"How to reassign a task if the server executing it dies? If the server dies, it can't mark the task as open. What are efficient ways to accomplish this?"

If decentralized, you have to work with consensus, if a central (master) server instance is in place, it is the one that is the master who is distributing jobs. So this server is the one to decide if a job has completed or not right?

Simplify the complex problem as much as you can, in other words what you're doing is simply scheduling tasks to nodes. Add:

• Heartbeat (to know if the executing node is up)
• (Optional) Execution timeout (to know if the task went endless loop)
• Keep track of jobs and their executing nodes, and simply round robin the tasks execution across the available nodes (heart beating)

Based on these, your central server will know if the node executing went offline, and ensure tasks execution is rotated across the available nodes.