Yes. I actually mean both. Thread safety does and doesn&#x27;t matter. MRI has a GIL so appending to an array across threads will &quot;just work.&quot; JRuby doesn&#x27;t and it won&#x27;t end well if you&#x27;re not giving it thought.<p>But yeah, multi-process web servers absolutely trample over each other. File access would need to be coordinated &#x2F; synchronized.