DCE
(distributed computing environment)
What is DCE
distributed computing environment
It is defined by the open software
foundation(OSF).
It is an architecture, a set of standard service,
and application programs, built on top of
the existing operating system which hides
the differences among individual computers.
used to support the development and usage
of distributed applications in a single
distributed system. It use client/server
model.
Goals of DCE
• 1) can run on many different computers, operating
systems (Unix, Os/2, VMS,windows) and networks
in a distributed system, Provide a coherent
seamless platform for running distributed
applications.
• 2) Provide a mechanism for synchronizing clocks on
different machines.
• 3) Provide tools to make it easier to write
distributed applications in which multiple users at
multiple locations can work together.
• 4)Provide extensive tools for authentication and
access protection
DCE architecture
• DCE cell: the basic unit of operation in the DCE.
A cell is a group of users, systems,and resources
that are typically centered around a common
purpose and that share a common DCE services.
It is an administrative domain that allows
users,machines, and resources to be managed
through functions distributed within the network
in which they are in. Members working on the
same project in an organization are likely belong
to the same cell.
•
DCE architecture(cont’d)
• DCE threads
• DCE remote procedure call
• DCE directory service: cell directory service; global
directory service
• DCE distributed time service
• DCE security service
• Distributed file service
Distributed services
provided by the DCE
• Thread services;
• RPC;
• time service;
• directory services;
• security service.
DCE threads
• DCE thread package: a collection of userlevel library procedures that allow processes
to create,delete, and manipulate threads.
multiple tasks could occur at the same time
to complete a process(work). It is designed
to minimize the impact on the existing
software: a single threaded program can be
converted into multithreaded one by setting
parameter indicating that more threads will
be used. All threads in a process share the
same address space, file system, and other
process resources. Each thread has its own
program counter, stack,and registers.
Scheduling
• determines how long a thread may run and which
thread will run next.
• Three algorithms:
•
FIFO: search and locate the priority queue with
one or more threads from highest to lowest, run
the first thread on this queue until finish by
blocking or exiting.
•
Round robin:the scheduler locates the highest
populated queue and runs each thread for a fixed
quantum.
•
Default algorithm: it uses a time-sliced roundrobin algorithm to run the threads on all queues,
the higher the priority the larger the quantum a
Synchronization
Mutexes: prevent multiple threads from
accessing the same resource at the same
time.
•
Condition variable: it is used with
mutexes to manage synchronization.e.g.: a
thread could use mutexes to gain exclusive
access to a resource, if this resource is not
available, the thread waits on a condition
variable that atomically suspends the
threads and releases the mutexes. Later,
when another thread signals the condition
variable, the waiting thread is restarted.
RPC
• DCE is based on the client/server model. RPC
package of DCE allows a client process to call a
procedure on a remote computer. It allows
application programmer to extend the local
procedure call to a distributed environment. The
application programmer doesn’t have to be
concerned with the detail of the network
communications between client and server nodes.
• RPC hides communication detail and removes
system and hardware dependencies. It can
automatically handle data type conversions
between the client and the server without
considering whether they run on the same or
different architecture, or have same or different
byte ordering.
DCE directory service
• Provide up-to-date addressing information
by keeping track of where all resources
(user, machine, file, server…) are located. So
users can identify these resources by names
and gain access to them without needing to
know where they are located
• It’s distributed service--- the information
that forms database is stored in different
places.
• A replicated service: the information is
replicated and stored in more than one
location which make it more readily
Time service
• It enables distributed applications on
different computers to determine event
sequencing, duration, and scheduling.
• Keeps clocks on separate computers
participating in a distributed system
synchronization.
• It uses universal coordinated time(UTC)
to synchronize DCE host’s time.
Security service
• Controls interactions between clients and servers.
It allows client and servers to authenticate each
other and perform authenticated RPC.
• Offers integrity and privacy of communications
without having password appearing on the network
.
• Controls access to resources by authorization;
• Principal: is a user or process that needs to
communicate securely. It could be a server,
computers, cells or users.
• Authentication: is the process of verifying a
principal’s network identity.
Distributed file service
• It’s a worldwide distributed file system;
• Allows users to access and share files stored
on a file server anywhere on the network
without knowing the physical location of the
file;
• The physical file system (DCE local file
system) is able to do several tasks:
Replicate data; log file system data, enable
quick recovering after a crash; Simple
administration by dividing the file system
into easily managed units; Associate access
control list with files and directory.
Advantages of DCE
• 1) The services provided by DCE is
much easier to use than the ones found
in other computer networking
environments: i.e.: the DCE remote
procedure call provide a much simpler
way for communicating between
software modules running on different
system than using socket calls.
• 2) the DCE security service provides a
reliable way to determine if a user in a
distributed system should be allowed to
perform certain action.
Advantages (cont’d)
• 3)supports portability and interoperability by
hiding differences among the various
hardware,software,and networking elements
in a large network.
• 4) supports distributed file service which
means files present on workstation in a
network are available to this network.
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