======================
Peering
======================

Concepts
--------

*Peering*
   the process of bringing all of the OSDs that store a Placement Group (PG)
   into agreement about the state of all of the objects in that PG and all of
   the metadata associated with those objects. Two OSDs can agree on the state
   of the objects in the placement group yet still may not necessarily have the
   latest contents.

*Acting set*
   the ordered list of OSDs that are (or were as of some epoch) responsible for
   a particular PG.

*Up set*
   the ordered list of OSDs responsible for a particular PG for a particular
   epoch, according to CRUSH. This is the same as the *acting set* except when
   the *acting set* has been explicitly overridden via *PG temp* in the OSDMap.

*PG temp* 
   a temporary placement group acting set that is used while backfilling the
   primary OSD. Assume that the acting set is ``[0,1,2]`` and we are
   ``active+clean``. Now assume that something happens and the acting set
   becomes ``[2,1,2]``. Under these circumstances, OSD ``3`` is empty and can't
   serve reads even though it is the primary. ``osd.3`` will respond by
   requesting a *PG temp* of ``[1,2,3]`` to the monitors using a ``MOSDPGTemp``
   message, and ``osd.1`` will become the primary temporarily. ``osd.1`` will
   select ``osd.3`` as a backfill peer and will continue to serve reads and
   writes while ``osd.3`` is backfilled. When backfilling is complete, *PG
   temp* is discarded. The acting set changes back to ``[3,1,2]`` and ``osd.3``
   becomes the primary.

*current interval* or *past interval*
   a sequence of OSD map epochs during which the *acting set* and the *up
   set* for particular PG do not change.

*primary*
   the member of the *acting set* that is responsible for coordination peering.
   The only OSD that accepts client-initiated writes to the objects in a
   placement group. By convention, the primary is the first member of the
   *acting set*.

*replica*
   a non-primary OSD in the *acting set* of a placement group. A replica has
   been recognized as a non-primary OSD and has been *activated* by the
   primary.

*stray*
   an OSD that is not a member of the current *acting set* and has not yet been
   told to delete its copies of a particular placement group.

*recovery*
   the process of ensuring that copies of all of the objects in a PG are on all
   of the OSDs in the *acting set*. After *peering* has been performed, the
   primary can begin accepting write operations and *recovery* can proceed in
   the background.

*PG info*
   basic metadata about the PG's creation epoch, the version for the most
   recent write to the PG, the *last epoch started*, the *last epoch clean*,
   and the beginning of the *current interval*. Any inter-OSD communication
   about PGs includes the *PG info*, such that any OSD that knows a PG exists
   (or once existed) and also has a lower bound on *last epoch clean* or *last
   epoch started*.

*PG log*
   a list of recent updates made to objects in a PG. These logs can be
   truncated after all OSDs in the *acting set* have acknowledged the changes.

*missing set*
   the set of all objects that have not yet had their contents updated to match
   the log entries. The missing set is collated by each OSD. Missing sets are
   kept track of on an ``<OSD,PG>`` basis.

*Authoritative History*
   a complete and fully-ordered set of operations that bring an OSD's copy of a
   Placement Group up to date.

*epoch*
   a (monotonically increasing) OSD map version number.

*last epoch start*
   the last epoch at which all nodes in the *acting set* for a given placement
   group agreed on an *authoritative history*.  At the start of the last epoch,
   *peering* is deemed to have been successful.

*up_thru*
   before a primary can successfully complete the *peering* process,
   it must inform a monitor that is alive through the current
   OSD map epoch by having the monitor set its *up_thru* in the osd
   map. This helps peering ignore previous *acting sets* for which
   peering never completed after certain sequences of failures, such as
   the second interval below:

   - *acting set* = [A,B]
   - *acting set* = [A]
   - *acting set* = [] very shortly after (e.g., simultaneous failure, but staggered detection)
   - *acting set* = [B] (B restarts, A does not)

*last epoch clean*
   the last epoch at which all nodes in the *acting set* for a given placement
   group were completely up to date (this includes both the PG's logs and the
   PG's object contents). At this point, *recovery* is deemed to have been
   completed.

Description of the Peering Process
----------------------------------

The *Golden Rule* is that no write operation to any PG
is acknowledged to a client until it has been persisted
by all members of the *acting set* for that PG.  This means
that if we can communicate with at least one member of
each *acting set* since the last successful *peering*, someone
will have a record of every (acknowledged) operation
since the last successful *peering*.
This means that it should be possible for the current
primary to construct and disseminate a new *authoritative history*.

It is also important to appreciate the role of the OSD map
(list of all known OSDs and their states, as well as some
information about the placement groups) in the *peering*
process:

   When OSDs go up or down (or get added or removed)
   this has the potential to affect the *active sets*
   of many placement groups.

   Before a primary successfully completes the *peering*
   process, the OSD map must reflect that the OSD was alive
   and well as of the first epoch in the *current interval*.

   Changes can only be made after successful *peering*.

Thus, a new primary can use the latest OSD map along with a recent
history of past maps to generate a set of *past intervals* to
determine which OSDs must be consulted before we can successfully
*peer*.  The set of past intervals is bounded by *last epoch started*,
the most recent *past interval* for which we know *peering* completed.
The process by which an OSD discovers a PG exists in the first place is
by exchanging *PG info* messages, so the OSD always has some lower
bound on *last epoch started*.

The high level process is for the current PG primary to:

  1. get a recent OSD map (to identify the members of the all
     interesting *acting sets*, and confirm that we are still the
     primary).

  #. generate a list of *past intervals* since *last epoch started*.
     Consider the subset of those for which *up_thru* was greater than
     the first interval epoch by the last interval epoch's OSD map; that is,
     the subset for which *peering* could have completed before the *acting
     set* changed to another set of OSDs.

     Successful *peering* will require that we be able to contact at
     least one OSD from each of *past interval*'s *acting set*.

  #. ask every node in that list for its *PG info*, which includes the most
     recent write made to the PG, and a value for *last epoch started*.  If
     we learn about a *last epoch started* that is newer than our own, we can
     prune older *past intervals* and reduce the peer OSDs we need to contact.

  #. if anyone else has (in its PG log) operations that I do not have,
     instruct them to send me the missing log entries so that the primary's
     *PG log* is up to date (includes the newest write)..

  #. for each member of the current *acting set*:

     a. ask it for copies of all PG log entries since *last epoch start*
	so that I can verify that they agree with mine (or know what
	objects I will be telling it to delete).

	If the cluster failed before an operation was persisted by all
	members of the *acting set*, and the subsequent *peering* did not
	remember that operation, and a node that did remember that
	operation later rejoined, its logs would record a different
	(divergent) history than the *authoritative history* that was
	reconstructed in the *peering* after the failure.

	Since the *divergent* events were not recorded in other logs
	from that *acting set*, they were not acknowledged to the client,
	and there is no harm in discarding them (so that all OSDs agree
	on the *authoritative history*).  But, we will have to instruct
	any OSD that stores data from a divergent update to delete the
	affected (and now deemed to be apocryphal) objects.

     #. ask it for its *missing set* (object updates recorded
	in its PG log, but for which it does not have the new data).
	This is the list of objects that must be fully replicated
	before we can accept writes.

  #. at this point, the primary's PG log contains an *authoritative history* of
     the placement group, and the OSD now has sufficient
     information to bring any other OSD in the *acting set* up to date.

  #. if the primary's *up_thru* value in the current OSD map is not greater than
     or equal to the first epoch in the *current interval*, send a request to the
     monitor to update it, and wait until receive an updated OSD map that reflects
     the change.

  #. for each member of the current *acting set*:

     a. send them log updates to bring their PG logs into agreement with
	my own (*authoritative history*) ... which may involve deciding
	to delete divergent objects.

     #. await acknowledgment that they have persisted the PG log entries.

  #. at this point all OSDs in the *acting set* agree on all of the meta-data,
     and would (in any future *peering*) return identical accounts of all
     updates.

     a. start accepting client write operations (because we have unanimous
	agreement on the state of the objects into which those updates are
	being accepted).  Note, however, that if a client tries to write to an
        object it will be promoted to the front of the recovery queue, and the
        write willy be applied after it is fully replicated to the current *acting set*.

     #. update the *last epoch started* value in our local *PG info*, and instruct
	other *active set* OSDs to do the same.

     #. start pulling object data updates that other OSDs have, but I do not.  We may
	need to query OSDs from additional *past intervals* prior to *last epoch started*
	(the last time *peering* completed) and following *last epoch clean* (the last epoch that
	recovery completed) in order to find copies of all objects.

     #. start pushing object data updates to other OSDs that do not yet have them.

	We push these updates from the primary (rather than having the replicas
	pull them) because this allows the primary to ensure that a replica has
	the current contents before sending it an update write.  It also makes
	it possible for a single read (from the primary) to be used to write
	the data to multiple replicas.  If each replica did its own pulls,
	the data might have to be read multiple times.

  #. once all replicas store the all copies of all objects (that
     existed prior to the start of this epoch) we can update *last
     epoch clean* in the *PG info*, and we can dismiss all of the
     *stray* replicas, allowing them to delete their copies of objects
     for which they are no longer in the *acting set*.

     We could not dismiss the *strays* prior to this because it was possible
     that one of those *strays* might hold the sole surviving copy of an
     old object (all of whose copies disappeared before they could be
     replicated on members of the current *acting set*).

Generate a State Model
----------------------

Use the `gen_state_diagram.py <https://github.com/ceph/ceph/blob/master/doc/scripts/gen_state_diagram.py>`_ script to generate a copy of the latest peering state model::

        $ git clone https://github.com/ceph/ceph.git
        $ cd ceph
        $ cat src/osd/PeeringState.h src/osd/PeeringState.cc | doc/scripts/gen_state_diagram.py > doc/dev/peering_graph.generated.dot
        $ sed -i 's/7,7/1080,1080/' doc/dev/peering_graph.generated.dot
        $ dot -Tsvg doc/dev/peering_graph.generated.dot > doc/dev/peering_graph.generated.svg

Sample state model:

.. graphviz:: peering_graph.generated.dot