Choosing Your Network Recovery Model in .NET framework

Drawing QR in .NET framework Choosing Your Network Recovery Model
Choosing Your Network Recovery Model
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We ve covered a few cases in which scripts or system administrator help is necessary to keep the network chugging along with its packets. Unlike systemlevel failover involving disks, network reconnection involves multiple points of control and many moving parts unless you use the static route driven failover pairs provided by VRRP. There are several configurations that make the case for manual failover as opposed to fully automated but very complex operations. We assume automated failover as the default mode of operation through most of this book. However, network failover requires careful coordination of all hosts that need to be reconfigured, and partial failovers are often harder to sort out than complete shutdowns of a single network. We use this rule of thumb: If there s one point of control and one set of resources, automation is your friend. Multiple points of control, coordination of multiple resources, and multiple steps that have to coordinate with each other, and occur in order, argue for manual intervention. Some of the network redundancy techniques that rely on router information or route changes are ideal for automated care and feeding; others that require changing host routes, or ensuring that all hosts have switched to an alternate rail, are best handled by the network operations staff. The key question to address is the MTTR. With active, aggressive monitoring and an on-call network staff, you can limit your manual failover time to the window in which the staff is alerted, problems found, and repairs made. With a 24 7 staff, this window gets smaller, and with well-documented failover processes you can be reasonably sure that the failovers will happen the right way the first time. Partial failovers are to be expected in the automated network management world. Some hosts end up stuck on the primary network, because not all of them noticed the failure, or because scripts that triggered the failover failed on the dying network. If you account for the typical time required to dig through these failure modes, you can argue for manual intervention in the case of a complete network meltdown. To repeat a bit of our management dogma,
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aggressive, active network monitoring will aid early detection and repair, possibly eliminating the corner cases that require interruption of someone s beauty sleep.
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Load Balancing and Network Redirection
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In addition to explicitly making the intra-server room network highly available, you ll need to deal with a highly demanding user base that wants to be able to reach your servers, with low latency and high predictability. When using redundant servers to increase reliability, you need some way to steer traffic to all of them, in an equitable way, and then to redirect that traffic in the event of a failure. We look at two high-level approaches, one that uses DNS to have the clients do the work and a switch-based approach that gives you finegrain control over load balancing and failure detection.
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Round-Robin DNS
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Round-robin DNS is useful in rack-and-stack clusters, or shared-nothing clusters where you achieve redundancy by building out multiple instances of web servers (we cover rack-and-stack, or horizontal, clustering in 14). Each web server has its own filesystem, its own IP address and hostname, and its own interface to the world. DNS does the heavy lifting by associating multiple IP addresses with the virtual hostname; for example, you may assign four IP addresses to www.blah.com so that incoming traffic will go (fairly evenly) to all four addresses over time. Some implementations of DNS do this automatically, circulating through all available IP addresses for a given hostname. In other configurations, you re going to need to get a copy of the publicly available load-balancing BIND daemon (lbbindd). When a client gets an IP address via round-robin DNS, it will use it until the DNS lifetime has expired. Sites using round-robin DNS typically shrink the DNS lifetime to several minutes, forcing clients to rebind the hostname to an address more regularly and reducing the average window of an outage. Done well, round-robin DNS is a low-impact and low-budget solution with its own good and bad points, which are shown in Table 9.1
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