Data and Web Services in Visual Studio .NET

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Data and Web Services
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be records in a database or changes to a password file or even history files that are referenced in the future. Applications with state need to be run on deep (vertically scaled) SMP servers, since the state is very difficult to replicate across many small machines and frequently the operating system acts as a single, large cache for the on-disk state. Are clients server-agnostic If the clients don t care about the identity of the server providing them with data, they are suitable for a wide (horizontally scaled) strategy. If clients need to know the identity of a server (NFS file handles, for example), then you need to go deep with HA pairs. Databases can go either way a read-only database of stock quotes or a data mart has no persistent state, so clients can send their SQL to any available machine. Inserting records into a general ledger, on the other hand, creates state that is better handled by a single, large multiprocessor server in an HA pair. Is the code path ratio close to zero The closer you are to zero, the more the model fits the stack of boxes approach. With a small code path ratio, the operating system can become a rate-limiting factor in performance. Adding copies of the operating system through multiple servers is your best path to high performance and rapid failure recovery. This rule comes third because the server and state limiting requirements can make the code path ratio meaningless. NFS has a nearly ideal code path ratio (almost zero), but NFS relies on server-client relationships and the state represented by files to which those clients are writing. Of course, there are times when you can t build a bigger box, and you simply need more servers to meet performance demands. There are also recovery time windows smaller than the relatively uncontrolled database failover, typically where you re serving users on the Internet for whom the Stop button is hit at the first sense of web site slowness. Scaling web services web, application, and directory servers and making them reliable requires a mix of server and state replication.
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Web-Based Services Reliability
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Web-based services are the core functions that run web sites, whether for internal (intranet) or public-facing (Internet) use: web servers, application servers, directory servers, and the glue between them. Web services in the trade press refers to a specific set of protocols and application development standards that we address at the end of this section. We use the more generic term to cover the key functional areas that are used to build network-delivered applications, focusing on how to make them reliable and scalable.
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Web Server Farms
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Web servers are now the classic rack-and-stack appliance you scale the site by adding web servers and using network load balancing to distribute requests across all of them. Making a farm of web servers reliable requires that you ensure that each server has network connectivity and will receive incoming requests, that each server has access to the right set of HTML and other content files, and that user connections can be reestablished to another web server if one fails in the middle of a session. The following are some key Web server reliability design points:
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Treat all web servers as NFS clients, and put your content on one or more large NFS servers. Obviously, the NFS servers need to be made highly available, which can be achieved through standard clustering, as described in 15, Local Clustering and Failover. Further improvements in availability can be achieved by sharing critical filesystems between members of a SAN. If each web server has its own files, ensure that they are consistently updated so that all of the web servers appear the same to the outside world. Secure Socket Layer (SSL) requests can consume large portions of the web server s CPU. Consider using an SSL accelerator board, or a switch that offloads SSL processing from the web server and sends cleartext HTTP traffic to the web server. Make sure your connected TCP/IP sockets get connected to a surviving server when their original destination endpoint server crashes. If you re using a load-balancing switch, it should remove the failed server from its list of available servers and reroute requests destined for that server elsewhere. Some switches actually hide the web server failure by proxying TCP/IP connections between the public network side and the web server side. If the server failure is visible to the client in terms of a TCP/IP socket termination, then it s up to the application to reconnect to the server. Most applications are intelligent enough to recognize failures on a TCP/IP connection and to reestablish it.
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Many sites collect session information such as user login credentials, navigation histories, shopping carts, and other in-flight data. We refer to this collection of data as session state. Session state on the web server also includes HTTP connection information. State data used by objects on the web server typically gets embedded in a URL that is handed back to the client via a redirect operation, so the next client access (to the returned URL) returns enough information for the server to remember the client s state. The following is an example of a URL that contains state information:
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