Prefer the synchronous endpoint and use this Batch Cluster endpoint for long running problems only. The work flow is asynchronous:
send a POST request towards https://graphhopper.com/api/1/cluster/calculate?key=<your_key> and fetch the job_id.
poll the solution every 500ms until it gives status=finished. Do this with a GET request towards https://graphhopper.com/api/1/cluster/solution/<job_id>?key=<your_key>.