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Add replication notes

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Paul Dix 2013-10-09 11:21:52 -04:00
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@ -67,10 +67,16 @@ Modules
| | | | | | | |
+--------+-----------+ +-------+------------+ +--------+-----------+ +-------+------------+
Concensus Notes Replication & Concensus Notes
--------------- -----------------------------
Two state machines: Single raft cluster for which machines are in cluster and who owns which locations.
* 1 for the entire cluster of which machines are taking which portions of the ring 1. When a write comes into a server, figure out which machine owns the data, proxy out to that.
* 1 for each portion of the ring to replicate the operations 2. The machine proxies to the server, which assigns a sequence number
sequence number per ring location? that's the concensus, if they don't agree then request a replay from the last known sequence number 3. Each machine in the cluster asks the other machines that own hash ring locations what their latest sequence number is every 10 seconds (this is read repair)
For example, take machines A, B, and C. Say B and C own ring location #2. If a write comes into A it will look up the configuration and pick B or C at random to proxy the write to. Say it goes to B. B assigns a sequence number of 1. It keeps a log for B2 of the writes. It will also keep a log for C2's writes. It then tries to write #1 to C.
If the write is marked as a quorum write, then B won't return a success to A until the data has been written to both B and C. Every so often both B and C will ask each other what their latest writes are.
Taking the example further, if we had server D that also owned ring location 2. B would ask C for writes to C2. If C is down it will ask D for writes to C2. This will ensure that if C fails no data will be lost.