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+# Reading across collections
+
+Sometimes, it is useful for a user to be able to retrieve resources across
+multiple collections, or retrieve a single resource without needing to know
+what collection it is in.
+
+## Guidance
+
+There are two potential approaches for reading resources across multiple
+collections. Groups of related services **should**, as a unit, select a
+preferred approach and apply it consistently.
+
+### Wildcards
+
+Services **may** support reading resources across multiple collections by
+allowing users to specify a `-` (the hyphen or dash character) as a wildcard
+character in a standard [`List`][aip-132] operation:
+
+```
+GET /v1/publishers/-/books?filter=...
+```
+
+- The URI pattern **must** still accept a parameter to permit the collection to
+  be specified; a URI pattern **must not** hard-code the `-` character.
+- The operation **must** explicitly document that this behavior is supported.
+- Responses **must** populate the canonical parent ID rather than `-` if the
+  parent ID is normally provided in the resource in some way.
+  - If resources include the parent ID within the name of the resource, the
+    resources provided in the response **must** use the canonical name of the
+    resource, with the actual parent collection identifiers (instead of `-`).
+  - If resources include a field indicating the ID of the parent, resources
+    provided in the response **must** populate the canonical parent ID (instead
+    of `-`).
+- Services **may** support reading across collections on `List` requests
+  regardless of whether the identifiers of the child resources are guaranteed
+  to be unique.
+  - However, services **must not** support reading across collections on `Get`
+    requests if the child resources might have a collision.
+- Cross-parent requests **may** support `order_by`. However, if reading across
+  collections requires consulting multiple backends, databases, or locations,
+  cross-parent requests **should not** support `order_by`, and if they do, the
+  field **must** document that it is best effort. This is because cross-parent
+  requests introduce ambiguity around ordering, especially if there is
+  difficulty reaching a parent (see AIP-217).
+
+**Important:** If listing across multiple collections introduces the
+possibility of partial failures due to unreachable parents (such as when
+listing across locations), the operation **must** indicate this following the
+guidance in AIP-217.
+
+### Unique resource lookup
+
+Sometimes, a resource within a sub-collection has an identifier that is unique
+across parent collections. In this case, it may be useful to allow a
+[`Get`][aip-131] operation to retrieve that resource without knowing which
+parent collection contains it. In such cases, APIs **may** allow users to
+specify the wildcard collection ID `-` (the hyphen or dash character) to
+represent any parent collection:
+
+```
+GET https://example.googleapis.com/v1/publishers/-/books/{book}
+```
+
+- The URI pattern **must** still be specified with `*` and permit the
+  collection to be specified; a URI pattern **must not** hard-code the `-`
+  character.
+- The operation **must** explicitly document that this behavior is supported.
+- The resource name in the response **must** use the canonical name of the
+  resource, with actual parent collection identifiers (instead of `-`). For
+  example, the request above returns a resource with a name like
+  `publishers/123/books/456`, _not_ `publishers/-/books/456`.
+- The resource ID **must** be unique within parent collections.
+
+### Virtual Collections
+
+Services **may** create a _virtual collection_ (usually as an alternative to
+the use of the wildcard character), which exists alongside the erstwhile parent
+and provides child resources regardless of the parent they are under.
+
+The parent **must** be removed from the URI:
+
+```
+GET /v1/books?filter...
+```
+
+The service **must** still recognize the full path to the resource (e.g.
+`/publishers/123/books/les-mis`) as the canonical path. Services **should**
+send `308 Permanent Redirect` to the canonical path if a user attempts to
+access a resource directly within the virtual collection (for example,
+`/v1/books/les-mis`).
+
+**Note:** The child IDs **must** be unique across all parents for
+`308 Permanent Redirect` to be appropriate; if they are not, the service
+**must** send `404 Not Found` in this situation instead.
+
+## Further reading
+
+- For partial failures due to unreachable resources, see AIP-217.
diff --git a/aip/general/0159/aip.yaml b/aip/general/0159/aip.yaml
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+---
+id: 159
+state: approved
+created: 2019-07-26
+placement:
+  category: design-patterns
+  order: 70