XLIB Programming Manual, Rel. 5, Third Edition

15.5 Close Down Mode

Normally all resources associated with a client will be destroyed when the connection between the client and the server closes. This can happen without prior warning to either the server or the client when, for example, the network cable is accidentally pulled out of one of the machines or the machine running the server crashes. Therefore, robust applications need a way of recovering from that occurrence. XSetCloseDownMode() helps implement one method of recovery.

Clients in the default DestroyAll close down mode will have all their resources killed when the connection to the server dies. XSetCloseDownMode() can set two other modes, RetainPermanent and RetainTemporary, which allow client resources to live on for a time. A client may want its resources to live on to assist in the process of recovering from a broken connection with the server, usually caused by a network failure. When next run after the problem has been corrected, the application could somehow determine which resources were its own and continue operating where it left off. The “somehow” is the crux of the problem. The only way we can think of to allow the client to find out the IDs of its resources after the client is resurrected is for the client to save all the resource IDs in a file (or perhaps in a property, but this would not survive a server crash) immediately after they are created. Then upon startup, it can read this information and see if the specified resources still exist. If they do, ...