Scalable web sessions were a significant improvement brought by 4D v18 R6. They allow you to use 4D tags, 4D actions, and REST API in preemptive processes, even in interpreted mode, on a 4D server. However, to debug such programs, you needed to open your development environment on the server to make it switch to cooperative mode, as the debugger window cannot be opened with preemptive processes. This way, until v19 R2, you could debug REST, 4D actions, or 4D tags. From v19 R3, all of this has become much easier, and you can debug on the server-side simply by attaching the debugger to it, as usual.