Listers allow us to find new origins from sources of data we already know.

Instances of listers have the following characteristics:

• a type, corresponding to the module where the lister is defined (e.g. swh.lister.github)
• parameters, that allow sharing the same code for different sources (e.g. the URL of a forge, or of a webpage containing software)

Listers should be able to operate in two modes:

• incremental, supposed to run frequently
• full, supposed to run the first time then infrequently

Both modes of listers will:

• for new origins
  • Create the origin
  • Create associated entities
  • Schedule the recurring task in the scheduler database and link it to the origin
• for disappeared origins
  • Mark the origin as inactive
  • Unschedule the recurring task in the scheduler database

Lister can use ad-hoc storage facilities (e.g. their own database) to keep state between incrementals.