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ASF GitHub Bot commented on PHOENIX-1734: ----------------------------------------- Github user chrajeshbabu commented on the pull request: https://github.com/apache/phoenix/pull/135#issuecomment-160454376 Thanks for review @JamesRTaylor. Please find my answers to the comments. > 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. -- This message was sent by Atlassian JIRA (v6.3.4#6332)