According to the XL language specification, the XL queries are applied in parallel. In order to make sure the queries and rules are applied in parallel without concurrency access, GroIMP implementation use transactions and queues.
GroIMP transactions work as buffers of operations on the graph. A transaction includes a set of queues that contains the list of modifications to be applied on the graph.
When applied, each RGG method get a transaction automatically (created if needed) and all modifications are pushed into the queues of the current transaction.
There are four main queues: