doc: add adr regarding full text search

docs/decisions/0008-full-text-search.md

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+# Full Text Search
+
+## Context and Problem Statement
+
+Users should be able to find things by name / label. The PodOS navigation bar
+should be enhanced to a search bar, where users can enter plain text instead of
+full URIs. Also users shall be able to link things to one another and there need
+to find them by a name they remember instead of a crypting URI.
+
+## Considered Options
+
+### Search execution
+
+- Comunica Link Traversal
+- SPARQL search via rdflib.js
+- SPARQL search via Comunica
+- Lunr.js
+
+### Search index
+
+- index the whole Pod
+- index anything the users visits
+- build index manually by users choice
+
+## Decision Outcome
+
+Build a manual index by users choice and perform a search via Lunr.js.
+
+The index could be persisted in a turtle document linked from the users profile.
+PodOS can find and load this document on startup and create a Lunr index from it.
+
+### Reasoning
+
+#### Comunica Link Traversal
+
+This is very powerful, since it is able to traverse search the whole Pod and
+even follow links to other Pods, but it is two slow to be practically usable for
+a type-ahead search. It can be considered for advanced searches later on.
+
+#### SPARQL search via rdflib.js
+
+rdflib.js only supports a subset of SPARQL and cannot do e.g. filtering by
+regex, therefore it is not feasible for the task.
+
+#### SPARQL search via Comunica
+
+Comunica can do SPARQL with filter by regex, but the rdflib.js store we are
+currently using is not compatible to the current RDF/JS Source interface
+Comunica requires. Also, SPARQL in general seems not to support fuzzy matching
+to ignore e.g. small typos. Search resuls do not have a score.
+
+#### Lunr.js
+
+This is not an RDF specific solution, but a quite powerful client-side search
+tool. A Lunr index could be created out of RDF data. It also turned out to be
+quite fast and bulit an index of 10.000 documents in 1 second. Also Lunr search
+results contain a score to be able to order resuls by how good they match the
+query.
+
+#### Index the whole Pod
+
+This would take a long time. Also, there might be things users do not want to
+have in the index.
+
+#### Index anything the users visits
+
+The index would basically contain the whole browsing history of the user, which
+raises privacy issues. Also, things might be indexed that are irrelevant for a
+later search.
+
+#### Build index manually by users choice
+
+This gives the user most control over what to index and what not, and it prevents
+finding irrelevant stuff later.
+
+### Positive Consequences
+
+- Fast search
+- Find only things that are relevant
+
+### Negative Consequences
+
+- User has to add things manually to the search index
+- Lunr.js has an immutable index which needs to be re-created anytime things change.
+
+## Links
+
+- [Lunr.js](https://lunrjs.com)
+- [Comunica](https://comunica.dev)
+- [RDF/JS Source Interface](https://rdf.js.org/stream-spec/#source-interface)
+- [Comunica Link Traversal](https://comunica.dev/research/link_traversal/)