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Software architecture
What is it ?
The software architecture of a program or computing
system is the structure or structures of the system,
which comprise software elements, the externally visible
properties of those elements, and the relationships
among them.
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Software architecture
● Considerations about architecture:
● Impressive-sounding word. To talk about something is
important.
● Many definitions. Not an agreement.
● Three key ideas:
– High level design
– Difficult to change
– Many architectures in one systemwww.libreplan.com
Architectural patterns
● Definition: Description of elements and relation types
together with a set of constraints on how they may be
used.
● Some traits:
● They guide how to implement new features.
● They exhibit known quality attributes.
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Layering pattern
● Layering: It is a technique to deal with complexity.
● Several layers of software (source code) like in a
cake.
● Each layer rests on a lower layer.
● The lower layer is unaware of the upper layer
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Layering pattern
● Benefits:
● You can understand each layer in isolation.
● You can substitute each layer by an alternative
implementation.
● They promote reuse.
● They minimize dependencies between structural
parts.
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Layering pattern
● Downsides:
● Two much layers can harm performance.
● Desired encapsulation many times is imperfect.
Cascade changes.
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Layering pattern
● LibrePlan architecture uses the layering pattern.
● LibrePlan is a three layer (tier) application.
● Layer 0.
– Purpose = Persistence
– Components = Hibernate framework +
Repositories.
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Layering pattern
● Layer 1.
– Purpose = Domain logic
– Components = Domain Entities, Behavioral
classes, Conversational services
● Layer 2.
– Purpose = Interfaces (Human + Machine)
– Components = ZK web pages, Web services.
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Client - server pattern
● Definition: A program is a client-server application if
is a piece of software which runs split it two
machines.
– Client computer. It makes request to a remote
server computer.
– Server computer. It does the calculations asked
by clients and answers them.
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Client - server pattern
● Some features:
● It implies the existence of a network.
● Client-server applications can be cross-platform.
● Some nodes can be clients and servers at the same
time depending on the analyzed role.
● They are related with the layering. Where do you run
the layers?
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Client - server pattern
● LibrePlan is a client-server application.
● LibrePlan clients:
● User browser.
– Example: Firefox, Google chrome.
– Runs part of the interface tier.
● Outer WS client. Example: ERP importing into LibrePlan
employees.
● LibrePlan WS client. Example: outsourcing module
● LibrePlan persistence tier. Its server is the RDMS. www.libreplan.com
Client - server pattern
● LibrePlan server.
● Runs part of the ZK interface tier, the domain logic
and part of the persitence layer.
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Domain model pattern
● What is the domain? Subject area to which the user
applies the program.
● LibrePlan domain is project planning, monitoring
and control.
● In a few words: Build a model of the domain that both
incorporates logic and data.
● What implies? Inserting a whole layer of objects.
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Domain model pattern
● Alternative pattern (more frequently used):
Transaction script.
● It what does it consist of? It organizes business logic
by procedures where each procedure handles a
single request from the presentation.
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Domain model pattern
● Advantages:
● It allows logic reuse on large systems. Typical
transaction script architectures promote duplication
● It allows to change easily the behavior of complex
domains. It uses the power of the OO model to
accomplish it.
● It can be used for knowledge crunching (analysis)
with domain experts. Domain driven design.
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Domain model pattern
● Downsides:
● It requires effort. Assimilate the paradigm shift.
● It requires initially more effort to implement.
● LibrePlan uses domain model pattern.
● How is it implemented?
● Hibernate entities are related each other.
● They are POJOs. They can live outside the Hibernate
session.www.libreplan.com
IoC pattern
● Inversion of Control – Pattern of object-oriented
programming where the object coupling is done at run
time by an assembler object and is typically not known
at compile time by static analysis.
● How is done in traditional programming?
● The flow and the business logic is determined by
objects that are statically assigned to one another.
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IoC pattern
● Benefits:
● Decouples the execution of some tasks from
specific implementation. It allows to replace
components.
● It allows to reuse components more easily (among
different programs).
● It allows to focus the components in what they are
designed for (cohesion).
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IoC pattern
● Techniques to implement Inversion of Control
● Factory method pattern (GoF).
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IoC pattern
● Using a Service Locator.
– An object who knows how to get hold of all the
possible services an application can need.
– The client component makes explicit calls to the
Service Locator
● Using Dependency Injection.
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DI pattern
● Concept: Software design pattern where an
element, the injector, places dependent elements to
the destination according to the destination
requirements.
● Three elements:
– A dependent consumer.
– A declaration of a component's dependencies.
– An injector (aka container or provider)
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DI pattern
● Three types of DI:
– Interface injection. The dependent component provides
an interface that the consumer must implement in order
to get the dependencies at run time.
– Setter injection. The dependent consumer exposes a
setter method that the injector uses.
– Constructor injection. The dependencies are placed in the
constructor of the dependent consumer.
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IoC - DI LibrePlan
● LibrePlan uses Inversion of Control.
● LibrePlan implements Inversion of Control with the
pattern Dependency Injection.
● LibrePlan uses the Setter Dependency Injection Type.
● LibrePlan uses Spring framework as injector
(container)
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IoC – DI LibrePlan
● Spring Framework has about 20 modules.
● Core Container.
● Inside the Core Container the module for IoC and DI
is the Core and Beans.
● Where is it used IoC in LibrePlan architecture?
● Getting models from ZK page controllers.
● Getting repositories (DAO) from models.
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MVC pattern
● Model-View-Controller: It is a presentation pattern that
consists of splitting the user interface interaction in
three roles (Model, View and Controller)
● Model Role. Non-visual object containing all the
information and behavior from the application domain.
● View Role. Represents the display of the model in the
UI. Example: UI widgets or HTML. It only displays
information.
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MVC pattern
● Controller Role. It takes the user input, manipulates
the model and causes the view to update accordingly.
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MVC pattern
● Two separations:
● Presentation from the model
● Controller from the view
● Reasons separation presentation from model:
● They are different concerns.
● Depending on context you can use model in different ways.
● Non visual objects are easier to test than visual ones
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MVC pattern
● Separation controller from the view:
● Less important separation.
● This separation is usually not done.
● LibrePlan uses MVC pattern:
● It does not separate controller from view. Many times in the
controller is rendered the view (HTML).
● View and the controller split:
– Some part runs in the client browser (HTML + JS)
– Other part runs in the server (Java objects).
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MVC pattern
● LibrePlan uses the MVC pattern
● The models are injected (DI) with the Spring
container into the controllers.
● The models belong to the domain layer and are
decoupled by DI from the presentation layer.
● The models are reused sometimes:
– HTML view
– Web Services
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ZK architecture
● Component based framework.
● It abstracts widgets that are reused in different web
pages. Other: Tapestry, Wicket
● Components have two representations:
● DOM in the browser.
● Java objects in the server (POJOs)
● They are synchronized automatically by ZK
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ZK architecture
● It is a RIA framework
● Very productive.
● You do not need to program in JS.
● You have not to worry about AJAX for
synchronization.
● The best place to manage complex business logic is
the server. With ZK you have the domain accessible
from the presentation easily.
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LibrePlan and ZK
● LibrePlan uses ZK for the web interface.
● LibrePlan uses the components of ZK CE.
● LibrePlan has developed new ZK components mainly
related with the Gantt chart. Examples:
● Task component.
● Dependency component
● Milestone component
● Timetracker componentwww.libreplan.com
Object relational mapping
● Object relational mapping is a technique used for
converting the data in a relational database to the
object oriented paradigm.
● Hibernate is Java ORM that is the de facto standard in
the Java platform for persistence.
● LibrePlan uses Hibernate as ORM in the persistence
layer.
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Layers data exchange
● What is an entity?
● An entity is an object defined primarily by its
identification.
● It is anything that has continuity through a life-cycle
and distinction independent of the attributes that are
important for the application's user. Example:
Person, city, lottery ticket, etc.
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Repository pattern
● A repository represents all objects of a certain type as
a conceptual set.
● Collection with a more elaborate querying capability.
● Clients requests objects from the repository using
query methods based on criteria specified by the
client.
● Repository retrieves the requested objects,
encapsulating the machinery of database queries.
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Repository pattern
● LibrePlan uses the repository pattern.
● They are Java classes called XXXDAO
● They are implemented with Hibernate (HQL, Criteria API).
● They are managed by DI with the Spring container.
● They are injected in the domain Model classes (used in the
MVC presentation pattern)
● They are used also in the domain entities but cannot be injected
there. They are got from a Registry where they are injected.
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Repository pattern
● In LibrePlan entities are loaded in memory by two
ways:
● Using the repositories.
● By traversing the entity relationships. Two
conditions:
– Hibernate session has to be open.
– Objects must be attached.
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Layers data exchange
● Bottom layers offer services to the upper layers by
returning data.
● There are two approaches to exchange data between
subsystems: Use Data Transfer Objects or return the
own data objects of the bottom layer.
● Data Transfer Objects.
● They do not have behavior.
● Only getters and setters.
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Layers data exchange
● Data Transfer Objects:
● They are designed according to client needs.
● Usually aggregate data from several classes of the
subsystem that is returning them.
● Analyzing DTOs:
● They are expensive and time-consuming because of
the translation code (in - out)
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Layers data exchange
● Analyzing DTOs:
● They are recommendable if the data objects of the
service subsystem have inner dependencies. Most
of all, for network communications.
● They are comfortable and easy to use by the clients
because they are prepared for them.
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Layers data exchange
● LibrePlan use of DTOs in:
● Web services. They are easily converted in XML to be
send in the web services body with JAXB (Java
Architecture for XML Binding).
● Reports done with JasperReports. The reports require
to receive DTOs because of the framework internals.
● LibrePlan uses the domain objects (entities) as layer
data exchange.
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Layers data exchange
● Perils of using domain entities (Hibernate mapping
objects) as exchange data: Beware of the life cycle.
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Layers data exchange
● Perils of using domain entities (Hibernate mapping
objects) as data exchange: Being aware of the graph
depth of the objects retrieved.
● You cannot retrieve the full database (performance
issues).
● The client has to know the extent of the graph
retrieved. If not, LazyInitializationException maybe is
thrown.
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Conversations
● LibrePlan is an stateful system.
● Stateless system. At any point the output only depends on
the input.
● Stateful system. At a point in time the output depends on
the input and internal state.
● You cannot keep the transaction open between user
interactions.
● They take a long time and consume resources.
● Maybe they do not finish.www.libreplan.com
Conversations
● Conversations are designed in LibrePlan with the
pattern session-per-request-with-detached-objects.
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Conversations
● Things to take into account with conversations with
detached objects:
● Beware of reattaching objects on asking operations
from the controller.
● Avoid having duplicated objects in the session.
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Conversations
● In LibrePlan session-per-request-with-detached
objects conversations:
● They happen in the XXXModel objects.
● Model objects are injected by DI in the ZK UI
controllers.
● They have prototype scope (not singleton).
– The dependent object (XXXModel) is created by
Spring before its injection.
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Conversations
● In LibrePlan session-per-request-with-detached
objects conversations:
● There is a convention for the conversations:
– prepareForOperation()
– confirmOperation()
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OCC Pattern
● OCC (Optimistic Concurrency Control) is a pattern
that assumes that everything will be OK and that
conflicting data modifications are rare.
● OCC raises an error only at the end of a unit of work,
when data is written.
● It sets the strategy: First commit wins.
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OCC Pattern
● LibrePlan uses OCC.
● It is got help of Hibernate.
● A version field is used, which is incremented each
new update operation.
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Aggregate Pattern
● Problem: In a complex domain it is difficult to
guarantee the consistency of changes to objects with
complex associations.
● Invariants needs to be maintained in a closed group
of objects, not just discrete objects.
● Use the aggregate pattern: An aggregate is a cluster of
assocated objects that is treated as a unit for data
change operations.
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Aggregate Pattern
● The definition of an aggregate implies:
● To define a boundary.
– Defines what is inside the aggregate
● To define the root of the aggregate.
– It is a simple specific entity contained in the
aggregate
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Aggregate Pattern
● Rules of the pattern:
● The entities outside the aggregate only can hold
references to the root aggregate.
● Objects inside the root aggregate can hold
references to each other.
● Aggregates root can be got directly with
repositories. All other objects must be found by
traversal of associations.
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Aggregate Pattern
● Rules of the pattern:
● A delete operation must erase everything inside the
aggregate.
● LibrePlan uses Aggregate pattern.
● The XXXModel need to be aware of the aggregates.
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Domain Model Validation
● Problem: Many applications implement validations in
many layers. This has two drawbacks:
● Duplication.
● You may not be aware on the validation code you
have implement if the application is large.
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Domain Model Validation
● Domain Model Validation in entities: It consists of
implementing the validation logic inside domain model
entities.
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Domain Model Validation
● LibrePlan uses validation inside domain model
entities.
● LibrePlan uses Hibernate Validator to do this process
simpler.
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