Choosing Server-Hosts in Java

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3.9.3 Choosing Server-Hosts
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Choosing the best host for a service is an issue with several themes. The main principles have to do with efficiency and security, and can be summarized by the following questions:
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Does traffic have to cross subnet boundaries Do we avoid unnecessary network traffic Have we placed insecure services on unimportant hosts
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Service requests made to servers on different subnets have to be routed. This takes time and uses up switching opportunities which might be important on a heavily loaded network. Some services (like DNS) can be mirrored on each subnet, while others cannot be mirrored in any simple fashion. Unnecessary network traffic can be reduced by eliminating unnecessary dependencies of one service on another. For example, suppose we are setting up a file server (WWW or FTP). The data which these servers will serve to clients lie on a disk which is physically attached to some host. If we place the file-server on a host which does not have direct physical access to the disks, then we must first use another network service (e.g. NFS) as a proxy in order to get the data from the host with the disk attached. Had we placed the file server directly on the host with the disk, the intermediate step would have been unnecessary and we could halve the amount of network traffic. Principle 11 (Inter-dependency) Avoid making one service reliant on another. The more independent a service is, the more efficient it will be, and the fewer possibilities there will be for its failure. Some services are already reliant on others by virtue of their design. For example, most services are reliant on the DNS. Suggestion 1 (File servers with common data) Place all file servers which serve the same data on a common host, e.g. WWW, FTP and NFS serving user files. Place them on the host which physically has the disks attached. This will save an unnecessary doubling of network traffic and will speed up services. A fast host with a lot of memory and perhaps several CPUs should be used for this. 3.9.4 Distributed File Systems and Mirroring
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The purpose of a network is to share resources amongst many hosts. Making files available to all hosts from a common source is one of the most important issues in setting up a network community. There are three types of data which we have to consider separately: Users' home directories. Software or binary data (architecture specific). Other common data (architecture unspecific).
Since users normally have network accounts which permit them to log onto any host in the network, user data clearly have to be made available to all hosts. The same is not true of software, however. Software only needs to be shared between hosts running comparable operating systems. A windows program will not run under GNU/Linux (even though they share a common processor and machine code), nor will a SCO Unix program run under Free BSD. It does not make sense to share binary file systems between hosts, unless they share a common architecture. Finally, sharable data, such as manual information or architecture-
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Exercises
independent databases, can be shared between any hosts which specifically require access to them. How are network data shared There are two strategies: Use of a shared file system (e.g. NFS or Novell Netware). Disk mirroring.
Using a network file system is always possible, and it is a relatively cheap solution, since it means that we can minimize the amount of disk space required to store data, by concentrating the data on just a few servers. The main disadvantage with the use of a network file system is that network access rates are usually much slower than disk access rates, because the network is slow compared to disks, and a server has to talk to many clients concurrently, introducing contention or competition for resources. Even with the aggressive caching schemes of NFS, there is a noticeable difference in loading files from the network and loading files locally. Bearing in mind the principles of the previous section, we would if possible like to minimize load to the network. A certain amount of network traffic can be avoided by mirroring software rather than sharing with a network file system. Mirroring means copying every file from a source disk to the local disk of another host. This can be done during the night when traffic is low and, since software does not change often, it does not generate much traffic for upgrades after the initial copy. Mirroring is cheap on network traffic, even during the night. During the daytime, when users are accessing the files, they collect them from the mirrors. This is both faster and requires no network bandwidth at all. Mirroring cannot apply to users' files since they change too often, while users are logged onto the system, but it applies very well to software. If we have disk space to spare, then mirroring software partitions can relieve the load of sharing. There are various options for disk mirroring. On Unix hosts we have rdist, r sync and cf engine; variations on these have also been discussed [224, 267, 89, 72]. The use of rdist can no longer be recommended (see section 6.4.6) for security reasons. Cfengine can also be used on NT. Network file systems can be used for mirroring, employing only standard local copy commands; file systems are first mounted and then regular copy commands are used to transfer the data as if they were local files. The benefits of mirroring can be considerable, but it is seldom practical to give every workstation a mirror of software. A reasonable compromise is to have a group of file servers, synchronized by mirroring from a central source. One would expect to have at least one file server per subnet, to avoid router traffic, money permitting.
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