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diff --git a/discovery/dnssd/public/querier.h b/discovery/dnssd/public/querier.h
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+++ b/discovery/dnssd/public/querier.h
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+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef DISCOVERY_DNSSD_PUBLIC_QUERIER_H_
+#define DISCOVERY_DNSSD_PUBLIC_QUERIER_H_
+
+#include "absl/strings/string_view.h"
+#include "discovery/dnssd/public/instance_record.h"
+
+namespace openscreen {
+namespace discovery {
+
+class DnsSdQuerier {
+ public:
+  // TODO(rwkeane): Add support for expiring records in addition to deleting
+  // them.
+  class Callback {
+   public:
+    virtual ~Callback() = default;
+
+    // Callback fired when a new InstanceRecord is created.
+    virtual void OnInstanceCreated(const DnsSdInstanceRecord& new_record) = 0;
+
+    // Callback fired when an existing InstanceRecord is updated.
+    virtual void OnInstanceUpdated(
+        const DnsSdInstanceRecord& modified_record) = 0;
+
+    // Callback fired when an existing InstanceRecord is deleted.
+    virtual void OnInstanceDeleted(const DnsSdInstanceRecord& old_record) = 0;
+  };
+
+  virtual ~DnsSdQuerier() = default;
+
+  // Begins a new query. The provided callback will be called whenever new
+  // information about the provided (service, domain) pair becomes available.
+  // The Callback provided is expected to persist until the StopQuery method is
+  // called or this instance is destroyed.
+  virtual void StartQuery(const absl::string_view& service,
+                          const absl::string_view& domain,
+                          Callback* cb) = 0;
+
+  // Stops an already running query.
+  virtual void StopQuery(const absl::string_view& service,
+                         const absl::string_view& domain,
+                         Callback* cb) = 0;
+};
+
+}  // namespace discovery
+}  // namespace openscreen
+
+#endif  // DISCOVERY_DNSSD_PUBLIC_QUERIER_H_