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With a lack of clearly decoupled extension points, one is left to modify the core of Geode to add new services. With no clear and consistent bootstrapping the startup dependencies are not clear. New services must not depend on different or conflicting implementations of common SPIs or different versions of common third party libraries. Without a single source of configuration management, extending configuration to a new model requires modification or extension of multiple services in Geode. Such constraints make extending Gemfire cumbersomeGeode cumbersome.
End users deploying applications into Geode have clearly defined extension points but lack the isolation from the core that would allow their application to depend on competing SPI implementations and third party library versions. This situation requires coordination between the Geode platform owner and the application owner to make sure such conflicts don’t exist. When deploying multiple applications coordination must be made between all application owners.
While Geode is not an application server in terms of J2EE it does share many of the traits that define an application server. At the very core is the concept of controlled execution of user-provided logic. While Geode blurs this line by also being a storage engine this should not exclude it from the need for many of the design patterns of an application server. Such patterns solve many of the issues we face when extending and deploying into Geode.
Goals
The main goals goal of modularization is to have a loosely coupled system with high cohesion. What this means is that modules are not tightly coupled to other modules (interchangeability), with all . All code inside of a single module is specifically there to complete a task.
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A module is a component that performs a distinct function. Each module will define/implement a public interface that allows for the interchangeability with another component. Modules can be assembled into new modules differing in size, complexity, and functionality.
Application
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A method to isolate collections of classes related to a specific deployable unit, either application or module. It guarantees that third-party dependencies of different versions or SPI implementations may be loaded within into a deployable unit without conflict with another deployable unit.
Dependency management
Dependency management is management the management of the logical relationship relationships between different activities or modules. At the simplest case, it is the management of dependencies that a module’s implementation has on the libraries it uses. It is also the management of the relationship between different modules. Relationships are usually defined as as "depends-on" or "has-dependants-of". Note: A module can never be both the dependency or dependantdependent. This is called a cyclical dependency and should be avoided.
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Dependency injection is a form of Inversion of Control. It aids with the decoupling of modules in manner a manner that a dependent service is only injected into the module at runtime. Allowing for the injection of different modules satisfying the interface to be interchanged. I.e no tight coupling to services
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A module is a self-contained unit or item. Each modules will module will be composed of the following:
Module application code
Public module services and interfaces
Dependant Dependent libraries (incl. version)
Dependant Dependent module references (incl. version)
The benefit for modularity is that each module can be interchanged with another implementing module, provided it adheres to the publically exposed interface. In addition to this, each module can be tested as a black box. This allows for better, more precise and targeted testing.
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Each module will be isolated from any other module by a lightweight container that provides class loader isolation. A module may contain all third party libraries it depends on or may call out dependencies on libraries provided commonly shipped with the platform, like Log4J or Commons Collections. Such dependencies will include a version of range versions that must be met to satisfy the external dependency to better avoid dependency version incompatibility. Two or more versions of the same third party module may be deployed. A module will also contain the classes that make of up the core of the module’s functionality.
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As can be seen in the diagram, ModuleA, ModuleB, and ModuleC each have different library dependencies and different library version dependencies. Due to the isolation, the different libraries or versions do not affect one another.
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Not only will a module list implementation dependencies described by isolation but also dependencies of instantiation. If a module depends on the the on the instance of another module this dependency will be described such that the bootstrapper can instantiate and inject a at the appropriate time. Dependencies will be injected into the instantiation instance of the module.
Configuration
Each module shall expose a single authoritative view of its configuration. All configuration services within the platform shall manipulate the exposed configuration of the module and not require modification to the configuration services. This means that if module A is deployed then the XML config, cluster config, rest admin, JMX and gfsh GFSH interfaces should all expose the configuration of A without explicit modification to either of those services. The configuration should communicate instance dependencies within the global and application domains.
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A module shall have lifecycle states as either stopped or started as seen from any other module. A module in a started state is guaranteed that all it’s its dependencies are also in the started stated. If any dependency is being transitioned to the stopped state then the dependent module will be transitioned to a stopped state first. The lifecycle manager shall maintain the state of each module and it’s its dependencies. A module must not signal completion of its started state transition until all internal state is in a consistent state such that a call to any method on the service provides an expected and determinate outcome.
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