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Consider the following scenario: Coordinator had started a createTable operation and wrote the first entry corresponding to that operation in log. After this, some master, say M1 crashes. It may or may not have already started recovering in parallel. What to do when we get to the part where we have to actually add tablet map entries and assign tablets?
Possible solution 1: Recompute mapping.
Recompute T1 mapping, and log entire mapping. (Might be a little tricky implementation-wise.)
Problem: What if, before crash, T1 had already been successfully assigned to M1, but the entire createTable had not completed so we hadn't logged the operation to be done. Since it had been successfully assigned, a (timely) recovery of M1 would've recovered T1 on some other master.
Possible solution 2: Just add entries to tabletMap and recovery will take care of it. (Solution for previous failure scenario.)
Problem: This works only if recovery for M1 will start after we have added this entry to our tabletMap. However, the recovery could've already started or even completed. Adding this entry would basically refer to a server that will never exist or be recovered.
Possible solution 3: Add entries to tabletMap before logging to LogCabin.
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Problem: Suppose coordinator crashes after first part, and then M1 crashes. That means that M1 will not be recovered before the new coordinator starts and completes its recovery. When we get to recovery of this createTable, we have T1 -> M1 and M1 doesn't exist, and if we skip this without doing anything, then T1 will never be assigned to anyone.
Possible solution 4: Hold lock on serverlist for a short while such that we can use solution 2.
Hold lock on serverlist, compute mappings, add mappings to local coordinator state, release lock, log to logcabin. D says that we probably don't need timestamps for a new table, need to ask S. Even if we do, its probably not imp, and we can put in a placeholder in the local mappings while i hold a lock on serverlist.
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