Serverless (Deep Workflow)

Serverless shifts complexity to permissions, quotas, observability, and state at the edges. Guide the user to explicit trade-offs: simplicity vs cold starts, synchronous vs async, and least privilege IAM that is still operable.

When to Offer This Workflow

Trigger conditions:

• Choosing between containers vs functions, or decomposing a service into functions
• Cold starts, timeouts, memory sizing, or concurrency throttling
• “Works locally, fails in Lambda”—IAM, VPC, DNS, or env differences
• Cost spikes, recursive invocation, or DLQ backlogs

Initial offer:

Use six stages: (1) workload fit & constraints, (2) triggers & contract, (3) IAM & networking, (4) runtime performance, (5) observability & ops, (6) cost & governance. Confirm cloud and language/runtime.

Stage 1: Workload Fit & Constraints

Goal: Decide if functions are appropriate and what boundaries look like.

Fit Criteria (heuristics)

• Good: event-driven, spiky traffic, small well-defined units, short execution, state externalized
• Hard: long CPU-heavy jobs, large in-memory state, strict low-latency p99 without provisioned concurrency, complex socket protocols

Clarify

• SLAs: sync API vs async pipeline
• Payload limits, execution time cap, tmp storage
• Stateful needs: DB, queue, cache, workflow engine

Exit condition: Clear yes/no/partial with escape hatch (container, batch, ECS/Fargate, Step Functions).

Stage 2: Triggers & Contract

Goal: Define inputs, idempotency, retry semantics, and output side effects.

Map

• Triggers: HTTP, queue, schedule, object storage, streams, webhooks
• At-least-once delivery → idempotent handlers and dedupe keys
• Partial failure in batch: what gets retried vs poison messages

Design

• Event schema versioning; backward-compatible consumers
• DLQ or failed-letter path with replay procedure

Exit condition: Written contract: success criteria, retry policy, dead-letter ownership.

Stage 3: IAM & Networking

Goal: Least privilege that is debuggable; correct VPC when needed.

IAM

• One role per function family; resource-scoped policies
• Avoid * actions on * resources except where cloud forces it—then narrow ASAP
• Cross-account and KMS decrypt permissions explicit

Networking

• Public vs VPC-attached functions (cold start + ENI trade-offs)
• Egress for third-party APIs: NAT costs and security groups / NACLs
• Private API Gateway / internal ALB patterns if applicable

Exit condition: IAM policy review with least privilege checklist; network path diagram for dependencies.

Stage 4: Runtime Performance

Goal: Meet latency and throughput within platform limits.

Tactics

• Memory tuning: CPU scales with memory on many clouds—profile
• Provisioned concurrency / min instances for critical sync paths—cost trade-off
• Connection reuse (HTTP, DB) outside handler global where safe
• Cold start: trim dependencies, ARM Graviton if supported, lazy init discipline
• Timeouts set below client expectations; avoid infinite hangs

Concurrency

• Reserved concurrency vs account limits; avoid starving other functions

Exit condition: Load test or trace evidence for p95/p99; documented limits and mitigations.

Stage 5: Observability & Operations

Goal: Debuggable serverless—correlation across async hops.

Practices

• Structured logging with request IDs; PII redaction
• Tracing (X-Ray, OpenTelemetry) across queue → function → DB
• Metrics: throttles, errors, duration, iterator age for streams
• Alarms on error rate, DLQ depth, duration approaching timeout

Runbooks

• Replay DLQ safely (idempotency!)
• Blue/green or canary if using traffic shifting features

Exit condition: Dashboard + alerts + on-call steps for top failure modes.

Stage 6: Cost & Governance

Goal: Predictable spend and guardrails.

Levers

• Right-size memory; eliminate unnecessary VPC; async where sync not needed
• Recursive patterns and accidental infinite loops—billing alerts
• Tagging for cost allocation; budgets and anomaly detection

Governance

• Approved runtimes; dependency scanning; org-level deny policies for public buckets, etc.

Final Review Checklist

• Workload fit validated; boundaries documented
• Idempotency + DLQ + replay story clear
• IAM minimal; network path understood
• Latency/cold start addressed for critical paths
• Observability and alarms in place
• Cost and recursion risks acknowledged

Tips for Effective Guidance

• Always state at-least-once and what breaks if handlers are not idempotent.
• When user says “Lambda slow,” separate cold start vs downstream vs code.
• Prefer Step Functions / workflows when logic is long-running branching—not nested Lambdas calling Lambdas ad hoc.

Handling Deviations

• “We only have one function”: still document IAM, retries, and logs—future you will thank you.
• Edge workers: emphasize CPU time limits, geography, and cache semantics.