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zhaozhao.zz authored
Durability of database is a big and old topic, in this regard Redis use AOF to support it, and `appendfsync=alwasys` policy is the most strict level, guarantee all data is both written and synced on disk before reply success to client. But there are some cases have been overlooked, and could lead to durability broken. 1. The most clear one is about threaded-io mode we should also set client's write handler with `ae_barrier` in `handleClientsWithPendingWritesUsingThreads`, or the write handler would be called after read handler in the next event loop, it means the write command result could be replied to client before flush to AOF. 2. About blocked client (mostly by module) in `beforeSleep()`, `handleClientsBlockedOnKeys()` should be called before `flushAppendOnlyFile()`, in case the unblocked clients modify data without persistence but send reply. 3. When handling `ProcessingEventsWhileBlocked` normally it takes place when lua/function/module timeout, and we give a chance to users to kill the slow operation, but we should call `flushAppendOnlyFile()` before `handleClientsWithPendingWrites()`, in case the other clients in the last event loop get acknowledge before data persistence. for a instance: ``` in the same event loop client A executes set foo bar client B executes eval "for var=1,10000000,1 do end" 0 ``` after the script timeout, client A will get `OK` but lose data after restart (kill redis when timeout) if we don't flush the write command to AOF. 4. A more complex case about `ProcessingEventsWhileBlocked` it is lua timeout in transaction, for example `MULTI; set foo bar; eval "for var=1,10000000,1 do end" 0; EXEC`, then client will get set command's result before the whole transaction done, that breaks atomicity too. fortunately, it's already fixed by #5428 (although it's not the original purpose just a side effect : )), but module timeout should be fixed too. case 1, 2, 3 are fixed in this commit, the module issue in case 4 needs a followup PR.
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