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<hr><p align="left"><small>发信人: strayli (stray), 信区: Delphi <br>
标 题: A Technical View of Borland MIDAS (2) <br>
发信站: BBS 水木清华站 (Thu Nov 12 21:51:42 1998) WWW-POST <br>
<br>
An Overview of Borland’s Multi-tier Computing Technology <br>
In this section I make a few broad statements about multi-tier computing. My <br>
goal is to give a general definition of the technology, particularly for the <br>
sake of users who are new to the subject. In later portions of the paper, the <br>
discussion will be increasingly technical and increasingly specific. <br>
<br>
Though distributed datasets can be based on DCOM, there are parallels between <br>
multi-tier computing and the Web. In particular, a person can use a browser <br>
to view a dataset on a remote machine without having any database tools on <br>
the client. The same is true of Borland’s multi-tier implementation of <br>
distributed datasets. <br>
<br>
Unlike Borland distributed datasets, browsers are severely limited in terms <br>
of functionality. For instance, without a powerful third-party tool such as <br>
IntraBuilder, it is difficult to enforce constraints, to program a browser to <br>
perform a join, or to set up tables in a one-to-many relationship. These <br>
chores are simple to execute inside a Delphi multi-tier application. Delphis <br>
high performance compiled applications are also much faster and much more <br>
responsive than HTML-based applications. <br>
<br>
Distributed datasets allow you to use all the standard Delphi components, <br>
including database tools in your client side applications, but the client <br>
side does not have to include the Borland Database Engine, ODBC, or any <br>
client database libraries (e.g., Oracle SQL*NET, Sybase CT-Lib, etc.) <br>
Somewhere on the network the BDE or a similar engine needs to exist, but you <br>
don’t have to have it on the client side. In short, you now need only one <br>
set of server side database tools where before you needed database tools, on <br>
each client machine. <br>
<br>
Distributed datasets are one means of cutting down on network traffic. After <br>
you have download data from the server, you can manipulate it on the client <br>
side, without initiating any more network traffic until you are ready to <br>
update the server. This means you can edit, insert, and delete multiple <br>
records without causing network traffic. When it comes time to update the <br>
server, you can send multiple data packets over the network at one specific, <br>
prechosen time. <br>
<br>
Furthermore, using what is called the“briefcase model”, you can disconnect <br>
the client from the network and still access the data. Here’s how it works. <br>
You can: <br>
<br>
<br>
Save a remote dataset to disk, shut down your machine, boot up again, and <br>
edit your data without connecting to the network. <br>
You can then reconnect to the network and update the database. All of this <br>
occurs without the presence of large database tools on the client machine. <br>
All of this adds up to the fact that you don’t have to actually be able to <br>
reach the server at all times in order to work with your data. This is ideal <br>
for laptop users, or sites where you want to keep database traffic to a <br>
minimum. There is a small (about 150 Kb) file called DBCLIENT.DLL that is <br>
needed on the client side, but that is very little when compared with the <br>
many Mbs of files required by the BDE or other database middleware. <br>
<br>
Access to database constraints are another important aspect of the MIDAS <br>
technology. When you download data from the server, you can simultaneously <br>
download a set of constraints that will be automatically enforced. The <br>
constraints can help programmers ensure that the user enters only valid data. <br>
When you are reconnected to the network, your data can then be updated <br>
without mishap. <br>
<br>
If perchance an error does occur while you are updating the dataset, then <br>
there are built-in mechanisms that can aid the programmer in reporting and <br>
handling the error. For instance, if a second user has updated a record that <br>
you are trying to update, then that fact can be surfaced, and the user can be <br>
given a choice of options on how to proceed. A prebuilt form that ships in <br>
the Delphi Object Repository makes it simple to implement error handling in <br>
your application. <br>
<br>
Yet another important feature of Borland抯 multi-tier computing includes <br>
distributing the load borne by a database over multiple servers, as well as <br>
providing fail-over capabilities in case of an error. This subject will be <br>
explored in more depth in the next section, and also in the technical <br>
sections found in the latter half of the paper. <br>
<br>
<br>
-- <br>
※ 来源:·BBS 水木清华站 bbs.net.tsinghua.edu.cn·[FROM: 202.38.72.60] <br>
</small><hr>
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