MaxScale should detect if two or more instances of MaxScale are running on the same computer and refuse to start if they are started in a way that causes conflicts between them.

Case in point; MXS-1661 describes a situation where when there was an authentication error, MaxScale would log database is locked. The reason was that two MaxScale instances were running; both using the same cache directory and thus the same sqlite user database.

When a second MaxScale instances is started, it should detect if there is such a conflict, log a descriptive error message, and refuse to start.