FileLink (Hortonworks Data Platform Apache HBase Java API Reference)

java.lang.Object
- org.apache.hadoop.hbase.io.FileLink

Direct Known Subclasses:

HFileLink, WALLink
```
@InterfaceAudience.Private
public class FileLink
extends Object
```
The FileLink is a sort of hardlink, that allows access to a file given a set of locations.
The Problem:
- HDFS doesn't have support for hardlinks, and this make impossible to referencing the same data blocks using different names.
- HBase store files in one location (e.g. table/region/family/) and when the file is not needed anymore (e.g. compaction, region deletion, ...) moves it to an archive directory.
If we want to create a reference to a file, we need to remember that it can be in its original location or in the archive folder. The FileLink class tries to abstract this concept and given a set of locations it is able to switch between them making this operation transparent for the user. HFileLink is a more concrete implementation of the FileLink.
Back-references: To help the CleanerChore to keep track of the links to a particular file, during the FileLink creation, a new file is placed inside a back-reference directory. There's one back-reference directory for each file that has links, and in the directory there's one file per link.
HFileLink Example
- /hbase/table/region-x/cf/file-k (Original File)
- /hbase/table-cloned/region-y/cf/file-k.region-x.table (HFileLink to the original file)
- /hbase/table-2nd-cloned/region-z/cf/file-k.region-x.table (HFileLink to the original file)
- /hbase/.archive/table/region-x/.links-file-k/region-y.table-cloned (Back-reference to the link in table-cloned)
- /hbase/.archive/table/region-x/.links-file-k/region-z.table-2nd-cloned (Back-reference to the link in table-2nd-cloned)

Field Summary

Fields
Modifier and Type Field and Description

static String BACK_REFERENCES_DIRECTORY_PREFIX
Define the Back-reference directory name prefix: .links-/

Fields
Modifier and Type	Field and Description
`static String`	`BACK_REFERENCES_DIRECTORY_PREFIX` Define the Back-reference directory name prefix: .links-/

Constructor Summary

Constructors
Modifier Constructor and Description

protected FileLink()

FileLink(Collection<Path> locations)

FileLink(Path originPath, Path... alternativePaths)

Constructors
Modifier	Constructor and Description
`protected`	`FileLink()`
	`FileLink(Collection<Path> locations)`
	`FileLink(Path originPath, Path... alternativePaths)`

Method Summary

All Methods Static Methods Instance Methods Concrete Methods
Modifier and Type	Method and Description
`boolean`	`equals(Object obj)`
`boolean`	`exists(FileSystem fs)`
`Path`	`getAvailablePath(FileSystem fs)`
`static String`	`getBackReferenceFileName(Path dirPath)` Get the referenced file name from the reference link directory path.
`static Path`	`getBackReferencesDir(Path storeDir, String fileName)` Get the directory to store the link back references
`FileStatus`	`getFileStatus(FileSystem fs)` Get the FileStatus of the referenced file.
`Path[]`	`getLocations()`
`int`	`hashCode()`
`static boolean`	`isBackReferencesDir(Path dirPath)` Checks if the specified directory path is a back reference links folder.
`FSDataInputStream`	`open(FileSystem fs)` Open the FileLink for read.
`FSDataInputStream`	`open(FileSystem fs, int bufferSize)` Open the FileLink for read.
`protected void`	`setLocations(Path originPath, Path... alternativePaths)` NOTE: This method must be used only in the constructor! It creates a List with the specified locations for the link.
`String`	`toString()`

Methods inherited from class java.lang.Object
clone, finalize, getClass, notify, notifyAll, wait, wait, wait

- Field Detail
  - BACK_REFERENCES_DIRECTORY_PREFIX
```
public static final String BACK_REFERENCES_DIRECTORY_PREFIX
```
    Define the Back-reference directory name prefix: .links-/
    
    See Also:
    
    Constant Field Values
- Constructor Detail
  - FileLink
```
protected FileLink()
```
  - FileLink
```
public FileLink(Path originPath,
                Path... alternativePaths)
```
    Parameters:
    
    originPath - Original location of the file to link
    
    alternativePaths - Alternative locations to look for the linked file
  - FileLink
```
public FileLink(Collection<Path> locations)
```
    Parameters:
    
    locations - locations to look for the linked file
- Method Detail
  - getLocations
```
public Path[] getLocations()
```
    Returns:
    
    the locations to look for the linked file.
  - toString
```
public String toString()
```
    Overrides:
    
    toString in class Object
  - exists
```
public boolean exists(FileSystem fs)
               throws IOException
```
    Returns:
    
    true if the file pointed by the link exists
    
    Throws:
    
    IOException
  - getAvailablePath
```
public Path getAvailablePath(FileSystem fs)
                      throws IOException
```
    Returns:
    
    the path of the first available link.
    
    Throws:
    
    IOException
  - getFileStatus
```
public FileStatus getFileStatus(FileSystem fs)
                         throws IOException
```
    Get the FileStatus of the referenced file.
    
    Parameters:
    
    fs - FileSystem on which to get the file status
    
    Returns:
    
    InputStream for the hfile link.
    
    Throws:
    
    IOException - on unexpected error.
  - open
```
public FSDataInputStream open(FileSystem fs)
                       throws IOException
```
    Open the FileLink for read.
    It uses a wrapper of FSDataInputStream that is agnostic to the location of the file, even if the file switches between locations.
    
    Parameters:
    
    fs - FileSystem on which to open the FileLink
    
    Returns:
    
    InputStream for reading the file link.
    
    Throws:
    
    IOException - on unexpected error.
  - open
```
public FSDataInputStream open(FileSystem fs,
                              int bufferSize)
                       throws IOException
```
    Open the FileLink for read.
    It uses a wrapper of FSDataInputStream that is agnostic to the location of the file, even if the file switches between locations.
    
    Parameters:
    
    fs - FileSystem on which to open the FileLink
    
    bufferSize - the size of the buffer to be used.
    
    Returns:
    
    InputStream for reading the file link.
    
    Throws:
    
    IOException - on unexpected error.
  - setLocations
```
protected void setLocations(Path originPath,
                            Path... alternativePaths)
```
    NOTE: This method must be used only in the constructor! It creates a List with the specified locations for the link.
  - getBackReferencesDir
```
public static Path getBackReferencesDir(Path storeDir,
                                        String fileName)
```
    Get the directory to store the link back references
    To simplify the reference count process, during the FileLink creation a back-reference is added to the back-reference directory of the specified file.
    
    Parameters:
    
    storeDir - Root directory for the link reference folder
    
    fileName - File Name with links
    
    Returns:
    
    Path for the link back references.
  - getBackReferenceFileName
```
public static String getBackReferenceFileName(Path dirPath)
```
    Get the referenced file name from the reference link directory path.
    
    Parameters:
    
    dirPath - Link references directory path
    
    Returns:
    
    Name of the file referenced
  - isBackReferencesDir
```
public static boolean isBackReferencesDir(Path dirPath)
```
    Checks if the specified directory path is a back reference links folder.
    
    Parameters:
    
    dirPath - Directory path to verify
    
    Returns:
    
    True if the specified directory is a link references folder
  - equals
```
public boolean equals(Object obj)
```
    Overrides:
    
    equals in class Object
  - hashCode
```
public int hashCode()
```
    Overrides:
    
    hashCode in class Object

Class FileLink

Field Summary

Constructor Summary

Method Summary

Methods inherited from class java.lang.Object

Field Detail

BACK_REFERENCES_DIRECTORY_PREFIX

Constructor Detail

FileLink

FileLink

FileLink

Method Detail

getLocations

toString

exists

getAvailablePath

getFileStatus

open

open

setLocations

getBackReferencesDir

getBackReferenceFileName

isBackReferencesDir

equals

hashCode