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+// Labels provide a way to associate user-defined metadata with various
+// objects.  Labels may be used to organize objects into non-hierarchical
+// groups; think metadata tags attached to mp3s.  For more details, see
+// http://go/exosphere:labels
+
+syntax = "proto2";
+
+package tech.label;
+
+// A key-value pair applied to a given object.
+message Label {
+  // The key of a label is a syntactically valid URL (as per RFC 1738) with
+  // the "scheme" and initial slashes omitted and with the additional
+  // restrictions noted below.  Each key should be globally unique.  The
+  // "host" portion is called the "namespace" and is not necessarily
+  // resolvable to a network endpoint.  Instead, the namespace indicates what
+  // system or entity defines the semantics of the label.  Namespaces do not
+  // restrict the set of objects to which a label may be associated.
+  //
+  // Keys are defined by the following grammar:
+  //
+  //   key          = hostname "/" kpath
+  //   kpath        = ksegment *[ "/" ksegment ]
+  //   ksegment     = alphadigit | *[ alphadigit | "-" | "_" | "." ]
+  //
+  // where "hostname" and "alphadigit" are defined as in RFC 1738.
+  //
+  // Example key:
+  //   spanner.google.com/universe
+  required string key = 1;
+
+  // The value of the label.
+  oneof value {
+    // A string value.
+    string str_value = 2;
+    // An integer value.
+    int64 num_value = 3;
+  }
+}
+
+// A collection of labels, such as the set of all labels attached to an
+// object.  Each label in the set must have a different key.
+//
+// Users should prefer to embed "repeated Label" directly when possible.
+// This message should only be used in cases where that isn't possible (e.g.
+// with oneof).
+message Labels {
+  repeated Label label = 1;
+}