A V A I L A B I L I T Y in Visual Studio .NET

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A V A I L A B I L I T Y
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Services and Applications Client Management Local Environment Networking Disk and Volume Management Reliable Backups Good System Administration Practices INVESTMENT
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Figure 13.1 Level 7 of the Availability Index.
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There are two aspects to application recovery: First, how does the application react when a data service fails, and second, what else might go wrong within the application code that would cause a failure Sometimes it s the application to blame, dumping core or behaving erratically when the network and all of its services are just fine. In this chapter, we add to our analysis of design rules and building blocks with an investigation of application-level recovery, starting with a list of failure modes and application reactions, then we take a brief look at internal application failures, some process-level replication techniques. We conclude with an overview of good developer habits that improve overall system reliability.
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Application Recovery Overview
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Service availability only ensures that a data management service like a database is at the ready on the network. In the previous chapter, we looked at what might happen to client systems that don t tolerate failures well. On the server side, your user-level applications may not be able to access these data management services if they are not configured to deal with redundancy and get confused after a server failover. Applications may not deal with failures in a gentle manner, resulting in application crashes or endless streams of errors while they try to recover or reconnect to the wrong server. Application-level reliability is a trap for finger-pointing: The front-end team blames the backend team for making their part of the application crash; the web designers insist they didn t do anything on the edge that would make a browser seize up; the database team blames the middleware selection for failing to route
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Application Design
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transactions after a failover, while the middleware team insists it s a database recovery problem. Developing good detection and prevention techniques will help you sort through these situations.
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Application Failure Modes
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From a network application s perspective, a server failover is indistinguishable from a fast reboot. (For more on failovers, see 17, Local Clustering and Failover. ) The client needs to reconnect, restart any in-flight operations, and continue from where it left off. If the application happens to be running on the machine that crashes, obviously it will be restarted when the machine comes back up. These are the two extreme cases, where there s a clear failure and recovery at the system level. But what about nonfatal system or software logic faults These can impair or crash an application, even while the system is still running. Commercial failover management systems often let you try to restart a crashed application at least once before taking down the entire server, so you ll need to make sure your applications tolerate these soft crashes well. The following are the application failure modes we dissect in detail in this chapter: Improper handling of a server restart. Networked applications must be able to detect the corner cases when data management services restart or failover, such as a long break in service or an aborted transaction. Many of these conditions are the same for server reboot and failover, so ensuring proper restart behavior is a good idea even without high availability or other server management software in place. Nonfatal system or external failures. How does your user application react if it can t find a remote server What if a disk quota is exceeded and a write operation fails None of these conditions is strong enough to cause a failover immediately, but they may have adverse affects on applications. When a disk fills up, failover isn t going to help, as the overflowing disk is going to be imported on the other side of the HA pair. Persistent failures, such as a network segment that fails and requires redundant routing, or a disk that presents media failures, may be handled automatically, but you want to be sure your application will tolerate the intermediate failures well. There s little point in having automated recovery if the user-level application has already failed. Internal memory and state management. In the vernacular, this category is filed under bugs. We restrict our discussion to memory-corruption problems and their detection. Memory-related problems are probably one of the most common sources of application-level failures. Developer difficulty with memory management was one of the key drivers behind the memory access constraints in Sun s Java environment.
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