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kustomize/cmd/config/docs/api-conventions/merge3.md
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# Merge (3-way)
3-way merge identifies changes between an original source + updated source and merges the result
into a destination, overriding the destination fields where they have changed between
original and updated.
### Resource MergeRules
- Resources present in the original and deleted from the update are deleted.
- Resources missing from the original and added in the update are added.
- Resources present only in the dest are kept without changes.
- Resources present in both the update and the dest have their fields merged with the destination.
### Field Merge Rules
Fields are recursively merged using the following rules:
- scalars
- if present in either dest or updated and `null`, clear the value
- if unchanged between original and updated, keep dest value
- if changed between original and updated (added, deleted, changed), take the updated value
- non-associative lists -- lists without a merge key
- if present in either dest or updated and `null`, clear the value
- if unchanged between original and updated, keep dest value
- if changed between original and updated (added, deleted, changed), take the updated value
- map keys and fields -- paired by the map-key / field-name
- if present in either dest or updated and `null`, clear the value
- if present only in the dest, it keeps its value
- if not-present in the dest, add the delta between original-updated as a field
- otherwise recursively merge the value between original, updated, dest
- associative list elements -- paired by the associative key
- if present only in the dest, it keeps its value
- if not-present in the dest, add the delta between original-updated as a field
- otherwise recursively merge the value between original, updated, dest
### Associative Keys
Associative keys are used to identify "same" elements within 2 different lists, and merge them.
The following fields are recognized as associative keys:
[`mountPath`, `devicePath`, `ip`, `type`, `topologyKey`, `name`, `containerPort`]
Any lists where all of the elements contain associative keys will be merged as associative lists.