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From James Taylor <jamestay...@apache.org>
Subject Re: LOCAL vs TRANSACTIONAL indexes
Date Sat, 17 Sep 2016 23:26:59 GMT
On Fri, Sep 16, 2016 at 7:22 PM, Matthew Van Wely <mvanwely@salesforce.com>
wrote:

> All,
>
> I would like some guidance on LOCAL vs TRANSACTIONAL indexes and I
> cannot quite get the details I need from the Phoenix site:
> https://phoenix.apache.org/secondary_indexing.htm
>
> Transactional Tables
>
> <snip>
> transactional tables with secondary indexes potentially lowers your
> availability of being able to write to your data table, as both the data
> table and its secondary index tables must be availalbe as otherwise the
> write will fail
> </snip>
>
> 1) What is the likelihood that an index is not available?
>
This is rare and unlikely. If a region server goes down, HBase relocates
the regions it was hosting to another region server. If you write data
exactly when this happens, it's possible that you'll get an exception back
if this relocation takes longer than your # of retries and timeout settings.


>
> 2) If rebuilding, is this on the order of minutes, hours?
>
Not sure what rebuilding you're asking about. For mutable, non
transactional secondary indexes, Phoenix has the ability to partially
rebuild them if a write failure occurs. This will be relatively faster
because it only rebuilds index rows that were added after the writes began
failing. See the options listed under
https://phoenix.apache.org/secondary_indexing.html#Mutable_Tables

If on the other hand you're asking how long does it take to completely
rebuild the index, then that depends on how much data the table has (so
then you're really asking how fast does HBase write).


>
> 3) Does Phoenix give an indication the write failed due to unavailable
> table/index (bc if so client could handle this with other write options)?
>

Yes, Phoenix throws an exception if the write fails. It never fails
silently. If your data is immutable, then it's up to you to handle the
write failure (usually by just continually retrying the failed write). If
mutable, then Phoenix has some options that can automate catching the index
up with the data table (see
https://phoenix.apache.org/secondary_indexing.html#Consistency_Guarantees).
If your table is transactional, then it cannot get out of sync with the
index.


>
> Local Indexes
>
> <snip>
> all local index data in the separate shadow column families in the
> same data table. At read time when the local index is used, every region
> must be examined for the data as the exact region location of index data
> cannot be predetermined. Thus some overhead occurs at read-time.
> </snip>
>
> 4) Are there any requirements on table PK and index key regarding key
> ordering?
>
No


>
> 5) How is something locally indexed if the keys are completely mismatched?
> I get the sense that it doesn't matter given that "every region must be
> examined".
>

The rows of a local index are sorted in each region. The client just has to
do a merge sort between all the data it gets back for the scans over each
region. This is very fast, so not too much overhead here.


>
> Mutable Tables
>
> <snip>
> indexes on non transactional mutable tables are only ever a single
> batch of edits behind the primary table
> </snip>
>
> 6) If my use case updates a table and then reads from an index, it seems a
> likely race condition that I can read-my-write.
>

>From the same client, there is no race condition. The upsert statement is
synchronous, so when control returns back to you, all of your data has been
written (both to the data and index table(s)).

If the read happens from a different client than the write, with global,
mutable, non transactional indexes, it's possible that a read could occur
after the write to the data table but before the write to the index
table(s) (since the with global indexes, the regions for the index table
are potentially on different region servers than the regions of the data
table).

With local indexes the above is even more unlikely because the writes are
all occurring to the same region server, but in theory it's still possible.
With the fix that was made as part of HBASE-15600, this wouldn't be
possible at all, though.

With transactional tables, this scenario isn't possible.


>
> 7) Would you be willing to bet that most reads are consistent with the
> table and only in rare scenarios is the table/index out of sync?
>
Yes

>
> I appreciate your help and feedback on these questions.  Thanks,
> --Matthew
>

Thanks,
James

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