System and method for an optimistic database access

• US 7,774,319 B2
• Filed: 08/11/2004
• Issued: 08/10/2010
• Est. Priority Date: 08/11/2004
• Status: Active Grant

• Current Assignee: SAP SE
• Original Assignee: SAP AG (SAP SE)
• Inventors: Wenner, Tobias, Ziegler, Torsten, Drach, Guenther, Haas, Frank-Martin, Schweigkoffer, Rainer
• Primary Examiner(s): Vital; Pierre M
• Assistant Examiner(s): Raab; Christopher J
• Application Number:
  US10/917,548
• Publication Number:
  US 20060036574A1
• Time in Patent Office: 2,190 Days
• Field of Search: 707/8, 707/201, 707/2, 709/203
• US Class Current: 707/690
• CPC Class Codes: G06F 16/2322 using timestamps
• CPC Class Codes: G06F 16/2329 using versioning
• CPC Class Codes: G06F 16/2343 Locking methods, e.g. distributed locking or locking implementation details

Abstract

In an embodiment, a database interface performs a three-step optimistic database access. The database interface initially performs an optimistic read of a database object. The database object is checked for consistency after the optimistic read. If the database object is inconsistent, a first pessimistic read is performed with an isolation level that does not guarantee consistency (e.g., with an isolation level of committed read). The database object is checked again for consistency after the first pessimistic read. If the database object is inconsistent, a second pessimistic read is performed with an isolation level that may guarantee consistency (e.g., with an isolation level of repeatable read).

Claims

1. A method for reading data from a database object in a database, wherein the method comprises:

performing an uncommitted read including an optimistic read of the database object during a transaction, wherein the optimistic read is performed without enforcing consistency requirements of data that is not required to be committed to the database;

executing a first consistency check of the database object to determine whether the data is consistent with the database;

performing a committed read including a first pessimistic read of the database object during the transaction when the first consistency check indicates the data as being inconsistent with the database, wherein the performing of the committed read includes committing the data to the database and allowing concurrent modification of the database object;

executing a second consistency check of the database object to determine whether the data is consistent with the database, in response to the first pessimistic read; and

performing a guaranteed read including a second pessimistic read of the database object during the transaction when the second consistency check indicates the data not being consistent with the database, wherein the performing of the guaranteed read includes committing the data to the database and disallowing concurrent modification of the database object.

2. The method of claim 1, wherein:

performing the uncommitted read that results in the data being inconsistent with the database comprises reading data that is not committed to the database when the database object is read, and is later abandoned; and

wherein

performing the committed read that results in the data being inconsistent with the database comprises reading data that is committed to the database when the database object is read, and subsequently or concurrently modified.

3. The method of claim 1, wherein performing the uncommitted read that results in the data being inconsistent with the database comprises the database identifying the uncommitted read as one of a dirty read, a non-repeatable read, or a phantom read of the database object.

4. The method of claim 1, wherein performing the committed read that results in the data being inconsistent with the database comprises performing a consistency check of the data that indicates the data is inconsistent with the database.

5. The method of claim 1, wherein performing the uncommitted read that results in the data being inconsistent with the database comprises performing a consistency check of the data that indicates the data is inconsistent with the database.

6. The method of claim 5, wherein performing the consistency check of the data that indicates that the data is inconsistent with the database comprises comparing an access timestamp of the database object with a write agent timestamp for the database object.

7. The method of claim 1, wherein performing the uncommitted read that results in the data being inconsistent with the database comprises receiving, from the database, an error indicating the data is not consistent with the database.

8. The method of claim 1, wherein performing the uncommitted read that results in the data being inconsistent with the database comprises determining that a cursor used to read the data in the database object is corrupted.

9. The method of claim 1, wherein performing the guaranteed read that disallows concurrent modification of the database object further comprises performing a consistency check on data read from the database object.

10. The method of claim 1, wherein performing the guaranteed read that disallows concurrent modification of the database object comprises exclusively locking the database object, excluding any other concurrent operation on the database object.

11. The method of claim 1, wherein performing the guaranteed read that disallows concurrent modification of the database object comprises creating a shared lock on the database object, excluding any other concurrent operation from modifying, updating, deleting, or writing to the database object.

12. The method of claim 11, wherein creating the shared lock comprises allowing concurrent read operations of the database object.

13. The method of claim 1, wherein:

performing the guaranteed read of the database object comprises performing a repeatable read or performing a serialized read of the database object.

14. The method of claim 1, wherein:

performing the committed read of the database object comprises performing a database read at an isolation level greater than an uncommitted read isolation level; and

wherein

performing the guaranteed read of the database object comprises performing the database read at an isolation level greater than or equal to a consistent object isolation level.

15. The method of claim 14, wherein performing the guaranteed read that requires the data be committed to the database and disallows concurrent modification of the database object comprises setting a flag in a shared library of the database specifying a database read isolation level greater than or equal to a repeatable read isolation level.

16. The method of claim 1, wherein:

performing the committed read of the database comprises performing the committed read of the database object responsive to performing the uncommitted read that results in the data being inconsistent with the data base object; and

wherein

performing the guaranteed read of the database object comprises performing the guaranteed read of the database object responsive to performing the committed read that results in the data being inconsistent with the data base object.

17. A database system having a database, a memory, and a processor coupled to the memory, the database system further having a database interface system including a database interface represented via executable content stored in the memory and executed by the processor to read data from a database object stored in the database, the database interface system comprising:

means for performing an uncommitted read including an optimistic read of the database object during a transaction, wherein the optimistic read is performed without enforcing consistency requirements of data that is not required to be committed to the database;

means for executing a first consistency check of the database object to determine whether the data is consistent with the database;

means for performing a committed read including a first pessimistic read of the database object during the transaction when the first consistency check indicates the data as being inconsistent with the database, wherein the performing of the committed read includes committing the data to the database and allowing concurrent modification of the database object;

executing a second consistency check of the database object to determine whether the data is consistent with the database, in response to the first pessimistic read; and

means for performing a guaranteed read including a second pessimistic read of the database object during the transaction when the second consistency check indicates the data not being consistent with the database, wherein the performing of the guaranteed read includes committing the data to the database and disallowing concurrent modification of the database object.

18. The database system of claim 17, wherein performing the committed read that results in the data being inconsistent with the database comprises means for performing a consistency check of the data that indicates the data is inconsistent with the database.

19. The database system of claim 17, wherein performing the guaranteed read that disallows concurrent modification of the database object comprises means for exclusively locking the database object, excluding any other concurrent operation on the database object.

20. The database system of claim 17, wherein performing the guaranteed read that disallows concurrent modification of the database object comprises means for creating a shared lock on the database object, excluding any other concurrent operation from modifying, updating, deleting, or writing to the database object.

21. The database system of claim 20, wherein creating the shared lock comprises means for allowing concurrent read operations of the database object.

22. The database system of claim 17, wherein:

performing the committed read of the database object comprises means for performing a database read at an isolation level greater than an uncommitted read isolation level; and

wherein

performing the guaranteed read of the database object comprises means for performing the database read at an isolation level greater than or equal to a consistent object isolation level.

23. The database system of claim 17, wherein:

performing the committed read of the database comprises means for performing the committed read of the database object responsive to performing the uncommitted read that results in the data being inconsistent with the data base object; and

wherein

performing the guaranteed read of the database object comprises means for performing the guaranteed read of the database object responsive to performing the committed read that results in the data being inconsistent with the data base object.

24. A machine-readable storage medium having instructions stored thereon that, when executed by a processor, cause the processor to perform the following operations, comprising:

performing an uncommitted read including an optimistic read of the database object during a transaction, wherein the optimistic read is performed without enforcing consistency requirements of data that is not required to be committed to the database;

executing a first consistency check of the database object to determine whether the data is consistent with the database;

performing a committed read including a first pessimistic read of the database object during the transaction when the first consistency check indicates the data as being inconsistent with the database, wherein the performing of the committed read includes committing the data to the database and allowing concurrent modification of the database object; and

executing a second consistency check of the database object to determine whether the data is consistent with the database, in response to the first pessimistic read; and

performing a guaranteed read including a second pessimistic read of the database object during the transaction when the second consistency check indicates the data not being consistent with the database, wherein the performing of the guaranteed read includes committing the data to the database and disallowing concurrent modification of the database object.

25. The machine-readable storage medium of claim 24, wherein:

the uncommitted read that results in the data being inconsistent with the database comprises reading data that is not committed to the database when the database object is read, and is subsequently abandoned; and

wherein

the committed read that results in the data being inconsistent with the database comprises reading data that is committed to the database when the database object is read, and subsequently or concurrently modified.

26. The machine-readable storage medium of claim 24, wherein the uncommitted read that results in the data being inconsistent with the database comprises the database identifying the uncommitted read as one of a dirty read, a non-repeatable read, or a phantom read of the database object.

27. The machine-readable storage medium article of claim 24, wherein the uncommitted read that results in the data being inconsistent with the database comprises receiving, from the database, an error indicating the data is not consistent with the database.

28. The machine-readable storage medium of claim 24, wherein the guaranteed read that disallows concurrent modification of the database object comprises creating a shared lock on the database object, excluding any other concurrent operation from modifying, updating, deleting, or writing to the database object, and wherein creating the shared lock allows concurrent read operations of the database object.

29. The machine-readable storage medium of claim 24, wherein:

the guaranteed read of the database object comprises a repeatable read or a serialized read of the database object.

30. The machine-readable storage medium of claim 24, wherein:

the committed read of the database comprises the committed read of the database object to be performed responsive to the uncommitted read that results in the data being inconsistent with the data base object; and

wherein

the guaranteed read of the database object comprises the guaranteed read of the database object to be performed responsive to the committed read that results in the data being inconsistent with the data base object.