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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 614
SEC. 8.3
DISTRIBUTED SYSTEMS
583
When a CORBA object is created, a reference to it is also created and returned
to the creating process. This reference is how the process identifies the object for
subsequent invocations of its methods. The reference can be passed to other proc-
esses or stored in an object directory.
To invoke a method on an object, a client process must first acquire a reference
to the object. The reference can come either directly from the creating process or,
more likely, by looking it up by name or by function in some kind of directory.
Once the object reference is available, the client process marshals the parameters to
the method calls into a convenient structure and then contacts the client ORB.
In
turn, the client ORB sends a message to the server ORB, which actually invokes
the method on the object. The whole mechanism is similar to RPC.
The function of the ORBs is to hide all the low-level distribution and commu-
nication details from the client and server code.
In particular, the ORBs hide from
the client the location of the server, whether the server is a binary program or a
script, what hardware and operating system the server runs on, whether the object
is currently active, and how the two ORBs communicate (e.g., TCP/IP, RPC, shar-
ed memory, etc.).
In the first version of CORBA, the protocol between the client ORB and the
server ORB was not specified. As a result, every ORB vendor used a different pro-
tocol and no two of them could talk to each other.
In version 2.0, the protocol was
specified. For communication over the Internet, the protocol is called
IIOP
(
Inter-
net InterOrb Protocol
).
To make it possible to use objects that were not written for CORBA with
CORBA systems, every object can be equipped with an
object adapter
.
This is a
wrapper that handles chores such as registering the object, generating object refer-
ences, and activating the object if it is invoked when it is not active. The arrange-
ment of all these CORBA parts is shown in Fig. 8-36.
Client
Client stub
Operating system
Client ORB
Client
code
Object
adapter
Operating system
Server
code
Server
Skeleton
IIOP protocol
Network
Server ORB
Figure 8-36.
The main elements of a distributed system based on CORBA. The
CORBA parts are shown in gray.
Page 615
584
MULTIPLE PROCESSOR SYSTEMS
CHAP. 8
A serious problem with CORBA is that every object is located on only one ser-
ver, which means the performance will be terrible for objects that are heavily used
on client machines around the world. In practice, CORBA functions acceptably
only in small-scale systems, such as to connect processes on one computer, one
LAN, or within a single company.
8.3.6 Coordination-Based Middleware
Our last paradigm for a distributed system is called
coordination-based mid-
dleware
.
We will discuss it by looking at the Linda system, an academic research
project that started the whole field.
Linda
is a novel system for communication and synchronization developed at
Yale University by David Gelernter and his student Nick Carriero (Carriero and
Gelernter, 1986; Carriero and Gelernter, 1989; and Gelernter, 1985). In Linda, in-
dependent processes communicate via an abstract
tuple space
.
The tuple space is
global to the entire system, and processes on any machine can insert tuples into the
tuple space or remove tuples from the tuple space without regard to how or where
they are stored.
To the user, the tuple space looks like a big, global shared memo-
ry, as we have seen in various forms before, as in Fig. 8-21(c).
A
tuple
is like a structure in C or Java.
It consists of one or more fields, each
of which is a value of some type supported by the base language (Linda is imple-
mented by adding a library to an existing language, such as C).
For C-Linda, field
types include integers, long integers, and floating-point numbers, as well as com-
posite types such as arrays (including strings) and structures (but not other tuples).
Unlike objects, tuples are pure data; they do not have any associated methods.
Fig-
ure 8-37 shows three tuples as examples.
("abc", 2, 5)
("matrix-1", 1, 6, 3.14)
("family", "is-sister", "Stephany", "Roberta")
Figure 8-37.
Three Linda tuples.
Four operations are provided on tuples. The first one,
out
, puts a tuple into the
tuple space. For example,
out("abc", 2, 5);
puts the tuple ("abc", 2, 5) into the tuple space. The fields of
out
are normally con-
stants, variables, or expressions, as in
out("matrix
−
1", i, j, 3.14);
which outputs a tuple with four fields, the second and third of which are deter-
mined by the current values of the variables
i
and
j
.
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