An SQL database is designed to handle complex workloads and perform quickly and consistently even when dealing with a multitude of concurrent requests.Of course the issue of two processes trying to take advantage of the same server resource simultaneously is a common one, and fledgling DBAs might be concerned when conflicts of this kind arise.
Here is a look at why concurrent calls trying to reach the same tables can create performance problems, as well as why it doesn’t have to be a bad thing in every scenario.
A brief introduction to SQL locking & blocking
In an SQL ecosystem, preserving data integrity is of paramount importance, hence the reason that processes can be assigned locks to determine whether or not they can access and modify data within a table at a given instant.
There are several lock modes to consider, and in general for read-only tasks a shared lock will be applied, meaning that the chances of problematic blocking arising are minimal.
Read: How to install MySQL on Ubuntu 18.04
Conversely if an exclusive lock is in place, that resource will be reserved for just one process, thus preventing others from accessing or altering the data in question until the lock is removed.
As hinted at above, this so-called blocking is not just an entirely normal part of the way SQL servers function, but indeed integral to why they are able to perform so effectively as concurrency systems. Without blocking, tables could become messy and inconsistent as multiple calls attempted to access and modify entries at the same time.
Problems worth troubleshooting
While SQL locks and the blocking they bring about are not intrinsically a bad thing, it is still worth monitoring this aspect of a server to make sure that flaws which do create disruption can be dealt with.
Concurrent calls which result in longer than expected blocks should be a priority for anyone tasked with troubleshooting an SQL database. The duration which defines an unacceptably long lasting block will vary, but anything upwards of five seconds may be deserving of your attention.
This is not just about improving database performance, but also about keeping end users happy, since ultimately it will be the apps and services which rely on a server that suffer if requests time out unexpectedly.
Especially severe concurrency complications are known as deadlocks, and result in one of the two processes that is attempting to access the same table being terminated. This is especially suboptimal and, while it does preserve that all-important data integrity, it is a symptom of bigger problems lying beneath the surface.
The importance of monitoring
You can keep tabs on SQL server performance and pinpoint any concerning aspects with monitoring software built for this purpose, and indeed it is sensible to do so because contemporary solutions can automate the detection of problems to streamline a DBA’s duties.
The more experienced you become, the better equipped to manage the quirks of SQL servers and other database platforms you will be, but that is no reason to hobble yourself by not using software to your advantage.
Author: rob@thecopywriter.io
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