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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 (3) <br>
发信站: BBS 水木清华站 (Thu Nov 12 21:53:17 1998) WWW-POST <br>
<br>
Terminology <br>
I want to spend a few paragraphs on terminology, since this is a new <br>
technology, and one that is being interpreted in various ways by different <br>
companies. This paper is about something called, at various times: <br>
<br>
distributed computing <br>
client/server computing <br>
multi-tier computing <br>
n-tiered computing <br>
remote datasets <br>
distributed datasets <br>
and a host of other terms, many of which are often used out of context, or in <br>
a very loosely defined manner. The specifics of the definition of multi-tier <br>
computing used in this paper were outlined in the last section, entitled "An <br>
Overview of Borland抯 Multi-tier Computing Technology." The definition is <br>
further expanded and refined in this section. <br>
<br>
Multi-tiered computing is an industry-wide generic term for this kind of <br>
technology. In particular, multi-tier is the term for this technology that <br>
has been adopted by Borland. <br>
<br>
Borland supports a three-tier technology, which in its classic form consists <br>
of: <br>
<br>
A database server on one machine, <br>
An application server on a second machine, <br>
And a thin client on a third machine. <br>
The server would be a tool such as InterBase, Oracle, Sysbase, MS SQL server, <br>
etc. The application server and the thin client would be built in Delphi. In <br>
most scenarios, the database access software (e.g BDE, SQL*NET, etc.) would <br>
run on the same machine as the application server. Remember that this is <br>
simply the classic case, and there are many other configurations possible. <br>
<br>
The Delphi team refers to their specific tools for implementing this <br>
technology as “distributed datasets”. In other words, Delphi implements a <br>
multi-tier technology via a set of components, and our docs refer to the <br>
technology supported by these components as distributed datasets. <br>
<br>
Borland promotes this technology under the name MIDAS. Midas was a mythical <br>
king of Greece who received from Dionysus a gift enabling him to turn all he <br>
touched into gold. After a period in which all his food, and key members of <br>
touched into gold. After a period in which all his food, and key members of <br>
his family, were turned into gold, he grew weary of the gift and was released <br>
from it by washing his hands in a river. The sands of that river were then <br>
turned into golden-colored sand, as visitors to Greece can still see today. <br>
<br>
There are at least two distinct aspects to Borland’s distributed dataset <br>
technology: <br>
<br>
The built-in Delphi components that make this possible. <br>
The OLEnterprise product that provides extra support for distributed <br>
computing, and for load balancing. <br>
Both the Delphi Components and OLEnterprise are included in MIDAS. <br>
<br>
The built-in Delphi components make it easy for you to use DCOM to connect <br>
two machines and pass datasets back and forth between them. <br>
<br>
The OLEnterprise tools enhance the core Delphi technology. In particular, the <br>
OLEnterprise tools provide an alternative to DCOM that simplifies the task of <br>
connecting two machines, and particularly of connecting two Windows 95 <br>
machines. <br>
<br>
OLEnterprise comes from Borland’s recent purchase of the Open Environment <br>
Corporation. The purchase of OEC also gave Borland access to an Object Broker <br>
that allows you to randomly distribute the load of a task across several <br>
servers. In particular, you can load your server tools up on several <br>
machines; then the broker will chose one of these machines each time you make <br>
a connection. For example, if you had 100 clients and three servers, then the <br>
Object Broker would randomly divide the load across the three servers so that <br>
each had (approximately) 33 clients. <br>
<br>
The broker also provides support for those occasions when a server is forced <br>
to shut down unexpectedly. By writing a few lines of code, you can provide <br>
fail-over services that would switch clients of a downed server over to a <br>
running server. Furthermore, the broker would never attempt to connect a new <br>
client to a server that has gone down, but would instead automatically <br>
connect them to one of the servers that is still running. I include a sample <br>
20-line procedure later in this paper that demonstrates how to implement this <br>
fail-over process. <br>
<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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