How does an RB decide where to send jobs?
A popular misconception is that the Resource Broker has a built-in algorithm to decide where to send jobs. In fact the job distribution algorithm is determined by the user via the Requirements and Rank expressions (and also any data requirements) in the JDL. All CEs are filtered against the Requirements, and the Rank is calculated for all the CEs which match. The job will go to the CE with the maximum Rank, with the following caveats: • If several CEs have the same maximum Rank, the job is sent randomly to one of them. Thus specifying “Rank = 1” will send jobs randomly to all matching CEs. • The JDL has a “fuzzy rank” option, see the manual for details. In this case the randomisation is extended over more of the highest-ranking CEs. • Some extra Requirements are added automatically: the CE must allow access to the selected VO, and the batch system behind the CE must be accepting and running jobs. • VOs can also add their own restrictions to the sites considered for matching via the Fre
