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diff --git a/src/crimson/osd/backfill_facades.h b/src/crimson/osd/backfill_facades.h
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+++ b/src/crimson/osd/backfill_facades.h
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+// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
+// vim: ts=8 sw=2 smarttab
+
+#pragma once
+
+#include "crimson/osd/backfill_state.h"
+#include "crimson/osd/pg.h"
+#include "osd/PeeringState.h"
+
+namespace crimson::osd {
+
+// PeeringFacade -- main implementation of the BackfillState::PeeringFacade
+// interface. We have the abstraction to decuple BackfillState from Peering
+// State, and thus cut depedencies in unit testing. The second implemention
+// is BackfillFixture::PeeringFacade and sits in test_backfill.cc.
+struct PeeringFacade final : BackfillState::PeeringFacade {
+  PeeringState& peering_state;
+
+  hobject_t earliest_backfill() const override {
+    return peering_state.earliest_backfill();
+  }
+
+  const std::set<pg_shard_t>& get_backfill_targets() const override {
+    return peering_state.get_backfill_targets();
+  }
+
+  const hobject_t& get_peer_last_backfill(pg_shard_t peer) const override {
+    return peering_state.get_peer_info(peer).last_backfill;
+  }
+
+  const eversion_t& get_last_update() const override {
+    return peering_state.get_info().last_update;
+  }
+
+  const eversion_t& get_log_tail() const override {
+    return peering_state.get_info().log_tail;
+  }
+
+  void scan_log_after(eversion_t v, scan_log_func_t f) const override {
+    peering_state.get_pg_log().get_log().scan_log_after(v, std::move(f));
+  }
+
+  bool is_backfill_target(pg_shard_t peer) const override {
+    return peering_state.is_backfill_target(peer);
+  }
+  void update_complete_backfill_object_stats(const hobject_t &hoid,
+                                             const pg_stat_t &stats) override {
+    peering_state.update_complete_backfill_object_stats(hoid, stats);
+  }
+
+  bool is_backfilling() const override {
+    return peering_state.is_backfilling();
+  }
+
+  PeeringFacade(PeeringState& peering_state)
+    : peering_state(peering_state) {
+  }
+};
+
+// PGFacade -- a facade (in the GoF-defined meaning) simplifying the huge
+// interface of crimson's PG class. The motivation is to have an inventory
+// of behaviour that must be provided by a unit test's mock.
+struct PGFacade final : BackfillState::PGFacade {
+  PG& pg;
+
+  const eversion_t& get_projected_last_update() const override {
+    return pg.projected_last_update;
+  }
+
+  PGFacade(PG& pg) : pg(pg) {}
+};
+
+} // namespace crimson::osd