in-memory-compaction logging
Adds logging of CompactingMemStore configuration on construction.
Add logging of detail about Store on creation including memstore type.
Add chapter to refguide on new in-memory compaction feature.
When prefetch on open is specified, there is a deadlocking case
where if the prefetch is cancelled, the PrefetchExecutor interrupts
the threads if necessary, when that happens in FileIOEngine, it
causes an ClosedByInterruptException which is a subclass of
ClosedChannelException. If we retry all ClosedChannelExceptions,
this will lock as this access is expected to be interrupted.
This change removes calling refreshFileConnections for
ClosedByInterruptExceptions.
Signed-off-by: Andrew Purtell <apurtell@apache.org>
The Master asks a RegionServer whether a Region can be split or not, primarily to
verify that the region is not closing, opening, etc. This change has the RegionServer
also consult the configured RegionSplitPolicy.
Signed-off-by: Josh Elser <elserj@apache.org>
Remove commons-cli and commons-collections4 use. Account
for the newer internal protobuf version of 3.5.1.
Signed-off-by: Michael Stack <stack@apache.org>
Signed-off-by: Mike Drob <mdrob@apache.org>
Changes for HBASE-20027 seem to cause UI not showing up on default port in standalone mode. For concurrent
unit test execution, individual tests can set hbase.localcluster.assign.random.ports to true or modify
test/resources/hbase-site.xml.
Changes for HBASE-20027 seem to cause UI not showing up on default port in standalone mode. For concurrent
unit test execution, individual tests can set hbase.localcluster.assign.random.ports to true or modify
test/resources/hbase-site.xml.
Adds a prepare step to RecoverMetaProcedure in which we test for
cluster up and master being up. If not up, we fail the run.
Modified hbase-server/src/main/java/org/apache/hadoop/hbase/master/cleaner/HFileCleaner.java
Modified hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/ChunkCreator.java
Minor log cleanup.
Modified hbase-server/src/main/java/org/apache/hadoop/hbase/master/procedure/RecoverMetaProcedure.java
Add pepare step.
Modified hbase-server/src/test/java/org/apache/hadoop/hbase/master/TestAssignmentManagerMetrics.java
Debug for the failing test....
Added hbase-server/src/test/java/org/apache/hadoop/hbase/master/procedure/TestRecoverMetaProcedure.java
Test the prepare step goes down if master or cluster are down.
in-memory compactions)
Log less. Log using same format as used elsewhere in log.
Align logs in HFileArchiver with how we format elsewhere. Removed
redundant 'region' qualifiers, tried to tighten up the emissions so
easier to read the long lines.
M hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/ChunkCreator.java
Add a label for each of the chunkcreators we make (I was confused by
two chunk creater stats emissions in log file -- didn't know that one
was for data and the other index).
M hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/CompactSplit.java
Formatting. Log less.
M hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/MemStoreCompactionStrategy.java
Make the emissions in here trace-level. When more than a few regions,
log is filled with this stuff.
HBASE-19435 implements a fix for reopening file channels when they are unnexpected closed
to avoid disabling the BucketCache. However, it was missed that the the channels might not
actually be completely closed (the write or read channel might still be open
(see https://docs.oracle.com/javase/7/docs/api/java/nio/channels/ClosedChannelException.html)
This commit closes any open channels before creating a new channel.
M hbase-client/src/main/java/org/apache/hadoop/hbase/client/DoNotRetryRegionException.java
M hbase-client/src/main/java/org/apache/hadoop/hbase/exceptions/MergeRegionException.java
Allow passing cause to Constructor.
M hbase-protocol-shaded/src/main/protobuf/MasterProcedure.proto
Add prepare step to move procedure.
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/MergeTableRegionsProcedure.java
Add check that regions to merge are actually online to the Constructor
so we can fail fast if they are offline
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/MoveRegionProcedure.java
Add prepare step. Check regions and context and skip move if not right.
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/SplitTableRegionProcedure.java
Add check parent region is online to constructor.
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/procedure/AbstractStateMachineTableProcedure.java
Add generic check region is online utility function for use by subclasses.
M hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestRegionMove.java
Add test that we fail if we try to move an offlined region.
Add Fail-Fast to Procedures by throwing exception out of Procedure
constructor so if move but table is disabled or if master is going
down, etc., we can give notice before the procedure is scheduled.
Will help guard against scheduling Procedures that will have a hard
time succeeding; e.g. a move when table is offline.
Also fixed bug around table state where we presumed ENABLED though no
entry in hbase:meta (we were using this mechanism for whether a table
existed or not).
M hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestRegionMove.java
Test stolen from HBASE-20131
M hbase-client/src/main/java/org/apache/hadoop/hbase/client/TableState.java
Add convenience isEnabled/isDisabled
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/HMaster.java
Promote assert state to throw exception.
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/MasterServices.java
Add isClusterUp
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/AssignmentManager.java
Move constructor now throws exception
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/MoveRegionProcedure.java
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/MergeTableRegionsProcedure.java
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/SplitTableRegionProcedure.java
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/procedure/DisableTableProcedure.java
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/procedure/ModifyTableProcedure.java
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/procedure/RestoreSnapshotProcedure.java
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/procedure/TruncateTableProcedure.java
Do environment check at construction and fail-fast if hostile.
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/procedure/AbstractStateMachineTableProcedure.java
Add preflightCheck utility method.
M hbase-client/src/main/java/org/apache/hadoop/hbase/MetaTableAccessor.java
Removed setting time setting table state; broke when using other than
default environment edge masked by presumption that no state meant
active.
Allow that DisableTableProcedue can grab a region lock before
ServerCrashProcedure can. Cater to this cricumstance where SCP
was not unable to make progress by running the search for RIT
against the crashed server a second time, post creation of all
crashed-server assignemnts. The second run will uncover such as
the above DisableTableProcedure unassign and will interrupt its
suspend allowing both procedures to make progress.
M hbase-protocol-shaded/src/main/protobuf/MasterProcedure.proto
Add new procedure step post-assigns that reruns the RIT finder method.
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/AssignmentManager.java
Make this important log more specific as to what is going on.
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/UnassignProcedure.java
Better explanation as to what is going on.
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/procedure/ServerCrashProcedure.java
Add extra step and run handleRIT a second time after we've queued up
all SCP assigns. Also fix a but. SCP was adding an assign of a RIT
that was actually trying to unassign (made the deadlock more likely).
We assumed that we can run for loop from 0 to lastStep sequentially. MergeTableRegionProcedure skips step 2. So, when i is 0 the procedure is already at step 3.
Added a method StateMachineProcedure#getCurrentStateId that can be used from test code only.
On failed RPC we expire the server and suspend expecting the
resultant ServerCrashProcedure to wake us back up again. In tests,
TestRSGroup hung because it failed to schedule a server expiration
because the server was already expired undergoing processing (the
test was shutting down). Deal with this case by having expire
servers return false if unable to expire. Callers will then know
where a ServerCrashProcedure has been scheduled or not.
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/ServerManager.java
Have expireServer return true if successful.
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/RegionTransitionProcedure.java
The log that included an exception whose message was the current
procedure as a String totally baffled me. Make it more obvious what
exception is.
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/UnassignProcedure.java
If failed expire of a server, wake our procedure -- do not suspend --
and presume ok to move region to CLOSED state (because going down or
concurrent crashed server processing ongoing).
This de-couples the snapshot size calculation from the
SpaceQuotaObserverChore into another API which both the periodically
invoked Master chore and the Master service endpoint can invoke. This
allows for multiple sources of snapshot size to reported (from the
multiple sources we have in HBase).
When a file is archived, snapshot sizes can be more quickly realized and
the Master can still perform periodical computations of the total
snapshot size to account for any delayed/missing/lost file archival RPCs.
Signed-off-by: Ted Yu <yuzhihong@gmail.com>
* rely on git plumbing commands when checking if we've built the site for a particular commit already
* switch to forcing '-e' for bash
* add command line switches for: path to hbase, working directory, and publishing
* only export JAVA/MAVEN HOME if they aren't already set.
* add some docs about assumptions
* Update javadoc plugin to consistently be version 3.0.0
* avoid duplicative site invocations on reactor modules
* update use of cp command so it works both on linux and mac
* manually skip enforcer plugin during build
* still doing install of all jars due to MJAVADOC-490, but then skip rebuilding during aggregate reports.
* avoid the pager on git-diff by teeing to a log file, which also helps later reviewing in the case of big changesets.
Signed-off-by: Michael Stack <stack@apache.org>
Signed-off-by: Misty Stanley-Jones <misty@apache.org>
Master never left waitForMasterActive because it never checked state of
the clusterUp flag. The test here was aborting regionserver and then
just exiting. The minihbasecluster shutdown sets the cluster down flag
but we were never looking at it so Master thread was staying up.
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/HMaster.java
The tableOnMaster check in waitForMasterActive looks wrong. It was
making it so a 'normal' Master was getting stuck in here. This is not
the place to worry about tablesOnMaster. That is for the balancer to be
concerned with. There is a problem with Master hosting
system-tables-only. After further study, Master can carry regions like a
regionserver but making it so it carries system tables only is tricky
given meta assign happens ahead of all others which means that the
Master needs to have checked-in as a regionserver super early... It
needs work. Punted for now.
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/AssignmentManager.java
Mostly renaming so lists and maps of region infos have same name as they
have elsewhere in code base and cleaning up confusion that may arise
when we talk of servers-for-system-tables....It is talking about
something else in the code changes here that is other than the normal
understanding. It is about filtering regionservers by their version
numbers so we favor regions with higher version numbers. Needs to go
back up into the balancer.
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/balancer/BaseLoadBalancer.java
It was possible for the Master to be given regions if no regionservers
available (as per the failing unit test in this case).
M hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/HRegionServer.java
Minor reordering moving the waitForMasterActive later in the initialize
and wrapping each test in a check if we are to keep looping (which
checks cluster status flag).
M hbase-server/src/test/java/org/apache/hadoop/hbase/master/TestAssignmentManagerMetrics.java
This was an old test from the days when Master carried system tables.
Updated test and fixed metrics. Metrics count the hbase:meta along with
the userspace region so upped expected numbers (previously the
hbase:meta was hosted on the master so metrics were not incremented).
M hbase-server/src/test/java/org/apache/hadoop/hbase/master/balancer/TestRegionsOnMasterOptions.java
I took a look at this test again but nope, needs a load of work still to
make it pass.
M hbase-zookeeper/src/main/java/org/apache/hadoop/hbase/zookeeper/ZKUtil.java
Stop being so whiney.
Certain operations in HBase are known to directly affect
the utilization of tables on HDFS. When these actions
occur, we can circumvent the normal path and notify the
Master directly. This results in a much faster response to
changes in HDFS usage.
This requires FS scanning by the RS to be decoupled from
the reporting of sizes to the Master. An API inside each
RS is made so that any operation can hook into this call
in the face of other operations (e.g. compaction, flush,
bulk load).
Signed-off-by: Ted Yu <yuzhihong@gmail.com>
Allow OPEN as a possible state when update region transition state.
Usually state is OPENING but if crash before finish step is completed,
on replay, master may have read that the state is OPEN from meta table
and so will think it open... When we replay the procedure finish, allow
that the region is already OPEN.
Fix MoveRandomRegionOfTableAction. It depended on old AM behavior.
Make it do explicit move as is required in AMv3; w/o it, it was just
closing region causing test to fail.
Fix pom so hadoop3 profile specifies a different netty3 version.
Bunch of logging format change that came of trying trying to read
the spew from this test.
This reverts commit f1a81618fd.
Conflicts:
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/HStore.java
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/Segment.java
patch reverted changes that happened in parallel without explanation. see jira.
Negative Remaining KVs and progress percent greater than 100 is because CompactionProgress#totalCompactingKVs is sometimes less than CompactionProgress#currentCompactedKVs.
Changes add a getter to CompactionProgress#totalCompactingKVs and from inside getter warning is logged. currentCompactedKVs are return when totalCompactingKVs are less than current.
Signed-off-by: Michael Stack <stack@apache.org>
Revert "Revert "HBASE-2004 TestClientClusterStatus is flakey""
This is a revert of a revert, i.e. a reapplication, just so I can fix
the JIRA number.
This reverts commit 6796b8e21f.
Negative Remaining KVs and progress percent greater than 100 is because CompactionProgress#totalCompactingKVs is sometimes less than CompactionProgress#currentCompactedKVs.
Changes add a getter to CompactionProgress#totalCompactingKVs and from inside getter warning is logged. currentCompactedKVs are return when totalCompactingKVs are less than current.
LocalHBaseCluster forces random port assignment for sake of concurrent unit test
execution friendliness, but we still need a positive test for RPC and info port
assignment.
The 1.x functionality of Master DDL operations is that "post" observer hooks
are only invoked when the DDL action was successful. With the async-ness of
ProcV2, we find ourselves in a case where the post-hook may be invoked before
the Procedure runs and fails. We need to introduce some blocking to wait and
see if the Procedure is going to fail on a precondition before invoking the hook.
Signed-off-by: Michael Stack <stack@apache.org>
- Added ADMIN permission check for following rpc calls:
normalize, setNormalizerRunning, runCatalogScan, enableCatalogJanitor, runCleanerChore,
setCleanerChoreRunning, execMasterService, execProcedure, execProcedureWithRet
- Moved authorizationEnabled check to start of AccessChecker's functions. Currently, and IDK why,
we call authManager.authorize() first and then discard its result if authorizationEnabled is false. Weird.
Remove assert in splittableregionprocedure. It was in the prepare.
Was causing fail in legit case where a region split follows a
table split BEFORE the parent has been GC'd. The region split
finds the parent in SPLIT state which is right. The assert was
having us fail. No need.
Also disabled TestHTrace since not supported in 2.0.0 and flakey.
Only call server.checkIfShouldMoveSystemRegionAsync if a node has been
added. Do not call it if only one regionserver in cluster. Make it
so ServerCrashProcedure runs before it. Add logging if
server.checkIfShouldMoveSystemRegionAsync was responsible for
MOVE (Previous was a mystery when it cut in).
Previous we'd call it when there was a nodeChildrenChanged. These
happen before nodeDeleted. If a server crashed,
checkIfShouldMoveSystemRegionAsync could run first, find the
server that had not yet registered as crashed, find system
tables on it and then try to move them. It would fail because
server would not respond to RPC. The region move would then
be waiting on the servercrashprocedure to wake it up when
done processing but this move had locked the region so
SCP couldn't run....
Serializing, if appropriate, write the hbase-1.x version of the
Comparator to the hfile trailer so hbase-1.x files can read hbase-2.x
hfiles (they are the same format).
This patch adds mirroring of table state out to zookeeper. HBase-1.x
clients look for table state in zookeeper, not in hbase:meta where
hbase-2.x maintains table state.
The patch also moves and refactors the 'migration' code that was put in
place by HBASE-13032.
D hbase-client/src/main/java/org/apache/hadoop/hbase/CoordinatedStateException.java
Unused.
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/HMaster.java
Move table state migration code from Master startup out to
TableStateManager where it belongs. Also start
MirroringTableStateManager dependent on config.
A hbase-server/src/main/java/org/apache/hadoop/hbase/master/MirroringTableStateManager.java
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/TableStateManager.java
Move migration from zookeeper of table state in here. Also plumb in
mechanism so subclass can get a chance to look at table state as we do
the startup fixup full-table scan of meta.
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/AssignmentManager.java
Bug-fix. Now we create regions in CLOSED state but we fail to check
table state; were presuming table always enabled. Meant on startup
there'd be an unassigned region that never got assigned.
A hbase-server/src/test/java/org/apache/hadoop/hbase/master/TestMirroringTableStateManager.java
Test migration and mirroring.
Value of 'htd' is null as it is initialized in the constructor but when the object is deserialized its null. Got rid of member variable htd and made it local to method.
* Added a comment in MergeTableRegionsProcedure and SplitTableRegionProcedure explaining specific rollbacks has side effect that AssignProcedure/s are submitted asynchronously and those procedures may continue to execute even after rollback() is done.
* Updated comments in tests with correct rollback state to abort
* Added overloaded method MasterProcedureTestingUtility#testRollbackAndDoubleExecution which takes additional argument for waiting on all procedures to finish before asserting conditions
* Updated TestMergeTableRegionsProcedure#testRollbackAndDoubleExecution and TestSplitTableRegionProcedure#testRollbackAndDoubleExecution to use newly added method
Signed-off-by: Michael Stack <stack@apache.org>
Fix two issues:
# Meta Replicas can all be assigned to the same server. This
will call the test to hang when we do our kill of the server
hosting meta because there'll be no replicas to read from
as test intends. Check is to look for this condition on
startup and adjust if we come across it. Replicas cross-cut
assignment. They need work.
# Other issue was shutdown. The master started toward the
end of the test may not have come up fully by the time
shutdown is called. We could be stuck assigning the
meta replicas. Have shutdown shutdown the procedure
executor engine.
There is other cleanup and notes in the below.
M HMaster
Remove the silly stops in startup now we have real
means of shutting down Master during init.
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/MasterMetaBootstrap.java
This replica stuff was doing stuff it shouldn't be doing
like setting core Master state flags. It may have made
sense once but now meta is assigned by a Pv2 Procedure
so the flag setting in here is meddlesome. Clear out
methods no longer needed.
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/AssignmentManager.java
Remove unused methods.
Changes local variable names so they align w/ our naming elsewhere in
code base.
M hbase-server/src/test/java/org/apache/hadoop/hbase/client/TestMetaWithReplicas.java
Check for all replicas on the one server.
On Master@shutdown, close the shared Master connection to kill any
ongoing RPCs by hosted clients.
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/HMaster.java
Call close ont the Master shared clusterconnection to kill any ongoing
rpcs.
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/ServerManager.java
Remove guts of close; we were closing the Masters connection....not
our responsibility.
Added unit test written by Duo Zhang which demonstrates the case where
Master will not go down.
Signed-off-by: zhangduo <zhangduo@apache.org>
Kill backup master first
Add some cleanup around NamespaceManager
Shorten the timeout waiting on namespace manager as workaround
until we have better soln for interrupting ongoing client rpcs.
Do it in general for all tests.
Signed-off-by: zhangduo <zhangduo@apache.org>
Rename the PE Worker threads.
Send an interrupt if worker taking a long time to go down
(it may be RPC'ing out to a dead server, retrying so
interrupt). Also join on the ProcedureExecutor shutting down.
This will make problems shutting down more obvious.
Disable TestRegionsOnMasterOptions. Master carrying Regions is broke.
Set the ProcedureExcecutor worker threads as daemon.
Ditto for the timeout thread.
Remove hack from TestRegionsOnMasterOptions that was
put in place because the test would not go down.
M dev-support/make_rc.sh
Disable checkstyle building site. Its an issue being fixed over in HBASE-19780
M hbase-server/src/main/java/org/apache/hadoop/hbase/master/HMaster.java
The clusterid was being set into the process only after the
regionserver registers with the Master. That can be too late for some
test clients in particular. e.g. TestZKAsyncRegistry needs it as soon
as it goes to run which could be before Master had called its run
method which is regionserver run method which then calls back to the
master to register itself... and only then do we set the clusterid.
HBASE-19694 changed start order which made it so this test failed.
Setting the clusterid right after we set it in zk makes the test pass.
Another change was that backup masters were not going down on stop.
Backup masters were sleeping for the default zk period which is 90
seconds. They were not being woken up to check for stop. On stop
master now tells active master manager.
M hbase-server/src/test/java/org/apache/hadoop/hbase/TestJMXConnectorServer.java
Prevent creation of acl table. Messes up our being able to go down
promptly.
M hbase-server/src/test/java/org/apache/hadoop/hbase/master/balancer/TestRegionsOnMasterOptions.java
M hbase-server/src/test/java/org/apache/hadoop/hbase/client/TestMultiParallel.java
M hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestRegionServerReadRequestMetrics.java
Disabled for now because it wants to run with regions on the Master...
currently broke!
M hbase-server/src/test/java/org/apache/hadoop/hbase/client/TestZKAsyncRegistry.java
Add a bit of debugging.
M hbase-server/src/test/java/org/apache/hadoop/hbase/master/TestDLSAsyncFSWAL.java
Disabled. Fails 40% of the time.
M hbase-server/src/test/java/org/apache/hadoop/hbase/master/TestDLSFSHLog.java
Disabled. Fails 33% of the time.
Disabled stochastic load balancer for favored nodes because it fails on
occasion and we are not doing favored nodes in branch-2.
Become active Master before calling the super class's run method. Have
the wait-on-becoming-active-Master be in-line rather than off in a
background thread (i.e. undo running thread in startActiveMasterManager)
Purge the fragile HBASE-16367 hackery that attempted to fix this issue
previously by adding a latch to try and hold up superclass RegionServer
until cluster id set by subclass Master.
In some situations, Runtime.getRuntime().getAvailableProcessors()
may return 0 which would result in calculatePoolSize returning 0
which will trigger an exception. Guard against this case.
Signed-off-by: Reid Chan <reidddchan@outlook.com>
Signed-off-by: Chia-Ping Tsai <chia7712@gmail.com>
Signed-off-by: Ted Yu <yuzhihong@gmail.com>
Ashish Singhi
Michael Stack
zhangduo
Jan Hentschel
Mike Drob
tedyu
Vasudevan
Yu Li
Josh Elser
Zach York
Rajeshbabu Chintaguntla
Toshihiro Suzuki
Yuki Tawara
Reid Chan
anoopsamjohn
Sakthi
Chia-Ping Tsai
Umesh Agashe
Peter Somogyi
Andrew Purtell
BELUGA BEHR
Apekshit Sharma
Chance Li
huzheng
Mike Grimes
Sahil Aggarwal
Guangxu Cheng
haxiaolin
Sean Busbey
Sergey Soldatov
Vladimir Rodionov
gsheffi
Esteban Gutierrez
anastas
eshcar
Thiruvel Thirumoolan
Pankaj Kumar
Guanghao Zhang
Aman Poonia
Balazs Meszaros
kewang
Rahul Gidwani
binlijin
Artem Ervits
lixiang
Scott Wilson
Rahul Gidwani
Janos Gub