Stephen Yuan Jiang created HBASE-12464:
------------------------------------------
Summary: meta table region assignment stuck in the FAILED_OPEN
state due to region server not fully ready to serve
Key: HBASE-12464
URL: https://issues.apache.org/jira/browse/HBASE-12464
Project: HBase
Issue Type: Bug
Components: Region Assignment
Affects Versions: 0.99.1, 1.0.0, 2.0.0
Reporter: Stephen Yuan Jiang
Assignee: Stephen Yuan Jiang
Fix For: 1.0.0, 2.0.0
meta table region assignment could reach to the 'FAILED_OPEN' state, which
makes the region not available unless the target region server shutdown or
manual resolution. This is undesirable state for meta tavle region.
Here is the sequence how this could happen (the code is in
AssignmentManager::assign()):
Step 1: Master detects a region server (RS1) that hosts one meta table region
is down, it changes the meta region state from 'online' to 'offline'
Step 2: In a loop (with configuable maximumAttempts count, default is 10, and
minimal is 1), AssignmentManager tries to find a RS to host the meta table
region. If there is no RS available, it would loop forver by resetting the
loop count (!!BUG#1 from this logic - a small bug!!)
if (region.isMetaRegion()) {
- try {
- Thread.sleep(this.sleepTimeBeforeRetryingMetaAssignment);
- if (i == maximumAttempts) i = 1; // ==> BUG: if maximumAttempts
is 1, then the loop will end.
- continue;
- } catch (InterruptedException e) {
- ...
- }
Step 3: Once a new RS is found (RS2), inside the same loop as Step 2,
AssignmentManager tries to assign the meta region to RS2 (OFFLINE, RS1 =>
PENDING_OPEN, RS2). If for some reason that opening the region in RS2 failed
(eg. the target RS2 is not ready to serve - ServerNotRunningYetException),
AssignmentManager would change the state from (PENDING_OPEN, RS2) to
(FAILED_OPEN, RS2). then it would retry (and even change the RS server to go
to). The retry is up to maximumAttempts. Once the maximumAttempts is reached,
the meta region will be in the 'FAILED_OPEN' state, unless either (1). RS2
shutdown to trigger region assignment again or (2). it is reassigned by an
operator via HBase Shell.
Based on the document ( http://hbase.apache.org/book/regions.arch.html ), this
is by design - "17. For regions in FAILED_OPEN or FAILED_CLOSE states , the
master tries to close them again when they are reassigned by an operator via
HBase Shell.".
However, this is bad design, espcially for meta table region (it is arguable
that the design is good for regular table - for this ticket, I am more focus on
fixing the meta region availablity issue).
I propose 2 possible fixes:
Fix#1 (band-aid change): in Step 3, just like Step 2, if the region is a meta
table region, reset the loop count so that it would not leave the loop with
meta table region in FAILED_OPEN state.
Fix#2 (more involved): if a region is in FAILED_OPEN state, we should provide a
way to automatically trigger AssignmentManager::assign() after a short period
of time (leaving any region in FAILED_OPEN state or other states like
'FAILED_CLOSE' is undesirable, should have some way to retrying and auto-heal
the region).
I think at least for 1.0.0, Fix#1 is good enough. We can open a task-type of
JIRA for Fix#2 in future release.
--
This message was sent by Atlassian JIRA
(v6.3.4#6332)
