Sentinel's main safety argument is that there are no two configurations
for the same master with the same version (configuration epoch).
For this to be true Sentinels require to be authorized by a majority.
Additionally Sentinels require to do two important things:
* Never vote again for the same epoch.
* Never exchange an old vote for a fresh one.
The first prerequisite, in a crash-recovery system model, requires to
persist the master->leader_epoch on durable storage before to reply to
messages. This was not the case.
We also make sure to persist the current epoch in order to never reply
to stale votes requests from other Sentinels, after a recovery.
The configuration is persisted by making use of fsync(), this is
considered in the context of this code a good enough guarantee that
after a restart our durable state is restored, however this may not
always be the case depending on the kind of hardware and operating
system used.
Now the way HELLO messages are received is unified.
Now it is no longer needed for Sentinels to converge to the higher
configuration for a master to be able to chat via some Redis instance,
the are able to directly exchanges configurations.
Note that this commit does not include the (trivial) change needed to
send HELLO messages to Sentinel instances as well, since for an error I
committed the change in the previous commit that refactored hello
messages processing into a separated function.
Example: if the user will try to configure a cluster with 9 nodes,
asking for 1 slave for master, redis-trib will configure a 4 masters
cluster with 1 slave each as usually, but this time will assign the
spare node as a slave of one of the masters.
By manually modifying nodes configurations in random ways, it is possible
to create the following scenario:
A is serving keys for slot 10
B is manually configured to serve keys for slot 10
A receives an update from B (or another node) where it is informed that
the slot 10 is now claimed by B with a greater configuration epoch,
however A still has keys from slot 10.
With this commit A will put the slot in error setting it in IMPORTING
state, so that redis-trib can detect the issue.
The new "error" subcommand of the DEBUG command can reply with an user
selected error, specified as its sole argument:
DEBUG ERROR "LOADING please wait..."
The error is generated just prefixing the command argument with a "-"
character, and replacing newlines with spaces (since error replies can't
include newlines).
The goal of the command is to help in Client libraries unit tests by
making simple to simulate a command call triggering a given error.
getKeysFromCommand() is designed to be called with the command arguments
passing the basic arity checks described in the command table.
DEBUG CMDKEYS must provide the same guarantees for calling
getKeysFromCommand() to be safe.
Examples:
redis 127.0.0.1:6379> debug cmdkeys set foo bar
1) "foo"
redis 127.0.0.1:6379> debug cmdkeys mget a b c
1) "a"
2) "b"
3) "c"
redis 127.0.0.1:6379> debug cmdkeys zunionstore foo 2 a b
1) "a"
2) "b"
3) "foo"
redis 127.0.0.1:6379> debug cmdkeys ping
(empty list or set)
There is the exception of a "constant" BY pattern that is used in order
to signal to don't sort at all. In this case no lookup is needed so it
is possible to support this case in Cluster mode.
Previously we used zunionInterGetKeys(), however after this function was
fixed to account for the destination key (not needed when the API was
designed for "diskstore") the two set of commands can no longer be served
by an unique keys-extraction function.
This API originated from the "diskstore" experiment, not for Redis
Cluster itself, so there were legacy/useless things trying to
differentiate between keys that are going to be overwritten and keys
that need to be fetched from disk (preloaded).
All useless with Cluster, so removed with the result of code
simplification.
The code was already correct but it was using that bindaddr[0] is set to
NULL as a side effect of current implementation if no bind address is
configured. This is not guarnteed to hold true in the future.
When node-timeout is too small, in the order of a few milliseconds,
there is no way the voting process can terminate during that time, so we
set a lower limit for the failover timeout of two seconds.
The retry time is set to two times the failover timeout time, so it is
at least 4 seconds.
The previous implementation wasn't taking into account
the storage key in position 1 being a requirement (it
was only counting the source keys in positions 3 to N).
Fixesantirez/redis#1581
This value needs to be set to zero (in addition to
stat_numcommands) or else people may see
a negative operations per second count after they
run CONFIG RESETSTAT.
Fixesantirez/redis#1577
