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:
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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