A method performed by a central server node is provided. The method includes: receiving local model weights and corresponding key from a local client node; and updating a model pool having a plurality of central models and corresponding keys associated with each of the central models. Updating the model pool is based on the local model weights, and one or more of the key corresponding to the local client node and the keys collectively corresponding to each of the central models. Updating the model pool comprises updating at least two of the plurality of central models contained in the model pool.