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The Persistence Plugin Architecture is intended to address decoupling persistence implementations from the Core Data code base. EdgeX Foundry code base. The core-data, core-metadata, export-client, support-logging and support-notifications services all currently rely on database persistence. Today (Delhi release), persistence implementations are part and parcel of the EdgeX Golang Services repo. As a result, adding a new persistence implementation or updating an existing implementation requires a full test of the platform. For a future (Edinburgh) release, the goal is to move persistence implementations into their own repos so they can be maintained independently of the EdgeX code base. 

The architecture depends on Go Plugins. 

all team members to have knowledge of every supported persistence platform as well as access to an installation of each in order to fully test any changes. New capabilities must be added to every persistence implementation before they can be called complete. This is not scalable. For a future (Edinburgh) release, the goal is to move persistence implementations into their own repos so they can be maintained independently of the EdgeX code base. 

The proposed architectural solution utilizes Go Plugins.

Within each service, a configuration value will drive which persistence plugin to load, similar to what we do now. This value can be the same across all services or even different for every service should a customer feel that one or another platform is best suited to the role the service plays in their environment. Each service defines an interface for the necessary database operations (core-data, core-metadata) and then a vendor's plugin must conform to and support that interface.

User Stories

1. As a persistence implementation provider (a provider), I want a mechanism so that I can add a new persistence implementation to the EdgeX platform.
2. As a provider, I want a mechanism for sustaining a persistence implementation so that I do not rely on EdgeX platform releases.
3. As a provider, I want a mechanism for configuring my implementation without knowing the rest of the EdgeX platform configuration.
4. As a provider, I want the EdgeX platform build scripts to consolidate my build-time and run-time requirements so that I can maintain my implementation independently of the EdgeX platform.
5. As a provider, I want an API contact so that I know what to implement.
6. As a provider, I want an API contract so that I can unit test.
7. As a provider, I want EdgeX platform testing to be able to certify implementation so that EdgeX platform users will the confidence to use my implementation.
8. As an EdgeX platform maintainer (a maintainer), I want a default persistence implementation so that I can maintain CI/CT.
9. As a maintainer, I want providers to create reasonable defaults so that I can add them into CI/CT.
10. As an EdgeX platform user (a user), I want to select one or more persistence implementations for use by the various Core Data services.
11. As a user, I want the option to run the platform along with the selected persistence implementation(s) directly on the OS.
12. As a user, I want the option to run the platform along with the selected persistence implementation(s) in a containerized deployment.

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The basis of the requirements came out of looking at how others have solved this challenge. . Also, note Vladimir Vivien's helpful article on "Writing Modular Go Programs with Plugins".

Azure IoT Edge

Azure IoT Edge is organized around independently configured modules (aka containers) that are grouped to form a solution. Each module is described independently and then combined into a solution with a deployment manifest. For example, the RedisEdge module is described using the following module.json. 

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