Tasks in the RasterAnalysisTools service execute asynchronously, meaning that you submit a request (or job) and poll the service for the status. When the status is esriJobSucceeded, you can access the results of the task. When you submit a request, the RasterAnalysisTools service returns to you a unique job ID for the transaction.
http://<server url>/arcgis/rest/services/System/RasterAnalysisTools/GPServer/<task name>/jobs/<jobId>
For example, with a server named myserver on esri.com and a Web Adaptor called server that was running CalculateDensity and the job ID returned was jbe5013b63142432dac207dcff51b68ds, the URL for checking the job status would be http://myserver.esri.com/server/rest/services/System/RasterAnalysisTools/GPServer/CalculateDensity/jobs/jbe5013b63142432dac207dcff51b68ds.
You can use the job ID to periodically check the status of the job and messages. Additionally, if the job has successfully completed, you can use the job ID to retrieve the results.
The jobStatus property can have the following values: