org.h2.store

Class LobStorageBackend

java.lang.Object

org.h2.store.LobStorageBackend

All Implemented Interfaces:

LobStorageInterface

public class LobStorageBackend
extends java.lang.Object
implements LobStorageInterface

This class stores LOB objects in the database, in tables. This is the back-end i.e. the server side of the LOB storage.

Using the system session

Why do we use the system session to store the data? Some LOB operations can take a very long time. If we did them on a normal session, we would be locking the LOB tables for long periods of time, which is extremely detrimental to the rest of the system. Perhaps when we shift to the MVStore engine, we can revisit this design decision (using the StreamStore, that is, no connection at all).

Locking

Normally, the locking order in H2 is: first lock the Session object, then lock the Database object. However, in the case of the LOB data, we are using the system session to store the data. If we locked the normal way, we see deadlocks caused by the following pattern:

  Thread 1:
     locks normal session
     locks database
     waiting to lock system session
  Thread 2:
      locks system session
      waiting to lock database.
 

So, in this class alone, we do two things: we have our very own dedicated session, the LOB session, and we take the locks in this order: first the Database object, and then the LOB session. Since we own the LOB session, no-one else can lock on it, and we are safe.

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
class	LobStorageBackend.LobInputStream An input stream that reads from a LOB.

Field Summary

Fields

Modifier and Type	Field and Description
static java.lang.String	LOB_DATA_TABLE The name of the lob data table.

Constructor Summary

Constructors

Constructor and Description
LobStorageBackend(Database database)

Method Summary

Modifier and Type	Method and Description
ValueLobDb	copyLob(ValueLobDb old, int tableId, long length) Copy a lob.
Value	createBlob(java.io.InputStream in, long maxLength) Create a BLOB object.
Value	createClob(java.io.Reader reader, long maxLength) Create a CLOB object.
java.io.InputStream	getInputStream(ValueLobDb lob, byte[] hmac, long byteCount) Get the input stream for the given lob.
void	init() Initialize the lob storage.
boolean	isReadOnly() Whether the storage is read-only
void	removeAllForTable(int tableId) Remove all LOBs for this table.
void	removeLob(ValueLobDb lob) Delete a LOB (from the database, if it is stored there).

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

LOB_DATA_TABLE

public static final java.lang.String LOB_DATA_TABLE

The name of the lob data table. If this table exists, then lob storage is used.

See Also:

Constant Field Values

Constructor Detail

LobStorageBackend

public LobStorageBackend(Database database)

Method Detail

init

public void init()

Description copied from interface: LobStorageInterface

Initialize the lob storage.

Specified by:

init in interface LobStorageInterface

removeAllForTable

public void removeAllForTable(int tableId)

Description copied from interface: LobStorageInterface

Remove all LOBs for this table.

Specified by:

removeAllForTable in interface LobStorageInterface

Parameters:

tableId - the table id

removeLob

public void removeLob(ValueLobDb lob)

Description copied from interface: LobStorageInterface

Delete a LOB (from the database, if it is stored there).

Specified by:

removeLob in interface LobStorageInterface

Parameters:

lob - the lob

getInputStream

public java.io.InputStream getInputStream(ValueLobDb lob,
                                          byte[] hmac,
                                          long byteCount)
                                   throws java.io.IOException

Description copied from interface: LobStorageInterface

Get the input stream for the given lob.

Specified by:

getInputStream in interface LobStorageInterface

Parameters:

lob - the lob id

hmac - the message authentication code (for remote input streams)

byteCount - the number of bytes to read, or -1 if not known

Returns:

the stream

Throws:

java.io.IOException

isReadOnly

public boolean isReadOnly()

Description copied from interface: LobStorageInterface

Whether the storage is read-only

Specified by:

isReadOnly in interface LobStorageInterface

Returns:

true if yes

copyLob

public ValueLobDb copyLob(ValueLobDb old,
                          int tableId,
                          long length)

Description copied from interface: LobStorageInterface

Copy a lob.

Specified by:

copyLob in interface LobStorageInterface

Parameters:

old - the old lob

tableId - the new table id

length - the length

Returns:

the new lob

createBlob

public Value createBlob(java.io.InputStream in,
                        long maxLength)

Description copied from interface: LobStorageInterface

Create a BLOB object.

Specified by:

createBlob in interface LobStorageInterface

Parameters:

in - the input stream

maxLength - the maximum length (-1 if not known)

Returns:

the LOB

createClob

public Value createClob(java.io.Reader reader,
                        long maxLength)

Description copied from interface: LobStorageInterface

Create a CLOB object.

Specified by:

createClob in interface LobStorageInterface

Parameters:

reader - the reader

maxLength - the maximum length (-1 if not known)

Returns:

the LOB