DSpace Service Manager Tutorial
The objectives of this tutorial are to provide the general DSpace Developer with an understanding of what the DSpace Service Manager is and what we are attempting to attain through its usage and some basic software development practices and design principles. The tutorial will break these practices and the overall development platform down into developer centric terms that should, if successful, give the developer a reference for how to best design their code. Goals of the Service Manager are to assist in separating the dependencies of individual functional areas of DSpace on one another, by eliminating significant dependencies on other parts of the codebase and the prevalence of "StaticManager" Classes.
The DSpace Services Framework is a back-porting of the DSpace 2.0 Development Group's work in creating a reasonable and simple "Core Services" layer for DSpace. The Services Framework provides a means for application developers to both lookup and register their own "services" or JAVA objects that can be referred to by the application.
What are services?
Answer: services are a generic term for the business actions that provide functionality that will complete a specific task in the application.
In DSpace Services are conceptually similar to OSGi Services , where an addon library (a OSGi Bundle) delivers a singleton instance of a class as a service for other application code to utilize. In OSGi the Service often has a Java Interface class and constitutes a "Contract" for the application.
From a Design Standpoint, The Service Manager is a Service Locator Pattern. This shift represents a "best practice" for new DSpace architecture and the implementation of extensions to the DSpace application. DSpace Services are best described as a "Registry" of Services that are delivered to the application for use by the use of a Spring Application Context. The original (DSpace 2.0 ) core services are the main services that make up a DSpace Service Manager system. These include services for the application "Configuration", "Transactional Context", "Requests" and user "Session", "Persistence" things like user and permissions management and storage and caching. These services can be used by any developer writing DS2 plugins (e.g. statistics), providers (e.g. authentication), or user interfaces (e.g. JSPUI).
What is OSGi?
An OSGi service is a java object instance, registered into an OSGi framework with a set of properties. Any java object can be registered as a service, but typically it implements a well-known interface. Considerable has evolved in both the Spring and OSGi Communities.
What is Spring?
Spring is an Inversion of Control (IoC) container that utilizes Dependency Injection (a fancy way of just saying that Spring creates and hands the JAVA objects you would normally have had to create in your code).
Example of Not Using Spring
As a brief example Here is a class that does not use Dependency Injection, it calls "new SomeOtherClass()" directly in its constructor.
Example Using Spring
Here is an example of a better coding practice, where we give up the responsibility for the creation of the class and allow the IoC/DI container to be responsible for its creation.
This immediately opens the door for "SomeOtherClass" to be changed out with new/other subclasses of "SomeOtherClass". In Spring, the definition "example.xml" file might look like this:
Where, when the ServiceManager is started, two beans are instantiated by Spring, and one is used in the constructor argument of the other. The simplest instantiation of a Spring Container might look like the following.
The Case For Spring
This tutorial focuses on adoption of Spring as a best practice for many aspects of DSpace development, from Core Library definition and instantiation to Application Developer implementation of customizations and addons.
- Spring focuses around providing a way to manage your business objects. (DSpace currently lacks this capability).
- Spring is both comprehensive and modular. Spring has a layered architecture, you can choose to use just about any part of it in isolation.
- It is easy to introduce Spring incrementally into existing projects. (The Latest DSpace WebMVC, REST and XMLUI development efforts already leverage Spring WebMVC in the application tier).
- Spring is designed from the ground up to help you write code that's easy to test. Spring is an ideal framework for test driven projects. (DSpace has only just introduced a JUnit Test Suite, which does not leverage Spring in its solution. However, the DSpace Service Manager already delivers a testing suite leverages Spring to support testing configuration).
- Spring is an increasingly important integration technology, its role recognized by several large vendors. By utilizing Spring, DSpace will be able to incrementally improve its architecture to be more robust, and more "enterprise grade".
The Service Manager
The ServiceManager provides the DSpace Application with the above Spring ApplicationContext so that the developer does not need to be responsible for its creation when developing against DSpace. Thus, to extend the pervious example, the DSpace class and its underlying Service Manager can be utilized to get at any object that has been instantiated as a service bean by core or addon application code.
The DSpace Service Manager implementation manages the entire lifecycle of a running DSpace application and provides access to services by Applications that may be executing external to this "kernel" of DSpace Services. Via Spring and loading of individual dspace.cfg properties the ServiceManager manages the configuration of those services, (either through providing those properties to the Spring Application Context where they can be injected in Spring definition xml files and/or "annotations" or by exposing those properties via the injection of the DSpace ConfigurationService.
The ServiceManagerSystem abstraction allows the DSpace ServiceManager to use different systems to manage its services. The current implementation is Spring Framework based. The original design intent of the Service Manager was to support more than one IoC/DI Solution, however, as work has progressed with DSpace, it has become clear that there are trade offs to consider in its usage.
- Spring Injection, when used properly, means we do not reallyneed to make "lookup calls" to a central "ServiceManager" or "DSpace" object to acquire the service beans we our own code to work with.
- Fewer lookups mean less centralized dependencies
- Fewer Centralized Dependencies means fewer bottlenecks in source Code Dependency Management.
- Fewer Bottlenecks means greater modularity and encapsulation, less need to carry around all the source code when overriding and customizing "dspace".
- More use of binary distributions means greater ease in upgrading DSpace.
It is clear that Spring has become quite dominant a solution in DSpace with the adoption of Apache Cocoon 2.2 for the Manakin XMLUI and Spring MVC for the Freemarker Prototype Webapplication currently under Development in the community.
This leads us to our first best practice:
Best Practice 1: If possible, do use Dependency Injection to acquire your services. This will be of benefit down the road if we make changes to the service manager architecture.
In creating classes that work within the system, the ServiceManager will inject those services you need for you.
Where in our spring configuration we would register this Service via the following configuration:
Spring AutoWiring looks for other bean of our specific type elsewhere in our configuration and injects them into our service. This is the basic mechanism whereby Addon Modules can reuse existing services or even services provided by other third party modules without having to explicitly depend on any specific implementation of those services.
The DSpace Application Lifecycle
The life cycle of the container and the services therein is controlled by the Web-application context or the DSpace CLI ScriptLauncher main executable in which the servlet has been deployed. The Lifecycle assures that when the Service managers (and specifically Spring in this case) are initialized, the required core and addon services are wired and made available to your application.
The ServiceManager Request Cycle
The ServiceManager Request Cycle is similar to the Webapplication Request Cycle. In the case of the Service Request, There is an incoming "Request" object with the state of the a call from the client, this Request is then enterpreted by the Container and mapped to a specific Servlet, which executes to completion, On Completion, the Serlvet Generates a
When a request is made by either the Webapplication or the CLI initialization, then the Request Lifecycle is engaged:
To use the Framework you must begin by instantiating and starting a DSpaceKernel. The kernel will give you references to the ServiceManager and the ConfigurationService. The ServiceManager can be used to get references to other services and to register services which are not part of the core set. For standalone applications, access to the kernel is provided via the Kernel Manager and the DSpace object which will locate the kernel object and allow it to be used.
The DSpace launcher ([dspace]/bin/dspace) initializes a kernel before dispatching to the selected command.
The Service Manager Interface
ConfigurationService contributed to DSpace 1.7.1 (Service Manager Version 2.0.3) And maintains Parity with the existing DSpace ConfigurationManager in supporting "dspace.cfg" and modular "config/modules/[module].cfg" configuration.
The ConfigurationService controls the external and internal configuration of DSpace 2. It reads Properties files when the kernel starts up and merges them with any dynamic configuration data which is available from the services. This service allows settings to be updated as the system is running, and also defines listeners which allow services to know when their configuration settings have changed and take action if desired. It is the central point to access and manage all the configuration settings in DSpace.
Manages the configuration of the DSpace ServiceManager system. Can be used to manage configuration for any Service Bean within the ServiceManager
Acquiring the Configuration Service
The ConfigurationService API
Benefits over the Legacy DSpace ConfigurationManager
Type Casting and Array Parsing
- Type Casting: Common Configuration Interface supports type casting of configuration values of the type required by the caller.
- Array Parsing: As part of this type casting, the Configuration Service will split comma separated values for you when you request the property as type "Array"
Modular Default Configuration
Any service can provide sane defaults in a java properties configuration file. These properties will be able to be looked up directly using a prefix as syntax.
Example of Usage:
Modularization of Configuration Not Bound to API signature.
Any service can provide overrides in the DSpace home configuration directory sane defaults in a java properties configuration file. These properties will be able to be looked up directly using a prefix as syntax.
Example of Usage:
In DSpace 1.7.0 enhanced capabilities were added to the ConfigurationManager to support the separation of of properties into individual files. The name of these files is utilized as a "prefix" to isolate properties that are defined across separate files from colliding.
Example of Usage:
Use commas for lists of values, use lookups (If you end up thinking you want to create maps in your properties, your doing it in the wrong place look instead at Spring Configuration and objectifying your configuration)
Objectifying Configuration... If you Configuration is too complex, then it probably should be an Object Model
A request is an atomic transaction in the system. It is likely to be an HTTP request in many cases but it does not have to be. This service provides DSpace with a way to manage atomic transactions so that when a request comes in which requires multiple things to happen they can either all succeed or all fail without each service attempting to manage this independently.
In a nutshell this simply allows identification of the current request and the ability to discover if it succeeded or failed when it ends. Nothing in the system will enforce usage of the service, but we encourage developers who are interacting with the system to make use of this service so they know if the request they are participating in with has succeeded or failed and can take appropriate actions.
The DSpace Session Service
The Session represents a user's session (login session) in the system. Can hold some additional attributes as needed, but the underlying implementation may limit the number and size of attributes to ensure session replication is not impacted negatively. A DSpace session is like an HttpSession (and generally is actually one) so this service is here to allow developers to find information about the current session and to access information in it. The session identifies the current user (if authenticated) so it also serves as a way to track user sessions. Since we use HttpSession directly it is easy to mirror sessions across multiple servers in order to allow for no-interruption failover for users when servers go offline.
Do not pass Http Request or Session Objects in your code. Use Dependency Injection to make the RequestService, SessionService and Configuration Service Available in your Service Classes. Or use ServiceManager lookups if your work is out of scope of the ServiceManager.
DSpace Context Service COMING SOON
The DSpace Context Service is part of the DSpace Domain Model refactoring work and provides an easy means for any Service Bean to gain access to a DSpace Context object that is in scope for the current user request cycle. This Context will be managed by the ServiceManager RequestService and represents a means to maintain a "Transactional Envelope" for attaining "Atomic" changes to DSpace (Add Item, Update Item, Edit Metadata, etc).
DSpace Legacy DataSource Service COMING SOON
Similar to the Context Service, The DSpace Legacy DataSource Service is part of the Domain Model refactoring work and bring the preexisting DSpace DataSource instantiated within the the DSpace DatabaseManager into the Spring Application Context. The exposure of the DataSource will enable Service Beans in the DSpace ServiceManager to utilize popular tools for ORM such as Hibernate, JPA2, ibatis, Spring Templates, or your own custom persistence support to be used when developing your Services for DSpace.
Test Driven Development
Test-driven development (TDD) is a software development process that relies on the repetition of a very short development cycle: first the developer writes a failing automated test case that defines a desired improvement or new function, then produces code to pass that test and finally refactors the new code to acceptable standards. Kent Beck, who is credited with having developed or 'rediscovered' the technique, stated in 2003 that TDD encourages simple designs and inspires confidence.[http://en.wikipedia.org/wiki/Test-driven_development]
We want to clarify that that the Testing Framework in the DSpace Services Module Predated the actual JUnit testing support that was added to dspace-api. Testing is a very beneficial practice where the developer writes small java based test of the code they are going to produce. The
Using the Service Manager Testing Framework
This is an abstract class which makes it easier to test execution of your service within a DSpace "Request Cycle" and includes an automatic request wrapper around every test method which will start and end a request, the default behavior is to end the request with a failure which causes a rollback and reverts the storage to the previous values