The purpose of this service is to provide a disk-backed cache for time-windowed state data extracted from backend dCache components, most importantly, pools.
The service executes collection updates periodically to its write-through cache so that the data is both stored on disk and held in memory. In the case of restart, the existing files are read back.
The Frontend service contacts the history service using normal dCache messaging in order to serve this data through the RESTful API.
Currently, pool state information is saved by the History service into a set of .json files; their default location is
on the file system local to the node running the service. This location, along with the collection intervals, can be configured from properties; see /usr/share/dcache/defaults/history.properties.
Running this service is essential for the delivery of histogram data from the frontend. It can be run out of the box with no special properties set:
While it is not required to run this service in its own domain, it is usually good practice to isolate a non-essential service this way, allowing a restart which does not affect other services, should that be necessary.