Note: The JIRA issue for this work is FCREPO-648

Problem Statement

Fedora's original Server and Module classes were designed in 2002, and provided a common way for major functional components ("modules") of the repository to be plugged in, configured, initialized, and stopped. Problems with the existing framework include:

• It's homegrown. Better, more widely-understood frameworks have come along.
• Unit testing is unnecessarily complicated. The base Module class depends on a Server instance being available in order to function, and the Server base class is not easily mocked.

Requirements

Use standard, well-known frameworks/libraries to:

• Resolve inter-module dependencies via dependency injection
• Provide hooks to initialize/de-initialize modules when the webapp container starts and stops
• Allow re-configuration and plugging in of alternative modules without re-compiling

Non-requirements

This work will NOT attempt to:

• Provide a way to dynamically re-configure modules without restarting
• Provide the ability to run modules in their own classloader space

These capabilities may be added in the future, possibly with the help of OSGi.

Framework Choice

After analyzing the available options, we have selected the Spring framework, version 3.

Popular frameworks that support the dependency injection pattern include Spring, PicoContainer, and Guice.

How do they compare? Several articles have been written comparing Spring and Guice, as well as all three. As many have pointed out, Spring and Guice are more than DI frameworks. For our purposes, we considered the attributes of each that are most relevant to the problem at hand:

Spring was selected because:

• It provides an out-of-box and commonly-used way to wire and configure modules ("beans"), outside of code.
• It supports JSR-250 annotations for module lifecycle hooks
• Its OSGi-friendliness is well-documented

Implementation Strategy/Principles

• Prefer constructor injection to setter injection
• Minimize coupling to DI framework
  • Use JSR-250 @PostConstruct/@PreDestroy lifecycle hooks when needed
  • Avoid use of framework-specific interfaces, classes, and annotations
• Minimize changes to existing Fedora functionality

Implementation Plan

Overview + Discussion

View presentation from March 16th, 2010 Special Topic Meeting

Phase I - Prepare

• List all dependents of existing module interfaces
• Identify and remove unused modules and classes with module dependencies.
  • Remove ReportServlet - FCREPO-646
  • Remove ThreadMonitor - FCREPO-647
• Identify circular module dependencies (noted with on above page).

Modify existing modules to accept injected dependencies and config values

• Decouple module interface impl from Module abstract class where needed
• Push param validation responsibility down to each impl (not in Module)
• Use constructor injection if possible. For those with circular dependencies that can't be refactored easily, provide setters.
• Where existing modules look at configuration of other modules, get the configuration value from a getter in the interface, not the configuration.
• Where existing modules look at global fcfg values, make those available via bean-style class, GlobalConfig.
• Where existing modules look at datastore fcfg values, inject the connectionpool or config values directly.
• Constructors for impls should do as much arg validation/setup as they can.  If they can't do it all, it should be done in a @PostConstruct void init() method.  In either case, if validation or setup fails, an unchecked exception should be thrown, as per JSR-250.
• Where de-initialization is needed, a @PreDestroy void destroy() method should be used. Errors encountered during de-initialization should be logged by this method, and an unchecked exception should be thrown, as per JSR-250.

  Tasklist
  Modifications Needed Modifications Needed
  ||Completed||Priority||Locked||CreatedDate||CompletedDate||Assignee||Name|| |F|M|F|1268080202735| |cwilper|Add and populate GlobalConfig| |F|M|F|1268080236272| |cwilper|Update org.fcrepo.server.access.DefaultAccess| |F|M|F|1268080257979| |cwilper|Update org.fcrepo.server.access.DynamicAccessModule| |F|M|F|1268080268001| |cwilper|Update org.fcrepo.server.journal.Journaler| |F|M|F|1268080276055| |cwilper|Update org.fcrepo.server.management.BasicPIDGenerator| |F|M|F|1268080285519| |cwilper|Update org.fcrepo.server.management.ManagementModule| |F|M|F|1268080327906| |cwilper|Update org.fcrepo.server.messaging.MessagingModule| |F|M|F|1268080339553| |cwilper|Update org.fcrepo.server.oai.FedoraOAIProviderModule| |F|M|F|1268080350944| |cwilper|Update org.fcrepo.server.resourceIndex.ResourceIndexModule| |F|M|F|1268080374403| |cwilper|Update org.fcrepo.server.search.FieldSearchSQLModule| |F|M|F|1268080381223| |cwilper|Update org.fcrepo.server.security.DefaultAuthorization| |F|M|F|1268080399963| |cwilper|Update org.fcrepo.server.security.DefaultBackendSecurity| |F|M|F|1268080425721| |cwilper|Update org.fcrepo.server.storage.ConnectionPoolManagerImpl| |F|M|F|1268080433280| |cwilper|Update org.fcrepo.server.storage.DefaultDOManager| |F|M|F|1268080461067| |cwilper|Update org.fcrepo.server.storage.DefaultExternalContentManager| |F|M|F|1268082094738| |cwilper|Update org.fcrepo.server.storage.lowlevel.akubra.AkubraLowlevelStorageModule| |F|M|F|1268082154104| |nigel thomas|Update org.fcrepo.server.storage.lowlevel.DefaultLowlevelStorageModule| |F|M|F|1268082165864| |cwilper|Update org.fcrepo.server.storage.translation.DOTranslationModule| |F|M|F|1268082180490| |cwilper|Update org.fcrepo.server.validation.DOValidatorModule|

Phase II - Swap

• Decide on DI framework: Spring 3
• Convert fcfg to DI configuration and update installer to populate it instead
• Trigger initialization of module singletons via DI framework in context initialization
• Use injected module dependencies wherever possible, avoiding use of Module and Server at runtime

Phase III - Cleanup

• Get rid of Module, Server, and subclasses
• Get rid of everything else that parses/looks at fcfg