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Without a logical server name, the repository would have no idea how POA names map onto server processes and would promptly start the same server twice, one for each POA For servers designed for shared activation mode, this is typically very bad news For example, the server may use the file system as a simple database If two server processes are running side by side, they may write to the same file in parallel without locking, and that usually results in corrupted data on disk The logical server name prevents start-up of multiple server processes in shared activation mode Whenever the implementation repository receives a request that requires starting a server, it maps the POA name in the request to the logical server name If the repository has already started that logical server and is waiting for the server to enter its event loop, the repository delays all other binding requests for POAs in the same server until the server has initialized itself and can accept requests When the server process is ready, the repository returns its LOCATION_FORWARD reply to all clients that are currently binding a reference to any object in the server This behavior effectively prevents the repository from accidentally starting the same server multiple times 1472 Race Conditions during Shutdown Another race condition can arise during server shutdown Consider a running server whose event loop has just terminated If the server caches updates, it must flush these updates to its database after the event loop terminates but before the server exits As soon as the server's event loop has terminated, the server can no longer accept requests, so the implementation repository must start another server instance for new requests that arrive from clients This creates a potential race condition, because the first server may still be flushing data to files while a second server instance, started by the repository, concurrently reads the same files before entering its event loop To get around this problem, most implementation repositories monitor the server processes they create and do not start a second server process until after the first process has exited; the repository delays binding requests from clients while a server is shutting down until the server physically exits When you write a server, you should therefore make an effort to quickly exit after the event loop terminates; otherwise, you will unduly delay binding requests from clients You need to consult your ORB documentation to determine exactly how server shutdown is handled by your repository Some repositories make no effort to deal with the shutdown race, in which case you must synchronize server processes yourself (for example, by using a lock file) 1473 Server Shutdown and Rebinding After a server's event loop terminates, the server-side run time closes all open connections after sending a CloseConnection message on each connection The CloseConnection messages inform clients that they must rebind using the
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implementation repository before sending more requests to the server This rebind is necessary because a new instance of the server may listen on a different port Rebinding is handled by the ORB run time and therefore is transparent to the client application code Rebinding prevents the client application from receiving spurious exceptions just because a server terminated at an inconvenient moment If a client is waiting for a reply to a request and detects a broken connection, this means that connection shutdown was disorderly, either because the server crashed or because of a network failure In either case, the client run time raises a COMM_FAILURE exception with a completion_status of COMPLETED_MAYBE (because the client cannot know whether the server crashed just before it accepted the request or after the request completed) If a client detects disorderly connection shutdown while it does not have any replies outstanding, the behavior depends on the ORB Most ORBs try to rebind at least once before propagating an exception to the application code Some ORBs permit you to configure the number of times and the intervals at which the run time will attempt to rebind before giving up Occasionally, retry attempts are combined with exponential back-off, which increases the time between retries by a constant factor on every attempt For example, the ORB may double the amount of time it waits between retries until the maximum number of retries is reached Exponential back-off is useful if a large number of clients are connected to a server that has terminated abnormally, because it prevents clients from flooding the network with retry attempts Often, exponential back-off is combined with a small amount of random variation on each retry period Again, this is to prevent avalanche effects if many clients are confronted with an unreachable server The random variation stops large numbers of clients from attempting to rebind all at the same time
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