2 - Architectures Flashcards by Josh Davidson

Architectural Style: Definition

Components with well defined interfaces.
Relationships between components

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Connector: Definition

Components

Mechanism that mediates comms between components

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Architectural Styles: List of styles

Layered
Object based
Data centred
Event based

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What is an example of a layered style?

The Internet Protocol stack

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Object Based: Definition

Components are objects
Objects encapsulate data and offer methods
Objects may be on different machines

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Object based: Communication method

RPC (Remote Procedure Call)

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Object based: Examples

Common Object Request Broker Architecture
Java Remote Method Invocation

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Object based: CORBA

Common Object Request Broker Architecture

Middleware that supports RPCs
Support of different platforms and languages
OOP approach but other programs possible

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Object Based: RMI

Java Remote Method Invocation

Allows method calls between JVMs.
Locally or multiple computers
Platform independent

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Data centred: Definition

Shared data approach

Communicate via a repository
Common in web apps

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Data centred: Examples

RESTful Architecture
Amazon Simple Storage Service

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Data centred: RESTful Architecture

Collection of resources managed by components.

Resources added, removed, retrieved and modified by remote apps

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Data centred: Amazon S3

Simple Storage Service

Objects stored in buckets
Operations sent as HTTP requests

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Event based: Definition

(pub/sub)

Publish/Subscriber systems (queues)

Components publish messages on an event bus
Components subscribe to event bus while specifying desired type

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Event based: Do components/processes have explicit references?

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Event based: Example

OpenStack Cloud Software

Data centred (Database)
Event Based (Message Broker)

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Middleware: Definition

Software layer on top of the OS

Adds additional services LIKE comms and I/O routines
Can handle replication-related tasks
Makes using different (heterogeneous) hardware easier

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Middleware: Transparency

OS and hardware hidden by middleware

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Middleware: Design Patterns

(two different design types)

Wrapper - Offers client app interface
Interceptor - Breaks flow of comms to allow other code to exec

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Middleware: Example

Object-based system

App calls method doit of obj B. App is unaware of B’s location.

Call intercepted and sent to object B

Centralised Arch: Definition

Uses a central coordination system to direct services

Centralised Arch: Examples:

Application Layering
Client server and multi- architectures
Two tiered configurations

Peer to Peer (P2P): Definition

In terms of the load

Horizontal distribution of components and load (not vertical)

P2P: Asymmetric or symmetric?

Symmetric. Processes are servers and clients simultaneously

P2P: Centralised or decentralised?

Decentralised

P2P: Structured or unstructured?

Either!

Hypercube: Definition

Structured P2P which can be used to store data

Hypercube: Common practice for location identification

Use a hash funciton to map data items to keys identifying the node location Key(item) = hash(item's value)

Chord: Organisation shape

Ring

Chord: Data location (eg for virtual nodes)

Data with key k is stored at the node with the smallest id >= k (successor)

Chord: Successor

Next node with the smallest identifier greater than or equal to the key number

Overlay Network: Definition

Virtual Network on top of a physical network. Links usually not physical but path of physical links

Unstructured P2P: Nodes maintain what? What does the overlay resemble?

Each node maintains a list of neighbours and the resulting overlay resembles a random graph

Unstructured P2P: Search Types

Random Walks Flooding

Unstruct. P2P: Random Walks

Send request to random neighbour, v if v has data then send back and stop else repeat for another neighbour

Unstruct. P2P: Flooding

Send request to all neighbours (v) with reduced hop counter If v has data, has seen request before or hop counter = 0 then stop and send back data (if applic.) If not v forwards to all neighbours

If there are N nodes in a unstruct. P2P net with r nodes having the correct data, what is the expected number of nodes to be checked? Random walk vs Flooding

Random = N/r Flooding = Smallest k that fulfills d*(d-1)k-1>=N/r

Flooding vs Random Walk

Flooding is faster but sends more messages. Data may be found but child processes only stop when hop counter = 0 and the hop counter must be larger than the expected number of steps (wasted messages)

Superpeer: What problem are they trying to solve?

Searching in unstructured networks is resource-intensive

Superpeer: Concept

Introduce partial-structures which form a backbone net. Each (weak) peer is assigned to a superpeer that keeps an index All comms are via superpeers

Superpeer: Problem

Failure of a superpeer can be fatal so backups are required.

Adaptive Systems: List

Feedback Control Model Content Delivery Networks

Feedback Control Model

Used to automatically adjust a distributed system by analysing metrics which are compared to a reference output. If necessary, corrections are issued

Content Delivery Network

Large, automatically adjusting media delivery systems System decides whether to replicate content and where.

Three tiered approach. Layers

Interface processing data

Flooding how not to waste messages

Set the initial hop counter to exactly the diameter of the network (otherwise to ensure finding the data, it must be at least the diameter)

What kind of overlay network might you choose for broadcasts/multicasts?

Acyclic Graph - a tree so there's only single paths to nodes.

Middleware in terms of layers

Above transport but below application layer