Application Recovery from System Failures in .NET

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External events such as a disk filling up can cause application failures. In this section, we look at system-level failures that create headaches for applications, and some common workarounds or early detection steps. We also cover network programming interactions and dependencies, since some network-based applications don t restart well after they have failed and been restarted. We cover the easy ones (memory exhaustion and I/O errors) first, then look at how network failures impact application flow.
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Virtual Memory Exhaustion
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Even with large physical memories, systems can exhaust their virtual memory. In some operating systems, this happens when swap space or paging files are full; in others, it s when the sum of physical memory and swap space is allocated completely. In either case, virtual memory exhaustion is an external, system-level failure that can crash, confuse, or otherwise impair the reliability of an application. The two most common causes of virtual memory depletion are memory leaks, in which one or more processes grow in memory consumption until they occupy all of memory, and endless forking (spawning) of new processes due to a bug in a script. Even forking a large application can cause a temporary shortfall as the system cleans up the copy of the large application to make a process container for a new job. When you run out of memory, normal processes start to act like denizens of the twilight zone. Checking all return values from malloc() is the first step in reducing memory-related failures, but that only tells an application if there s enough virtual memory left to satisfy a request of the stated size. An application may try to dynamically allocate a full megabyte of space, and fail,
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but the system has 512KB free. On the other hand, the system may be down to only a few tens of kilobytes of available memory, and out-of-memory errors start to pop up in many places:
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Attempts to create new processes fail because there is not enough memory left to copy the parent process s state. Attempts to dynamically allocate memory, even in small cell sizes, also fail because there is not enough virtual memory left. Network applications attempting to create sockets to the machine cannot complete their socket connections, because there is not enough memory left for TCP data buffers and socket control block structures. Similarly, attempts to create new sockets on the memory-starved machine also fail because the necessary kernel data structures cannot be allocated. System calls fail, often producing the EAGAIN error on Unix systems, indicating that the error is temporary and may be recoverable, but at the time of the call there were not sufficient resources to complete it. Generally this indicates a lack of space for I/O buffers.
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How do you resolve a memory shortfall If the problem is due to a memory leak, killing the offending process should restore order to the system. When memory exhaustion becomes a chronic problem, it s time to look at resource management tools as described in 19, Virtual Machines and Resource Management. If it s an overall load issue, or a process that has gone out of control forking copies of itself, then system administrator help is often required to reboot the machine or kill the root process in an attempt to clean up all of the rampant children processes. Remote help isn t always a possibility, as a machine in a memory contention fight may not have the resources to open up a new telnet or rlogin session. This is a corner case that may best be handled with automated failover management systems, adding memory exhaustion to the list of monitored conditions that make a machine go out of service.
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