A toy model of the factor graph of metabolic modules, flux balance conditions, and the flux model for the module R₂ (top right). In the factor graph, each C (metabolites) corresponds to one flux balance condition and serves as a factor, and each R (can be a reaction or a module) is a variable. For example, $C^0(R_0,R_1,R_2|L_{c^0})$ simply represents that the metabolite C⁰ is determined by the flux balance loss of R₀, R₁, R₂, where $L_{c^0}$ is the flux balance term of C⁰. Import and export/degradation reactions are considered as having no input or output substrates.