A method of maintaining a data structure comprising: creating a data block; transmitting a message to validator devices requesting permission to add said data block to the data structure; determining that consensus is reached, and in response: forming a first sub-chain in the data-structure by adding the data block to the data structure, the first sub-chain starting with a genesis block and ending with said data block, wherein the data block comprises a block number indicating the number of blocks in the data structure from the genesis block, and creating a further data block; transmitting a further message to the validator devices requesting permission to add said further data block to the data structure; determining that consensus is reached, and in response, forming a second sub-chain by adding the further data block to the data structure after said data block, the second sub-chain starting with said further data block.