However, when blocking occurs, using the appropriate query, we will be able to get a tree view of blocking like the following one:
DEADLOCK COATINGS UPDATE
This situation will end as soon as the UPDATE query has been completed and UserA has committed their transaction.Īs we may expect, we can encounter a lot more complex situations, involving a session holding multiple locks on multiple resources. The situation can be graphically summarized as follows: Until that happen, we can say that UserB’s session is blocked by UserA’s session. This means that thread taking care of UserB’s session has to wait for UserA’s session to release this X lock. However, shared and exclusive locks are incompatible (even semantically). Among them, the page with an X lock acquired by UserA. Try to attempt a shared (S) lock on the pages needed to display the list. This will work as IX and IS on a table are compatible. This lock is used to establish a lock hierarchy in order to perform read-only operations. UserB’s thread will:Īcquire an Intent Shared (IS) lock on the Invoice table. This means that the session will be the only one allowed to modify that row until it releases this lock.Īt the same time, UserB wants to get a list of the invoices for current month and unfortunately, the invoice UserA is editing sits in that list. This lock is used to establish a lock hierarchy in order to perform data modifications.Īn Exclusive (X) lock on the row userA is editing. To perform this operation, the thread associated to that session inside SQL Server database engine has to acquire and hold:Īn Intent-Exclusive (IX) lock on the Invoice table and on the page, that contain the row userA is editing. UserA is currently editing an invoice, which implies an UPDATE statement against an Invoice table with a WHERE clause that restricts to a particular value of InvoiceId column of that table. Let’s assume that we have two database users already connected to SQL Server using an application: UserA and UserB. We will take some time to understand blocking before talking about deadlocks as they seem to be the worst case of blockingĪ blocking situation can be understood with the following example. This second case is commonly referred to as “blocking”. Still, intuitively, we could say that a deadlock falls into the second case, the one that tells other sessions to wait for a resource, but this wait might never end.
For that reason, Microsoft provided a documentation page about what they call lock compatibility. Let’s notice that all locking modes are not compatible. Choosing a mode instead of another to access a given resource in a session will either let other sessions (or transaction) access the same resource or will make other sessions wait for that resource to be unlocked. Actually, we can acquire a lock on different kind of resources (row identifier, key, page, table…) and using different modes (shared, exclusive…).
In relational database management systems, locking is a mechanism that happens every time. We could intuitively understand it as a lock that leads to a dead end… So, what is a “deadlock”? Etymologically, it can be divided into two root words: “dead” and “lock”. Then we will see that SQL Server actually has some tools to help detect their occurrences and get everything we need to find and fix it, although the solution might be simpler said than done… So, in the following sections, we will try to understand, basically, what a deadlock is and how it occurs with a practical example in T-SQL.
These actions would lead to either a lower occurrence frequency or a total disappearance of deadlock conditions. After reading this article you will be able to explain how a deadlock can occur and how we can get extended information about this deadlock so that we will be able to diagnose and take the appropriate actions.
DEADLOCK COATINGS SERIES
This article is the first one of a series that will focus on deadlock issues and reporting. Due to their nature and like any similar blocking situation, deadlocks can have a direct impact on user experience and the more they occur, the bigger the impact.
DEADLOCK COATINGS CODE
Sometimes, poorly written code or lack of indexes will generate blocking conditions that we refer to as a“Deadlock”. As a DBA, I’ve been, more than, confronted with performance problems.
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