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From "Daniel Vimont (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-17257) Add column-aliasing capability to hbase-client
Date Thu, 15 Dec 2016 05:22:58 GMT

    [ https://issues.apache.org/jira/browse/HBASE-17257?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15750429#comment-15750429
] 

Daniel Vimont commented on HBASE-17257:
---------------------------------------

Here is a partial list of outstanding questions regarding this JIRA issue (to add a column-aliasing
capability to HBase):

*META-ASSESSMENT*: Is this proposed feature (i.e. column-aliasing) of sufficient potential
importance/usefulness to the HBase ecosystem to justify people taking the time to assess its
technical viability via this JIRA entry? ([~aoxiang] commented above "I think it is a very
useful feature." This is helpful, and others have anecdotally given me similar comments in
other venues in the past few months, but input from others would still be welcome, particularly
from those who might take a contrary viewpoint.)

*QUESTION 1* -- Aside from the question of where to persist alias-mapping data (that question
is below) -- _Is the overall architecture for aliasing as presented in patch-v3 (and as outlined
in the specifications above)..._
(a) ... viable?
(b) ... optimal?

*QUESTION 2a* -- _Where should alias-mapping data (referred to hereafter as "hbase:alias")
be persisted?_
I currently see three options:
(1) An existing system table (hbase:meta has been suggested). This might entail scalability
problems, since system tables apparently cannot be split into more than one region.
(2) A separate table in the "hbase" namespace, but created/administered as a "normal" HBase
table on RegionServer(s). [This is currently what is offered in patch-v3: "hbase:alias" is
in the system namespace, but it is a standard HBase table created on a "normal" RegionServer.
In my original design-phase, I went with this option after realizing the scalability limitations
inherent in Option 1, above.]
(3) A separate aliasMapping table for every alias-enabled Table.

*QUESTION 2b* -- _How to increase/improve fault-tolerance of hbase:alias table:_
Given that read/write access to all alias-enabled column-families in a cluster is completely
dependent upon unfettered read/write access to hbase:alias, are there heightened "fault tolerance"
options available to decrease the chances of an unscheduled unavailability of hbase:alias?
(Concerns of this have been relayed to me in offline discussions.)

*QUESTION 3* -- _Does basic read/write access to alias-enabled column-families need to be
guaranteed when/if the Master server becomes unavailable?_
It seems to be a publicized feature of HBase that basic read/write access to HBase tables
(i.e. to RegionServers) can continue uninterrupted even when the Master server becomes unavailable.
However, in the current, patch-v3 architecture, column-aliasing requires access to the Master
server (to fulfill #getTableDescriptor RPC invocations to look-up aliasing metadata for a
table and its families). I have experimented with a potential "patch-v4" architecture in which
all required metadata for a column-family is stored in hbase:alias, negating the need for
invocation of #getTableDescriptor, and thus making alias-enabled tables still ostensibly accessible
in the event of an outage of the Master server. I can finalize and submit this architectural
variation upon request.

*THANKS TO ALL WHO HAVE READ TO THIS POINT!! I VERY MUCH APPRECIATE YOUR TIME!!!*

> Add column-aliasing capability to hbase-client
> ----------------------------------------------
>
>                 Key: HBASE-17257
>                 URL: https://issues.apache.org/jira/browse/HBASE-17257
>             Project: HBase
>          Issue Type: New Feature
>          Components: Client
>    Affects Versions: 2.0.0
>            Reporter: Daniel Vimont
>            Assignee: Daniel Vimont
>              Labels: features
>         Attachments: HBASE-17257-v2.patch, HBASE-17257-v3.patch, HBASE-17257.patch
>
>
> Review Board link: https://reviews.apache.org/r/54635/
> Column aliasing will provide the option for a 1, 2, or 4 byte alias value to be stored
in each cell of an "alias enabled" column-family, in place of the full-length column-qualifier.
Aliasing is intended to operate completely invisibly to the end-user developer, with absolutely
no "awareness" of aliasing required to be coded into a front-end application. No new public
hbase-client interfaces are to be introduced, and only a few new public methods should need
to be added to existing interfaces, primarily to allow an administrator to designate that
a new column-family is to be alias-enabled by setting its aliasSize attribute to 1, 2, or
4.
> To facilitate such functionality, new subclasses of HTable, BufferedMutatorImpl, and
HTableMultiplexer are to be provided. The overriding methods of these new subclasses will
invoke methods of the new AliasManager class to facilitate qualifier-to-alias conversions
(for user-submitted Gets, Scans, and Mutations) and alias-to-qualifier conversions (for Results
returned from HBase) for any Table that has one or more alias-enabled column families. All
conversion logic will be encapsulated in the new AliasManager class, and all qualifier-to-alias
mappings will be persisted in a new aliasMappingTable in a new, reserved namespace.
> An informal polling of HBase users at HBaseCon East and at the Strata/Hadoop-World conference
in Sept. 2016 showed that Column Aliasing could be a popular enhancement to standard HBase
functionality, due to the fact that full column-qualifiers are stored in each cell, and reducing
this qualifier storage requirement down to 1, 2, or 4 bytes per cell could prove beneficial
in terms of reduced storage and bandwidth needs. Aliasing is intended chiefly for column-families
which are of the "narrow and tall" variety (i.e., that are designed to use relatively few
distinct column-qualifiers throughout a large number of rows, throughout the lifespan of the
column-family). A column-family that is set up with an alias-size of 1 byte can contain up
to 255 unique column-qualifiers; a 2 byte alias-size allows for up to 65,535 unique column-qualifiers;
and a 4 byte alias-size allows for up to 4,294,967,295 unique column-qualifiers.
> Fuller specifications will be entered into the comments section below. Note that it may
well not be viable to add aliasing support in the new "async" classes that appear to be currently
under development.



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