Document initial architectural decisions related to GDT

docs/architecture-decisions/0002-extend-timdex-ui-for-gis-data.md

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+# 2. Extend TIMDEX UI for GIS data
+
+Date: 2024-01-24
+
+## Status
+
+Accepted
+
+## Context
+
+The [GIS Data Tool (GDT) project](https://mitlibraries.atlassian.net/jira/software/c/projects/GDT/boards/225)
+requires a discovery interface for GIS data. The project team has made the decision to extend TIMDEX UI for this
+purpose.
+
+This approach necessarily adds some complexity to the TIMDEX UI codebase. Generally, the UI design changes proposed for
+GDT fall into three categories:
+
+* Changes that are unique to the GDT app (e.g., app name in the header, 'Ask GIS' sidebar widget);
+* Changes that should apply to all TIMDEX UI apps (e.g., filter sidebar redesign, IA enhancements); and
+* Changes that are currenty unique to the GDT app, but may later apply other other TIMDEX UI apps (e.g., geospatial
+querying, filters that are specific to geospatial data).
+
+General UI enhancements should be straightforward to implement across all TIMDEX UI applications. However, we will also
+need to develop the GDT UI without disrupting existing TIMDEX UI implementations, while considering the future
+possibility of integrating geospatial search into those applications.
+
+These are the potential solutions we have considered:
+
+### Feature flags
+
+Using the [`flipflop` gem](https://github.com/voormedia/flipflop), we can add a `GDT` feature that is toggled by an
+environment variable. We've implemented this in many of our Rails apps and found that it's an effective way to manage
+multiple versions of an application (most recently, in TIMDEX API). The downside of using `flipflop` is that it does
+not appear to be especially well maintained, but we're familiar with it and have used it successfully in the past.
+
+### Index detection
+
+Since geospatial records will be in a separate index at first, we can check for the index the app is using (currently
+defined in in an environment variable) to determine which UI elements to display and which type of query to construct.
+This would solve the problem similarly to feature flags. It does not account for the possible future state of geospatial
+data existing in other indices, but it could solve the immediate need of maintaining GDT features separately.
+
+### Search params
+
+Bento uses a `target` param to identify which source is being queried. We could implement a similar design pattern in
+TIMDEX UI to distinguish between geospatial queries and 'general' queries. This would likely include a boolean in the
+search form that would trigger a geospatial query. In addition to providing the query target, we could use the param
+value to evaluate whether to add GIS-specific UI elements to the results view.
+
+Like index detection, this is duplicative of the feature flags approach, but it would allow us to achieve a similar
+effect without using an external dependency. However, this would not be a consistent solution for the full record view,
+as direct linking is a common use case for full records, and we may not be able to guarantee the presence of a `gis=true`
+param in these links.
+
+One use case where search params could be useful is the potential future state in which `search.libraries.mit.edu` (or
+another TIMDEX UI app) includes GIS data in search results. In that instance, it may be valuable to allow users to
+choose whether they want geospatial data queried in their search.
+
+### Data detection
+
+We could check for the presence of certain fields in the API response to determine which UI elements to display. If
+geospatial records are in the results, then GDT-specific fields would appear in the brief and full record, and
+GDT-specific filters would appear in the sidebar. This would help address the use case of adding geospatial data to
+indices searched by non-GDT applications. 
+
+### Theme gem
+
+We can use the theme gem to conditionally modify certain views that are shared across our Rails properties. This would
+solve the problem of customizing the header, but not views that are specific to the GDT UI.
+
+## Decisions
+
+We will extend TIMDEX UI for GIS data using a combination of the solutions described above:
+
+* We will create a separate Heroku app in the TIMDEX UI pipeline for GDT.
+* Where possible and reasonable, local header customizations will be controlled by the theme gem.
+* We will use `flipflop` to implement a GDT feature flag, which will toggle GDT-specific features.
+* We will add universal UI improvements to all TIMDEX UI implementations.
+* In the future, we will consider adding a `gis` search param and/or a data detection enhancer as needed.
+
+If we decide to add geospatial data  to other TIMDEX UI applications in the future, we can decide whether a search param
+or data detection approach will be more effective. (Note that this decision will likely require us to modify how our
+feature flag is implemented, or to replace it altogether.)
+
+We will not use index detection, as it does not address any additional use cases.
+
+## Consequences
+
+Using the theme gem for header customizations may be useful for other applications. We can minimize repetition by
+managing these changes in environment variables read by the theme gem, but we should be wary of overabstraction. We will
+try this with adding the app name to the header to start, and carefully consider whether it may be useful for other UI
+elements.
+
+The feature flag will serve as an initial solution to isolating GDT UI features. Because we have used `flipflop` before,
+it should not significantly impact development time, which is an important consideration at this point in the project.
+
+There may be additional use cases we haven't yet considered that could impact this decision. We should remain open to
+changing our approach as the project develops.