Mercurial > lbo > hg > clusterconsensus
view README.md @ 5:3f7cb34bdc63
Implement Add/RemoveParticipant on client side
author | Lewin Bormann <lbo@spheniscida.de> |
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date | Fri, 07 Oct 2016 23:38:17 +0200 |
parents | 23272f45c9f4 |
children | a8a747e3e568 |
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# clusterconsensus A library implementing a replicated state machine with generic state machine, transport and state changes. For example, a using application could use this package with protocol buffers, leveldb and gRPC; and another one might use JSON, in-memory state and HTTP. ## Protocol The protocol is a custom Paxos implementation. It works like this, basically: ### Bootstrapping * A set of participants is started. Participant `A`'s `ConsensusServer` receives a StartParticipation request. This request is `StartParticipation(0, 0, Member{addr: "A"}, [Member{addr: "A"}], []byte{})` (i.e., an empty StartParticipation request) * Now one participant is running, and sees that it itself is the master. It now finds out in some way which other participants to add, and calls `AddMember()` on itself. The library will arrange a process by which a new member is added to the consensus round. * Repeat this for other members ### Normal operation We assume that the current instance is 12, and the sequence number in that instance is 34. * One participant wants to modify the state. It calls `Submit()` on the `Participant` with one or more changes. * If the Participant is not master, it will send the request to the master using the `Submit()` method on the stub. * If the Participant is the master, or has received a `Submit` request, it will proceed by coordinating the change. * First, all non-master participants are sent `Accept(12, 35, []Change{ *some change* })`. * This leads to the non-master participants *staging* that change. * The next time the master wants to apply a change, it sends `Accept(12, 36, []Change{ *some new change* })`. This leads to non-master participants "committing" (i.e. applying) all staged changes before sequence `36` to the state machine. ### Master has crashed Current instance: 22 Current sequence: 44 * A participant, "B", wants to modify the state, but can't reach the master to `Submit()` a change. It proceeds by becoming a candidate. It sends `Prepare(23, Member{ addr: "B" })` to all known members. If a majority replies within the deadline with a positive vote, then the `B` is the new master. * In order for all participants to know about this change, `B` submits a request `Accept(23, 1, []Change{})` which confirms the previous `Prepare()` call (because of the new instance in the `Accept` request) * Now everyone knows of the new master, and the cluster continues in *Normal operation*. ### Adding/removing members * Membership changes are relatively straight-forward; they are just special changes that don't use `Accept()`, but rather the `AddMember()` and `RemoveMember()` methods in `ParticipantStub`.