ColumnTracker (Apache HBase 1.2.12 API)

All Known Implementing Classes:

ExplicitColumnTracker, ScanWildcardColumnTracker
```
@InterfaceAudience.Private
public interface ColumnTracker
```
Implementing classes of this interface will be used for the tracking and enforcement of columns and numbers of versions and timeToLive during the course of a Get or Scan operation.
Currently there are two different types of Store/Family-level queries.
- ExplicitColumnTracker is used when the query specifies one or more column qualifiers to return in the family.
- ScanWildcardColumnTracker is used when no columns are explicitly specified.
This class is utilized by ScanQueryMatcher mainly through two methods:
- checkColumn(byte[], int, int, byte) is called when a Put satisfies all other conditions of the query.
- getNextRowOrNextColumn(byte[], int, int) is called whenever ScanQueryMatcher believes that the current column should be skipped (by timestamp, filter etc.)
These two methods returns a ScanQueryMatcher.MatchCode to define what action should be taken.
This class is NOT thread-safe as queries are never multi-threaded

Method Summary

Methods
Modifier and Type	Method and Description
`ScanQueryMatcher.MatchCode`	`checkColumn(byte[] bytes, int offset, int length, byte type)` Checks if the column is present in the list of requested columns by returning the match code instance.
`ScanQueryMatcher.MatchCode`	`checkVersions(byte[] bytes, int offset, int length, long ttl, byte type, boolean ignoreCount)` Keeps track of the number of versions for the columns asked for.
`boolean`	`done()`
`ColumnCount`	`getColumnHint()` Used by matcher and scan/get to get a hint of the next column to seek to after checkColumn() returns SKIP.
`ScanQueryMatcher.MatchCode`	`getNextRowOrNextColumn(byte[] bytes, int offset, int qualLength)` Retrieve the MatchCode for the next row or column
`boolean`	`isDone(long timestamp)` Give the tracker a chance to declare it's done based on only the timestamp to allow an early out.
`void`	`reset()` Resets the Matcher

- Method Detail
  - checkColumn
```
ScanQueryMatcher.MatchCode checkColumn(byte[] bytes,
                                     int offset,
                                     int length,
                                     byte type)
                                       throws IOException
```
    Checks if the column is present in the list of requested columns by returning the match code instance. It does not check against the number of versions for the columns asked for. To do the version check, one has to call checkVersions(byte[], int, int, long, byte, boolean) method based on the return type (INCLUDE) of this method. The values that can be returned by this method are ScanQueryMatcher.MatchCode.INCLUDE, ScanQueryMatcher.MatchCode.SEEK_NEXT_COL and ScanQueryMatcher.MatchCode.SEEK_NEXT_ROW.
    
    Parameters:
    bytes -
    offset -
    length -
    type - The type of the KeyValue
    
    Returns:
    The match code instance.
    
    Throws:
    
    IOException - in case there is an internal consistency problem caused by a data corruption.
  - checkVersions
```
ScanQueryMatcher.MatchCode checkVersions(byte[] bytes,
                                       int offset,
                                       int length,
                                       long ttl,
                                       byte type,
                                       boolean ignoreCount)
                                         throws IOException
```
    Keeps track of the number of versions for the columns asked for. It assumes that the user has already checked if the keyvalue needs to be included by calling the checkColumn(byte[], int, int, byte) method. The enum values returned by this method are ScanQueryMatcher.MatchCode.SKIP, ScanQueryMatcher.MatchCode.INCLUDE, ScanQueryMatcher.MatchCode.INCLUDE_AND_SEEK_NEXT_COL and ScanQueryMatcher.MatchCode.INCLUDE_AND_SEEK_NEXT_ROW. Implementations which include all the columns could just return ScanQueryMatcher.MatchCode.INCLUDE in the checkColumn(byte[], int, int, byte) method and perform all the operations in this checkVersions method.
    
    Parameters:
    type - the type of the key value (Put/Delete)
    ttl - The timeToLive to enforce.
    ignoreCount - indicates if the KV needs to be excluded while counting (used during compactions. We only count KV's that are older than all the scanners' read points.)
    
    Returns:
    the scan query matcher match code instance
    
    Throws:
    
    IOException - in case there is an internal consistency problem caused by a data corruption.
  - reset
```
void reset()
```
    Resets the Matcher
  - done
```
boolean done()
```
    Returns:
    true when done.
  - getColumnHint
```
ColumnCount getColumnHint()
```
    Used by matcher and scan/get to get a hint of the next column to seek to after checkColumn() returns SKIP. Returns the next interesting column we want, or NULL there is none (wildcard scanner). Implementations aren't required to return anything useful unless the most recent call was to checkColumn() and the return code was SKIP. This is pretty implementation detail-y, but optimizations are like that.
    
    Returns:
    null, or a ColumnCount that we should seek to
  - getNextRowOrNextColumn
```
ScanQueryMatcher.MatchCode getNextRowOrNextColumn(byte[] bytes,
                                                int offset,
                                                int qualLength)
```
    Retrieve the MatchCode for the next row or column
  - isDone
```
boolean isDone(long timestamp)
```
    Give the tracker a chance to declare it's done based on only the timestamp to allow an early out.
    
    Parameters:
    timestamp -
    
    Returns:
    true to early out based on timestamp.

Interface ColumnTracker

Method Summary

Method Detail

checkColumn

checkVersions

reset

done

getColumnHint

getNextRowOrNextColumn

isDone