It’s crucial to continuously monitor any errors that occur in your application. You also have to be sure to diligently fix the errors you detect and implement processes to ensure that the same vulnerabilities will not reoccur. A good way of doing this is by introducing a rigorous testing methodology.

Moreover, adding new features at the maintenance stage often means using a new API or library – you should have a mechanism in place that will allow you to verify that any given technology won’t compromise your security profile.

Finally, as your application grows, you might need to change hosting providers or expand your infrastructure by using a new provider or service. It is critical that this process is conducted in a well-thought-out, planned, and conscientious manner so as not to introduce any weaknesses which may compromise your application’s security.

Well it depends. But there are certain rules to follow:

• Keep your code clean
• Leverage 12factor
• Take care of your database
• Prevent problems with queries
• Choose the right hosting
• Track caching
• Prepare for load balancing
• Relieve the backend
• Test and monitor
• Optimize
• Separate code
• Update on a regular basis