se464.txt

* What is the Software Architecture?

The set of principal design decisions about the system.

* Generally, what is architecture?

The parts of a system that would be difficult to change once the system is
built.

* What are the 3 primary dimensions that architectures capture?

- Structure
- Communication
- Nonfunctional requirements

* What is Software Architecture about?

- Communication
- "What parts are there?"
- "How the parts fit together."

* What is Software Architecture not about?

- Development
- Algorithms
- Data structures

* What are the 5 steps of the design process?

1. Ideation
2. Analysis
  a. Determine Criteria
  b. Apply Criteria
3. Selection
4. Elaboration/Refinement
5. Iteration

* Why is building software hard?

- Young field
- High user expectations
- Software cannot execute independently

* What are the two types of difficulties?

Incidential and essential.

* What are incidental difficulties?

Problems that can be overcome.

* What are essential difficulties?

Problems that cannot be easily overcome.

* What are some examples of essential difficulties?

- Complexity: Grows non-linearly with program size.
- Conformity: System is dependent on its environment
- Changeability: Perception that software is easily modified.
- Intangibility: Not constrained by physical laws.

* What are some of the ways to tackle the problem of complexity?

- High-level languages
- Development tools and environments
- Component-based reuse
- Development strategies
- Emphasizing design

# Properties

* What are "principal" design decisions?

Decisions so important that they are granted "architectural status".

* What is a prescriptive architecture?

The design decisions made prior to the system's construction.
(As-conceived or as-intended architecture.)

* What is descriptive architecture?

Describes how the system has been built.
The "as-implemented" or "as-realized" architecture.

* What are some reasons why a system's descriptive architecture is directly modified?

- Developer sloppiness.
- Perception of short deadlines which prevent thinking through and documenting.
- Lack of documented prescriptive architecture.
- Need or desire for code optimizations.
- Inadequate techniques or tool support.

* What are the two ways that architectural degradation can occur?

- Architectural drift
- Architectural erosion

* What is architectural drift?

Introduction of principal design decisions into a system's descriptive
architecture that:
  - Are not included in, encompassed by, or implied by the prescriptive
    architecture.
  - But which do not violate any of the prescriptive architecture's design
    decisions.

* What is architectural erosion?

Introduction of principal design decisions into a system's descriptive
architecture that violate its prescriptive architecture.

* What is architectural recovery?

The process of determining a software system's architecture from its
implementation-level artifacts.

* What are some examples of implementation-level artifacts w.r.t architectural recovery?

- Source code
- Executable files
- Java .class files

* What 3 elements should a software system's architecture be composed of?

- Processing
- Data, aka state
- Interaction

* What are software components?

Elements that encapsulate processing and data in a system's architecture.

* What is the definition of a software component?

An architectural entity that:
  - Encapsulates a subset of the system's functionality and/or data.
  - Restricts access to that subset via an explicitly defined interface.
  - Has explicitly defined dependencies on its required execution context.

* What do software components usually provide?

Application-specific services.

* What, in complex systems, can become more important and challenging?

Interaction

* What is a software connector?

An architectural building block tasked with effecting and regulating
interactions among components.

* What do software connectors typically provide?

Application-independent interaction facilities.

* What are some examples of connectors?

- Procedure call
- Shared memory
- Message passing
- Streaming
- Distribution
- Wrapper/adaptor

* What are software configurations?

A set of specific associations between the components and connectors of a
software system's architecture.

Also called topology.

* What are possible assessment dimensions for the deployment view of a system?

- Available memory
- Power consumption
- Required network bandwidth

* What are the two main topological goals?

- Minimize coupling between components.
  - The less components know about each other, the better.
- Maximize cohesion within each component.
  - Components should be responsible for a single, logical service.

* What is abstraction?

Focusing on key details while eliding extraneous detail.

* What are some ways abstraction is used in software?

- Control abstraction (e.g. structured programming)
- Data abstraction (e.g. abstract data types)

* What is decomposition?

- Break problem into independent components
- Describe each component

Top-down abstraction.

* What are some criteria for decomposition?

- Implementing teams
- Application domains
- Parallelization

* What is Conway's Law?

The structure of a software system reflects the structure of the organization
that built it.

* What is software architecture fundamentally about?

Facilitating technical communication between stakeholders.

* What are the 3 properties of architectural representations?

- Ambiguity: Open to more than one interpretation?
- Accuracy: Correct with tolerances
- Precision: Consistent but not necessarily correct

* What is an architecture model?

An artifact documenting some or all of the architectural design decisions
about a system.

* What is an architecture visualization?

A way of depicting some or all of the architectural design decisions about a
system to a stakeholder.

* What is an architecture view?

A subset of related architectural design decisions.

* What does a component diagram capture or record?

Components and relationships.
Required and provided APIs.

* What does a sequence diagram focus on or capture?

Inter-component collaboration.
Behaviour of specific runtime scenarios.

* What does a deployment diagram provide?

Mapping between components and physical devices.

* What is a statechart diagram?

More formal description of system behaviour.
Poor mapping between states and components.

# F and NF properties

* What are functional properties?

What the system is supposed to do. ("The system shall do X")

* What are non-functional properties?

What the system is supposed to be ("The system shall be X");

* What are some examples of non-functional properties?

- Efficiency
- Scalability
- Adaptability
- Complexity
- Heterogeneity
- Dependability

* What is the relation between FPs and NFP w.r.t success of a product?

Engineering products are generally sold based on their FPs.
The system's success ultimately rests on its NFPs.

* What are the challenges of designing NFPs?

- Only partially understood in many domains.
- Qualitative vs quantitative.
- Frequently multi-dimensional
- Non-technical pressures

* What are the 5 design guidelines for ensuring NFPs?

- Only guidelines, not laws or rules.
- Promise but do not guarantee a given NFP.
- Necessary but not sufficient for a given NFP.
- Have many caveats and exceptions.
- Many trade-offs are involved.

* What is efficiency w.r.t NFPs?

A quality that reflects a system's ability to meet its performance requirements.

* How can one design components to be efficent?

- Keep them "small"
- Simple and compact interfaces.
- Allow multiple interfaces to the same functionality
- Separate data from processing components.
- Separate data from metadata.

* How can one design connectors to be efficient?

- Carefully select connectors.
- Be careful of broadcast connectors
- Encourage asynchronous interaction.
- Be wary of location/distribution transparency.

* How can one design the topology to be efficient?

Keep frequent collaborators "close".
Consider the efficiency impact of selected styles.

* What is scalability?

The capacity of a system to be adapted to meet new size/scope requirements.

* What is heterogeneity?

The system's ability to be composed of, or execute within, disparate parts.

* What is portability?

The ability of a system to execute on multiple platforms while retaining
their functional and non-functional properties.

* How can one design components to be scalable/heterogenic?

- Keep components focused.
- Simplify interfaces.
- Avoid unnecessary heterogeneity
- Distribute data sources
- Replicate data

* How can one design connectors to be scalable/heterogenic?

- Use explicit connectors
- Choose the simplest connectors
- Direct vs. indirect connectors

* How can one design the topology to be scalable/heterogenic?

- Avoid bottlenecks
- Place data close to consumer
- Location transparency

* What is evolvability?

The ability to change to satisfy new requirements and environments.

* How can one design components for evolvability?

- Same as for complexity
- Reduce risks by isolating modifications

* How can one design connectors for evolvability?

- Clearly define responsibilities
- Make connectors flexible

* How can one design the topology for evolvability?

- Avoid implicit connectors
- Encourage location transparency

* What are the three parts of dependability?

- Reliability: The probability a system will perform within its design limits
  without failure over time.
- Availability: The probability the system is available at a particular instant
  in time
- Robustness: The ability of a system to respond adequately to unanticipated
  runtime conditions.

* What is fault-tolerance?

The ability of a system to respond gracefully to failures at runtime.

* Where can faults arise from at runtime?

- Environment
- Components
- Connectors
- Component-connector mismatches

* What is survivability?

The ability to resist, recover, and adapt to threats.

* What are sources of threats w.r.t survivability?

- Attacks
- Failures
- Accidents

* What are the steps for combatting threats?

Resist, recognize, recover, adapt.

* What is safety w.r.t dependability?

The ability to avoid failures that will cause loss of life, injury, or loss
to property.

* How can one design components for dependability?

- Control external component dependencies
- Support reflection
- Support exception handling

* How can one design connectors for dependability?

- Use explicit connectors
- Provide integration guarantees

* How can one design the topology for dependability?

- Avoid single points of failure
- Enable back-ups
- Support system health monitoring
- Support dynamic adaptation

* FP or NFP: "Clicking the approve button moves the request to the Approval Workflow"

FP

* FP or NFP: "Field 2 only accepts dates before the current date."

FP

* FP or NFP: "The system must handle 1,000 transactions per second."

NFP

* FP or NFP: "Screen 1 can print on-screen data to the printer."

FP

* FP or NFP: "The system must have less than 1hr downtime per 3 months."

NFP

* FP or NFP: "Source code should contain no more than X bugs per 10K LOC."

NFP



# Architectural Styles

* What is an architectural pattern?

A set of architectural design decisions that are applicable to a recurring
design problem, and parameterized to account for different software development
contexts in which that problem appears.

* What is the three-tiered architectural pattern?

Composed of three tiers:
  - Front tier: Contains the user interface functionality to access the system's
    services
  - Middle tier: Contains the application's major functionality
  - Back tier: Contains the application's data access and storage functionality.

* What is the Model-View-Controller architectural pattern?

Model: Encapsulation of information.
View: Encapsulation of display choices.
Controller: Encapsulation of interaction semantics.

* What is the Sense-Compute-Control architectural pattern?

1. Read all sensor values
2. Compute control outputs
3. Send controls to all actuators

* What is an architectural style?

A named collection of architectural design choices that:
- Are applicable in a given development context.
- Constrain architectural design decisions that are specific to a particular
  system with that context.
- Elicit beneficial qualities in each resulting system.

* What are 4 good properties of an architecture?

- Result in a consistent set of principled techniques.
- Resilient in the face of (inevitable) changes.
- Source of guidance through product lifetime.
- Reuse of established engineering knowledge.

* What is the difference between "pure" architectural styles and their use in practice?

Pure architectural styles are rarely used. Instead, systems in practice:
- Regularly deviate from pure styles
- Typically feature many architectural styles.

* What are the benefits of using architectural styles?

- Design reuse
- Code reuse
- Understandability of system organization
- Interoperability
- Style-specific analyses
- Visualizations

* What are some architectural style analysis dimensions?

- What is the design vocabulary? (Component and connector types)
- What are the allowable structural properties?
- What is the underlying computational model?
- What are the essential invariants of the style?
- What are common examples of its use?
- What are (dis)advantages of using the style?
- What are the style's specializations?

* What are 4 architectural styles?

- Language based
- Layered
- Dataflow
- Peer-to-Peer
- Shared Memory
- Interpreter
- Implicit Invocation

* What is the Layered architectural style?

- Hierarchical system organization
  - Each layer exposes an API to the above layers.
- Each layer is a:
  - Client to layers below.
  - Server to layers above.

* What are the connectors in the layered architectural style?

Protocols of layer interaction.

* What is an example of a system that uses the layered architectural style?

Operating systems.

* What are the advantages of the layered architectural style?

- Increasing abstraction levels
- Evolvability
- Changes in a layer affect at most the adjacent two layers -> Reuse
- Different implementations of layers
- Standardized layer interfaces

* What are the disadvantages of the layered architectural style?

- Not universally applicable
- Performance

* What is the Client-Server architectural style?

- Components are clients and servers.
- Servers do not know number or identities of clients.
- Clients know servers' identities.

* What are the connectors in the client-server architectural style?

RPC-based network interaction protocols.

* What is an example of a system that uses the client-server architectural style?

Games.

* What are 2 types of data-flow architectural styles?

- Batch Sequential
- Pipe and Filter

* What is the batch sequential data-flow architectural style?

Separate programs are executed in order; data is passed as an aggregate from
one program to the next.

* What are the connectors in the batch sequential data-flow architectural style?

The mechanisms that carry information between each program.

* What are data elements in the batch sequential data-flow architectural style?

Explicit, aggregate elements passed from one component to the next upon
completions of the producing program's execution.

* What is an example of system that uses the batch sequential data-flow architectural style?

Transaction processing in financial systems.

* What is the pipe and filter data-flow architectural style?

Components are filters that transform input data streams into output data
streams.

Allows for incremental production of output and feedback loops.

* What are connectors in the pipe and filter data-flow architectural style?

Conduits for data streams.

* What are the invariants in the pipe and filter data-flow architectural style?

Filters are independent (no shared state)
Filter has no knowledge of up- or down-stream filters.

* What are some examples of systems that use the pipe and filter data-flow architectural style?

- UNIX shell
- Distributed systems
- Signal processing
- Parallel programming

* What is a common shared-memory architectural style?

Blackboard.

* What is the blackboard architectural style?

2 components:
- Central data structure (i.e. blackboard)
- Components operating on the blackboard

System control is entirely driven by the blackboard state.

* What are some examples of systems that use the blackboard architectural style?

- AI systems
- Integrated software environments
- Compiler architecture

* What is the rule-based architectural style?

- Inference engine parses user input and determines if it is a fact/rule or a
query.
- If it is a rule, it adds the entry to its knowledge base.
- Otherwise, if queries the knowledge base to try to resolve the query.

* What is the interpreter architectural style?

Interpreter parses and executes input commands, updating the state maintained by
the interpreter.

* What are the typical components in the interpreter architectural style?

- Command interpreter
- Program/interpreter state
- User interface

* What is the implicit invocation architectural style?

Use event announcement instead of method invocation.

- Listeners register interest in and associate methods with events.
- System invokes all registered methods implicitly.

* What are the invariants in the implicit invocation architectural style?

- Announcers are unaware of their events' effects.
- No assumption about processing in response to events.

* What is the peer-to-peer architectural style?

State and behaviour are distributed among peers which can act as either clients
or servers.

* What are peers, connectors, and data elements w.r.t the peer-to-peer architectural style?

Peers: Independent components, having their own state and control thread.
Connectors: Network protocols.
Data Element: Network messages.

# Design Recovery

* What is the task of design recovery?

- Examining the existing code base.
- Determining what the system's components, connectors, and overall topology
  are.

* What is a common approach to design recovery?

Clustering of the implementation-level entities into architectural elements.

* What are the two types of clustering w.r.t design recovery?

Syntactic clustering, or semantic clustering.

* What is syntactic clustering w.r.t design recovery?

- Focusing exclusively on the static relationships in code.
- Can be performed without executing the system
- Embodies inter-component (coupling) and intra-component (cohesion) connectivity

* What is a problem of syntactic clustering w.r.t design recovery?

May ignore or misinterpret subtle relationships, because dynamic information is
missing.

* What is semantic clustering w.r.t design recovery?

- Includes all aspects of a system's domain knowledge and information about the
behavioural similarity of its entities.

* What are some problems of semantic clustering w.r.t design recovery?

- Requires interpreting the system entities, meaning that one many need to
execute the system.
- Difficult to automate
- May also be difficult to avail oneself of it.

* How does one tackle greenfield design?

- Analogy searching
- Brainstorming
- "Literature" searching
- Morphological charts
- Removing mental blocks

# Back to NFPs

* What are the 3 parts of security?

- Confidentiality: Preventing unauthorized parties from accessing the information.
- Integrity: Preventing unauthorized parties to manipulate the information.
- Availability: Allowing authorized parties to view the information.

* What are some design principles for security?

- Least privilege
- Fail-safe defaults
- Economy of mechanism
- Complete mediation
- Open Design
- Separation of Privilege
- Least Common Mechanism
- Psychological Acceptability
- Defense in Depth

* What is the least privilege design principle w.r.t security?

Give each component only the privileges it requires.

* What is the fail-safe defaults design principle w.r.t security?

Deny access if explicit permission is absent.

* What is the economy of mechanism design principle w.r.t security?

Adopt simple security mechanisms.

* What is the complete mediation design principle w.r.t security?

Ensure every access is permitted.

* What is the open design design principle w.r.t security?

Do not rely on secrecy for security.

* What is the separation of privilege design principle w.r.t security?

Introduce multiple parties to avoid exploitation of privileges.

* What is the least common mechanism design principle w.r.t security?

Limit critical resource sharing to only a few mechanisms.

* What is the psychological acceptability design principle w.r.t security?

Make security mechanisms usable.

* What is the defense in depth design principle w.r.t security?

Have multiple layers of countermeasures.

* What are architectural access control models?

Models for deciding whether access to a protected resource should be granted
or denied.

* What are the two types of architectural access control models?

Discretionary and mandatory.

* What is the discretionary access control model?

Restricting access based on the identity of the user.
Users can define access for their own resources.

* What is the mandatory access control model?

There is a higher-level service that determines whether the access is granted.
(e.g. Operating systems)

* What are some problems that affect security?

- Impersonation
- Fraudulent actions
- Misrepresentation
- Collusion
- Addition of unknowns

* What is impersonation w.r.t security?

Pretending to be someone else.

* What is an example of a fraudulent action w.r.t security?

Buyer pays for an item but seller does not ship anything.

* What is misrepresentation w.r.t security?

Lying about the facts around someone else.

* What is collusion w.r.t security?

Multiple people spreading false information.

* What is an example of addition of unknowns w.r.t security?

A new user joins the system.

* What is trust?

A particular level of the subjective probability with which an agent assesses
that another agent will perform a particular action.

* What is reputation?

The expectation about an entity's behaviour based on past behaviour.

* What are the 2 types of trust management systems?

- Credential or policy-based
- Reputation-based

* What can online content compromise w.r.t security?

Confidentiality: Leak user data.
Integrity: Read/write data to disk.
Availability: Crash host.

* What 3 main security design principles does Chrome rely on?

- Least privilege
- Separation of privilege
- Defence in depth

# Architecture modelling

* What is architectural modelling?

The reification and documentation of architectural models.

* What is an architectural modelling notation?

A language or means of capturing design decisions.

* What basic architectural elements should be modelled?

- Components
- Connectors
- Interfaces
- Configurations
- Rationale

* What elements of the architectural style should be modelled?

- Inclusion of specific basic elements (e.g. components, connectors)
- Component, connector, and interface types
- Constraints and interactions
- Behavioural constraints
- Concurrency constraints

* What are the different aspects w.r.t architectural modelling?

Static and Dynamic.
Function and Non-functional.

* What are static aspects w.r.t architectural modeling?

Aspects of a system that do not change as a system runs.

* What are examples of static aspects w.r.t architectural modeling?

Topologies, assignment of components/connectors to hosts.

* What are dynamic aspects w.r.t architectural modeling?

Aspects that change as a system runs.

* What are examples of dynamic aspects w.r.t architectural modeling?

State of individual components or connectors.
State of dataflow.

* What are the 3 important characteristic of architectural models?

- Ambiguity
- Accuracy
- Precision

* What is ambiguity w.r.t architectural models?

The model is open to more than one interpretation.

* What is accuracy w.r.t architectural models?

The model is accurate if it is correct, conforms to fact, or deviates from
correctness within acceptable limits.

* What is precision w.r.t architectural models?

The model is precise if it is sharply exact or delimited.

* What are views and viewpoints w.r.t architectural models?

Since it is not feasible to documentation everything at once, you define
several models that each capture a subset of the design decisions.

The subset model is a "view" and the concern is a "viewpoint".

* What are 5 commonly-used viewpoints?

- Logical
- Physical
- Deployment
- Concurrency
- Behavioural

* What is consistency w.r.t architectural models?

Views are consistent if the design decisions they contain are compatible.

* When are views inconsistent w.r.t architectural models?

When two views assert design decisions that cannot simultaneously be true.

* What are direct inconsistencies w.r.t architectural models?

Direct contradictions, such as the number of hosts a system runs on.

* What are refinement inconsistencies w.r.t architectural models?

Conflicts between the high-level and low-level views.

* What are some common types of inconsistencies?

- Dynamic vs dynamic aspect inconsistencies
- Functional vs non-functional inconsistencies.

* What are the various ways to evaluate modeling approaches?

- Scope and purpose
- Basic elements
- Style
- Static and dynamic aspects
- Dynamic modeling
- Non-functional aspects
- Ambiguity
- Accuracy
- Precision
- Viewpoints
- Viewpoint consistency

* What is UML?

Unified Modelling Language. 13 loosely-interconnected notations that capture
static and dynamic aspects of software-intensive systems.

* What are the advantages of using UML to model?

- Supports diverse array of viewpoints.
- Ubiquitous.
- Extensive documentation and tool support.

* What are the disadvantages of using UML to model?

- Needs customization through profiles to reduce ambiguity.
- Difficult to access consistency among views.
- Difficult to capture foreign concepts or views.

* How are static structures modelled in UML?

- Component diagrams
- Deployment diagrams
- Class diagrams

* How is dynamic behaviour modelled in UML?

- Sequence diagrams
- Use-case diagrams
- State machine diagrams

# Design

* What is analysis?

- Domain-level modelling of "real world" objects
- Addresses functional challenges
- Captures what a system does
- Provides implementation guidance

* What is design?

- Modelling of implementation objects
- Address implementation challenges
- Captures how the system realizes its OOA
- Assigning responsibilities to objects.

* What are the 3 steps of the design process?

Build, Measure, Learn.

* Why is design-level reuse valuable?

- Matches problems to design experience.
- Avoid previously-encountered problems.

* What are the 4 main aspects that good design tries to promote?

- Abstraction
- Flexibility
- Modularity
- Elegance

* Why is abstraction important?

Prevents designers from being distracted by implementation details.

* What are the STUPID principles?

- Singleton
- Tight Coupling
- Untestable
- Premature optimization
- Indescriptive Naming
- Duplication

* What are the SOLID principles?

- Single Responsibility
- Open/Closed
- Liskov Substitution
- Interface Segregation
- Dependency Inversion

* What is the single responsibility SOLID principle?

Classes should have only one major task.

* What is the advantage of the single responsibility SOLID principle?

Classes are insulated from one another.

* What is the open/close SOLID principle?

Classes should be open for extension, but closed to modification.

* How would does one extend classes, following the open/close SOLID principle?

Use subclassing to minimize impact on existing clients.

* What is the Liskov substitution SOLID principle?

Subtypes should behave as their parent types.

* What is the interface segregation SOLID principle?

Only place key methods in interfaces.

* What is a disadvantage of the interface segregation SOLID principle?

There may be a larger number of smaller interfaces to maintain.

* What is the dependency inversion SOLID principle?

High-level methods should not depend on lower-level modules.

* What does the dependency inversion SOLID principle try to do?

Minimize direct coupling between concrete classes.

* What are some lower-level design principles?

- Encapsulate what varies.
- Program to abstractions, not implementations
- Favour composition over inheritance
- Strive for loose coupling

* What are the 3 quality attributes of design?

- Simplicity
- Coupling
- Cohesion

* How can one strive for the simplicity quality attribute?

- Meet goals without extraneous embellishment
- Don't repeat yourself (DRY)
- Keep It Simple, Stupid (KISS)

* What are the 6 types of coupling?

- Content
- Global
- Control
- Data
- Message
- None

* What is content coupling?

When one depends on the content of the implementation. (e.g. reflection.)

* What is global coupling?

When one depends on global variables.

* What is control coupling?

When the control flow of (e.g. a function) depends on the parameters passed.

* What is data coupling?

When one shares raw data. (e.g. passing integer parameters)

* What is message coupling?

When one passes around objects/messages to only communicate.

* What are the 6 types of cohesion?

- Coincidental
- Logical
- Temporal
- Communication
- Sequential
- Functional

* What is coincidental cohesion?

Parts randomly grouped.

* What is logical cohesion?

Parts grouped by logical categories. (e.g. input handling)

* What is temporal cohesion?

Parts grouped by when they are processed.

* What is communication cohesion?

Parts grouped by what data they operate on.

* What is sequential cohesion?

Parts grouped close to their inputs and outputs.

* What is functional cohesion?

Parts grouped by their task.

* What are 2 ways of spotting incoherency?

- An operation's description is full of 'and' clauses.
  - Indicates temporal cohesion, logical cohesion.
- An operation's description is full of 'if..then..else' clauses.
  - Indicates control coupling, coincidental cohesion, logical cohesion.

* What are the 4 cognitive dimensions?

- Premature commitment
- Hidden dependencies
- Secondary notation
- Viscosity

* What is premature commitment?

Decisions made with insufficient data that constrains future choices.

* What are hidden dependencies?

Latent (e.g. temporal) dependencies.

* What is secondary notation w.r.t cognitive dimensions?

Non-obvious relationships that may be meaningful.

* What is viscosity w.r.t cognitive dimensions?

Resistance to change.

* What are 4 reasons to use design patterns?

- Ease communication by using a shared vocabulary.
- Leverage existing design knowledge.
- Enhance flexibility for future change.
- Increase reusability of developed code.

* What are design patterns?

- Common solutions to recurring design problems.
- Abstract recurring structures.

* What are design patterns comprised of?

Class and/or object:
- Dependencies
- Structures
- Interactions
- Conventions

* What 4 main parts do design patterns have?

1. Name
2. Problem
3. Solution
4. Consequences / trade-offs

* What is composition?

Creating objects with other objects as members.

* When should composition be used?

When a has-a relationship applies.

* What is inheritance?

The concept of classes automatically containing the variables and methods
defined in their supertypes.

* What is contra-variance and co-variance?

Co-variant: The type of variable changes in the same direction as the type of
the class.
contra-variant: Opposite directions.

* What is co-variant or contra-variant w.r.t the Liskov substitution principle?

The return value is co-variant, as it can be more specific.
The parameters are contra-variant, as it can only be less specific.

* What are the 3 "by purpose" categories in the Gang of Four design patterns?

- Creational
- Structural
- Behavioural

* What are the 2 "by scope" categories in the Gang of Four design patterns?

- Class
- Object

* What are the creational class design patterns in the Gang of Four?

- Factory method

* What are the creational object design patterns in the Gang of Four?

- Abstract Factory
- Builder
- Prototype
- Singleton

* What are the structural class design patterns in the Gang of Four?

- Adapter (class)

* What are the structural object design patterns in Gang of Four?

- Adapter (object)
- Bridge
- Composite
- Decorator
- Facade
- Flyweight
- Proxy

* What are the behavioural class design patterns in the Gang of Four?

- Interpreter
- Template Method

* What are the behavioural object design patterns in the Gang of Four?

- Chain of Responsibility
- Command
- Iterator
- Mediator
- Memento
- Observer
- State
- Strategy
- Visitor

* What are creational design patterns used for?

- Adding indirection to object creation.
- Defering decisions about class instantiations until runtime.
- Encapsulating logic of construction.

* What is the singleton design pattern?

There is only one instance of the object, that you get from the singleton.

* What is the factory method design pattern?

- Creator and Product interface.
- Creator interface has a "create object" method.
- Concrete creator class implements the abstract class.
- Calling the concrete "create object" methods returns a new object.

* What is the abstract factory design pattern?

Abstract factories produce abstract products.
Concrete implementations produce subtypes of abstract products.

* What is the builder pattern?

Two different function calls to get a new object.

BuildPart(), which builds a bit needed to get the final result.
GetResult(), which returns the finished, new object.

* What is the prototype pattern?

Objects have Clone() methods, which return copies of itself.

* What is refactoring?

The process of restructing existing computer code without changing its external
behaviour.

* What can refactoring improve?

- Abstraction
- Code reuse
- Cohesion
- Coupling
- Naming
- Usage of design patterns

* What are some code smells?

- Large classes and methods
- Deep nesting of control structures
- Duplicate code
- Static state
- No information hiding, bad encapsulation
- Many parameters
- Casts to implementation classes

* What are structural design patterns used for?

Making relationships/structure of code more flexible.

* What is the class adapter pattern?

Adaptor class that extends all adaptees.
Calling an Adaptor method may call methods that the Adaptor class inherits from.

* What is the object adapter pattern?

Adapter class holds an instance of the adaptee.
Calling an Adaptor method may call methods of the adaptee instance.

* What is the bridge pattern?

The implementation is held in a separate class.
An instance of the implementation class is held by the main abstraction.

* What is the decorator pattern?

A decorator wraps a component so it can extend the component without modifying
the component directly.

* What is the facade pattern?

A Facade object interfaces with other packages/libraries instead of the
clients.

* What is the proxy pattern?

A Proxy class functions as a delegate to something else.
The Proxy class will usually have the same type as its delegate.
A wrapper that serves as an agent to a real object.

* What is the composite pattern?

Treating a group of objects in the same way as a single instance.

* What is the flyweight pattern?

A flyweight object tries to share as much data as possible with similar objects.

* What are behavioural patterns used for?

Identifying common patterns between objects to increase flexibility.

* What is the interpreter pattern?

Implementing a specialize language to rapidly solve a specific set of problems.

* What is the template method pattern?

Defining a program skeleton of an algorithm.
Calling subclasses that could redefine certain steps of an algorithm.

* What is the visitor pattern?

Visitor class has methods which take an object and run new operations.
Add new operations without needed to actually modify the original classes.

* What is the observer pattern?

Observers can register themselves to a subject.
The subject will periodically notify all registered observers.

# Things from midterm review

* Layered architectures are designed to reduce coupling. Why is reduced coupling useful?

- Modifability: Changes can be made to one layer without affecting others.
- Reusability: Layers can be reused in similar systems.
- Understandabiity: Easier to understand how the software works.

* What are the basic characters of using client/server architecture?

- Scalability: Can scale horizonally and vertically.
- Clients are typically heterogeneous and multi-vendor.

* What design pattern is useful for a file system?

Composite.

* What design pattern is useful for extending utility without modifying the original interface?

Visitor.

# Behavioural design patterns

* What do behavioural design patterns do?

Identify common communication patterns between objects and realize these patterns.

* What are 2 class-scoped behavioural design patterns?

1. Interpreter pattern.
2. Template method pattern.

* What are the 4 main motivations of MVC?

1. UI changes more frequently than business requirements.
2. The same data is often displayed in different ways.
3. Designers and developers are often different people.
4. Testing UI code is difficult and expensive.

* What are the 3 main goals of MVC?

1. Decouple models and views.
2. Increase maintainability/testability of the system.
3. Permit new views to be developed.

* What are some properties of the "model" in MVC?

- Container of application data.
- Does not know how to present itself.
- Domain independent.

* What are models called in the observer pattern?

Subjects.

* What are some properties of the "view" in MVC?

- Presents the model to the user.
- Allows the user to manipulate the data.
- Does not store data.
- Configurable to display different data.

* What are some properties of the "controller" in MVC?

- Glues the model and view together.
- Updates the view when the model changes.
- Updates the model when the user interacts with the view.
- Houses the application logic.
- Loose coupling between model and others.
- Tightly coupled with the views.

* What are the interaction mechanisms w.r.t MVC?

- User interacts with the view.
- View notifies controller of changes.
- Controller performs actions on the model.
- Model fires change events.
- View responds to the change events.

* What are the benefits of MVC?

- Decoupled view and model.
  - Multiple views.
  - Maintainability.
  - Split teams.
- Testability

* What are the drawbacks of MVC?

- Complexity
- Efficiency

* What basic design patterns are used in MVC?

- Strategy pattern (view/controller)
- Composite pattern (view)
- Observer pattern (view/model)

* What is the difference with the model between MVC and MVP?

The model fires events into an event bus, instead of using the observer pattern.

* What are the characteristics of the view in MVP?

- Thin UI front-end for controller.
- Can be interchanged easily.
- Does not ever see or manipulate model objects.
- Interacts only with primitives.

* What are the characteristics of the presenter in MVP?

- Glues the model and view together.
- Updates the view when the model changes.
- Updates the model when the user manipulates the view.
- Houses the application logic.

* What are the improvements of MVP over MVC?

- Decoupling of views from the model.
- Increase testability.
- A little less complex than MVC.

# Dependency Injection

* What is dependency inversion also known as?

"Hollywood principle" or "inversion of control".

* What is the principle of dependency inversion?

High-level methods should not depend on lower-level methods.

* What are the benefits of dependency inversion?

Minimizes direct coupling between concrete classes.

* How does direct coupling between concrete classes usually manifest?

Object creation.

* What is the dependency inversion principle?

Program to interfaces not to implementations.

* What is a key separation when using ghe dependency inversion principle?

Separate objects from their assemblers.

* What are the goals of dependency injection?

To eliminate initialization statements.

* What are the 4 tasks of dependency injection?

1. Takes a set of components
2. Adds a set of configuration metadata
3. Provides the metadata to an injection framework.
4. Bootstraps object creation with a configured injector.

* What is Google Guice?

An inversion of control framework for alleviating some of the boilerplate code.

* What does the @Inject statement do in Google Guice?

Tells Google Guice to automatically instantiate the correct objects.

* What determines what gets injected in Google Guice?

The module.

# Cloud/REST architectures

* What is grid computing?

Combining computing resources from multiple domains to do a single task.

* What is grid computing usually used for?

Creating "supercomputers" to work on specific parallel computation tasks.

* What is cloud computing?

A model of enabling convenient, on-demand network access to a shared pool of
configurable computing resources.

These resources can be rapidly provisioned and released with minimal management
effort or service provider interaction.

* What does the cloud model promote?

Availability.

* What are the 5 essential characteristics of cloud computing?

1. On-demand self-service.
2. Broad network access.
3. Resource pooling.
4. Rapid elasticity.
5. Measured service.

* What is the on-demand self-service characteristic of cloud computing?

Provisioning is possible without human interaction.

* What is the "broad network access" characteristic of cloud computing?

Capabilities are available over the network through standard mechanisms.

* What is the "resource pooling" characteristic of cloud computing?

Computing resources are pooled to serve multiple consumers.
Location independence.

* What is the "rapid elasticity" characteristic of cloud computing?

Resources can be easily added or removed.

* What is the "measured service" characteristic of cloud computing?

Metering of storage, processing, bandwidth.

* What are some benefits of cloud computing?

- Agility. One can quickly respond to changes.
- Scalability. Resources can be quickly added.
- Cost.
- Reliability.
- Security.

* What technologies are used to make cloud computing possible?

- Thick and thin clients.
- Broadband internet.
- Large, global datacenters
- APIs for administration, development, and resource migration.

* What is the goal of virtualization?

Decoupling of physical and computing resources.

* What are the various cloud layers?

- Software as a Service
- Platform as a Service
- Infrastructure as a Service

* What are SaaSes?

Vendor-controlled remote applications.

* What is an example of a SaaS?

Google Docs.

* What are some concerns one may have about using SaaSes?

Control, performance, security, privacy.

* What are PaaSes?

Vendor-controlled environments.

* What is an example of a PaaS?

Google AppEngine.

* What are some concerns one may have about using PaaSes?

Control, performance, security, privacy, and limited technology choices.

* What are IaaSes?

Vendor-provided resources.

* What is an example of a IaaS?

Amazon EC2.

* What are some concerns one may have about using IaaSes?

More expertise needed to leverage flexibility.

* What are the NFPs w.r.t cloud security?

- Confidentiality
- Integrity
- Authenticity
- Anonymity
- Privacy

* What is a key concern about cloud security?

Data remanence. How can you purge data from the cloud?

* What does REST stand for?

Representational state transfer.

* What are the key constraints of REST?

- Client-server architecture.
- Servers do not maintain session state.
- Clients must not depend on direct server access.
- Clients communicate using a uniform interface.

* What are the four REST operations?

GET, POST, PUT, DELETE.

* What does a GET REST command usually do?

List or retrieve a resource.

* What does a POST REST command usually do?

Append a resource.

* What does a PUT REST command usually do?

Replace a resource.

* What does a DELETE REST command usually do?

Delete a resource.

* What is a common property of REST URIs?

They are usually versioned.

* What are some problems of Java's type system?

Does not prevent enough errors, such as runtime exceptions.
Some of these errors can also be silent.

* How does one use a pluggable type system?

1. Design a custom type system to solve a specific problem.
2. Writing type qualifiers in code.
3. Using the type checker to warn about violations.

* What is the benefit of type systems?

- Finds bugs in programs.
- Guarantees the absence of errors.
- Improvements documentation.
- Aids compilers and static analysis tools.

* What is the CLIMB-to-top defaulting rule of type systems?

Top type is the default for:
- Casts
- Local variables
- Instanceof
- Implicit Bounds

* What are the 4 steps for defining a type system?

1. Qualifier hierarchy.
2. Type introduction rules.
3. Type rules.
4. Flow-refinement.

* What is the "qualifier hierarchy" step of defining a type system?

Using @SubtypeOf and other stuff.

* What is the "type introduction rules" step of defining a type system?

Defining whether a type is for expressions, declarations, or something else.

* What is the "type rules" step of defining a type system?

Defining methods to check types.