# ADR 0005. Phased Event-Log Infrastructure

## Status
Accepted.

Phase 2 is triggered by spec §35.4 measurements, not a calendar date:
sustained inbox write/worker saturation, independent consumers that need
replay, lag that must be isolated, material ordering/partitioning,
backfills competing with live ingest, or operational benefit that exceeds
running another stateful platform. Dagster is the initial batch/backfill
scheduler; Airflow is not the default (spec §35.1).

## Context
PlayerPal must be cheap to run at low volume and scale to high volume
without a redesign. Committing to Kafka/Redpanda on day one adds
operational overhead (cluster management, JVM footprint or equivalent,
monitoring, minimum viable cluster size) that isn't justified at low
traffic. By contrast, an ordinary application log file lacks replication,
consumer offsets, replay tooling, and backpressure handling. It isn't
a substitute for a real durable log at higher volume.

## Decision
Two-phase approach:

**Phase 1 (lean MVP):** `API -> Postgres append table -> workers`, or
for batch-heavy activity, `API -> object storage append/batches ->
workers`. Appropriate while volume is low/moderate and operational
simplicity matters most.

**Phase 2 (real event log):** `API -> Redpanda/Kafka-compatible log ->
reward worker / analytics writer / archival`. Move here when traffic
growth is sustained, more independent consumers appear, replay becomes
important, or Postgres ingestion becomes a bottleneck.

Redpanda is the leading Kafka-compatible candidate (C++, no JVM, no
ZooKeeper, generally lower operational overhead) but its licensing for
managed/enterprise tiers is **not confirmed free** and must be checked
against current terms before committing. Self-hosting vs. managed is
a real cost decision, not just a technical one.

## Consequences
- No premature complexity at low volume.
- Phase 2 starts when a §35.4 condition is observed in production metrics,
  not when a date or guessed events/sec target arrives. Record the measured
  evidence in a new ADR before introducing Redpanda. Confirm Redpanda
  licensing at that time; it is still unchecked.