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From Gary Helmling <ghelml...@gmail.com>
Subject Re: [DISCUSS] No regions on Master node in 2.0
Date Tue, 12 Apr 2016 18:22:14 GMT
Sorry to be late to the party here.  I'll sprinkle my comments over the
thread where they make the most sense.

> Currently in the master branch, HMaster hosts 'system tables': e.g.
> hbase:meta. HMaster is doing more than just gardening the cluster,
> bootstrapping and keeping all up and serving healthy as in branch-1; in
> master branch, it is actually in the write path for the most critical
> system regions.
I think it's important to point out that this feature exists and is usable
in branch-1 as well, including in all 1.x releases.  It just disabled by
default branch-1 and enabled by default in master. So this is really a
comparison of an existing, shipping, feature that does work, and is being
used vs. ongoing development work in master.

> Let me provoke discussion by making the statement that we should undo
> HMaster carrying any regions ever; that the HMaster function is work enough
> for a single dedicated server and that it important enough that it cannot
> take a background role on a serving RegionServer (I could go back from this
> position if evidence HMaster role could be backgrounded). Notions of a
> Master carrying system tables only are just not on given system tables will
> be too big for a single server especially when hbase:meta is split (so we
> can scale).

If we really think that normal master housekeeping functions are work
enough that we shouldn't combine with region serving, then why do we think
that those will _not_ have to be scaled by splitting the metadata space
across multiple servers when we encounter meta-scaling issues that require
splitting meta to distribute it across multiple servers?  If we really want
to scale, then it seems like we need to tackle scaling the region metadata
in general across multiple active masters, in which case meta-on-master is
not really an argument either way.

> This simple distinction of HMaster and RegionServer roles is
> also what our users know and have gotten used to so needs to be a good
> reason to change it (We can still pursue the single binary that can do
> HMaster or HRegionServer role determined at runtime).

The distinction in roles in HBase has long been used as a criticism of
HBase's operational complexity.  I think we would be doing our users a
service by simplifying this and making it a detail they do not need to
worry about. If we can truly make this transparent to users and improve
operability at the same time, I think that would be the best outcome.

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