# Principles of developing robust applications
Kotlin edition
[TOC levels=2-6]
## Making illegal state unrepresentable
### Union types
### Validate at the boundary
### Null handling
### Enumerating all cases
### Specific types with value classes
## Exceptions and how to handle them
## Logging
Log as much as necessary but as little as possible. Messages should have enough context to be valuable. This means if you read a log message it should be useful. Who did what, when and where: "Document updated" is less useful than: "User A updated document D, changing the title from Y to Z."
### Log Levels
* Verbose: Use this to print raw input / output from external sources.
* Debug: Provide context that is useful for debugging, i.e. making internal state visible, log the path taken in important conditionals
* Info: Use this to log (user) and system actions that were done purposefully, i.e. triggered by an external event.
* Warning: Use this for errors that can be recovered from.
* Error: Use this for unrecoverable errors.
### Log rotation
Don't forget to enable log rotation, otherwise your application will go down unexpectedly because of a full disk.
## Memory usage
If possible, try to keep the memory usage constant when working with external resources. Use streams instead of loading them into memory if the size is undefined.
For example, this takes as much memory as the file is big. If the file's bigger than the JVMs heap allowance, it dies with an OOM.
```kotlin
// read file content into memory
val content = Files.readAllBytes(Paths.get("source.jpg"))
// write file content to disk
Files.write(Paths.get("target.jpg"), content)
```
On the other hand, the following code:
```kotlin
FileInputStream("source.jpg").use { inputStream ->
FileOutputStream("target.jpg").use { outputStream ->
inputStream.copyTo(outputStream, 4096)
}
}
```
takes roughly 4096 bytes at once, no matter how big the file is.
Often, this can't be avoided when you need the whole object in-memory to process it. Even in this case, though, you can often avoid having multiple representations of the same object (in different states) in memory. This means loading the whole document from a stream instead of a byte array.