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From Jesse Yates <jya...@apache.org>
Subject Re: secondary index feature
Date Mon, 23 Dec 2013 19:10:43 GMT
The work that James is referencing grew out of the discussions Lars and I
had (which lead to those blog posts). The solution we implement is designed
to be generic, as James mentioned above, but was written with all the hooks
necessary for Phoenix to do some really fast updates (or skipping updates
in the case where there is no change).

You should be able to plug in your own simple index builder (there is
an example
in the phoenix codebase<https://github.com/forcedotcom/phoenix/tree/master/src/main/java/com/salesforce/hbase/index/covered/example>)
to basic solution which supports the same transactional guarantees as HBase
(per row) + data guarantees across the index rows. There are more details
in the presentations James linked.

I'd love you see if your implementation can fit into the framework we wrote
- we would be happy to work to see if it needs some more hooks or
modifications - I have a feeling this is pretty much what you guys will need

-Jesse


On Mon, Dec 23, 2013 at 10:01 AM, James Taylor <jtaylor@salesforce.com>wrote:

> Henning,
> Jesse Yates wrote the back-end of our global secondary indexing system in
> Phoenix. He designed it as a separate, pluggable module with no Phoenix
> dependencies. Here's an overview of the feature:
> https://github.com/forcedotcom/phoenix/wiki/Secondary-Indexing. The
> section that discusses the data guarantees and failure management might be
> of interest to you:
> https://github.com/forcedotcom/phoenix/wiki/Secondary-Indexing#data-guarantees-and-failure-management
>
> This presentation also gives a good overview of the pluggability of his
> implementation:
> http://files.meetup.com/1350427/PhoenixIndexing-SF-HUG_09-26-13.pptx
>
> Thanks,
> James
>
>
> On Mon, Dec 23, 2013 at 3:47 AM, Henning Blohm <henning.blohm@zfabrik.de>wrote:
>
>> Lars, that is exactly why I am hesitant to use one the core level generic
>> approaches (apart from having difficulties to identify the still active
>> projects): I have doubts I can sufficiently explain to myself when and
>> where they fail.
>>
>> With "toolbox approach" I meant to say that turning entity data into
>> index data is not done generically but rather involving domain specific
>> application code that
>>
>> - indicates what makes an index key given an entity
>> - indicates whether an index entry is still valid given an entity
>>
>> That code is also used during the index rebuild and trimming (an M/R Job)
>>
>> So validating whether an index entry is valid means to load the entity
>> pointed to and - before considering it a valid result - validating whether
>> values of the entity still match with the index.
>>
>> The entity is written last, hence when the client dies halfway through
>> the update you may get stale index entries but nothing else should break.
>>
>> For scanning along the index, we are using a chunk iterator that is, we
>> read n index entries ahead and then do point look ups for the entities. How
>> would you avoid point-gets when scanning via an index (as most likely,
>> entities are ordered independently from the index - hence the index)?
>>
>> Something really important to note is that there is no intention to build
>> a completely generic solution, in particular not (this time - unlike the
>> other post of mine you responded to) taking row versioning into account.
>> Instead, row time stamps are used to delete stale entries (old entries
>> after an index rebuild).
>>
>> Thanks a lot for your blog pointers. Haven't had time to study in depth
>> but at first glance there is lot of overlap of what you are proposing and
>> what I ended up doing considering the first post.
>>
>> On the second post: Indeed I have not worried too much about
>> transactional isolation of updates. If index update and entity update use
>> the same HBase time stamp, the result should at least be consistent, right?
>>
>> Btw. in no way am I claiming originality of my thoughts - in particular I
>> read http://jyates.github.io/2012/07/09/consistent-enough-
>> secondary-indexes.html a while back.
>>
>> Thanks,
>> Henning
>>
>> Ps.: I might write about this discussion later in my blog
>>
>>
>> On 22.12.2013 23:37, lars hofhansl wrote:
>>
>>> The devil is often in the details. On the surface it looks simple.
>>>
>>> How specifically are the stale indexes ignored? Are the guaranteed to be
>>> no races?
>>> Is deletion handled correctly?Does it work with multiple versions?
>>> What happens when the client dies 1/2 way through an update?
>>> It's easy to do eventually consistent indexes. Truly consistent indexes
>>> without transactions are tricky.
>>>
>>>
>>> Also, scanning an index and then doing point-gets against a main table
>>> is slow (unless the index is very selective. The Phoenix team measured that
>>> there is only an advantage if the index filters out 98-99% of the data).
>>> So then one would revert to covered indexes and suddenly is not so easy
>>> to detect stale index entries.
>>>
>>> I blogged about these issues here:
>>> http://hadoop-hbase.blogspot.com/2012/10/musings-on-
>>> secondary-indexes.html
>>> http://hadoop-hbase.blogspot.com/2012/10/secondary-indexes-part-ii.html
>>>
>>> Phoenix has a (pretty involved) solution now that works around the fact
>>> that HBase has no transactions.
>>>
>>>
>>> -- Lars
>>>
>>>
>>>
>>> ________________________________
>>>   From: Henning Blohm <henning.blohm@zfabrik.de>
>>> To: user <user@hbase.apache.org>
>>> Sent: Sunday, December 22, 2013 2:11 AM
>>> Subject: secondary index feature
>>>
>>> Lately we have added a secondary index feature to a persistence tier
>>> over HBASE. Essentially we implemented what is described as "Dual-Write
>>> Secondary Index" in http://hbase.apache.org/book/secondary.indexes.html.
>>> I.e. while updating an entity, actually before writing the actual
>>> update, indexes are updated. Lookup via the index ignores stale entries.
>>> A recurring rebuild and clean out of stale entries takes care the
>>> indexes are trimmed and accurate.
>>>
>>> None of this was terribly complex to implement. In fact, it seemed like
>>> something you could do generically, maybe not on the HBASE level itself,
>>> but as a toolbox / utility style library.
>>>
>>> Is anybody on the list aware of anything useful already existing in that
>>> space?
>>>
>>> Thanks,
>>> Henning Blohm
>>>
>>> *ZFabrik Software KG*
>>>
>>> T:     +49 6227 3984255
>>> F:     +49 6227 3984254
>>> M:     +49 1781891820
>>>
>>> Lammstrasse 2 69190 Walldorf
>>>
>>> henning.blohm@zfabrik.de <mailto:henning.blohm@zfabrik.de>
>>> Linkedin <http://www.linkedin.com/pub/henning-blohm/0/7b5/628>
>>> ZFabrik <http://www.zfabrik.de>
>>> Blog <http://www.z2-environment.net/blog>
>>> Z2-Environment <http://www.z2-environment.eu>
>>> Z2 Wiki <http://redmine.z2-environment.net>
>>>
>>
>>
>> --
>> Henning Blohm
>>
>> *ZFabrik Software KG*
>>
>> T:      +49 6227 3984255
>> F:      +49 6227 3984254
>> M:      +49 1781891820
>>
>> Lammstrasse 2 69190 Walldorf
>>
>> henning.blohm@zfabrik.de <mailto:henning.blohm@zfabrik.de>
>> Linkedin <http://www.linkedin.com/pub/henning-blohm/0/7b5/628>
>> ZFabrik <http://www.zfabrik.de>
>> Blog <http://www.z2-environment.net/blog>
>> Z2-Environment <http://www.z2-environment.eu>
>> Z2 Wiki <http://redmine.z2-environment.net>
>>
>>
>

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