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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.
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