System Architectures

Centralization vs. Decentralization

Centralized Organizations

• Simple Client-Server Architecture: client $\Longleftrightarrow$ server

  • Request-reply behavior

• Multitiered Architectures

  • two-tiered architecture: client machine and server machine

    • User interface

    • Application

    • Database

  • three-tiered architecture: a server also acts as a client

    • Client

    • Application server

    • Database server

Decentralized Organizations

Concepts

• Vertical Distribution

  • Placing logically different components on different machines.

• Horizontal Distribution

  • A client or server may be physically split up into logically equivalent parts, but each part is operating on its own share of the complete data set, thus balancing the load.

  • Peer-to-peer (P2P) systems support horizontal distribution.

• Peer-to-Peer (P2P) Systems

  • The processes that constitute a P2P system are all equal.

  • Each process will act as a client and a server at the same time (acting as a servant)

• Overlay Network

  • A network in which the nodes are formed by the processes and the links represent the possible communication channels (TCP connections).

Structured Peer-to-Peer Systems

• Nodes are organized in an overlay that adheres to a specific, deterministic topology: a ring, a binary tree, a grid, etc.

• The topology is used to look up data by its key

  • Distributed hash table (DHT)

  • key(data_item) = hash(data_item_value)

  • existing_node = lookup(key)

Unstructured Peer-to-Peer Systems

• Each node maintains an ad hoc list of neighbors.

• Data lookup requires searching

  • Flooding: a node sends the request to all its neighbors (expensive)

  • Random walks: a node asks a randomly chosen neighbor (takes longer time)