NServiceBus: is this an illusion or a best practice?
Given an NServiceBus microservice that uses MSMQ, When I deploy few instances of that service into the same machine, Am I scaling out my application?, Am I improving the performance? or one instance is enough. shall I instead have a more powerful machine to handle messages?
No, running multiple instances on a single machine will not make things run faster, it is only making execution less efficient.
However, it might be that a single instance isn’t giving you the expected performance even though your system monitoring indicates there are plenty of resources to spend but not used. In that case you might want tweak the configuration of your NServiceBus endpoint by configuration the amount of allowed parallel message execution.
On the following link you see how you can increase the concurrency:
You can further scaleout by actually using multiple machines but if all these endpoints share the same central database your network or database server can easily become the bottleneck. If you consider deploying or scaling out your endpoints across multiple machines make sure that any storage solutions are also scaled out for these not to become your bottleneck.
Zero downtime upgrades/deployments
The only reason to have multiple instance on the same box is for example when deploying a new version, you can temporarily run the current and the new version side-by-side to achieve zero downtime deployments.