For integrations, users have expressed a desire to register a HTTP callback so that when an event happens in Arvados (such as the completion of a workflow) other services can be notified. The advantage of the webhook style API compared to websockets or API polling is that services don't need to dedicate a thread or process to listening for events, they can receive events and process them as normal HTTP requests.

For more reading:

https://webhooks.pbworks.com/w/page/13385124/FrontPage