[jira] [Commented] (HBASE-15340) Partial row result of scan may return data violates the row-level transaction

["Jianwei Cui (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HBASE-15340?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15181663#comment-15181663 ] 

Jianwei Cui commented on HBASE-15340:
-------------------------------------

{quote}
The solution of having a client aware readPnt will solve even that(?)
{quote}
It seems [HBASE-13099|https://issues.apache.org/jira/browse/HBASE-13099] has proposed such solution: https://issues.apache.org/jira/browse/HBASE-13099?focusedCommentId=14337017&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14337017. However, there are cases the solution can't cover(if I am not wrong). For example:
1. the client holds the readPoint when the scanner is created on serverA and the client has read partial row data from serverA
2. move the region to another serverB before the whole row returned
3. before the client created a new scanner for the row with the readPoint on serverB: new mutations applied to the region, including deletes for the row, and a major compaction happens and completed.
The major compaction could delete the cells of the row because the new server can't get a proper smallestReadPoint for the compaction before all ongoing scan requests arrived. Then, the client can not read the remaining cells of the row after the compaction, and will break per-row atomicity for scan. 

> Partial row result of scan may return data violates the row-level transaction 
> ------------------------------------------------------------------------------
>
>                 Key: HBASE-15340
>                 URL: https://issues.apache.org/jira/browse/HBASE-15340
>             Project: HBase
>          Issue Type: Bug
>          Components: Scanners, Transactions/MVCC
>    Affects Versions: 2.0.0
>            Reporter: Jianwei Cui
>
> There are cases the region sever will return partial row result, such as the client set batch for scan or configured size limit reached. In these situations, the client may return data that violates the row-level transaction to the application. The following steps show the problem:
> {code}
> // assume there is a test table 'test_table' with one family 'F' and one region 'region'. 
> // meanwhile there are two region servers 'rsA' and 'rsB'.
> 1. Let 'region' firstly located in 'rsA' and put one row with two columns 'c1' and 'c2' as:
>     > put 'test_table', 'row', 'F:c1', 'value1', 'F:c2', 'value1'
> 2. Start a client to scan 'test_table', with scan.setBatch(1) and scan.setCaching(1). The client will get one column as : {column='F:c1' and value='value1'} in the first rpc call after scanner created, and the result will be returned to application.
> 3. Before the client issues the next request, the 'region' was moved to 'rsB' and accepted another mutations for the two columns 'c1' and 'c2' as:
>     > put 'test_table', 'row', 'F:c1', 'value2', 'F:c2', 'value2'
> 4. Then, the client  will receive a RegionMovedException when issuing next request and will retry to open scanner on 'rsB'. The newly opened scanner will higher mvcc than old data so that could read out column as : { column='F:c2' with value='value2'} and return the result to application.
>    Therefore, the application will get data as:
> 'row'    column='F:c1'   value='value1'
> 'row'    column='F:c2',  value='value2'
>    The returned data is combined from two different mutations and violates the row-level transaction.
> {code}
> The reason is that the newly opened scanner after region moved will get a different mvcc. I am not sure whether this result is by design for scan if partial row result is allowed. However, such row result combined from different transactions may make the application have unexpected state.



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