Optimistic Locking


With optimistic locking, you write your business logic allowing multiple users to read the same object at the same time, but allow only one user to update the object successfully. The assumption is that there will be a relatively large number of users trying to read the same object, but a low probability of having a small number of users trying to update the same object at the same time.

In the case of multiple users trying to update the same object at the same time, the one(s) that try to update a non-recent object version fail.

Optimistic locking relies on the idea that data remains unmodified while it is away from the server. As a simple example, consider how you update a customer details object. The customer details are stored within an object, and if a client application needs to update them, it first needs to get the object from the space. The data is not locked, and other client applications can have access to it simultaneously, thus ensuring a scalable system.

The problem is that while the customer details object is away from the space server, it may become stale. For example, a second client application can request the same customer details, update them, and commit them back to the space. The first client, unaware that it is dealing with a stale copy of the data, modifies and commits the data. Obviously, with no version checking mechanism to detect this conflict, the first client’s application changes, which commit last, are made permanent, thus overwriting the changes made by the second client application.

For optimistic locking to work effectively, you must be able to detect these update-update conflicts, and to make the client aware of them, so they can be dealt with appropriately.

GigaSpaces optimistic locking protocol:

  • Is best suited for environments with many read-only transactions, few read-update transactions, and a relatively low volume of objects that are changed.
  • Is more suitable for real-time systems than pessimistic locking, because the space runs best with short term transactions.
  • Has a big advantage when you want to read a large number of objects, but update only a few of them - or when it is unlikely that objects you want to work with are updated by other users.
  • Ensures that updated objects are the most recent ones, while improving the coherency of system behavior.

Using the Optimistic Locking Protocol

Here are the steps you should execute to update data, using the optimistic locking protocol:

Step 1 – Get a Space Proxy in Versioned Mode

Get a space proxy in versioned mode. This can be done using one of the options listed below. You may get remote or embedded space proxies. Make sure the proxy is in optimistic locking mode using the (versioned) option. This can be done using one of the options listed below:

public void createVersionedSpace()
{
	// Create the SpaceProxy to Embedded Space
	ISpaceProxy spaceProxy = new EmbeddedSpaceFactory("mySpace").Create();
	spaceProxy.OptimisticLocking = true;
}

Step 2 – Enable the Space Class to Support Optimistic Locking

You should enable the Space class to support the optimistic locking protocol, by including the [SpaceVersion] decoration on an int getter field. This field stores the current object version and is maintained by XAP. See below for an example:

	[SpaceClass]
	public class Account {
		[SpaceID]
		[SpaceRouting]
		private long? Id { set; get; }
		private String Number{ set; get; }
		private double? Receipts{ set; get; }
		private double? FeeAmount{ set; get; }
		private Nullable<EAccountStatus> Status{ set; get; }
		[SpaceVersion]
		private int Version{ set; get; }
		// ......
    }

Step 3 – Read Objects without using a Transaction

Read objects from the space without using a transaction. You may use the ReadMultiple method to get several objects in one call. Reading objects without using a transaction, allows multiple users to get the same objects at the same time, and allows them to be updated using the optimistic locking protocol. If objects are read using a transaction, no other user can update the objects until the object is committed or rolled back.

Step 4 – Modify and Update the Objects

Modify the objects you read from the space and call a Write space operation to update the object within the space. Use a transactional with your write operation. You must use a transaction when you update multiple space objects in the same context. When the write operation is called to update the object, the space does the following:

  • for each updated object, the Version ID in the updated object is compared with the Version ID of the corresponding object within the space. This is done at the space side.
  • if the Version ID of the updated object is the same as the Version ID of the corresponding object within the space, it is incremented by 1, and the object is updated within the space successfully.
  • if the Version ID of the updated object is different than the Version ID of the corresponding object within the space, the object is not updated within the space - i.e. the operation fails. In this case, a SpaceOptimisticLockingFailureException is thrown.

It is recommended that you call the update operation just before the commit operation. This minimizes the time the object is locked under a transaction.

optimistick_lock2.jpg

Step 5 – Update Failure

If you use optimistic locking and your update operation fails, an Exception is thrown. This exception is thrown when you try to write an object whose version ID value does not match the version of the existing object within the space - i.e. you are not using the latest version of the object. You can either roll back or refresh the failed object and try updating it again. This means you should repeat steps 3 and 4 - read the latest committed object from the space, back to the client side and perform the update again. For a fast refresh, you may re-read the object using ReadByID method. Make sure you also provide the SpaceRouting value.

Step 6 – Commit or Rollback Changes

At any time, you can commit or rollback the transaction. If you are using Spring automatic transaction demarcation, the commit is called implicitly once the method that started the transaction is completed. By following the above procedure, you get a shorter locking duration, that improves performance and concurrency of access among multiple users to the space object. The object version ID validation that is performed on Write, Take, and WriteMultiple requests, keeps the data within the space consistent.

Scenario Example

Suppose that you have two applications, Application_1 and Application_2, which are both working with the same Object A. The following sequence of events describes a simple optimistic locking scenario.

Time Action Application_1 Application_2
T=1 Initial conditions: both applications read the object from the space. Object A VersionID=1
Value1=X
Value2=Y
Object A
VersionID=1
Value1=X
Value2=Y
T=2 Both modify the same objects. Object A VersionID=1
value1=X
Value2=Y_1
Object A
VersionID=1
value1=X_2
Value2=Y
T=3 Application_2 updates object A and commits - commit is successful. Object A VersionID=1
Value1=X
Value2=Y_1
Object A
VersionID=2
Value1=X_2
Value2=Y
T=4 Application_1 tries to update object A, but fails due to invalid Version ID. Object A VersionID=1
Value1=X
Value2=Y_1
T=5 Application_1 refreshes object A (re-reads it). Object A VersionID=2
Value1=X_2
Value2=Y
T=6 Application_1 updates object A again and commits - commit is successful. Object A VersionID=3
value1=X_2
Value2=Y_1