Nearly every row in every database table is assigned an object_version_number. When a new row is inserted, the API usually sets the object version number to 1. Whenever that row is updated in the database, the object version number is incremented. The row keeps that object version number until it is next updated or deleted. The number is not decremented or reset to a previous value.
Note: The object version number is not unique and does not replace the primary key. There can be many rows in the same table with the same version number. The object version number indicates the version of a specific primary key row. Whenever a database row is transferred (queried) to a client, the existing object version number is always transferred with the other attributes. If the object is modified by the client and saved back to the server, then the current server object version number is compared with the value passed from the client.
• If the two object version number values are the same, then the row on the server is in the same state as when the attributes were transferred to the client. As no other changes have occurred, the current change request can continue and the object version number is incremented.
• If the two values are different, then another user has already changed and committed the row on the server. The current change request is not allowed to continue because the modifications the other user made may be overwritten and lost. (Database locks are used to prevent another user from overwriting uncommitted changes.) The object version number provides similar validation comparison to the online system.
Forms interactively compare all the field values and displays the "Record has been modified by another user" error message if any differences are found. Object version numbers allow transactions to occur across longer periods of time without holding long term database locks. For example, the client application may save the row locally, disconnect from the server and reconnect at a later date to save the change to the database. Additionally, you do not need to check all the values on the client and the server.
Example
Consider creating a new address for a Person. The create_person_address API automatically sets the object_version_number to 1 on the new database row. Then, two separate users query this address at the same time.
User A and user B will both see the same address details with the current object_version_number equal to 1. User A updates the Town field to a different value and calls the update_person_address API passing the current object_version_number equal to 1. As this object_version_number is the same as the value on the database row the update is allowed and the object_version_number is incremented to 2. The new object_version_number is returned to user A and the row is committed in the database.
User B, who has details of the original row, notices that first line of the address is incorrect. User B calls the update_person_address API, passing the new first line and what he thinks is the current object_version_number (1). The API compares this value with the current value on the database row (2). As there is a difference the update is not allowed to continue and an error is returned to user B.
To correct the problem, user B then re-queries this address, seeing the new town and obtains the object_version_number 2. The first line of the address is updated and the update_person_address API is called again. As the object_version_number is the same as the value on the database row the update is allowed to continue.
Therefore both updates have been applied without overwriting the first change.
2 comments:
Thats a very informative post.
Thanks very much for the info in your blog. It is really very helpful and answers the question exactly to which I was looking for.
When I try to terminate and reterminate an employee for couple of times and terminate it again, I get an error saying that, "This record already esists. You entered a duplicate value or sequence of values that must be unique for every record."
What does this error mean?
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