Networks in Visual Studio .NET

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incompatible, card on the same bus. This can be an issue with SCSIbased disks but will probably not be an issue in a SAN. Administration. This would be a very difficult situation for a system administrator, who would need to be proficient in the administration of both operating systems, scripting languages, and hardware environments. Work performed on one server would need to be translated to the operating environment of the other before it could be applied there. It is safe to say that this environment would more than double the amount of day-to-day tinkering required to keep the systems operating properly when compared to a more traditional cluster. Support. When there are problems, which vendor do you call It is not hard to imagine that one server might cause some sort of subtle problem on the other. Then what If, as we previously noted, it is unlikely that a vendor has tested every possible combination of its own servers in a failover environment, what are the chances that two competing vendors have tested every combination of their servers working together Despite all the negatives, it is likely that the introduction and widespread acceptance of storage area networks will turn out to be the first steps down the long path to make incompatible system failover a reality. From a hardware perspective, SANs solve the shared disk problem, the first step to more complete vendor partnerships that can solve the other problems, enabling incompatible system failover in limited combinations. But not any time soon.
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Networks
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Three different types of networks are normally found in a failover configuration: the heartbeats, the production or service network, and the administration network. For more detail on networks, please see 9, Networking. One caveat that applies to all of these network types: Some operating systems support virtual network interfaces, where more than one IP address can be placed on the same NIC. While these virtual IPs can be used in some failover scenarios, beware of introducing a single point of failure. For example, will the failure of a single NIC bring down both heartbeat networks
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Heartbeat Networks
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The heartbeat networks are the medium over which the systems in a failover pair communicate with each other. Fundamentally, the systems exchange Hi, how are you and I m fine, and you messages over the heartbeat links. In reality, of course, the heartbeat packets are much more complex, containing state information about each server and commands from one server directing the other to change states or execute some other function.
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Local Clustering and Failover
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Heartbeat networks can be implemented over any reliable network link. Most often, simple 100Base-T Ethernet is the medium of choice for heartbeats, since it is inexpensive and pervasive and it does not require any specialized hardware or cabling. Using a faster or more complicated network medium for your heartbeats will add absolutely no value to your cluster, so why spend the money Besides, these other media can be less reliable than good ol 100Base-T. Fast networks are ideal when sending large streams of data, but heartbeat messages are short and relatively infrequent (on a network traffic time scale). The primary purpose of heartbeats in a cluster is so that one server learns of the demise of one of its partners when the heartbeats stop. However, heartbeats can stop for several reasons besides a downed partner server. By running dual, parallel heartbeat networks, with no hardware at all in common, you can sidestep the impact of the most common causes of improperly stopped heartbeats, which include a failed NIC, a broken network cable, a network cable that has been pulled out, or the failure of a network hub. There are a few other events that might cause heartbeats to stop. These require somewhat more careful resolution but can also be handled: Heartbeat process has failed. If the process that sends the heartbeats fails on a node, heartbeats will stop on all networks, regardless of how many networks are involved. The process that sends the heartbeats must, therefore, be monitored and/or made locally redundant. If it stops, it should be automatically restarted by another process. This is a function that your FMS should provide. Sometimes FMS will offer an additional level of testing by sending ICMP packets (pings) to its partner host. Pings are not always useful (occasionally they will report success, even if the rest of the system is down) but can add value as a final test. Remote server is running too slowly. This is a trickier case. If server odds is running too slowly to report heartbeats to ends in a timely manner, ends may mistakenly assume that odds has failed. To reliably fix this, ends must leave enough time for odds to send heartbeats, thus slowing down detection. In return, odds must make sending heartbeats its highest priority, so that even if it is extremely bogged down, odds can still squeeze off one extra CPU cycle that goes to sending heartbeats. Again, minimizing the latency on the heartbeat network helps, as it gives ends more time to receive and process the heartbeat before it starts wondering what happened to its partner. A good FMS might be able to monitor not just the existence of heartbeat packets, but of the frequency of the packets. If odds senses that ends is slowing down, perhaps it can become a little more tolerant of longer durations between packets for a short period of time. Assuming that the operating system provides the functionality, a good FMS product will place itself on the real-time scheduling queue and offer an option to not be on it too.
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