REST Architectural Elements

Processing elements(Components)

→ focus on the roles of components, constrains on their interaction and their interpretation of significant data elements

Data elements

hypermedia architecture features

→ often move processing entity instead of moving data to processor.

options

1. render the data where it is located and send a fixed-format image to the recipient

   → the traditional client/server style (client implementation easier, but restrict)

2. encapsulate the data with a rendering engine and send both to the recipient

   → the mobile object style (while enabling processing of the data, but limiting the functionality of the recipient to what is anticipated within the unique engine)

3. send the raw data to the recipient along with metadata that describes the data type, so that the recipient can choose their own rendering engine

   → while minimizing the bytes transferred, but not hiding information and requiring both sender and recipient to understand the same data types

REST architecture - hybrid of the hypermedia architecture features

• client/server and mobile object style advantages

• focus on a shared understanding of data types with metadata

  → communicate by transferring a representation of the data in a format matching one of evolving set of standard data types, selected dynamically based on the capabilities or desires of the recipient and the nature of the data.

• limit the scope of what is revealed (the separation of concerns)

  → through an interface hiding implementation

Table 1: REST Data Elements

Data Element	Modern Web Examples
resource	the intended conceptual target of a hypertext reference
resource identifier	identifier URL, URN
representation	HTML document, JPEG image
representation metadata	media type, last-modified time
resource metadata	source link, alternates, vary
control data	if-modified-since, cache-control

Resources and Resource Identifiers

• A resource: a document, a image, a temporal service
• resource identifier: identify the particular resource

Representations and Representation metadata

• a representation: sequence of bytes
• a representation metadata: describe those bytes
• includes document, file, and HTTP message entity, instance or variant
• consists of data, metadata describing the data (name-value pairs)
• media type: data format for a representation → incrementally render a large HTML document

Control Data

• defines the purpose of a message

Connecting elements(Connectors)

encapsulate the activities of accessing resources and transferring resource representations

STATELESS Features

Each request contains all of the information necessary for a connector to understand the request.

1. removes any need for the connectors to retain application state between requests

   → reduces consumption of physical resources and improves scalability

2. allows interactions to be processed in parallel without requiring that the processing mechanism understand the interaction semantics
3. allows an intermediary to view and understand a request in isolation
4. forces all of the information that might factor into the reusability of a cached response to be present in each request application-level protocol

Connector Types

1. Client and Server

   e.g. client - libwww, libwww-perl / server- libwww, Apache API, NSAPI

   Client initiates communication by making a request, Server listens for connections and responds to requests

2. Cache

   e.g. browser cache, Akamai cache network

   located on the interface to a client or server connector in order to save cacheable responses to current interactions (server to avoid repeating the process of generating responses, reducing latency)

   A response indicates if the cache should not be shared

3. Resolver

   e.g. bind (DNS lookup library)

   translates partial or complete resource identifiers into network address information needed to establish an inter-component connection

   URI includes DNS hostname

   Web browser extracts the hostname from URI and make use of a DNS resolver to obtain the Internet Protocol address for that authority

4. Tunnel

   e.g. SOCKS, SSL after HTTP CONNECT

   relays communication across a connection boundary (e.g. firewall or lower-level network gateway)

   HTTP proxy that switches to a tunnel in response to a CONNECT method request, allowing its client to directly communicate with a remote server using a different protocol, such as TLS

REST Architectural views

how elements work together

Process View

• the path of data flow
• Services are implemented using a complex hierarchy of intermediaries and multiple distributed origin servers
• Each interaction to be independent of others → stateless
• Components act as either destination or intermediaries

Connector View

• a view on mechanics of the communication between components
• examine resource identifier in order to select an appropriate communication mechanism for each request (specific proxy)

Data View

• information flows through the components
• component interactions occur in the form of dynamically sized message ( request semantics e.g. GET)

References

Principled Design of the Modern Web Architecture

https://dl.acm.org/doi/pdf/10.1145/337180.337228