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Migration Playbook

Monolithic Application→Microservices with API Gateway

Very High complexityHigh riskComplex Rollback

Duration Estimate

3–6 months or more

Migration Stages

5

Blocking Prerequisites

2

Confidence

Strong

Readiness Checklist(2 blocking, 2 advisory)

blocking
Team

Team at senior maturity level

This migration requires senior engineering maturity. The team should have hands-on experience with the relevant technologies and proven ability to execute complex system changes safely.

Guidance: Build team skills through learning modules before attempting this migration.

blocking
Monitoring

Production observability baseline established

Comprehensive metrics, logs, and traces must be in place before the migration starts. You need a clear baseline to detect regressions during and after each migration stage.

Guidance: Instrument key metrics (latency p99, error rate, throughput, resource utilization) on all affected components.

Process

Rollback plan documented and tested

Rollback feasibility for this migration is 'complex'. Rollback is possible but may be complex, document the exact rollback steps and test them in a non-production environment.

Guidance: Run a full dry-run in a staging environment that mirrors production load characteristics.

Process

Migration trigger condition confirmed

Confirm that the actual trigger for this migration is present. Common triggers: Independent deployment requirement for specific high-velocity domains: one team's releases should not block all others; Regulatory data isolation requirement (PCI scope reduction, HIPAA PHI boundary) that demands a hard service boundary; Engineering organization exceeds 30 engineers: single deployment pipeline becomes a coordination bottleneck.

Guidance: Validate the trigger with production metrics or benchmarks before committing to the migration timeline.

Migration Stages

1Extraction Candidate IdentificationLow2–4 weeks

Audit the codebase for domains with clear ownership boundaries, low coupling to other domains, and a team or business driver for extraction. Score candidates by: number of cross-domain dependencies, data sharing surface area, and team ownership clarity. Reject candidates with deep cross-domain foreign key dependencies until those can be untangled.

Key tasks

  • ·Audit the codebase for domains with clear ownership boundaries, low coupling to other domains, and a team or business driver for extraction. Score candidates by: number of cross-domain dependencies, data sharing surface area, and team ownership clarity. Reject candidates with deep cross-domain foreign key dependencies until those can be untangled.
  • ·Validate rollback capability at this stage boundary before proceeding.

Exit criteria

  • ✓'Extraction Candidate Identification' validated in production with no regression in key metrics

Rollback: possible at this stage

2API Gateway in Pass-Through ModeLow1 week

Deploy the API gateway in front of the monolith with all traffic forwarded unchanged. Zero change to application behavior. Validate that latency overhead is acceptable (typically <2ms added per request). This establishes the routing infrastructure before it is needed.

Key tasks

  • ·Deploy the API gateway in front of the monolith with all traffic forwarded unchanged. Zero change to application behavior. Validate that latency overhead is acceptable (typically <2ms added per request). This establishes the routing infrastructure before it is needed.
  • ·Validate rollback capability at this stage boundary before proceeding.

Exit criteria

  • ✓'API Gateway in Pass-Through Mode' validated in production with no regression in key metrics

Rollback: possible at this stage

3First Service ExtractionHigh4–8 weeks

Extract the lowest-coupling, highest-team-ownership candidate domain into its own service. Provision its database. Implement dual-write from monolith to both old monolith tables and new service (to allow rollback). Route new traffic to service. Run both paths in parallel until confidence is established.

Key tasks

  • ·Extract the lowest-coupling, highest-team-ownership candidate domain into its own service. Provision its database. Implement dual-write from monolith to both old monolith tables and new service (to allow rollback). Route new traffic to service. Run both paths in parallel until confidence is established.
  • ·Validate rollback capability at this stage boundary before proceeding.

Exit criteria

  • ✓'First Service Extraction' validated in production with no regression in key metrics

Rollback: possible at this stage

4Shadow Traffic Validation and CutoverMedium2 weeks

Run shadow traffic comparison: requests hit both monolith path and new service path, responses compared for parity. Identify discrepancies and resolve before cutover. Cut over routing in the API gateway for the extracted domain's endpoints.

Key tasks

  • ·Run shadow traffic comparison: requests hit both monolith path and new service path, responses compared for parity. Identify discrepancies and resolve before cutover. Cut over routing in the API gateway for the extracted domain's endpoints.
  • ·Validate rollback capability at this stage boundary before proceeding.

Exit criteria

  • ✓'Shadow Traffic Validation and Cutover' validated in production with no regression in key metrics

Rollback: possible at this stage

5Repeat Extraction: Subsequent ServicesHigh4–8 weeks per service

Apply the established extraction pattern to each additional candidate domain. Each extraction becomes more predictable as the pattern matures and tooling is reused. Monitor the monolith's surface area shrinking over time.

Key tasks

  • ·Apply the established extraction pattern to each additional candidate domain. Each extraction becomes more predictable as the pattern matures and tooling is reused. Monitor the monolith's surface area shrinking over time.
  • ·Validate rollback capability at this stage boundary before proceeding.

Exit criteria

  • ✓'Repeat Extraction: Subsequent Services' validated in production with no regression in key metrics

Rollback: possible at this stage

Rollback Decision Points

After stage 3

Trigger: Any of the following in the 30 minutes after completing stage 3: p99 latency increases >50% from baseline, error rate exceeds 1%, or data consistency checks fail.

Procedure: Revert stage 3 changes. Rollback is complex, follow the documented rollback procedure for 'monolith-to-microservices'. Engage senior engineering support.

Recovery: 1–4 hours (complex rollback)

After stage 5

Trigger: Any of the following in the 30 minutes after completing stage 5: p99 latency increases >50% from baseline, error rate exceeds 1%, or data consistency checks fail.

Procedure: Revert stage 5 changes. Rollback is complex, follow the documented rollback procedure for 'monolith-to-microservices'. Engage senior engineering support.

Recovery: 1–4 hours (complex rollback)

After stage 5

Trigger: Final validation: confirm all success criteria are met within 24 hours of migration completion. If any success criterion fails, trigger rollback or remediation.

Procedure: If success criteria are not met within 24 hours, escalate to the architecture team. Decide between full rollback, partial rollback, or targeted remediation based on scope.

Recovery: 24–72 hours for full assessment and remediation

Success Criteria

  • ✓Target state 'Microservices with API Gateway' is stable in production for 72+ hours with no regressions.
  • ✓p99 latency on all affected services is within acceptable range (≤110% of pre-migration baseline).
  • ✓Error rate is at or below pre-migration baseline.

This migration playbook is derived from structured architecture knowledge. Duration estimates and risk levels are structural approximations based on documented migration complexity and operational risk, not measured execution data. Validate all stages against your specific system constraints before executing.