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permission before blindly sending a backed-out transaction again. Consider the case of a stock trading system, where a trade bookkeeping system is locked out for two minutes during a failover. When the client is connected to a live system again, the market may have moved away from its previous price point, making previously submitted trades less profitable. In this case, the database application should notify the user of the failure and let the user make the choice to resubmit or discard. In most cases, early notification results in the user solving the problem through another channel when time is critical. Less-than-perfect database applications abound. They cause user headaches in a variety of creative ways:
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Instead of reconnecting, they crash or sit disconnected from the real world. They force the user to log in again, or to provide other startup information. The surest way to annoy a time-stressed user during a failover is to require more user intervention. They remain strangely silent as soon as a disconnect or transaction abort is detected. When you re driven by transactions, it s best to let users know exactly where those transactions stand. If there s a sign of trouble, tell the user so that corrective action can be taken in other ways.
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We looked at issues for desktop applications that use databases in the previous chapter. On the server side, a database application needs to be able to recover itself, often independently of the database server. Server-based applications may need to notify the users of failures that require manual intervention, so recovery requires coordination of the server-resident and desktop-resident application components. If you re using a browser-based (HTML client) front end to your application, then generation of the appropriate error and user information pages is the responsibility of the server-side application alone.
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A well-functioning application that needs to use network services is at risk of network connectivity impacting its predictable behavior. To continue functioning normally, the networked application relies on routing, remote host availability, and name service configuration. The most common error is that a host or network is unreachable, meaning that either the network is partitioned because of a routing failure or some intermediate routing information is no longer valid, so that clients on one end of the network can t find the servers on the other end. Assuming that you ve done a bare minimum of redundant network design with multiple routers and rapid detection of outages, connectivity failures reflecting network outages should be short-lived. On the other hand, without network redundancy, you need an alternative plan either
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another server for the same service or some way to stall the requesting application. If you are merely waiting for the routers to reestablish end-to-end connectivity, a short wait-and-retry loop should make your application tolerant of these network hiccups. A second network connectivity problem is when your application and a host undergoing failover or reboot engage in a network footrace to see if the server can start listening on a port before the client s request arrives there. If the server wins, there s no visible impact; but if the client wins, it may see a port unreachable message indicating that the remote host was there but nobody was home on the desired port number. Again, if you assume that the remote service is on its way back to full health, then a short timeout and retry should establish a connection on the next attempt. Another set of network problems has to do with connections reset by peers. This is the networking equivalent of I don t like you anymore. Usually, a network connection reset occurs when one end of the connection reboots, telling the other end to drop as soon as it s able to do so. Resets can also be sent on connections that do not respond to any traffic for a long time, typically at the expiration of a keep-alive timer that sends periodic probes to the other end to be sure someone is still listening even if there s no traffic on the socket. When a database server reboots, all of its clients see their TCP connections reset by peers, and they have to reconnect. This should be handled transparently by applications; there aren t many worse ways to annoy a user than indicating that an application has to be restarted because the socket connection code is buried somewhere in the startup sequence. Finally, network applications have to be sensitive to connection request timeouts. If the TCP connection can t be built up when there s a valid route, one or more of the following problems is at work: The remote host has crashed; the remote side s connection queue is too small; there s a network latency problem or some piece of network hardware is dropping packets; or a bona fide denial-of-service attack is under way and absorbing all of the server s capacity. Your best approach, from an application perspective, is to try several times before declaring the remote server inaccessible. Consider the typical user reaction to 404-type errors from browsers. Users want applications that produce meaningful diagnostics when they have trouble reaching remote servers. Your tolerance for pain in this case is at the threshold where a user goes to click on the equivalent of the browser Stop button.
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