Server-available flow

The following figure shows the message flow of a Server-Available exchange, where the server attempts to connect to the client.

This flow only occurs when the server is already joined to the z/OS® cross-system coupling facility (XCF) group and recognizes that a client joins the group. A client should not wait for the server to recognize that it has joined the XCF group; the client should send its client-bid message as soon as it joins the group.

The client should ignore a Server-Available message after it has successfully completed its client-bid request and connected to the XCF group. The flow shown is:

  1. Server-available: MC, SD, SE
  2. Client-Bid: MC, SD, SE
  3. ACK: MC, SD, SE
Figure 1. Server-available flow
An arrow from the server to the client represents the first flow of the Server-Available exchange, the Server-Available protocol command message. A second arrow from the client to the server represents the second flow of the Server-Available exchange, the client-bid response. A third arrow represents the final flow of the Server-Available exchange: the ACK response.

The following table shows the contents of the message prefix. The flow step is listed, with the message flow type, message prefix section, and associated contents of the message prefix section for the prefixes MC, SD, and SE. The numbers used to show sequence in both the preceding figure and following table are not part of the actual message prefix.

Table 1. Contents of server-available flow message prefix
Flow step Message flow Message prefix section Content of prefix section
1 Server Available MC Architecture level = 1 Message type = command No response flag Command type = Server Available
SD The state data format for command messages applies to these fields: Length Member name Originator's token Destination token
2 Client-bid MC Architecture level = 1 Message type = command Response flag = response requested Command type = client-bid Prefix flag = state data + security data
SD The state data format for command messages applies to these fields: Length Member name Originator's token Destination token (DRU exit name) MaxBlocksize Aging value Hash table size
SE (Utoken)
3 ACK MC Architecture level = 1 Message type = command and response Response flag = ACK Command type = client-bid Prefix flag = state data + security data
SD The state data format for command messages applies to these fields: Length Member name Originator's token Destination token (DRU exit name) MaxBlocksize Aging value Hash table size
SE Utoken