Multimap API V1.2: Maintaining Service Availability

DNS Services

To maintain service availability, we provide our ASP services from multiple web server farms around the world. Multimap services are resilient because we make use of a dynamic Domain Name System (DNS) service that constantly checks the health of our server farms and responds to customers' DNS queries with the IP addresses of only those farms that are operating at optimal performance. While desktop web browsers do this without requiring action from the user, when using any of our XML services you should ensure that your application-layer software also correctly implements the DNS protocol (just like desktop browsers do).

When your browser visits www.multimap.com, it obtains the Multimap domain's IP address by performing lookups on DNS servers. The IP address is returned and cached locally in the user's browser for the remainder of its validity period (up to 30 seconds in the case of Multimap domains). During this time, any subsequent browser requests to www.multimap.com use that cached IP address directly. The first request to be submitted after the expiry of that validity period triggers the browser to make a fresh DNS lookup.

This is also how most web client libraries work. However, some older web client libraries do not support the DNS protocol fully and ignore the time-to-live (TTL) validity periods reported by DNS servers. They continue to use a (cached) IP address even when it can no longer be guaranteed to be available. If your web client library only caches DNS lookups for the duration of the validity period, you will immediately benefit from the resilience of our multiple server farms.

DNS address lookups for Multimap domains have a validity period of just 30 seconds, after which time any previously-reported server farm IP address cannot be guaranteed to be available. To ensure that your application can benefit from Multimap's resilience, your application-layer software should always re-query DNS servers for the IP address of an efficiently-functioning server farm before the 30-second validity period has expired. We also recommend that DNS servers be re-queried in the event of an interruption or failure of an internet connection and, if the IP address reported is different, to re-submit the service request.

DTD References

Multimap recommends that you configure your software to refer to our DTDs only when developing your systems; when you make your code live, you should remove all requests to these DTDs. There are two reasons for this: