The concept of state

The communication depicted in the preceding section could be looked at as message passing; find is passing on the filename as a message to the egrep process, or seq is passing messages (numbers) to the paste process. Generally speaking, a producer is sending messages to the consumer for consuming, as shown in the following diagram:

As shown in the preceding diagram, each process has its own state by design, and this state is hidden from other processes. The processes communicate with explicit messaging channels, in the same way that a pipe directs the flow of water. 

This notion of state is very important to understand the various upcoming concurrency patterns. We could look at the state as data in a certain stage of processing. For example, the paste process could be using program counters to generate the numbers. It could also be writing the numbers to the standard output (file descriptor 1; by default, the console). At the same time, the paste process is processing its input and writing data to its standard output. Both processes do not care about each other's state; in fact, they don't even know anything about the other process.

The real world is full of encapsulated states. The following diagram shows an example:

 

It defeats common sense to share the state (the need to buy milk) with the postal department employee. It is useless for him to know it, and it could create confusion.

Likewise, the employee will be going about his daily tasks and has a state of his own. Why do we, as consumers, need to know the internal details (state) of how he is going to manage his work (dispatch this big stack of letters)? The world is concurrent, and the various entities in it also hide unnecessary details from each other to avoid confusion. If we don't hide the internal details (that is, the state), it would create havoc. 

We could ask whether there is a global shared memory. If there is, then we could use it as a message channel. Using a shared data structure of our choice, the producer could put the data in it for subsequent consumption; that is, the memory is used as a channel of communication.