This document is for a development version of Ceph.

Monitor Elections

The Original Algorithm

Historically, monitor leader elections have been very simple: the lowest-ranked monitor wins!

This is accomplished using a low-state “Elector” module (though it has now been split into an Elector that handles message-passing, and an ElectionLogic that makes the voting choices). It tracks the election epoch and not much else. Odd epochs are elections; even epochs have a leader and let the monitor do its ongoing work. When a timeout occurs or the monitor asks for a new election, we bump the epoch and send out Propose messages to all known monitors. In general, if we receive an old message we either drop it or trigger a new election (if we think the sender is newly-booted and needs to join quorum). If we receive a message from a newer epoch, we bump up our epoch to match and either Defer to the Proposer or else bump the epoch again and Propose ourselves if we expect to win over them. When we receive a Propose within our current epoch, we either Defer to the sender or ignore them (we ignore them if they are of a higher rank than us, or higher than the rank we have already deferred to). (Note that if we have the highest rank it is possible for us to defer to every other monitor in sequence within the same election epoch!)

This resolves under normal circumstances because all monitors agree on the priority voting order, and epochs are only bumped when a monitor isn’t participating or sees a possible conflict with the known proposers.

The Problems

The original algorithm didn’t work at all under a variety of netsplit conditions. This didn’t manifest often in practice but has become important as the community and commercial vendors move Ceph into spaces requiring the use of “stretch clusters”.

The New Algorithms

We still default to the original (“classic”) election algorithm, but support letting users change to new ones via the CLI. These algorithms are implemented as different functions and switch statements within the ElectionLogic class.

The first algorithm is very simple: “disallow” lets you add monitors to a list of disallowed leaders. The second, “connectivity”, incorporates connection score ratings and elects the monitor with the best score.

Algorithm: disallow

If a monitor is in the disallowed list, it always defers to another monitor, no matter the rank. Otherwise, it is the same as the classic algorithm is. Since changing the disallowed list requires a paxos update, monitors in an election together should always have the same set. This means the election order is constant and static across the full monitor set and elections resolve trivially (assuming a connected network).

This algorithm really just exists as a demo and stepping-stone to the more advanced connectivity mode, but it may have utility in asymmetric networks and clusters.

Algorithm: connectivity

This algorithm takes as input scores for each connection (both ways, discussed in the next section) and attempts to elect the monitor with the highest total score. We keep the same basic message-passing flow as the classic algorithm, in which elections are driven by reacting to Propose messages. But this has several challenges since unlike ranks, scores are not static (and might change during an election!). To guarantee an election epoch does not produce multiple leaders, we must maintain two key invariants: * Monitors must maintain static scores during an election epoch * Any deferral must be transitive -- if A defers to B and then to C, B had better defer to C as well!

We handle these very explicitly: by branching a copy stable_peer_tracker of our peer_tracker scoring object whenever starting an election (or bumping the epoch), and by refusing to defer to a monitor if it won’t be deferred to by our current leader choice. (All Propose messages include a copy of the scores the leader is working from, so peers can evaluate them.)

Of course, those modifications can easily block. To guarantee forward progress, we make several further adjustments: * If we want to defer to a new peer, but have already deferred to a peer whose scores don’t allow that, we bump the election epoch and start() the election over again. * All election messages include the scores the sender is aware of.

This guarantees we will resolve the election as long as the network is reasonably stable (even if disconnected): As long as all score “views” result in the same deferral order, an election will complete normally. And by broadly sharing scores across the full set of monitors, monitors rapidly converge on the global newest state.

This algorithm has one further important feature compared to the classic and disallowed handlers: it can ignore out-of-quorum peers. Normally, whenever a monitor B receives a Propose from an out-of-quorum peer C, B will itself trigger a new election to give C an opportunity to join. But because the highest-scoring monitor A may be netsplit from C, this is not desirable. So in the connectivity election algorithm, B only “forwards” Propose messages when B’s scores indicate the cluster would choose a leader other than A.

Connection Scoring

We implement scoring within the ConnectionTracker class, which is driven by the Elector and provided to ElectionLogic as a resource. Elector is responsible for sending out MMonPing messages, and for reporting the results in to the ConnectionTracker as calls to report_[live|dead]_connection with the relevant peer and the time units the call counts for. (These time units are seconds in the monitor, but the ConnectionTracker is agnostic and our unit tests count simple time steps.)

We configure a “half life” and each report updates the peer’s current status (alive or dead) and its total score. The new score is current_score * (1 - units_alive / (2 * half_life)) + (units_alive / (2 * half_life)). (For a dead report, we of course subtract the new delta, rather than adding it).

We can further encode and decode the ConnectionTracker for wire transmission, and receive_peer_report()s of a full ConnectionTracker (containing all known scores) or a ConnectionReport (representing a single peer’s scores) to slurp up the scores from peers. These scores are of course all versioned so we are in no danger of accidentally going backwards in time. We can query an individual connection score (if the connection is down, it’s 0) or the total score of a specific monitor, which is the connection score from all other monitors going in to that one.

By default, we consider pings failed after 2 seconds (mon_elector_ping_timeout) and ping live connections every second (mon_elector_ping_divisor). The halflife is 12 hours (mon_con_tracker_score_halflife).

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