docs/book/kernel-process.md
# Introduction
In ZEUS, Process is defined as an instance of a PHP application that is being executed.
The execution of a single process must progress in a sequential fashion, therefore to achieve true concurrency, ZEUS may use _Process Scheduler_ to instantiate more than one such process at the same time.
# Process states
When a process executes, it passes through different states. In ZEUS, a process can have one of the following five states at a time:
Process can use hardware resources in only two states: waiting and running.
The following table describes each state in details:
| State | Description |
|--------------|---------------------------------------------------------------------------------------------------------------------------------------------|
| `STARTED` | An initial state when a process is first created |
| `WAITING` | Process is in a waiting state if it needs to wait for a resource, such as waiting for network connection, or some data to become available |
| `RUNNING` | Process sets the state to running just before it starts processing its data |
| `TERMINATED` | When process is terminated by the Process Scheduler, it is moved to the terminated state where it waits to be removed from the Task Pool |
| `EXITED` | This state is set when process finishes its execution, and it waits to be removed from the Task Pool |