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+====
+PG
+====
+
+Concepts
+--------
+
+*Peering Interval*
+ See PG::start_peering_interval.
+ See PG::acting_up_affected
+ See PG::PeeringState::Reset
+
+ A peering interval is a maximal set of contiguous map epochs in which the
+ up and acting sets did not change. PG::PeeringMachine represents a
+ transition from one interval to another as passing through
+ PeeringState::Reset. On PG::PeeringState::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::PeeringState::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::PeeringState::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.