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https://issues.apache.org/jira/browse/HBASE-5487?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13554176#comment-13554176
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Enis Soztutar commented on HBASE-5487:
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bq. FATE would violate one of our "Invariants", no permanent data in zk:
I think it depends on the definition of permanent. FATE keeps the stack of
operation states in zk, but when the whole thing finishes, no zk data is left.
You can still wipe out zk, if you are in a clean state.
bq. FATE's approach of serializing the steps along the way is easily replicated
via WAL. Write a step to the WAL to initiate, then write the next step, and the
next, &c as you go. When the whole thing is complete, write a completion record.
Agreed.
bq. I think using the WAL is not the ideal mechanism; it makes master failover
a very heavy-weight operation
Not sure why it is so heavy weight. It requires to replay only the non-finished
steps of the operation of undo the operation, so that we can clean state. As in
fate, we can divide every operation (create table, disable, etc) as a sequence
of steps, which are idempotent, and undo/redo'able. Before the operation, and
before every step master syncs the step to the WAL, after the step is done,
master does another sync to record a DONE state for that step. On replay, it
just buffers non-finished operations (ignores already finished ones), and
undos/redos the steps not finished.
We can do size based and periodic log rolling, and for every non-finished
operation keep the smallest seqNum. When operation is done, we can just release
that seqId, so that we can use the same fshlog infrastructure.
bq. An idea that Chris Trezzo just proposed (and I really like) is to move this
to a 'system level table'.
I must have missed this. This will still be a WAL I guess, but kept as an hbase
table. Considering megastore also keeps its WAL in bigtable,
I think it is doable.
bq. Why not write to a system table, (like META) using the existing wal? This
question came up for snapshot clone/restore recovery, and is a potential
solution for maintaining table enable/disable state (which is in zk and
violates the rule)
I never liked the table enabled/disabled state, and agreed that it is a
violation, as well as a major cause of bugs. I guess, regardless of where we
keep it (fshlog or system table), it is clear that we need a WAL.
> Generic framework for Master-coordinated tasks
> ----------------------------------------------
>
> Key: HBASE-5487
> URL: https://issues.apache.org/jira/browse/HBASE-5487
> Project: HBase
> Issue Type: New Feature
> Components: master, regionserver, Zookeeper
> Affects Versions: 0.94.0
> Reporter: Mubarak Seyed
> Assignee: Nick Dimiduk
>
> Need a framework to execute master-coordinated tasks in a fault-tolerant
> manner.
> Master-coordinated tasks such as online-scheme change and delete-range
> (deleting region(s) based on start/end key) can make use of this framework.
> The advantages of framework are
> 1. Eliminate repeated code in Master, ZooKeeper tracker and Region-server for
> master-coordinated tasks
> 2. Ability to abstract the common functions across Master -> ZK and RS -> ZK
> 3. Easy to plugin new master-coordinated tasks without adding code to core
> components
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