Working notes — 19 · Migrations projectsSUPPORTING MATERIAL
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Working notes — 19 · Migrations projects (2026-09-22)

Scratch notes for tiers/03-staff/19-migrations/projects.md and assets/diagrams/shadow-read.svg. Not part of the guide.

AWS facts verified against docs.aws.amazon.com this session (2026-09-22)

  • ALB forward action, weighted target groups — multiple target groups per forward action, weight 0–999, traffic distributed in proportion to weights. Caveat found: sticky sessions need target group stickiness (AWSALBTG cookie) or weight changes do not move cookied clients. Source: elasticloadbalancing/latest/application/rule-action-types.html
  • Route 53 weighted routing — traffic share = weight / sum of weights; "If you want to stop sending traffic to a resource, change the weight for that record to 0." Reversal is still bounded by clients honouring TTL. Source: Route53/latest/DeveloperGuide/routing-policy-weighted.html
  • CodeDeploy — predefined canary configs exist and are named as used in the text: CodeDeployDefault.LambdaCanary10Percent5Minutes (10% first increment, remaining 90% five minutes later), CodeDeployDefault.ECSCanary10Percent5Minutes. Automatic rollback "when a deployment fails or when a monitoring threshold you specify is met" (CloudWatch alarms on the deployment group). Sources: codedeploy/latest/userguide/deployment-configurations.html, deployments-rollback-and-redeploy.html
  • DMS — "perform one-time migrations or replicate ongoing changes to keep sources and targets in sync"; "supports fully heterogeneous data migrations between the supported engines." Source: dms/latest/userguide/Welcome.html

No prices anywhere in the section, so nothing to date-stamp on cost.

Decisions

  • Five projects = one migration's apparatus, in order: awkward case → block → counter → shadow → finish. Counter is two-sided (static + runtime) so that project 5's "zero" is defined by project 3.
  • Block is a ratchet (checked-in baseline that may only shrink), not a wall — a red build on day one gets the check deleted.
  • Shadow-read triage taxonomy: new-path bug / old-path bug / comparator bug. Old-path bugs are the staff-register moment (preserve or fix is a decision).
  • Distribution of the AWS patterns the task named: CloudTrail/Athena/X-Ray → p1; Config vs SCP, cfn-lint/Guard → p2; EMF, Athena vs Logs Insights, EventBridge Scheduler + Lambda + SNS → p3; metric filter, Kinesis vs SQS replay, Traffic Mirroring declined, dual-write reconciliation (EventBridge+Lambda+CloudWatch) → p4; ALB weights vs Route 53 weights, CodeDeploy canary, DMS vs read replica, snapshot expiry, Cost Explorer → p5.

Diagram

shadow-read.svg, 760×400. One request forks at a junction; old path row (green, serving) returns the answer to the user along the top line; new path row (dashed, shadow) feeds only the comparator; comparator emits to DIFF LOG; dashed warm stub with a stop bar marked "never returned". SMIL: request dot rides both branches, old answer returns, diff drops into the log, 9 s loop, no begin offsets; mid-cycle elements carry base opacity="0" so frame one reads correctly in static renderers. Rendered in chromium and inspected.