When Should I Add an Extra Database Node?

Ashraf Sharif

The fact that people are not easily convinced to have an additional database node in production due to cost is somewhat absurd and is an idea that should be put aside. While adding a new node would bring more complexity to the current database infrastructure, there is a plethora of automation and helper tools in the market that can help you manage the scalability and continuity of the database layer. 

There are diverse reasons that may influence this somewhat costly decision, and you will probably realize it only when something is going south or starting to fall apart. This blog post provides common reasons when you should add an extra database node into your existing database infrastructure, whether you are running on a standalone or a clustered setup.

Faster Recovery Time

The ultimate reason for having an extra database node for redundancy is to achieve better availability and faster recovery time when something goes wrong. It's a protection against malfunctions that could occur on the primary database node and you would have a standby node which is ready to take over the primary role from the problematic node at any given time. 

A standby node replicating to a primary node is probably the most cost-effective solution that you can have to improve the recovery time. When the primary database node is down, promote the standby node as the new master and change the database connection string in the applications to connect to the new master and you are pretty much back in business. The failover process can then be automated and fine tuned over time, or you could introduce a reverse proxy tier which acts as the gateway on top of the database tier.

Improved Performance

Application grows to be more demanding over time. The magnitude of growth could be exponential depending on the success of your business. Scaling out your database tier to cater for bigger workloads is commonly necessary to improve the performance and responsiveness of your applications. 

Database workloads can be categorized into two - reads or writes. For read-intensive workload, adding more database replicas will help to spread out the load to multiple database servers. For write-intensive workload, adding more database masters will likely reduce the contention that commonly happens in a single node and improve parallelism processing. Just make sure that the multi-master clustering technology that you use supports conflict detection and resolution, otherwise the application has to handle this part separately.

Approaching the Thresholds

As your database usage grows, there will be a point of time where the database node is fast approaching the defined threshold for the server and database resources. Resources like CPU clock, RAM, disk I/O and disk space are frequently becoming the limiting factors for your database to keep up with the demand.

For example, one would probably hit the limit of storage allocated for the database and also approaching the maximum connections allowed to the database. In this case, partitioning your data into multiple nodes would make more sense because you would get more storage space and I/O operations with the ability to process bigger write workloads for the database, just like killing two birds with one stone.

Upgrade Testing

Before upgrading to another major version, it's recommended to test out your current dataset on the new version just to make sure you can operate smoothly and eliminate the element of surprise later on. It's pretty common for the new major version to deprecate some legacy options or parameters that we have been using in the current version and some incompatibilities where application programming changes might be required. Also, you can measure the performance improvement (or regression) that you will get after upgrading, which could justify the reason for this exercise.

Major version upgrade commonly requires extra attention to the upgrade step, if compared to minor version patching which usually can be performed with few steps. Minor releases never change the internal storage format and are always compatible with earlier and later minor releases of the same major version number.

Generally, there are 3 ways to perform database major version upgrade:

  • In-place
  • Logical upgrade
  • Replication

In-place, where you use existing data directory against the new database major version, with just running upgrade script after binaries are upgraded. For logical upgrade, use the logical backup on an old version and then restore it on a new version. This usually requires an additional database node, unless you would like to restore the logical backup on the new version installed in the same server as the old one.

For replication, create a standby server with the updated database version and replicate from the old major version. Once everything is synced up, connect your application to the standby (or slave) server and verify if necessary adjustments are required. Then, you can promote the standby server as the new master and your database server is officially upgraded, with a very minimal downtime.

Backup Verification

We have stressed out this a couple of times in the older blog posts - backup is not a backup if it is not restorable. Backup verification is an important process to ensure you meet your RTO, which basically represents how long it takes to restore from the incident until normal operations are available to the mass users.

You can measure the amount of time it takes to recover by observing the backup verification process, which is the best to be performed on a separate node, as you don't want to increase the burden or put the production database servers under risks.

The ClusterControl backup verification feature allows you to estimate your total mean recovery time, with the extra database node used for verification process can be configured to shut down automatically right after verification process completes. Check out this blog post if you want to learn more about how ClusterControl performs this job.

Conclusion

As your database grows, scaling out your database nodes is going to be necessary and must be well-thought since the beginning. The actual cost of having more database nodes for your environment sometimes justify your requirements and could be more than worth it to keep up with the growth of your business.

 

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