Normally only one thread is allowed in the UltraLiteJ runtime at a time. An exception to this rule occurs during synchronization. While one connection is performing a synchronization operation, other connections can access the UltraLiteJ runtime, however no other thread can call the synchronize method for the database being synchronized while a synchronize operation is taking place.

A connection can access downloaded rows during the sync (that is, before they are committed) that may later vanish if the sync fails. If, during a synchronization, a connection modifies a row that the synchronization then attempts to change, the sync fails. During a synchronization, if a connection attempts to modify a row that the synchronization has changed, the attempt to modify fails.