A very commonly signaled operational problem with Redis master-replicas
sets is that, once the master becomes unavailable for some reason,
especially because of network problems, many times it wont be able to
perform a partial resynchronization with the new master, once it rejoins
the partition, for the following reason:
1. The master becomes isolated, however it keeps sending PINGs to the
replicas. Such PINGs will never be received since the link connection is
actually already severed.
2. On the other side, one of the replicas will turn into the new master,
setting its secondary replication ID offset to the one of the last
command received from the old master: this offset will not include the
PINGs sent by the master once the link was already disconnected.
3. When the master rejoins the partion and is turned into a replica, its
offset will be too advanced because of the PINGs, so a PSYNC will fail,
and a full synchronization will be required.
Related to issue #7002 and other discussion we had in the past around
this problem.
Before this commit, when upgrading a replica, expired keys will not
be loaded, thus causing replica having less keys in db. To this point,
master and replica's keys is logically consistent. However, before
the keys in master and replica are physically consistent, that is,
they have the same dbsize, if master got a problem and the replica
got promoted and becomes new master of that partition, and master
updates a key which does not exist on master, but physically exists
on the old master(new replica), the old master would refuse to update
the key, thus causing master and replica data inconsistent.
How could this happen?
That's all because of the wrong judgement of roles while starting up
the server. We can not use server.masterhost to judge if the server
is master or replica, since it fails in cluster mode.
When we start the server, we load rdb and do want to load expired keys,
and do not want to have the ability to active expire keys, if it is
a replica.
1. Call emptyDb even in case of diskless-load: We want modules
to get the same FLUSHDB event as disk-based replication.
2. Do not fire any module events when flushing the backups array.
3. Delete redundant call to signalFlushedDb (Called from emptyDb).
Because "keymiss" is "special" compared to the rest of
the notifications (Trying not to break existing apps
using the 'A' format for notifications)
Also updated redis.conf and module.c docs
