Thank you, Gavin, for the detailed explanations, helped me a lot.
I will follow your advice and not cache.
Best,
Kaspar
On Tue, Dec 17, 2013 at 6:42 PM, Gavin Lambert
On 18/12/2013 14:21, Quoth Kaspar Fischer:
Have others had any experiences with this? I want to avoid making a DNS
lookup every time I perform a (HTTP) request.
Typically you should. The OS has its own cache of recently queried domains and the corresponding Time-To-Live value (ie. "how long this is valid for") provided by the actual DNS server, and larger networks will often have a secondary DNS cache on the LAN that also respects this.
So a resolve doesn't always hit the Internet, but it's important to respect the TTL policy set by the target server to properly handle load balancing and dynamic addressing.
Also – and not strictly related to this question – why are DNS lookups
in Boost Asio tied to a service, as in resolve(query(host,"http"))?
Because the underlying getaddrinfo expects a service name/port. I think it's just a convenience to obtain the port number at the same time as the hostname, assuming both were provided as strings, rather than having to make two separate calls to resolve each.
Personally, I think it's better to use the actual numeric port instead of using a service name, unless you're obtaining the service name from user input (such as an URL) -- and even then typically a given app will only support specific service types anyway, so there isn't really much call for being able to look them up by name. Maybe there are historic reasons for it. (I know each client has a config file that contains the name->port mappings, but I can't think of any scenario in which editing this file to produce different results is useful.)
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