# What is a Distributed System?

## Definition

{% hint style="info" %}
A **distributed system** is a collection of autonomous computing elements that appears to its users as a single coherent system.
{% endhint %}

## Characteristics

1. **Collection of autonomous computing elements**
   * Nodes behave independently of each other.
   * Nodes are programmed to achieve common goals, which are realized by exchanging messages with each other.
2. **Single coherent system**
   * Users believe they are dealing with a single system
   * Distribution transparency

## Middleware

* **Middleware** is a layer that is logically placed **between OS and applications**.
* Middleware offers services that can also be found in most operating systems, including:
  * Facilities for **interapplication communication**
  * **Security** services
  * **Accounting** services
  * Masking of and recovery from **failures**


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