Models of distributed system

Models of a distributed system describe its main parts and their relationships. Coulouris classifies them as physical, architectural and fundamental models.

1. Physical model

Describes the hardware: computers, devices and network.

• Early distributed systems (1970s-80s): 10-100 nodes on a LAN.
• Internet-scale systems: many nodes on the Internet, heterogeneous.
• Contemporary systems: mobile, ubiquitous, cloud and IoT, with a large number of nodes.

2. Architectural model

Describes the structure, in terms of components and their interconnection.

 Client-server            Peer-to-peer
 C1 --\                   P1 --- P2
 C2 ---> Server           |  \  /  |
 C3 --/                   P4 --- P3

• Client-server: clients request, servers provide service (web, file server). Variations: multiple servers, proxy servers and caches, mobile code, mobile agents, thin clients.
• Peer-to-peer: all processes play similar roles and share resources (BitTorrent).
• Multi-tier (layered): presentation, application logic, data (three-tier web application).
• Cloud / service-oriented architecture.

3. Fundamental model

Abstract properties common to all systems.

• Interaction model: communication delays, clock drift, synchronous vs asynchronous.
• Failure model: omission, arbitrary (Byzantine) and timing failures.
• Security model: threats to processes and channels, defended by cryptography and access control.

Other classification (Tanenbaum): distributed computing systems (cluster, grid, cloud), distributed information systems (transaction processing, enterprise integration) and pervasive systems.