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The Booth Cluster Ticket Manager
=============
Booth manages tickets which authorize cluster sites located in
geographically dispersed locations to run resources. It
facilitates support of geographically distributed clustering in
Pacemaker.
Booth is based on the Raft consensus algorithm. Though the
implementation is not complete (there is no log) and there are a
few additions and modifications, booth guarantees that the ticket
is always available at just one site as long as it has exclusive
control of the tickets.
The git repository is available at github:
<https://github.com/ClusterLabs/booth>
github can also track issues or bug reports.
Description of a booth cluster
==============================
Booth cluster is a collection of cooperative servers
communicating using the booth protocol. The purpose of the booth
cluster is to manage cluster tickets. The booth cluster consists
of at least three servers.
A booth server can be either a site or an arbitrator. Arbitrators
take part in elections and so help resolve ties, but cannot hold
tickets.
The basic unit in the booth cluster is a ticket. Every
non-granted ticket is in the initial state on all servers. For
granted tickets, the server holding the ticket is the leader and
other servers are followers. The leader issues heartbeats and
ticket updates to the followers. The followers are required to
obey the leader.
Booth startup
------------
On startup, the booth process first loads tickets, if available,
from the CIB. Afterwards, it broadcasts a query to get tickets'
status from other servers. In-memory copies are updated from
the replies if they contain newer ticket data.
If the server discovers that itself is the ticket leader, it
tries to establish its authority again by broadcasting heartbeat.
If it succeeds, it continues as the leader for this ticket. The
other booth servers become followers. This procedure is possible
only immediately after the booth startup. It also serves as a
configuration reload.
Grant and revoke operations
------------
A ticket first has to be granted using the 'booth client grant'
command.
Obviously, it is not possible to grant a ticket which is
currently granted.
Ticket revoke is the operation which is the opposite of grant.
An administrative revoke may be started at any server, but the
operation itself happens only at the leader. If the leader is
unreachable, the ticket cannot be revoked. The user will need to
wait until the ticket expires.
A ticket grant may be delayed if not all sites are reachable.
The delay is the ticket expiry time extended by acquire-after, if
set. This is to ensure that the unreachable site relinquished the
ticket it may have been holding and stopped the corresponding
cluster resources.
If the user is absolutely sure that the unreachable site does not
hold the ticket, the delay may be skipped by using the '-F'
option of the 'booth grant' command.
If in effect, the grant delay time is shown in the 'booth list'
command output.
Ticket management and server operation
------------
A granted ticket is managed by the booth servers so that its
availability is maximized without breaking the basic guarantee
that the ticket is granted to one site only.
The server where the ticket is granted is the leader, the other
servers are followers. The leader occasionally sends heartbeats,
once every half ticket expiry under normal circumstances.
If a follower doesn't hear from the leader longer than the ticket
expiry time, it will consider the ticket lost, and try to acquire
it by starting new elections.
A server starts elections by broadcasting the REQ_VOTE RPC.
Other servers reply with the VOTE_FOR RPC, in which they record
its vote. Normally, the sender of the first REQ_VOTE gets the
vote of the receiver. Whichever server gets a majority of votes
wins the elections. On ties, elections are restarted. To
decrease chance of elections ending in a tie, a server waits for a
short random period before sending out the REQ_VOTE packets.
Everything else being equal, the server which sends REQ_VOTE
first gets elected.
Elections are described in more detail in the raft paper at
<https://ramcloud.stanford.edu/wiki/download/attachments/11370504/raft.pdf>.
Ticket renewal (or update) is a two-step process. Before actually
writing the ticket to the CIB, the server holding the ticket
first tries to establish that it still has the majority for that
ticket. That is done by broadcasting a heartbeat. If the server
receives enough acknowledgements, it then stores the ticket to
the CIB and broadcasts the UPDATE RPC with updated ticket expiry
time so that the followers can update local ticket copies. Ticket
renewals are configurable and by default set to half ticket
expire time.
Before ticket renewal, the leader runs an external program if
such program is set in 'before-acquire-handler'. The external
program should ensure that the cluster managed service which is
protected by this ticket can run at this site. If that program
fails, the leader relinquishes the ticket. It announces its
intention to step down by broadcasting an unsolicited VOTE_FOR
with an empty vote. On receiving such RPC other servers start new
elections to elect a new leader.
Split brain
------------
On split brains two possible issues arise: leader in minority and
follower disconnected from the leader.
Let's take a look at the first one. The leader in minority
eventually expires the ticket because it cannot receieve majority
of acknowledgements in reply to its heartbeats. The other
partition runs elections (at about the same time, as they find
the ticket lost after its expiry) and, if it can get the
majority, the elections winner becomes a new leader for the
ticket. After split brain gets resolved, the old leader will
become follower as soon as it receives heartbeat from the new
leader. Note the timing: the old leader releases the ticket at
around the same time as when new elections in the other partition
are held. This is because the leader ensures that the ticket
expire time is always the same on all servers in the booth
cluster.
The second situation, where a follower is disconnected from the
leader, is a bit more difficult to handle. After the ticket
expiry time, the follower will consider the ticket lost and start
new elections. The elections repeatedly get restarted until the
split brain is resolved. Then, the rest of the cluster send
rejects in reply to REQ_VOTE RPC because the ticket is still
valid and therefore couldn't have been lost. They know that
because the reason for elections is included with every REQ_VOTE.
Short intermittent split brains are handled well because the
leader keeps resending heartbeats until it gets replies from all
servers serving sites.
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