To configure use `database.backend` option
It can be a string value like `database.backend = 'native'`.
So it will always use same backend.
Or it can be a table value like
```Lua
database.backend = {
main = 'native';
base64 = '';
}
```
Role `database.backend.main` is predefined and it equal to `native` if not set.
If there no role when Database class creates or role unknown role `main` is used
```Lua
dbh = Database.new('system') -- uses role `main`
dbh = Database.new('system', 'main') -- same as previews
dbh = Database.new('system', 'base64') -- uses role `base64`
dbh = Database.new('system', 'test') -- uses role `main`
```
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