What is the first sign of a deadlock? Queries that should be fast start taking a long time to respond. That’s the first sign of a deadlock, but that’s also the first sign of a lot of other problems. Another sign of a deadlock is an error (error 1205 to be precise) and a very helpful error message: Transaction (Process ID %d) was deadlocked on {%Z} resources with another process and has been chosen as the deadlock victim. Rerun the transaction.SQL Server is telling you exactly how to solve the problem – re-run your transaction. Unfortunately, if the cause of the deadlock is still running, odds are that your transaction will fail. You can enable several trace flags to detect deadlocks (trace flag2 1204 and 1222), but they output the deadlock to the SQL Server error log and produce output that is difficult to read and analyze.

Once deadlocks show up, your database administrator might reach for a script to pull deadlocks out of Extended Events. Extended Events are a great source of data for analysis. Although they’re a relatively new feature to SQL Server, they first appeared in SQL Server 2008, Extended Events already provide an incredibly rich set of tools for monitoring SQL Server. Event data can be held in memory (which is the default) or written out to a file. It’s possible that to build a set of monitoring tools that log all deadlocks to a file and then analyze that file after the events happen.Just like newspapers help us find out what happened yesterday, Extended Events provide a great way to investigate deadlocks that have already occurred. If you don’t have any other monitoring tools in place, Extended Events are a great place to start. Once you start seeing deadlocks, you’ll want to start gathering more information about them. It takes some skill to read the XML from a deadlock graph, but it contains a great deal of information about what happened. You can find out which tables and queries where involved the deadlock process, which process was killed off, and which locks caused the deadlock to occur.

The flip side of the coin is that Extended Events will give you very fine grained information about every deadlock that has already happened. There’s nothing in Extended Events to help you stop deadlocks from happening or even to detect them right when they are happening. Much like a microscope lets you look at a prepared slide in excruciating detail, Extended Events let you look at a single point in time in excruciating detail. You can only find out about things after they happen, not as they happen.