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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (PHOENIX-1734) Local index improvements
Date Sat, 28 Nov 2015 23:52:11 GMT

    [ https://issues.apache.org/jira/browse/PHOENIX-1734?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15030709#comment-15030709
] 

ASF GitHub Bot commented on PHOENIX-1734:
-----------------------------------------

Github user JamesRTaylor commented on a diff in the pull request:

    https://github.com/apache/phoenix/pull/135#discussion_r46085866
  
    --- Diff: phoenix-core/src/main/java/org/apache/phoenix/coprocessor/UngroupedAggregateRegionObserver.java
---
    @@ -158,6 +162,10 @@ public RegionScanner preScannerOpen(ObserverContext<RegionCoprocessorEnvironment
                 throws IOException {
             s = super.preScannerOpen(e, scan, s);
             if (ScanUtil.isAnalyzeTable(scan)) {
    +            if (!ScanUtil.isLocalIndex(scan)) {
    +                scan.getFamilyMap().clear();
    +            }
    +
    --- End diff --
    
    Why this change?


> Local index improvements
> ------------------------
>
>                 Key: PHOENIX-1734
>                 URL: https://issues.apache.org/jira/browse/PHOENIX-1734
>             Project: Phoenix
>          Issue Type: Improvement
>            Reporter: Rajeshbabu Chintaguntla
>            Assignee: Rajeshbabu Chintaguntla
>         Attachments: PHOENI-1734-WIP.patch, PHOENIX-1734_v1.patch, PHOENIX-1734_v4.patch,
TestAtomicLocalIndex.java
>
>
> Local index design considerations: 
>  1. Colocation: We need to co-locate regions of local index regions and data regions.
The co-location can be a hard guarantee or a soft (best approach) guarantee. The co-location
is a performance requirement, and also maybe needed for consistency(2). Hard co-location means
that either both the data region and index region are opened atomically, or neither of them
open for serving. 
>  2. Index consistency : Ideally we want the index region and data region to have atomic
updates. This means that they should either (a)use transactions, or they should (b)share the
same WALEdit and also MVCC for visibility. (b) is only applicable if there is hard colocation
guarantee. 
>  3. Local index clients : How the local index will be accessed from clients. In case
of the local index being managed in a table, the HBase client can be used for doing scans,
etc. If the local index is hidden inside the data regions, there has to be a different mechanism
to access the data through the data region. 
> With the above considerations, we imagine three possible implementation for the local
index solution, each detailed below. 
> APPROACH 1:  Current approach
> (1) Current approach uses balancer as a soft guarantee. Because of this, in some rare
cases, colocation might not happen. 
> (2) The index and data regions do not share the same WALEdits. Meaning consistency cannot
be achieved. Also there are two WAL writes per write from client. 
> (3) Regular Hbase client can be used to access index data since index is just another
table. 
> APPROACH 2: Shadow regions + shared WAL & MVCC 
> (1) Introduce a shadow regions concept in HBase. Shadow regions are not assigned by AM.
Phoenix implements atomic open (and split/merge) of region opening for data regions and index
regions so that hard co-location is guaranteed. 
> (2) For consistency requirements, the index regions and data regions will share the same
WALEdit (and thus recovery) and they will also share the same MVCC mechanics so that index
update and data update is visible atomically. 
> (3) Regular Hbase client can be used to access index data since index is just another
table.  
> APPROACH 3: Storing index data in separate column families in the table.
>  (1) Regions will have store files for cfs, which is sorted using the primary sort order.
Regions may also maintain stores, sorted in secondary sort orders. This approach is similar
in vein how a RDBMS keeps data (a B-TREE in primary sort order and multiple B-TREEs in secondary
sort orders with pointers to primary key). That means store the index data in separate column
families in the data region. This way a region is extended to be more similar to a RDBMS (but
LSM instead of BTree). This is sometimes called shadow cf’s as well. This approach guarantees
hard co-location.
>  (2) Since everything is in a single region, they automatically share the same WALEdit
and MVCC numbers. Atomicity is easily achieved. 
>  (3) Current Phoenix implementation need to change in such a way that column families
selection in read/write path is based data table/index table(logical table in phoenix). 
> I think that APPROACH 3 is the best one for long term, since it does not require to change
anything in HBase, mainly we don't need to muck around with the split/merge stuff in HBase.
It will be win-win.
> However, APPROACH 2 still needs a “shadow regions” concept to be implemented in HBase
itself, and also a way to share WALEdits and MVCCs from multiple regions.
> APPROACH 1 is a good start for local indexes, but I think we are not getting the full
benefits for the feature. We can support this for the short term, and decide on the next steps
for a longer term implementation. 
> we won't be able to get to implementing it immediately, and want to start a brainstorm.



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