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

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 2.0.0
• Component/s: Scanners, Transactions/MVCC
• Assignee: Unassigned
• Reporter: Jianwei Cui
• Created: 26/Feb/16 07:56
• Updated: 05/Mar/16 10:08

Description

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:

// 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.

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.