Message ordering preserves campaign sequence

When multiple workers process a drip sequence concurrently, the order emails land in a recipient's inbox is determined by worker timing and network latency, not the sequence design. Email 3 can arrive before email 2 if workers race. For onboarding sequences where each message references the previous one, out-of-order delivery breaks the narrative and confuses recipients. CWE-362 covers race conditions on shared resources — concurrent workers competing for sequence jobs are precisely that. ISO-25010:2011 reliability.fault-tolerance requires the system to behave predictably under concurrent execution.

Schedule sequence jobs with explicit millisecond-separated delays to guarantee ESP ordering, or use BullMQ FlowProducer for strict job chains:

const DAY = 24 * 60 * 60 * 1000
await emailQueue.add('seq:step-1', { campaignId, recipientId }, { delay: 0 })
await emailQueue.add('seq:step-2', { campaignId, recipientId }, { delay: 3 * DAY })
await emailQueue.add('seq:step-3', { campaignId, recipientId }, { delay: 7 * DAY })

For same-day sequences where delay alone is insufficient, use FlowProducer in lib/flows.ts to enforce strict parent-child dependency between sequence steps.