There are two basic types of communication models for sharing information.

• shared memory model
• message passing model

In most cases, either high-level model can be implemented in terms of the other low-level model.

The term sharing refers to more than one having access to the same thing (either at the same time or at different times).

A shared memory model shares information on a storage device, such as a disk.

What are some examples of sharing information via a shared memory model? Some examples of sharing information via a shared memory model are the following.

• file sharing
• web page sharing
• global memory sharing (in programs)

You might share a web page with all your users.

If you change the page, every user gets to see the change.

On the web, the user must refresh their page.

What is the primary problem to be solved when using a shared memory model to share information?

The concurrent access problem is the primary problem to be solved when using a shared memory model to share information?

That is, what happens if two users/processes want access the the same resource at the same time?

Communication involves a sender sending a message to a receiver who can understand the message.

A message is information that is transmitted as data from a sender to a receiver.


All information can be considered a message originating from a sender to sent to a receiver - called a "call".

Often, the receiver sends a message back - called a reply or "return". The "return" can be just an acknowledgment of message receipt and action or that message may return something of value - additional data.

Note: Information in this context involves a arbitrary but consistent coded language system.

The ancient Egyptian written language is called hieroglyphics.

Eventually, no one knew how to read these hieroglyphics. The Rosetta stone is a stone tablet found in Egypt by Napoleon's army in the about 1800.


The Rosetta stone had on it the same inscription text in hieroglyphics, Greek, and Coptic (Egyptian with Greek influence). It took many years, but the ancient Egyptian hieroglyphics were eventually deciphered and could be read. Today, the term Rosetta stone is a term or symbol for the key to solving an important problem.

A message passing model shares information by passing messages between processes sharing information.

What are some examples of sharing information via a message passing model.

Here are some examples of sharing information via a message passing model.

• email messages

What is the primary problem to be solved when using a message passing model to share information?

The copy-update problem is the primary problem to be solved in using a message passing model to share information.

That is, the numerous copies of the same, or nearly the same, information, take space and make it difficult to manage and update.

You send a message to many users, and find a problem in the message.

You wanted to offer a discount of 10% but it got sent as 50%.

How do you retract the messages?

With email, a message passing system, it is not easy to retract messages, even if the recipient has not yet seen the message.

Within an organization, you may be able to recall messages that have not been accessed.

Shared memory model:

• Send a message with a link to your web page offering the discount.
• Users click to get to your web site to see the offer.

If you find a mistake, or want to make a change, update the (shared) page.

That is why it is often good to print or save a copy of a web site page that you think might change in the future.

When sharing, either shared memory or message passing, the authentication problem must often be solved.

The authentication problem is determining if the user (or process) is who the user (or process) claims to be.

Marketing terminology:

• push technology
• pull technology

Which is which?

• push technology (message passing, email)
• pull technology (shared memory, web pages)

Common marketing model:

• create web page (shared memory)
• send message with link (message passing)

Change the web page if necessary.

In programming the following are often encountered (as examples)

• shared memory: global and local variables and access via statements via control structures
• message passing: functions, functions in modules, functions/methods in classes and objects

Note: Non-functional systems that allow a call-by-reference use message passing to pass shared memory locations to the called routines.

Function calls can be thought of as a message passing system.

• The main routine passes values to the function
• The function receives values, computes values, and returns a value.
• The main routine receives the value from the function and uses it.

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