RATIS-1479.Make the min gap to take snapshot configurable

[codings-dan]

What changes were proposed in this pull request?

subtask of Support snapshot command: Make the min gap to take snapshot configurable

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/RATIS-1479

How was this patch tested?

UT

[codings-dan]

The CI test error seems to have nothing to do with this code change.

Error:  testMultipleStreamsMultipleServersStepDownLeader(org.apache.ratis.datastream.TestNettyDataStreamChainTopologyWithGrpcCluster)  Time elapsed: 100.012 s  <<< ERROR!
org.junit.runners.model.TestTimedOutException: test timed out after 100 seconds
	at sun.misc.Unsafe.park(Native Method)
	at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:215)
	at java.util.concurrent.FutureTask.awaitDone(FutureTask.java:426)
	at java.util.concurrent.FutureTask.get(FutureTask.java:204)
	at org.junit.internal.runners.statements.FailOnTimeout.getResult(FailOnTimeout.java:141)
	at org.junit.internal.runners.statements.FailOnTimeout.evaluate(FailOnTimeout.java:127)
	at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
	at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:298)
	at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:292)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.lang.Thread.run(Thread.java:748)

[szetszwo]

@codings-dan , thanks for working on this. Just have some comment on the conf.

    /** The log index gap between to two snapshot creations. */
    String CREATION_GAP_KEY = PREFIX + ".creation.gap";
    long CREATION_GAP_DEFAULT = 1024;
    static long creationGap(RaftProperties properties) {
      return getLong(properties::getLong,
          CREATION_GAP_KEY, CREATION_GAP_DEFAULT, getDefaultLog(), requireMin(1L));
    }
    static void setCreationGap(RaftProperties properties, long minGapTakeSnapshot) {
      setLong(properties::setLong, CREATION_GAP_KEY, minGapTakeSnapshot);
    }

BTW, we should replace the term "takeSanpshot" with "createSnapshot" since file systems (e.g. HDFS) use the term "create". Also, we may have "creation gap", "creation time", etc.

See if you want to do the rename in this pull request.

[szetszwo]

Let's call it "creation.gap".

[szetszwo]

The default should be larger. How about 1024?

[szetszwo]

The min should be 1L.

[codings-dan]

@szetszwo Thanks for reviewing the code, I don’t know much about the term on the file system. In Alluxio, it’s called takesnapshot/checkpoint. I have changed the code according to your comment. In addition, 1024 is really a good number, haha

[szetszwo]

+1 the change looks good.

[szetszwo]

... In Alluxio, it’s called takesnapshot/checkpoint. ...

@codings-dan , Good to know. Thanks.