A time-based consistency policy describes the amount of time that a replica node is allowed to lag behind the master node. These type of consistency policies matter only for read operations, because replica nodes can only service read operations.
When servicing a read operation, if the replica's data is
more than the specified amount of time out-of-date relative
to the master, then a
ConsistencyException is thrown. In
that event, you can either abandon the operation, retry it
immediately, or pause and then retry it.
In order for this type of a consistency policy to be effective, the clocks on all the nodes in the store must be synchronized using a protocol such as NTP.
In order to specify a time-based consistency policy, you use
the Consistency.Time class. The
constructor for this class requires the following
information:
permissibleLag
A long that describes the number
of TimeUnits the replica is
allowed to lag behind the master.
permissibleLagUnits
A TimeUnit that identifies the
units used by permissibleLag.
For example: TimeUnit.MILLISECONDS.
timeout
A long that describes how long the
replica is permitted to wait in an attempt to meet
the permissibleLag limit. That is,
if the replica cannot immediately meet the
permissibleLag requirement, then
it will wait this amount of time to see if it is
updated with the required data from the master. If
the replica cannot meet the permissibleLag
requirement within the timeout
period, a ConsistencyException
is thrown.
timeoutUnit
A TimeUnit that identifies the
units used by timeout.
For example: TimeUnit.SECONDS.
The following sets a default time-based consistency policy of 2 seconds. The timeout is 4 seconds.
package kvstore.basicExample;
import oracle.kv.Consistency;
import oracle.kv.KVStore;
import oracle.kv.KVStoreConfig;
import oracle.kv.KVStoreFactory;
import java.util.concurrent.TimeUnit;
...
KVStoreConfig kconfig = new KVStoreConfig("exampleStore",
"node1.example.org:5088, node2.example.org:4129");
Consistency.Time cpolicy =
new Consistency.Time(2, TimeUnit.SECONDS,
4, TimeUnit.SECONDS);
kconfig.setConsistency(cpolicy);
KVStore kvstore = KVStoreFactory.getStore(kconfig);