if it's a desired feature for this library in which case I can do a PR?

As background the requirement for the cache lookups is for a SIP (VoIP) library. In a similar manner to HTTP, SIP requests and responses are sent using SIP URI's in the form sip:1234@sipsorcery.com. A difference from HTTP is that SIP is normally sent over UDP and therefore has a built in retransmit mechanism. If a SIP message does not get a response it will retry the send at intervals of 0.5, 1s, 2s, 4s, 4s, 4s ... up to 11 retries.

How that's relevant for DNS cache lookups is that for high volume SIP server applications it's not feasible to block the main SIP transport thread waiting for DNS lookups. The desired approach is to get the initial request at t0, identify a DNS lookup is required and then discard the message. At t0.5 the same message will be retransmitted. If the DNS result is in the cache it gets used otherwise the message is again discarded. And so on at t1, t2, t4 etc.

[MichaCo]

@sipsorcery happy to hear the library helps to reduce lines of code, that's always good ;)

Interesting use-case.
Isn't it a bit risky though to rely on cache lifetime?
At least with this implementation of DnsClient, the ttl of a record is respected and if that ttl is very low, the response might be cached after your 10th try and then expire before your 11th try...
You could use min/max cache timeouts though to minimize that risk.

Another problem might be that you can get a valid response back which doesn't get cached at all. An empty response with no records for example, because the domain couldn't be resolved. Wouldn't that cause unnecessary retries?

It might be better if you just implement some custom cache on top where you have more control over and can decide what to do with DNS errors or empty responses for example.

Otherwise, yes, we could add a method on the LookupClient to check if the response of a Question is cached by generating the cache key for that question and checking if the key exists.
Feel free to send a PR for that ;)

[sipsorcery]

Otherwise, yes, we could add a method on the LookupClient to check if the response of a Question is cached by generating the cache key for that question and checking if the key exists.

Yes that's pretty much what I had in mind. I'll take a look.

For error and empty responses I still add them to the cache but with relatively short timeouts. Without that the same lookup failure can get repeated multiple times wasting resources. Applications with low volumes of lookups may not want that behaviour so it could be adjusted with a configuration setting on the client.

For the cache lifetime, what types of DNS records have a TTL of less than 4 seconds (the SIP retransmit interval)? That being said I do recall in the past putting a min/max lifetime on cache entries just in case.

[MichaCo]

/closing - PR is merged and will be in v1.4.0

Thanks again for your work 👍