This paper proposes an implementation of the data structure called bag or multiset used by descriptive programming languages (e.g. Gamma, Linda) over an open system. In this model, a succession of "chemical reactions" consumes the elements of the bag and produces new elements according to specific rules. This approach is particularly interesting as it suppresses all unneeded synchronization and reveals all the potential parallelism of a program. An efficient implementation of a bag provides an efficient implementation of the subsequent program. This paper defines a new communication and synchronization model adapted from workqueues used in parallel computing. The proposed model allows to benefit from the potential parallelism offered by this style of programming when only an approximate solution is needed.