THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
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type | zxid | data | notation | meaning |
|---|---|---|---|---|
SERVERINFO(11) aka FOLLOWERINFO | last zxid accepted | Info | SERVERINFO(lastZxid, info) | The follower has accepted up to lastZxid. |
NEWEPOCH(9) | newEpoch << 32 | n/a | NEWEPOCH(newEpoch) | A proposal to move to a new epoch. |
DIFF(13) | last committed zxid | n/a | DIFF(lastCommittedZxid) | lastCommittedZxid is the last zxid committed by the leader. |
TRUNC(14) | truncZxid | n/a | TRUNC(truncZxid) | Truncate the history to truncZxid |
SNAP(15) | n/a | n/a | SNAP | A state transfer (aka snapshot) will be sent to the follower. this will be a fuzzy state transfer that may include zxids being sent to the follower. The state transfer will immediately follow this packet. |
OBSERVERINFO(16) | last zxid accepted | n/a | OBSERVERINFO(lastZxid) | The observer has accepted up to lastZxid. |
NEWLEADER(10) | lastZxid | n/a | NEWLEADER(lastZxid) | Accept this leader as the leader of the epoch of lastZxid. |
UPTODATE(12) | n/a | n/a | UPTODATE | The follower is now uptodate enough to begin serving clients. |
PROPOSAL(2) | zxid of proposal | proposed message | PROPOSAL(zxid, data) | Propose a message. (Request that it be accepted into a followers history.) |
ACK(3) | zxid of proposal to ack | n/a | ACK(zxid) | Everything sent to the follower by the leader up to zxid has been accepted into its history (logged to disk). |
COMMIT(4) | zxid of proposal to commit | n/a | COMMIT(zxid) | Everything in the followers history up to zxid should be committed (aka delivered). |
INFORM(8) | zxid of proposal | data of proposal | INFORM(zxid, data) | Deliver the data. (Only used with observers.) i |
Zab servers have the following state:
Name | Meaning |
|---|---|
history | an on disk log of proposals accepted |
lastZxid | zxid of the last proposal in the history |
acceptedEpoch | the epoch number of the last NEWEPOCH packet accepted |
currentEpoch | the epoch number of the last NEWLEADER packet accepted |
Implementation assumptions
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- l The leader starts accepting connections from followers.
- f Followers connect the the leader and send SERVERINFO(lastZxid, info).
- l The leader sends SERVERINFO(lastZxid, info) to followers that connect.
- l The leader waits for a quorum of followers to connect all of whom has:
- f.currentEpoch <= l.currentEpoch
- if f.currentEpoch == l.currentEpoch, then f.lastZxid < l.lastZxid
- l Once the leader has quorum, it stops accepting connections, and sends NEWEPOCH(e) to all followers, where e is greater than all f.acceptedEpoch in the quorum.
- f Followers send ACK(e<<32) if e > f.acceptedEpoch. (If e < f.acceptedEpoch, the follower will abandon the leader. If e == f.acceptedEpoch, the follower will not send ACK(e<<32), but will still maintain the connection to the leader.)
- l The leader waits for all followers to ack NEWEPOCH(e)
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