feat(docs): add policy ADR for LIST limit and pagination (#1557)

Related to #55

---------

Co-authored-by: Ryan Schumacher <jschumacher@virtru.com>

service/policy/adr/0002-pagination-list-rpcs.md

@@ -0,0 +1,177 @@
+# Pagination in policy LIST RPCs
+
+## Table of Contents
+
+- [Background](#background)
+- [Chosen Option](#chosen-option)
+- [Considered Options](#considered-options)
+  - [LIMIT + OFFSET](#limit--offset)
+  - [Keyset Pagination](#keyset-pagination)
+  - [Cursor Pagination](#cursor-pagination)
+
+## Background
+
+At present, policy LIST RPCs are completely open-ended.
+
+Attribute Namespaces, Definitions, and Values LIST calls may be filtered by _active_ state.
+
+All Policy objects may be retrieved without quantity limits. This presents a challenge at scale if there
+are a very large number of any policy object in the platform database when responses become overwhelmingly
+large.
+
+Introduction of a `limit` on retrieved items in LIST procedure call responses necessitates the simultaneous introduction of
+pagination. This ADR clarifies the unified approach we will take within policy service LIST RPCs
+and at the database level for this pagination.
+
+## Chosen Option
+
+[LIMIT + OFFSET](#limit--offset)
+
+Because we do not know the likelihood of platforms running with Policy where any individual object has
+enough rows to experience the at-scale performance concerns of `offset` pagination, we will prefer
+this simple implementation for now and leave the door open for cursor-based pagination to solve the performance
+constraint should it be a realized problem in the future.
+
+## Considered Options
+
+### LIMIT + OFFSET
+
+The simplest approach is a simple update to the proto for LIST RPCs and db queries to take in `limit` and `offset` with default values.
+
+```proto
+message ListRequest {
+    // ...existing fields omitted
+    int32 limit = 3; // default depends on type of policy object
+    int32 offset = 4; // default: 0
+}
+message ListResponse {
+    // ...existing fields omitted
+    int32 total = 5; // indication of total available for pagination
+}
+```
+
+```sql
+-- subject-mappings example request:
+--   'limit' 100
+--   'offset' 100
+SELECT * FROM opentdf_policy.subject_mappings
+ORDER BY created_at
+LIMIT 100 OFFSET 100
+```
+
+#### Pros & Cons
+
+- :green_circle: Simple - support across any SQL database (just slightly different syntax)
+- :green_circle: Stateless - each request can independently paginate by specifying LIMIT / OFFSET
+- :green_circle: Flexibile - random-access pagination supported
+- :green_circle: Familiar - standard across LIST-type APIs
+- :yellow_circle: Create/Update/Delete of data between requests may throw off pages, but this is a relatively small concern when reads are exponentially more frequent than writes in Policy
+- :red_circle: Performance: large number of objects _or_ a high offset mean a lot of rows need to be scanned and discarded (skipped). However, (:yellow_circle:) we do not know how often the scale of policy objects will be large enough for this to be a problem
+
+> [!NOTE]
+> Pagination is roughly Big O(n) time complexity as offset increases
+
+### Keyset Pagination
+
+We would index a column (the most obvious would be `created_at`) to use as the pagination key for
+querying, and facilitate pagination before/after any arbitrary timestamp.
+
+```proto
+message ListRequest {
+    // ...existing fields omitted
+    int32 limit = 3; // default depends on type of policy object
+    google.protobuf.Timestamp after = 4; // default: start_of_time
+    int32 total = 5; // indication of total that can be paginated through
+}
+message ListResponse {
+    // ...existing fields omitted
+    int32 total = 5; // indication of total available for pagination
+}
+```
+
+```sql
+-- subject-mappings example request:
+--   'after' 2023-01-01
+--   'limit' 100
+SELECT * FROM opentdf_policy.subject_mappings
+WHERE created_at > '2023-01-01' ORDER BY created_at LIMIT 100;
+```
+
+#### Pros & Cons
+
+- :green_circle: Support - supported across any SQL database (just slightly different syntax)
+- :green_circle: Speed - much faster in deep pages than OFFSET due to reduced scan row count
+- :yellow_circle: Reliability - provisioned policy may contain the same `created_at` timestamp
+- :red_circle: Flexibility - pagination is only forward of the `created_at` timestamp
+- :red_circle: Complexity - client must maintain state since response timestamps are required to drive subsequent request timestamp pagination, and pagination backwards is not supported
+- :red_circle: Complexity - reliance on timestamps introduces timezone differential confusion unless a parameter is also employed to localize the query
+
+### Cursor Pagination
+
+We would index a column (the most obvious would be `created_at`) to use as the pagination key for
+querying, but we would utilize an encoded cursor approach.
+
+```proto
+message ListRequest {
+    // ...existing fields omitted
+    int32 limit = 3; // default depends on type of policy object
+    string cursor = 4; // defaulted in API layer to cursor for encoded start_of_time
+}
+
+message ListResponse {
+    // ...existing fields and response data ommitted
+    // cursors are encoded by the server as base64'd 'created_at' timestamps
+    string previous_cursor = 4;
+    string next_cursor = 4;
+    int32 total = 5; // indication of total available for pagination
+}
+```
+
+```sql
+-- subject-mappings example, request:
+--   'after_cursor' 2023-01-01 00:00:00.000000+00
+--   'limit' 100
+WITH Data AS (
+    SELECT *
+    FROM opentdf_policy.subject_mappings
+    WHERE created_at >= '2023-01-01 00:00:00.000000+00'
+    ORDER BY created_at
+    LIMIT 101
+),
+NextPage AS (
+    SELECT *
+    FROM Data
+    ORDER BY created_at
+    LIMIT 100
+),
+PreviousPage AS (
+    SELECT *
+    FROM opentdf_policy.subject_mappings
+    WHERE created_at < (SELECT MIN(created_at) FROM Data)
+    ORDER BY created_at DESC
+    LIMIT 101
+),
+CursorData AS (
+    SELECT
+        (SELECT MIN(created_at) FROM Data) AS first_item_created_at,
+        (SELECT MAX(created_at) FROM NextPage) AS next_cursor_created_at,
+        (SELECT MIN(created_at) FROM PreviousPage) AS previous_cursor_created_at
+)
+SELECT
+    (SELECT json_agg(row_to_json(NextPage)) FROM NextPage) AS data,
+    (SELECT json_build_object('created_at', next_cursor_created_at) FROM CursorData) AS next_cursor,
+    (SELECT json_build_object('created_at', previous_cursor_created_at) FROM CursorData) AS previous_cursor
+FROM CursorData;
+```
+
+#### Pros & Cons
+
+- :green_circle: Support - supported across any SQL database (just different syntax)
+- :green_circle: Speed - much faster in deep pages than OFFSET due to reduced scan row count
+- :green_circle: Flexibility - pagination _a single page_ backward made possible by response `previous_cursor` value
+- :green_circle: Complexity - timestamp timezone differential is not a problem as cursors are server-determined and an API concern
+- :yellow_circle:/:red_circle: Reliability - provisioned policy will sometimes contain the same `created_at` timestamp, making it less than 100% reliable
+- :red_circle: New index on the `created_at` timestamp required which adds overhead but little value for management with
+time pretty much irrelevant to attributes except if required for sorting
+- :red_circle: Complexity - SQL queries become significantly more complex to build and read into responses
+- :red_circle: Flexibility - random access is still not supported without client state management and prior knowledge of forward pagination's historical cursors