A method for upgrading software having steps (A) to (D) is disclosed. Step (A) may deny a first server access to read from and write to a database controlled by a second server while second software in the second server is being upgraded. The second software as upgraded may be incompatible with first software running in the first server. Step (B) may generate data in the first server in response to a current operation of the first software while the second software is being upgraded. Step (C) may update the database by transferring the data from the first server to the database through the second server after the upgrade of the second software has finished. The first server generally remains denied to read from the database. Step (D) may upgrade the first software to be compatible with the second software in response to finishing the current operation.