２。hbase CRUD--Lease in hbase

1.What is Lease

in general,between the communication of client and Region Server,there are RPCs in there.Lease is a mechanism to hold a reference object updated each client rpc. so Lease is similar to Hearbeat for server to know that client is alive always!(of course that is in a certain period in fact)

2.Why use Lease

at a first communication ,e.g. a scan operation,the client will get by batch to limit to size returned,then the client issue the second request to retrieve later data.during these requests,region Server will know to these are a scan operaction in fact,so it is needless to renew or create some new objects/caches/handles to locate the nexxt batch data.that is using a Lease will decrease some resourcces overhead!

so all theese cases will provide more stable,avilable.

u will say:why not use hearbeat instead of it?yes this is a good question.but if take a look carefully,the advantages are distingguished :

-Lease has NOT additional connection overhead,but Hearbeat does.in server,there are resouces created once per 'session',and ONLY 'update-on-requesting'

-easy to implement.the server can use some mechanism like lockid to archive Lease.and the client will only keep a lockid in mind for each request.also,the server will have a timeout strategy to cancel this Lease if over a time range that client has not requested something.

and if some exceptions occurs in server,the client will get them and use some utility methods(eg. scan#close() or htable#unlock() to release the lease immediately)

3.How to use Lease

as mentioned above ,there are two clients interacted with Lease:user client and server-side listener.

that is a 'double check ' for the lease in fact!

4.Compare:Mutation VS Get VS Scan

OP	use lease	through by	time to get	time to release	feature
Mutation	yes	row lock id	client:lockRow() server:internalObtainRowLock() if client not locked row before	client: htable#unlock server: RowLockListener#leaseExpired	client: manually to generate or remove lease server: automatically to generate or remove if timeout
Scan	yes	scannerId	client:openScanner (transparent to client)	client: scanner#close server: ScannerListener#leaseExpired	same as above
Gets	no

Questions:

while lock id is unique BUT scanner id ?this will causes some odd issues when multi clients use same scanner id.

lock id generateion:(unique)

while (true) {
        Integer lockId = lockIdGenerator.incrementAndGet();
        HashedBytes existingRowKey = lockIds.putIfAbsent(lockId, rowKey);
        if (existingRowKey == null) {
          return lockId;
        } else {
          // lockId already in use, jump generator to a new spot
          lockIdGenerator.set(rand.nextInt());
        }
      }

scanner id :

protected long addScanner(RegionScanner s) throws LeaseStillHeldException {
    long scannerId = -1L;
    scannerId = rand.nextLong();
    String scannerName = String.valueOf(scannerId);
    scanners.put(scannerName, s);	//直接put不考虑存在问题? TBD
    this.leases.createLease(scannerName, new ScannerListener(scannerName));
    return scannerId;
  }

any clues for me is appreciated!

Note

* invoke a scanner's close method will only remove the lease on region server,but NOT for connection to it.instead of Htable's close method does close all connections established to all servers!

Ref:

hbase源码学习.Lease