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+====
+PG
+====
+
+Concepts
+--------
+
+*Peering Interval*
+  See PG::start_peering_interval.
+  See PG::acting_up_affected
+  See PG::RecoveryState::Reset
+
+  A peering interval is a maximal set of contiguous map epochs in which the
+  up and acting sets did not change.  PG::RecoveryMachine represents a 
+  transition from one interval to another as passing through
+  RecoveryState::Reset.  On PG::RecoveryState::AdvMap PG::acting_up_affected can
+  cause the pg to transition to Reset.
+  
+
+Peering Details and Gotchas
+---------------------------
+For an overview of peering, see `Peering <../../peering>`_.
+
+  * PG::flushed defaults to false and is set to false in
+    PG::start_peering_interval.  Upon transitioning to PG::RecoveryState::Started
+    we send a transaction through the pg op sequencer which, upon complete,
+    sends a FlushedEvt which sets flushed to true.  The primary cannot go
+    active until this happens (See PG::RecoveryState::WaitFlushedPeering).
+    Replicas can go active but cannot serve ops (writes or reads).
+    This is necessary because we cannot read our ondisk state until unstable
+    transactions from the previous interval have cleared.