Example with sample data — read-only. Not a real team.
PRB-142 Decided
Add a Redis cache to the orders service?
Order lookups in the orders service take about 350 ms at p95 during peak hours, and support wants faster order pages. The team agreed on one rule for this quarter: no new infrastructure.
Options and arguments
Add a Redis cache
2 for2 against
- ForCached lookups would answer in a few milliseconds.
- ForMost lookups ask for the same recent orders.Evidence: APM trace
- AgainstBreaks the team rule: no new infrastructure this quarter.
- AgainstInvalidating the cache on every status change is a new way to fail.
Add a Postgres read replica
1 for1 against
- ForMoves read load off the primary database.
- AgainstThe time goes into the query plan, not into database load.Evidence: Load test, 12 Oct
Index the order-lookup queries
Added after the AI review
1 for0 against
- ForThe slow lookup scans the whole orders table; an index avoids that.Evidence: APM trace
AI review
Sample AI review — written for this example, no AI ran.
- Assumption“Reads are the bottleneck” — the load test points at the query plan instead.CitesDescriptionEvidence
- Contradiction“No new infrastructure this quarter” conflicts with the Redis option.CitesArgumentOption
- SuggestionMissing option: index the order-lookup queries.CitesDescriptionEvidence
Decision
Decided Recorded by the team, with its rationale
- Chosen
- Index the order-lookup queries first.
- Rationale
- The trace and the load test show the time going into a full scan of the orders table. An index fixes that within the team's rule, with no new system to run.
- Not chosen
- Add a Redis cache: no new infrastructure this quarter, and a cache would hide the slow query.
- Add a Postgres read replica: the slow part is the query, not the load.
- Reconsider if
- p95 stays above 200 ms after the indexes ship.
What a coding agent receives
The decision becomes a rule in the team's Agent Pack. An agent asks for it over MCP, or reads it from the downloaded pack.
--- name: rationode-prb-142 description: Use when changing how the orders service reads orders. --- PRB-142 · Decided No Redis in the orders service. Index the order-lookup queries first. Why: no new infrastructure this quarter, and the slow part is the query plan, not the read volume. Reconsider if p95 stays above 200 ms after the indexes ship.