LEARN. PRACTICE. GROW.An override may append instead of replacing
The lab base declares publication on 18080 and the override adds 19090, both targeting 8080 and bound to loopback. The model retains two entries. Reviewing only the override might incorrectly suggest the earlier port disappeared. In a fictional case with broad publication in the base, adding loopback does not restrict that inherited entry. The executor supports!override, which retained only the new list, and!reset, which cleared it. Confirm compatibility and effective results before application; do not infer actual exposure without observing the network.
Commands and mounts have their own rules
The same experiment replaces command and healthcheck.test without concatenating arguments. For volumes, entries targeting /work merge by that target identity and leave one bind with the replacement source and read_only=true. Avoid one mental rule for every YAML list. Read the model and confirm what disappeared, remained and changed. A healthcheck change may alter the evidence needed for healthy status. A read-only mount also does not establish that the source contains the correct data or that the consumer can use it.
The base path anchors overrides
With base/compose.yaml first and overlays/merge.yaml afterward,./replacement-data resolves to base/replacement-data. The experiment changes no project-directory and uses no include; it observes merging through -f. The source is not relative to the override folder in this context. For a fictional service reading closing files, that mistake may select missing data or data from another scope. Hold application while reviewing source, content and permissions. The lab only resolved the path and created no mounts or copies of files a real consumer would read.
Profiles select services rather than grant authorization
The model without profiles contains api and db. Enabling diagnostics adds debug and trace. This observation demonstrates configuration selection without executing any diagnostic operation. Documentation also distinguishes explicitly targeting one service from enabling the whole profile: other associated services do not start merely because they share that profile. This second claim is documentary, not a lab startup test. Review targets and dependencies before an operation and retain separate access controls. A profiled service still needs appropriate authorization, scope and criteria for its intended use.
Project names and data identity
Projects lab-one and lab-two produce different managed-volume names: lab-one_managed and lab-two_managed. The external volume with name=dr-shared-fixture retains the same name in both models. A project prefix is insufficient to establish isolation of that explicitly named resource. Before a migration rehearsal, verify effective destination, ownership and the authorized dataset. The local result proves neither volume existence, shared data contents nor mounting. It shows a common reference that needs resolution before allowing writes, with the actual daemon and environment included in that decision.
Declared dependencies and actual recovery
The final model requires healthy db and successful migration completion. It includes restart=true on the db dependency and restart=on-failure on the service, fields with different roles. The lab only parses those declarations. In a real release, observe what the healthcheck measures, migration outcome and the consumer journey. If the application produced data the previous image cannot read, swapping the image alone does not resolve recovery. Prepare handover with per-service state, affected data, pending decisions and owners without treating config’s zero exit as global success.
Ordinary merge: ports 18080 and 19090.!override: only 19090. Two projects: different managed volumes but the same external dr-shared-fixture reference. No resources were created.
Common pitfalls
Assuming the last list replaces every earlier one; resolving paths from the override; treating a profile as security; assuming external-volume isolation; confusing image rollback with data recovery.
Related topics: Build and distribution · Data and mounts · Diagnosis and recovery
Review the complete model and resource identity. Acceptance needs service and data evidence beyond valid configuration and declared dependency gates.
Reference: Merge Compose files: reference · Docker Engine Linux containers, BuildKit and Compose; official documentation consulted 2026-09-30; version-dependent behavior explicitly scoped