Checking the developed software product?

Question

Programming

So I developed some kind of software product
site, plugin, module, parser, some kind of functionality, and so on.
No matter what
How to properly check the performance of the product?
What is the general scheme?
Is it possible to automate this somehow?
Let's say I have developed a reporting system
. How to check the product for 100% performance?
That all errors are handled correctly, that 100% there will be no exceptions in the code.
That all restrictions and quotas are taken into account
. That the data shown in the report is 100% true.
That is, let's say that fractional numbers are not rounded, that everything is added up and divided as it should,
that there are no invalid formats, and so on.

Answer 1

You can't check 100%.
And never.
If this is something not quite trivial and no one needs it.
But the wings of your aspiration are clipped by the economy - for as more and more careful verification, its complexity increases radically.
Lots of methods.
Manual, automatic. Tested in parts and assemblies.
These are too different methods and too vast a topic to tell them all like this.
They decide individually - whether automatic or manual testing is needed, or to what extent in this particular task.
All more or less complex software systems contain errors.
You need to try to fix them, but even 99% of freedom from errors you get. Not that 100%
Anyone who finds a way to get rid of errors at least 99% will get rich.
To get rid of errors, they invent new programming languages, various work methodologies, etc., etc. - and you want to be told in a nutshell ................... .

Answer 2

Professional testing implies a separate direction.
The tester takes the TOR and writes test cases - that is, a description of how and what needs to be tested to make sure that it corresponds to the technical task.
Simple products can be tested for the main functionality on the knee.
Complicated - this is done by analysts with testers. The former refine the TK, the latter create tests and use it.
In general, read basic books and articles about testing.

Answer 3

How to properly check the performance of the product?

Test:
unit - the programmer usually writes on his own code, all dependencies get wet, most often in a white box. At 95+% coverage, a good 3/4 of your code is tests.
functional - also, usually written by a programmer, but with the involvement of external services, such as a database. The main thing here is not to overdo it, functional tests are written faster, but they are orders of magnitude slower than unit tests.
appearance - they write QA more often, sometimes they do it manually, the goal is to quickly understand that the product works in principle and nothing critical is
broken regration - they write QA more often, and they do it manually - this is a complete test that the product works according to the specification
1. Wrote the code for the task
2. Covered it with tests
3. Sent a review on the code
4. Made corrections, if necessary
5. Uploaded it to the working branch
In many ways - yes, of course, take and write tests.
Cover all possible problems with your product with tests. But, you won’t think of all of them, and you won’t protect yourself from them, so 100% is a rather ephemeral figure. For example, how do you guarantee that the product will work if all the DCs in which it is located fail at once?))
For each method, emulate the situation for each of the errors and check the correctness of these errors.