support multi-fields merge key

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+# Multi-fields Merge Key in Strategic Merge Patch
+
+## Abstract
+
+Support multi-fields merge key in Strategic Merge Patch.
+
+## Background
+
+Strategic Merge Patch is covered in this [doc](https://github.com/kubernetes/community/blob/master/contributors/devel/strategic-merge-patch.md).
+In Strategic Merge Patch, we use Merge Key to identify the entries in the list of non-primitive types.
+It must always be present and unique to perform the merge on the list of non-primitive types,
+and will be preserved.
+
+The merge key exists in the struct tag (e.g. in [types.go](https://github.com/kubernetes/kubernetes/blob/5a9759b0b41d5e9bbd90d5a8f3a4e0a6c0b23b47/pkg/api/v1/types.go#L2831))
+and the [OpenAPI spec](https://github.com/kubernetes/kubernetes/blob/master/api/openapi-spec/swagger.json).
+
+## Motivation
+
+The current implementation only support a single field as merge key.
+For some element Kinds, the identity is actually defined using multiple fields.
+[Service port](https://github.com/kubernetes/kubernetes/issues/39188) is an evidence indicating that
+we need to support multi-fields Merge Key.
+
+## Scope
+
+This proposal only covers how we introduce ability to support multi-fields merge key for strategic merge patch.
+It will cover how we support new APIs with multi-fields merge key.
+
+This proposal does NOT cover how we change the merge keys from one single field to multi-fields
+for existing APIs without breaking backward compatibility,
+e.g. we are not addressing the service port issue mentioned above.
+That part will be addressed by [#476](https://github.com/kubernetes/community/pull/476).
+
+## Proposed Change
+
+### API Change
+
+If a merge key has multiple fields, it will be a string of merge key fields seperated by ",", i.e. `patchMergeKey:"<key1>,<key2>,<key3>"`.
+
+If a merge key only has one field, it will be the same as before, i.e. `patchMergeKey:"<key1>"`.
+
+There are no patch format changes.
+Additional fields are just extra fields in the patch.
+
+E.g.
+foo and bar are the merge keys.
+
+Live list:
+```yaml
+list:
+- foo: a
+  bar: x
+  other: 1
+- foo: a
+  bar: y
+  other: 2
+- foo: b
+  bar: x
+  other: 3
+```
+
+Patch 1:
+```yaml
+list:
+- foo: a # field 1 of merge key
+  bar: x # field 2 of merge key
+  other: 4
+  another: val
+```
+
+Result after merging patch 1:
+```yaml
+list:
+- foo: a
+  bar: x
+  other: 4
+  another: val
+- foo: a
+  bar: y
+  other: 2
+- foo: b
+  bar: x
+  other: 3
+```
+
+Patch 2:
+```yaml
+list:
+- $patch: delete
+  foo: a # field 1 of merge key
+  bar: x # field 2 of merge key
+```
+
+Result after merging patch 2:
+```yaml
+list:
+- foo: a
+  bar: y
+  other: 2
+- foo: b
+  bar: x
+  other: 3
+```
+
+### Strategic Merge Patch pkg
+
+We will add logic to support
+- returning a list of fields instead of one single field when looking up merge key.
+- merging list respecting a list of fields as merge key.
+
+### Open API
+
+Open API will not be affected,
+since multi-fields merge key is still in one single string as an extension in Open API spec.
+
+### Docs
+
+Document that the developer should make sure the merge key can uniquely identify an entry in all cases.
+
+## Version Skew and Backward Compatibility
+
+It is fully backward compatibility,
+because there are no patch format changes and no changes to the existing APIs.