CI/CD Pipeline Service

Interview-Ready Approach

1) Clarify Scope and SLOs

• •Problem statement: Design a CI/CD platform (like GitHub Actions) that runs build pipelines in isolated containers with parallel steps and artifact caching.
• •Design for a peak load target around 300 RPS (including burst headroom).
• •Pipeline runs/day: ~100,000
• •Teams: ~5,000
• •Concurrent pipeline runs: ~2,000
• •Build step timeout: 30 minutes max
• •Log streaming latency: < 2 seconds

2) Capacity Planning Method

• •Convert traffic and growth constraints into request rate, storage growth, and concurrency budgets.
• •Keep at least 2-3x safety margin per tier (ingress, compute, storage, async workers).
• •Reserve explicit latency budgets per hop so p95 can be defended in review.

3) Architecture Decisions

• •Containerization: Run services/jobs in isolated containers with reproducible images and resource quotas.
• •Message Queues: Move non-blocking and retry-heavy work to async consumers with explicit retry and DLQ policies.
• •Databases: Define a clear system-of-record and design read/write paths separately before adding optimizations.
• •Storage: Use object storage for large blobs and keep metadata/authorization separate in the API tier.
• •Monitoring: Instrument golden signals (latency, traffic, errors, saturation) per tier and per tenant/domain.

4) Reliability and Failure Strategy

• •Use rolling deploys with readiness probes and fast rollback.
• •Guarantee idempotent consumers and trace every message with correlation IDs.
• •Use strong write constraints (transactions or conditional writes) and explicit backup/restore strategy.
• •Enforce lifecycle policies, retention tiers, and checksum validation.
• •Alert on user-impact SLOs, not only infrastructure metrics.

5) Validation Plan

• •Run one peak-load test, one dependency-degradation test, and one failover test.
• •Verify idempotency for all retried writes and async consumers.
• •Track user-facing SLOs first: p95 latency, error rate, and successful throughput.

6) Trade-offs to Call Out in Interviews

• •Containerization: Containerization standardizes environments but increases orchestration complexity.
• •Message Queues: Async pipelines absorb spikes well, but increase eventual-consistency complexity.
• •Databases: SQL gives stronger transactional guarantees; NoSQL often gives better write scaling and flexibility.
• •Storage: Object storage is cheap and durable, but random low-latency reads are weaker than databases/caches.
• •Monitoring: Deep observability speeds incident response but raises ingestion and tooling costs.

Practical Notes

• •A controller service dequeues pipeline requests and schedules steps onto worker nodes from a pool. Workers pull container images, run steps, and report status.
• •Artifact passing: store artifacts in an object store (S3) keyed by pipeline_run_id. Subsequent steps download from the same prefix.
• •Cache storage: key by (team + pipeline + hash(lockfile_contents)). Store in object storage with an LRU eviction policy.