as they arrive when the secondaryserver has received the expected number of writedone messages one writedone for each write in the batch it sends oktowrite messages for the next sequence number step 1 one of more of the i o processors issues a request for a sequence number to the primary client step 2 the primary client issues the request for a sequence number to the primary server step 3 the primary server issues a new batch request to the secondary server and batches up a plurality of requests that have been received from the primary clients step 4 the secondary server sends an u201c ok to write u201d message for the specified sequence number to the secondary client step 4 can only take place when there are no outstanding writes for a previous sequence number should one exist step 5 the primary server grants the request for a sequence number to the primary client step 6 the primary client passes the u201c request granted u201d message on to the or each requesting i o processor step 7 the or each i o processor in turn issues a write for the specified sequence number to the primary client step 8 the primary client issues the write request for the specified sequence number to the secondary client step 9 the write is issued to the i o handler which performs the write i o step 10 the i o handler returns a u201c write done u201d message to the secondary client step 11 the secondary client passes the u201c write done u201d message to the secondary server step 12 the secondary client passes the u201c write done u201d message to the primary client step 13 the primary client passes the u201c write done u201d message back to the relevant i.