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Deadlocks and their implications in databases

To enlighten you on all things deadlock-related, here is an overview of what they mean in a database context.

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When managing a database, it is expected that certain issues will arise over time that is deserving of your attention.

One of the most frequently encountered complications in an SQL server environment is deadlocking, a term that sounds superficially intimidating, especially to those not in the know.

Thankfully with a little bit of research and hands-on experience, you will see that deadlocks in databases are not a total catastrophe and that they can be remedied relatively easily. To enlighten you on all things deadlock-related, here is an overview of what they mean in a database context.

Brief deadlock definition

According to source SentryOne, a deadlock is an event that occurs when two or more processes that are initiated on a server hold the exclusive lock to specific resources and are unable to gain access to the next resource they need because of the stalemate this creates.

If that sounds a little complex, think of it like this; two frogs are sitting on two separate lily pads. They want to swap places, but in order to do so, they need their counterpart to move first, which of course is impossible. This is an illustration of just one of the different types of deadlocks that can occur within databases, but it provides a broad overview of the conflicts that can arise in such an environment.

How deadlocks are handled

If processes are competing over locked resources without being able to resolve this issue simply by waiting, the good news is that this will not cause the entire database to grind to a halt. This is because platforms like SQL Server are designed to dead with deadlocks by terminating one of the processes that are creating the logjam so that the other can be completed.

Of course, the result is that there is always at least one victim when a deadlock occurs, so you either need to put mechanisms in place to allow processes to be retried if they are terminated, or get to the root of the issue and fix the thing that caused the deadlock in the first place.

Why deadlocks are necessary

Deadlocks occur specifically in order to preserve the integrity of the data stored on the server. If there was no locking placed on resources, then the concurrent nature of most operations in such an environment would wreak havoc very quickly.

Because of this, deadlocking is part and parcel of a database’s normal operations and should not be seen as a disaster in its own right, because this type of event occurs for the very purpose of maintaining order and consistency.

What you can do to prevent deadlocks

It is worth noting that before we delve into the options you have for troubleshooting database deadlocks, there are some conflicts that are as unavoidable as they are rare and difficult to predict. As such, if a deadlock occurs very infrequently it may not be necessary to do anything about it.

For example, deadlocks may arise if unrelated processes happen to require identical database resources simultaneously on a one-off basis because the workload being handled at a given time is significant. This is why one way to avoid deadlocks is to make sure that you schedule maintenance and indexing tasks to occur during periods when the workload is light.

When it comes to deadlocks that can be detected and remedied, those which are based on the order in which operations are orchestrated to occur are amongst the simplest to fix. Adjusting the way that priorities are granted to processes and ensuring that the order in which they occur is well optimized should serve you well in this scenario.

It is best to focus on deadlocks that occur not only frequently, but also in such a way that the process which is designated as the victim does not get the opportunity to complete time after time. You can set trace flags to track this manually or make use of third party database monitoring solutions to automate some aspects of the troubleshooting process.

Ultimately it is best to be aware that while the implications of deadlocks in a database are not usually severe, you can be proactive in your approach to detecting them and making improvements as part of general server maintenance if you want a tip-top performance to be achieved.

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