Monday, September 19, 2005
Java Net, DNS Caching and Availability
Many organizations are deploying global load balancers to load balance across geographically distributed data centers. This also imporves service availability since one data center can be taken offline for maintainance without disruption to service. The GLSBs use DNS resolution to direct traffic to server farms. To ensure that the system that uses other backend web services is highly available, can handle failovers, and recover from failover without requiring a server re-start do the following:
The DNS name to IP address resolution capability is provided by InetAddress class (part of java.net package - core networking package for Java Platform). The default implementation is to cache DNS-to-IP resolution FOREVER. In fact InetAddress will also cache un-successful DNS-to-IP resolution for 10 seconds (default).
Java 1.4 and above versions provide system properties to modify DNS caching behavior by setting the the cache TTL (time-to-live) and negative cache TTL (i.e. failed resolution) documented here, http://java.sun.com/j2se/1.4.2/docs/guide/net/properties.html.
Unfortunately there is no standard or formally documented way of changing DNS caching behavior in versions Java 1.3 and below. However there is a non-standard Sun proprietary system property that can be set at Java command line to change the behavior (Java 1.4 documentation actually includes this property name). The property is
sun.net.inetaddr.ttl
The system property is specified at command line as:
java -Dsun.net.inetaddr.ttl=0
Values are interpreted as:
-1 (default) => Cache FOREVER
0 => Disable caching. This means every call to resolve address will require DNS query.
+integer => In seconds TTL for cache entry i.e. time after which cache entry is stale. After this time, a call for DNS-to-IP will result in DNS query.
- Use URL to connect to service endpoint, so that DNS lookup is used to determine service endpoint IP address.
- Java DNS cache TTL is set to a reasonable value. By DEFAULT Java's DNS resolution will cache DNS to IP resolution FOREVER. After the initial DNS-to-IP (successful) resolution only way to force Java to make DNS query is to re-cycle the JVM. Obviously this is not very good for building highly available system.
The DNS name to IP address resolution capability is provided by InetAddress class (part of java.net package - core networking package for Java Platform). The default implementation is to cache DNS-to-IP resolution FOREVER. In fact InetAddress will also cache un-successful DNS-to-IP resolution for 10 seconds (default).
Java 1.4 and above versions provide system properties to modify DNS caching behavior by setting the the cache TTL (time-to-live) and negative cache TTL (i.e. failed resolution) documented here, http://java.sun.com/j2se/1.4.2/docs/guide/net/properties.html.
Unfortunately there is no standard or formally documented way of changing DNS caching behavior in versions Java 1.3 and below. However there is a non-standard Sun proprietary system property that can be set at Java command line to change the behavior (Java 1.4 documentation actually includes this property name). The property is
sun.net.inetaddr.ttl
The system property is specified at command line as:
java -Dsun.net.inetaddr.ttl=0
Values are interpreted as:
-1 (default) => Cache FOREVER
0 => Disable caching. This means every call to resolve address will require DNS query.
+integer => In seconds TTL for cache entry i.e. time after which cache entry is stale. After this time, a call for DNS-to-IP will result in DNS query.
