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Please note that as of the Nolana release (Q3 2022) the Codex app was retired - these pages may still reference the app and associated modules. |
What is Codex?
Codex is a normalization and virtualizationlayer that allows Folio to integrate metadata about various resources regardless of format, encoding, or storage location. It is the piece that allows disparate resources to be surfaced using a common vocabulary and description.
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But in practice, domains, and the microservices they contain, form a system and there are necessary interactions between the microservices, including across domain boundaries. Those interactions create dependencies, and in a flat model, this leads to entanglement - thus breaking the promise of a loosely coupled system.
The Entanglement Problem
The interaction between microservices (i.e. modules) is an integration problem. It requires that individual modules be aware of other modules. For modules in the same domain, this is expected and not an issue. However, the same is not true across domain boundaries.
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In the general case, if a Folio system has N resource managing modules, the number of potential integration points is N(N-1)/2, which is not linear, but scales on the order of N2.
A different approach is to use a brokered integration. A new service can act as a coordinator between the different related modules. Each module in turn needs only to integrate with the brokering module. The problem of managing the state and availability of the different services can be achieved in one place. This allows the individual modules to “keep minding their own business” and remain blissfully unaware of other modules’ business. A brokering microservice does need to be aware of the other modules, because that is its business.
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Codex is a hierarchical domain that acts as a coordinator for resource management. As a coordination domain it is Codex’s business to know about the other domains.
A Hierarchical Domain Model
A significant responsibility of Folio is to manage resources, and more specifically the records that describe those resources. However, not all parts of Folio require the same level of detail for those resources. Nor do they have the same scope requirements: Circulation doesn’t much care about the purchase price of an item, but Acquisitions certainly does. The following diagram focuses on resource management specifically. As shown, the resource managing domains can be organized into layers based on the depth of detail that they require and the purpose they serve.
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