Flowable tasks are the most complex tasks to develop, and will usually be available from the Kestra core. You will rarely need to create a flowable task by yourself.
When developing such tasks, you must make it fault-tolerant as an exception thrown by a flowable task can endanger the Kestra instance and lead to inconsistencies in the flow execution.
Keep in mind that a flowable task will be evaluated very frequently inside the Executor and must be low CPU usage; no I/O should be done by this kind of task.
In the future, complete documentation will be available here. In the meantime, you can find all the actual Flowable tasks here to have some inspiration for Sequential or Parallel tasks development.