Strangler Fig Pattern: Migrate a Monolith to Serverless Step by Step

By Ravindra Bagale, trainer · LinkedIn · Last updated: 2 November 2026

1. Everyday example: city bus ticket counter

Omar in Amman (JOD) modernizes a gov portal. Citizens still hit portal.gov.jo. Week 1 only /forms/birth-cert goes to Lambda; everything else hits the old Tomcat box. Next sprint /forms/marriage. If Lambda misbehaves, flip that one path back. That is strangler fig — not rewriting the whole portal in a holiday weekend.

Edge skills: API Gateway. Pattern labs like REST API project and notes app show greenfield APIs — strangler adds the coexistence story.

2. Why strangle?

Why do we need this?

Big-bang rewrites slip. Risk concentrates in one cutover. Strangling spreads risk across sprints, proves each peel in production, and keeps revenue flowing in INR, USD, or EUR while you migrate.

What is it?

Strangler fig pattern = put a facade (often API Gateway, ALB, or reverse proxy) in front of the monolith; route selected paths to new serverless services; retire old paths when unused.

3. Vertical diagram

4. How: peel one route

How to do it (step by step)

1. Inventory routes by traffic and pain (errors, deploys).
2. Pick a leaf route with clear input/output (PDF form, not checkout core).
3. Put facade in front if not already (custom domain on API Gateway or ALB rules).
4. Implement Lambda + data access; contract tests vs old responses.
5. Shadow traffic or 1% canary; compare status and body hashes.
6. Raise to 100%; monitor; keep rollback switch one click.
7. Delete unused monolith code only after metrics prove zero hits.

5. Per-concept commands

List HTTP API routes

aws apigatewayv2 get-routes --region ap-south-1 --api-id "$API_ID" \
  --query 'Items[].{R:RouteKey,T:Target}' --output table

Meaning: Shows which paths already point at integrations (Lambda vs HTTP proxy).

ALB listener rules (if facade is ALB)

aws elbv2 describe-rules --region ap-south-1 --listener-arn "$LISTENER_ARN" \
  --query 'Rules[].{P:Priority,C:Conditions[0].PathPatternConfig.Values[0]}' --output table

Meaning: Path rules are your peel map — document before changing priority.

Compare canary error rates

aws cloudwatch get-metric-statistics --region ap-south-1 \
  --namespace AWS/Lambda --metric-name Errors --dimensions Name=FunctionName,Value=birth-cert-api \
  --start-time $(date -u -d '1 hour ago' +%Y-%m-%dT%H:%M:%S) --end-time $(date -u +%Y-%m-%dT%H:%M:%S) \
  --period 300 --statistics Sum

Meaning: If canary Errors rise vs baseline, flip route back before 100%.

Old host still getting peeled path?

# On monolith host (example): count hits to migrated path last hour
sudo grep '/forms/birth-cert' /var/log/nginx/access.log | tail -20

Meaning: After 100% peel, this should go quiet — proves facade owns the path.

6. Data coexistence

7. When NOT to strangle yet

• No facade possible (clients hardcode 20 IPs) — fix DNS/domain first.
• Team cannot test contracts — you will ship silent breaks.
• Regulated cut needs one audited switch — still can strangle behind flag with paperwork.
• Module is a tangled ball with no route boundary — extract a seam first.

8. Industries

• Gov (Omar, Amman, JOD): one PDF form per sprint.
• Bank (Aisha, Lagos, NGN): peel statement download before core ledger.
• Retail (Luis, Mexico City, MXN): peel store-finder API; POS stays.
• EdTech (Diego, São Paulo, BRL): peel certificate PDF gen.
• SaaS (Priya, Bengaluru, INR): peel /v1/webhooks receiver.

9. Lab

10. Lab - plan a peel (paper + gateway routes)

Do this in ap-south-1 (Mumbai) unless your org forbids it. Set a budget alert first.

How to do it (step by step)

1. Pick a real or sample monolith route list; mark leaf vs core.
2. Write facade choice: HTTP API proxy vs ALB path rule.
3. Define success metrics: p95, 5xx, business KPI.
4. Write rollback steps in five bullets.
5. Optional: create a lab HTTP API with two routes — one MOCK, one AWS_PROXY.

Cleanup (stop billing)

1. Delete lab APIs and Lambda.
2. Remove canary alarms you created.

11. Same pattern across industries

• Fintech (Aisha, Lagos): payment webhooks → SQS → Lambda; never block the bank callback thread.
• Ecommerce (Rohan, Pune): checkout API on API Gateway+Lambda; inventory fan-out on SNS.
• Healthcare (Mei, Singapore): appointment SMS via SNS; PHI stays in DynamoDB with fine-grained IAM.
• EdTech (Diego, São Paulo): nightly cleanup EventBridge → Lambda on student drafts.
• Logistics (Fatima, Casablanca): package scan events on EventBridge bus; Step Functions for customs saga.
• Media (Kenji, Osaka): S3 upload → Lambda thumbnails; fan-out to moderation + CDN invalidate.
• SaaS multi-tenant (Priya, Bengaluru): Cognito groups + DynamoDB partition per tenant.
• Gov portal (Omar, Amman): strangler fig — peel one PDF form route per sprint to Lambda.

12. Troubleshooting checklist

1. Confirm region (ap-south-1 vs us-east-1 for CloudFront/ACM quirks).
2. Check CloudWatch Logs for the function name and request id.
3. Verify IAM execution role allows the API you call.
4. For async: check DLQ depth and SQS age of oldest message.
5. For APIs: curl -v and compare status, CORS headers, body.
6. For workflows: open Step Functions execution event history.
7. Re-read the linked deep chapter before reinventing a service.
8. If cost spikes: check concurrency, retries, and provisioned capacity.

13. Common errors and fixes

Each row is a real mistake students and juniors hit. Read the fix before you panic.

14. FAQ

15. Interview Q&A (Beginner / Mid / Senior + MNC)

16. Tips in English, Marathi and Hindi

17. Architecture decision checklist

1. Write the user journey in five bullets with no AWS names.
2. Mark each step sync (user waits) or async.
3. Name failure mode: retry, DLQ, compensate, or alert-only.
4. Name the idempotency key for every write.
5. Pick data store from access patterns (link DynamoDB modeling).
6. Estimate monthly cost at 1x and 10x traffic (USD and INR).
7. Confirm account concurrency and service quotas in ap-south-1.
8. Decide auth: Cognito, IAM, Lambda authorizer, or none.
9. Name on-call and which CloudWatch alarm pages them.
10. Link the deep chapter you will not rewrite.
11. Only then open console or IaC.

18. Copy-paste quick commands (read-only first)

# Region and identity
aws configure get region
aws sts get-caller-identity

# Lambda landscape
aws lambda list-functions --region ap-south-1 --max-items 20 \
  --query 'Functions[].{N:FunctionName,R:Runtime,T:Timeout,M:MemorySize}' --output table
aws lambda get-account-settings --region ap-south-1

# API Gateway (HTTP APIs)
aws apigatewayv2 get-apis --region ap-south-1 --query 'Items[].{N:Name,Id:ApiId,Ep:ProtocolType}' --output table

# Queues and topics
aws sqs list-queues --region ap-south-1
aws sns list-topics --region ap-south-1 --query 'Topics[].TopicArn'

# Event buses and state machines
aws events list-event-buses --region ap-south-1
aws stepfunctions list-state-machines --region ap-south-1 --query 'stateMachines[].name'

# DynamoDB tables (names only)
aws dynamodb list-tables --region ap-south-1

Run these in a sandbox account. Prefer read-only IAM while learning.

19. More industry and stack stories

• Fintech (Aisha, Lagos, NGN): bank webhooks must return 200 fast — SQS absorbs spikes; ledger Lambda is idempotent on paymentId.
• Ecommerce (Rohan, Pune, INR): Black Friday — API Gateway throttle + SQS order intake; SNS fans inventory, email, analytics.
• Healthcare (Mei, Singapore, SGD): appointment reminders via SNS SMS; PHI only in DynamoDB with condition keys.
• EdTech (Diego, São Paulo, BRL): EventBridge nightly cleanup of draft quizzes; Lambda timeout 30s, not 15 min.
• Logistics (Fatima, Casablanca, MAD): package scans on custom EventBridge bus; Step Functions customs saga.
• Media (Kenji, Osaka, JPY): S3 → Lambda thumbnails; never write thumbs to the watched prefix.
• SaaS (Priya, Bengaluru, INR): Cognito groups + DynamoDB PK tenantId; one Lambda authorizer for all routes.
• Gov (Omar, Amman, JOD): strangler — peel one PDF form route per sprint from the monolith.
• Startup (Sofia, Sofia, EUR): start with Function URL for demos; move to HTTP API when partners need JWT.
• Retail (Luis, Mexico City, MXN): store POS sync via SQS FIFO when order matters; Standard when it does not.

• Python: handlers thin; put domain logic in plain modules for unit tests without Lambda.
• Node: prefer async/await; never block the event loop on sync DNS.
• Java: SnapStart for cold starts; keep dependencies lean.
• .NET: AspNetCoreServer for low-traffic tenants; watch package size.
• Go: single static binary; great for SQS workers.
• PHP: keep Laravel queues on ECS; Lambda for S3/webhooks glue.

20. Glossary (short)

21. Practice: explain out loud

1. 60 seconds: everyday story for strangler fig migration.
2. 60 seconds: sync vs async with a checkout example.
3. 60 seconds: when you refuse Lambda.
4. 60 seconds: DLQ + idempotency in one breath.
5. Record once; cut fillers; record again.

22. Safe lab hygiene

• Tag every lab resource with owner and expiry date.
• Set a monthly AWS Budget alarm in INR or USD before experiments.
• Prefer read-only commands first; mutate in a sandbox account.
• Delete queues, APIs, tables you created — DynamoDB and APIGW leave footprints.
• Never put production secrets in env vars or screenshots.
• Do not enable provisioned concurrency for a hello-world.

23. How this chapter links others

• Compute depth: Lambda.
• HTTP edge: API Gateway.
• Data modeling: DynamoDB.
• Messaging: SQS/SNS.
• Events: EventBridge.
• Workflows: Step Functions.
• Project chapters (build-*): worked assemblies — link, do not paste.

24. Myths to drop

25. Error budget thinking for serverless

If your monthly error budget is 0.1% on checkout, a fan-out that retries aggressively can burn the budget with duplicate emails while money stays correct. Separate business correctness SLOs from notification SLOs.

1. Write two SLOs: money-correct and notify-delivered.
2. Retries on money path: bounded + idempotent.
3. Retries on notify path: best-effort + DLQ.
4. Page humans on money DLQ; ticket on notify DLQ.

26. Regional notes (ap-south-1)

• Build labs in Mumbai (ap-south-1) unless your org mandates elsewhere.
• Some services launch first in us-east-1 — check feature availability.
• Data residency for INR fintech may require in-region only.
• Latency to end users in India usually favors ap-south-1.

27. Naming conventions that save incidents

# Suggested prefixes
# Lambda:  pay-intake-prod, pay-worker-prod
# SQS:     pay-orders.fifo / pay-orders-dlq
# SNS:     pay-order-events
# EB bus:  pay.domain
# SFN:     pay-refund-saga
# DDB:     pay-orders-prod

Include env (dev/stage/prod) and domain. Future you grepping CloudWatch will thank present you.

28. Pre-mortems (ten minutes)

1. Imagine tomorrow headline: double charge in AED. What broke?
2. Imagine DLQ depth 50,000 after a bad deploy. How do you drain?
3. Imagine Cognito outage. Do APIs fail closed or serve read-only?
4. Write answers in the runbook before go-live.

29. Currency and locale in examples

Rotate currencies in designs: INR, USD, EUR, AED, NGN, BRL, JPY, SGD. Rotate names: Aisha, Rohan, Mei, Diego, Fatima, Kenji, Priya, Omar, Sofia, Luis. This keeps mental models global — not one-country samples.

30. Worked story: Priya SaaS webhooks (INR)

1. Monolith /v1/webhooks overloaded on Black Friday.
2. Facade sends /v1/webhooks to Lambda+SQS.
3. 1% canary one day; errors flat; 100% next day.
4. Monolith handler deleted two weeks later after zero hits.

31. Anti-patterns

• Rewriting the database and the API on day one.
• No parity tests — silent field renames.
• Peeling checkout before login health is green.

Revision, interview and comparison cards

Revise this lesson in a few minutes. Save the cards on your phone or download them as PNG files. The QR code on each card opens this lesson.

More cards: Ravi Sir's Cards.