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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 608
SEC. 8.3
DISTRIBUTED SYSTEMS
577
The way the whole system hangs together is as follows. The Web is fundamen-
tally a client-server system, with the user being the client and the Website being the
server. When the user provides the browser with a URL, either by typing it in or
clicking on a hyperlink on the current page, the browser takes certain steps to fetch
the requested Web page.
As a simple example, suppose the URL provided is
http://www.minix3.org/getting-started/index.html
.
The browser then takes the fol-
lowing steps to get the page.
1.
The browser asks DNS for the IP address of
www.minix3.org
.
2. DNS replies with 66.147.238.215.
3.
The browser makes a TCP connection to port 80 on 66.147.238.215.
4.
It then sends a request asking for the file
getting-started/index.html
.
5. The
www.minix3.org
server sends the file
getting-started/index.html
.
6.
The browser displays all the text in
getting-started/index.html
.
7.
Manwhile, the browser fetches and displays all images on the page.
8.
The TCP connection is released.
To a first approximation, that is the basis of the Web and how it works. Many
other features have since been added to the basic Web, including style sheets, dy-
namic Web pages that are generated on the fly, Web pages that contain small pro-
grams or scripts that execute on the client machine, and more, but they are outside
the scope of this discussion.
8.3.4 File-System-Based Middleware
The basic idea behind the Web is to make a distributed system look like a giant
collection of hyperlinked documents.
A second approach is to make a distributed
system look like a great big file system.
In this section we will look at some of the
issues involved in designing a worldwide file system.
Using a file-system model for a distributed system means that there is a single
global file system, with users all over the world able to read and write files for
which they have authorization. Communication is achieved by having one process
write data into a file and having other ones read them back. Many of the standard
file-system issues arise here, but also some new ones related to distribution.
Transfer Model
The first issue is the choice between the
upload/download model
and the
remote-access model
.
In the former, shown in Fig. 8-33(a), a process accesses a
file by first copying it from the remote server where it lives. If the file is only to be
Page 609
578
MULTIPLE PROCESSOR SYSTEMS
CHAP. 8
read, the file is then read locally, for high performance.
If the file is to be written,
it is written locally.
When the process is done with it, the updated file is put back
on the server. With the remote-access model, the file stays on the server and the cli-
ent sends commands there to get work done there, as shown in Fig. 8-33(b).
2. Accesses are
done on the
client
3. When client is
done, file is
returned to server
New file
Old file
Server
Server
Client
Client
1. Client fetches file
File stays
on server
Reply
Request
(a)
(b)
Figure 8-33.
(a) The upload/download model. (b) The remote-access model.
The advantages of the upload/download model are its simplicity, and the fact
that transferring entire files at once is more efficient than transferring them in small
pieces. The disadvantages are that there must be enough storage for the entire file
locally, moving the entire file is wasteful if only parts of it are needed, and consis-
tency problems arise if there are multiple concurrent users.
The Directory Hierarchy
Files are only part of the story. The other part is the directory system.
All dis-
tributed file systems support directories containing multiple files. The next design
issue is whether all clients have the same view of the directory hierarchy. As an
example of what we mean, consider Fig. 8-34. In Fig. 8-34(a) we show two file
servers, each holding three directories and some files. In Fig. 8-34(b) we have a
system in which all clients (and other machines) have the same view of the distrib-
uted file system.
If the path
/D/E/x
is valid on one machine, it is valid on all of
them.
In contrast, in Fig. 8-34(c), different machines can have different views of the
file system.
To repeat the preceding example, the path
/D/E/x
might well be valid
on client 1 but not on client 2.
In systems that manage multiple file servers by re-
mote mounting, Fig. 8-34(c) is the norm.
It is flexible and straightforward to im-
plement, but it has the disadvantage of not making the entire system behave like a
single old-fashioned timesharing system.
In a timesharing system, the file system
looks the same to any process, as in the model of Fig. 8-34(b). This property
makes a system easier to program and understand.
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