System Design: Multi-tenant CI/CD Platform

Context

Design a hosted, multi-tenant CI/CD platform that supports many organizations, repositories, and regions. Assume cloud infrastructure, strong security requirements, and the need to scale elastically with predictable performance across tenants.

Requirements

Describe and justify an end-to-end architecture that addresses:

1. Queuing, Scheduling, and Partitioning
   • How to queue and schedule builds.
   • Partitioning by tenant, region, and repository.
   • Isolation to prevent noisy neighbors.
2. Runners/Executors and Scaling
   • Horizontal scaling of runners/executors.
   • Autoscaling policies and formulas.
   • Backpressure and admission control.
3. Storage and State
   • Storage for artifacts, logs, and metadata.
   • Secret management.
   • Per-tenant quotas and rate limits.
4. Deployment Flow Features
   • Rollbacks, canary releases, and approvals.
5. Reliability and Resilience
   • Retries, idempotency, deduplication.
   • Disaster recovery and multi-region failover.
6. APIs and Data Models
   • APIs and core data models for pipelines, stages/jobs, permissions, and runs.
7. Observability and Cost
   • Monitoring, alerting, SLOs.
   • Cost controls and chargeback/showback.

Assumptions (you may refine as needed)

• Multi-region deployment, regional data residency where applicable.
• Order-of-magnitude scale: up to 100k concurrent jobs at peak, thousands of tenants, millions of daily pipeline steps.
• Availability target ≥ 99.95% for control plane; artifact durability ≥ 11 nines.

Provide a clear architecture, key trade-offs, and operational considerations.