![]() UPDATE Book SET BookCode = '2021-01' WHERE BookId = 2 UPDATE Book SET AuthorId = 1 WHERE BookId = 2 - Process B UPDATE Author SET Email = WHERE AuthorId = 1 Here's an example of two SQL statements that would cause a deadlock: - Process A ![]() Introduction to Deadlocksĭeadlock is a special blocking scenario that occurs when two processes are blocked by each other because they are attempting to access a resource that it locked by the other process. Process B is said to have been "blocked" by Process A. The other process, i.e., Process B waits for Process A to complete so that the lock on the shared piece of data is released. Suppose Process A has locked the piece of shared data. This situation occurs when two processes (say Process A and Process B) need access to the same piece of data at the same point in time. Relational databases take advantage of locking when running concurrent operations to prevent data inconsistency and data loss. Locking protects data integrity during concurrent transactions. This prevents two SQL statements from updating the row at the same. In order to edit this row, the SQL statement would have to first acquire the lock. Imagine that each row in your table has a "lock". In this section, I'll discuss the concepts to understand why deadlocks occur. Looking for a modern SQL editor? Try Arctype's free SQL editor for easy queries, visualizations, and sharing. In this article I'll discuss why deadlocks occur, how different databases handle them, how to resolve them, and tips on how to prevent them. It occurs when two processes attempt to gain exclusive access to a resource, and each of them waits on the other to complete before moving forward. These may well however be risks you are willing to take and/or mitigate against in order to improve concurrency.Are you looking to improve your database performance? If you've already taken care of the low-hanging fruit like database indexes, then deadlocks in your database could be the culprit.ĭeadlocks create situations similar to a traffic jam at an intersection, where none of the cars can move. ![]() You'll find that less stringent isolation levels will allow the EF code to work, but at the risk of phantom records, non-repeatable reads etc. The Serializable isolation level can massively reduce concurrency and this example shows exactly why. The ExecuteSqlCommand only requires a single update statement and thus a deadlock does not occur. The other client itself then tries to obtain an exclusive lock and you have a deadlock scenario. Then when one or other of them first tries to perform the UPDATE they cannot get the requisite exclusive lock because the other client has a shared lock on it. ![]() With a serializable isolation level both client A and client B take a shared lock for the duration of the transaction on the record when the SELECT statement is run. and a subsequent UPDATE for the SaveChanges() call. This is due to the EF code generating two SQL statements: a SELECT for the line: var test = db.customer_table.Where(x => x.id = 38).FirstOrDefault() This is so confusing, what linq have done in behind making Transaction working inconsistent behavior? Var test = db.Database.ExecuteSqlCommand("Update customer_table set bank_holder_name = 'CLIENT XXXXX' where pu_id = 38") <= Client B is stop here and proceed after Client A is completedįinally, the transaction is working with code above (not linq function). I edited my code as below: using (myEntities db = new myEntities ()) Thus, I suspected this might caused by linq (incorrect row/ table lock) This is not what I expected where Client B should wait and not allowed to query any data about row id=38, but somehow it can proceed until SaveChanges and throws an error in the end. After CLIENT A finish commit, here will throw *Deadlock found error*" Test.bank_holder_name = "CLIENT NAME XXXX" ĭb.SaveChanges() <= CLIENT B stop here while client A still in progress. Var test = db.customer_table.Where(x => x.id = 38).FirstOrDefault()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |