• antirez's avatar
    Transactions: propagate MULTI/EXEC only when needed. · 611dcb56
    antirez authored
    MULTI/EXEC is now propagated to the AOF / Slaves only once we encounter
    the first command that is not a read-only one inside the transaction.
    
    The old behavior was to always propagate an empty MULTI/EXEC block when
    the transaction was composed just of read only commands, or even
    completely empty. This created two problems:
    
    1) It's a bandwidth waste in the replication link and a space waste
       inside the AOF file.
    
    2) We used to always increment server.dirty to force the propagation of
       the EXEC command, resulting into triggering RDB saves more often
       than needed.
    
    Note: even read-only commands may also trigger writes that will be
    propagated, when we access a key that is found expired and Redis will
    synthesize a DEL operation. However there is no need for this to stay
    inside the transaction itself, but only to be ordered.
    
    So for instance something like:
    
        MULTI
        GET foo
        SET key zap
        EXEC
    
    May be propagated into:
    
        DEL foo
        MULTI
        SET key zap
        EXEC
    
    While the DEL is outside the transaction, the commands are delivered in
    the right order and it is not possible for other commands to be inserted
    between DEL and MULTI.
    611dcb56
multi.c 11 KB