Add indexer ADR

docs/adrs/0022-query-service-indexer.md

@@ -0,0 +1,112 @@
+# 22. Query Service Indexer
+
+Date: 2023-08-18
+
+## Status
+
+Draft
+
+## Context
+
+Our solution for cross-cluster querying has evolved to support some powerful querying patterns (complex AND/OR logic, full-text search), but may be deficient for the next level of capabilities that our (internal) users are asking for.
+
+At present, we can do fairly complex query logic with **normalized fields**:
+
+```golang
+
+// Given these objects
+objects := []models.Object{
+    {
+        Cluster:    "management",
+        Name:       "podinfo-a",
+        Namespace:  "namespace-a",
+        Kind:       "HelmChart",
+        APIGroup:   "apps",
+        APIVersion: "v1",
+    },
+    {
+        Cluster:    "management",
+        Name:       "podinfo-b",
+        Namespace:  "namespace-a",
+        Kind:       "HelmRepository",
+        APIGroup:   "apps",
+        APIVersion: "v1",
+    },
+    {
+        Cluster:    "management",
+        Name:       "podinfo-d",
+        Namespace:  "namespace-b",
+        Kind:       "HelmRepository",
+        APIGroup:   "apps",
+        APIVersion: "v1",
+    },
+}
+
+
+qs := NewQueryService(...)
+
+
+q := &query{filters: []string{
+    // Regex-style query where we can add the OR logic within a field.
+    "kind:/(HelmChart|HelmRepository)/",
+    // Top-level fields are AND'd together.
+    "namespace:namespace-a",
+}}
+
+
+result := qs.RunQuery(q)
+// result will have "podinfo-a", "podinfo-b", but not "podinfo-d" since it is not in namespace-a
+```
+
+This works for enumerable fields, but there may be cases where we need to query against fields for which we do not know the key/value pairs. For example, if we want to query for labels on an object we run into a couple issues:
+
+- We don't know the fields for a given object's labels before adding them to the indexer
+- Each object's labels have their own key/value pairs that are only relevant in the context of one object, or more realistically, one `kind` of object
+
+An example of two normalized objects with different label pairs:
+
+```golang
+objects := []models.Object{
+    {
+        Cluster:    "management",
+        Name:       "podinfo-a",
+        Namespace:  "namespace-a",
+        Kind:       "HelmChart",
+        APIGroup:   "apps",
+        APIVersion: "v1",
+        Labels: map[string]string{
+            "weave.works/template": "true",
+        }
+    },
+    {
+        Cluster:    "management",
+        Name:       "podinfo-b",
+        Namespace:  "namespace-a",
+        Kind:       "HelmRepository",
+        APIGroup:   "apps",
+        APIVersion: "v1",
+        Labels: map[string]string{
+            "other.org.com/somekey": "othervalue",
+        }
+    },
+}
+```
+
+Given this data ^^, here is the type of query we would like to support:
+
+```golang
+q := &query{
+    labels: map[string]string{
+        "weave.works/template": "true",
+    }
+    filters: []string{
+        "kind:/(HelmChart|HelmRepository)/",
+    },
+}
+```
+
+The main issue with this query is that [bleve](https://blevesearch.com/), our current indexing/querying solution is not capable of doing this type of query, or if it is, the documentation available does not explain how it can be done.
+
+## Decision
+
+## Consequences