arlut.csd.ganymede.server

Class DBDeletionManager

java.lang.Object

arlut.csd.ganymede.server.DBDeletionManager

public final class DBDeletionManager
extends java.lang.Object

The DBDeletionManager class is used to handle deletion locking in the Ganymede DBStore.

This class was designed to remove the necessity of having all Invid link relationships be symmetrical. Previously, establishing a one-way link from object 'A' to object 'B' involved editing both A and B, and placing a reverse link in B that pointed to A.

This worked, but it meant that it was impossible for multiple users to be concurrently editing objects to establish or remove links to object B. This was especially troublesome for Owner Group objects, as it meant that only one user at a time could be creating or deleting objects in a given Owner Group.

With the introduction of the DBDeletionManager, we eliminate the need to be constantly editing targets of asymmetric links.

Instead, when an object 'A' is checked out for editing that contains asymmetric link to other objects, those objects are locked (using the DBDeletionManager addSessionInvids() method) so that they cannot be deleted. The session editing object A can remove the links to any of the linked objects if it wants, but if it doesn't, it can be confident that the targeted objects will still be there at the time the transaction commits.

Similarly, if object 'A' does not initially contain a link to 'B', the act of creating an asymmetric link from A to B will cause object B to be deletion locked at that time. See the source of the arlut.csd.ganymede.server.InvidDBField bind() method for details.

In either of the above scenarios, deletion locks established by a given DBSession are released when the session's transactions are committed or aborted in the session's DBEditSet.

Field Summary

Fields

Modifier and Type	Field and Description
private static java.util.Map<Invid,java.util.Set<DBSession>>	invids Invids will appear as keys in this Map when the DBObjects corresponding to those Invids have been delete locked by DBSessions in the server.
private static java.util.Map<DBSession,java.util.Set<Invid>>	sessions DBSession objects will appear as keys in this Map when those sessions have locked Invids in the database against deletion.

Constructor Summary

Constructors

Constructor and Description
DBDeletionManager()

Method Summary

Methods

Modifier and Type	Method and Description
static boolean	addSessionInvids(DBSession session, java.util.Set<Invid> invidSet) This method deletion-locks a vector of invids, returning false without changes if the deletion-locks could not all be performed.
static boolean	deleteLockObject(DBObject obj, DBSession session) This method is used by the Invid binding logic to attempt to block an object from being deleted.
static java.util.Set<Invid>	getSessionCheckpoint(DBSession session) This method returns a snapshot of what Invids are locked by a given session at the time the method is called.
static void	releaseSession(DBSession session) When a DBSession releases or commits a transaction, this method is called to clear any deletion locks asserted by it.
static void	revertSessionCheckpoint(DBSession session, java.util.Set<Invid> invidSet) This method rolls back the invids delete-locked by a session to an earlier state.
static boolean	setDeleteStatus(DBEditObject obj, DBSession session) This method is used by the DBSession deleteDBObject() method to safely convert a DBEditObject's status to a deletion state.

Methods inherited from class java.lang.Object

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

Field Detail

sessions

private static java.util.Map<DBSession,java.util.Set<Invid>> sessions

DBSession objects will appear as keys in this Map when those sessions have locked Invids in the database against deletion.

The Set of Invids mapped from the DBSession is the set of Invids that that DBSession has locked.

invids

private static java.util.Map<Invid,java.util.Set<DBSession>> invids

Invids will appear as keys in this Map when the DBObjects corresponding to those Invids have been delete locked by DBSessions in the server. The Set of DBSession objects is the list of DBSessions that have locked the Invid.

Constructor Detail

DBDeletionManager

public DBDeletionManager()

Method Detail

deleteLockObject

public static boolean deleteLockObject(DBObject obj,
                       DBSession session)

This method is used by the Invid binding logic to attempt to block an object from being deleted. If this method returns true, the object will be forbidden from being deleted until such time as the DBSession that asserted the lock clears it.

setDeleteStatus

public static boolean setDeleteStatus(DBEditObject obj,
                      DBSession session)

This method is used by the DBSession deleteDBObject() method to safely convert a DBEditObject's status to a deletion state. This method returns true if the object was able to be converted to a deletion state, or false if the object has been deletion locked by another DBSession.

releaseSession

public static void releaseSession(DBSession session)

When a DBSession releases or commits a transaction, this method is called to clear any deletion locks asserted by it.

addSessionInvids

public static boolean addSessionInvids(DBSession session,
                       java.util.Set<Invid> invidSet)

This method deletion-locks a vector of invids, returning false without changes if the deletion-locks could not all be performed.

getSessionCheckpoint

public static java.util.Set<Invid> getSessionCheckpoint(DBSession session)

This method returns a snapshot of what Invids are locked by a given session at the time the method is called. It is used by the DBEditSet checkpoint code to allow a transaction to checkpoint the objects that the transaction had delete-locked at a moment in time. With revertSessionCheckpoint(), below, a transaction can provisionally carry out some object linkages which delete-locks objects, then undo those additional locks if the transaction is rolled back.

revertSessionCheckpoint

public static void revertSessionCheckpoint(DBSession session,
                           java.util.Set<Invid> invidSet)

This method rolls back the invids delete-locked by a session to an earlier state. revertSessionCheckpoint() cannot cause more invids to be delete-locked.. the only effect it can have is to undelete-lock invids that were provisionally delete-locked by session between a checkpoint and rollback.