Transactional Outbox Pattern Has Hidden Throughput Limits, Load Tests Reveal

The transactional outbox pattern is widely used to ensure consistency between database writes and event publishing, but a load test of 1,000 requests exposed significant production gaps. A scheduled publisher configured with a 5-second delay and a batch size of 20 rows created a hard ceiling of just 4 events per second. During the test, inbound requests arrived at roughly 14 requests per second, causing a backlog of 720 rows and a peak event delay of 191 seconds. No data was lost, but the system was structurally incapable of meeting a 30-second event-freshness SLO at any input rate above 4 requests per second. The core finding is that throughput constraints set by arbitrary configuration values must be measured and documented explicitly in SLO catalogs, not left hidden in code.