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.
Redis refusing to run MULTI or EXEC during script timeout may cause partial
transactions to run.
1) if the client sends MULTI+commands+EXEC in pipeline without waiting for
response, but these arrive to the shards partially while there's a busy script,
and partially after it eventually finishes: we'll end up running only part of
the transaction (since multi was ignored, and exec would fail).
2) similar to the above if EXEC arrives during busy script, it'll be ignored and
the client state remains in a transaction.
the 3rd test which i added for a case where MULTI and EXEC are ok, and
only the body arrives during busy script was already handled correctly
since processCommand calls flagTransaction
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. server.repl_no_slaves_since can be set when a MONITOR client disconnects
2. c->repl_ack_time can be set by a newline from a MONITOR client
3. Improved comments
SELECT, and HELLO are commands that may be executed by the client
as soon as it connects, there's no reason to block them, preventing the
client from doing the rest of his sequence (which might just be INFO or
CONFIG, etc).
MONITOR, DEBUG, SLOWLOG, TIME, LASTSAVE are all non-data accessing
commands, which there's no reason to block.
Checking OOM by `getMaxMemoryState` inside script might get different result
with `freeMemoryIfNeededAndSafe` at script start, because lua stack and
arguments also consume memory.
This leads to memory `borderline` when memory grows near server.maxmemory:
- `freeMemoryIfNeededAndSafe` at script start detects no OOM, no memory freed
- `getMaxMemoryState` inside script detects OOM, script aborted
We solve this 'borderline' issue by saving OOM state at script start to get
stable lua OOM state.
related to issue #6565 and #5250.
So error message `ERR only (P)SUBSCRIBE / (P)UNSUBSCRIBE / PING / QUIT allowed in this context` will become
`ERR 'get' command submitted, but only (P)SUBSCRIBE / (P)UNSUBSCRIBE / PING / QUIT allowed in this context`
since the refactory of config.c, it was initialized from config_hz in initServer
but apparently that's too late since the config file loading creates objects
which call LRU_CLOCK
This message is there for ten years, but is hardly useful.
Moreover it is likely that it will fill an entire disk if log ratation
is not configured, for no good reasons.
Changes in behavior:
- Change server.stream_node_max_entries from int64_t to long long, so that it can be used by the generic infra
- standard error reply instead of "repl-backlog-size must be 1 or greater" and such
- tls-port and a few TLS booleans were readable (config get) even when USE_OPENSSL was off (now they aren't)
- syslog-enabled, syslog-ident, cluster-enabled, appendfilename, and supervised didn't have a get (now they do)
- pidfile was initialized to NULL in InitServerConfig but had CONFIG_DEFAULT_PID_FILE in rewriteConfig (so the real default was "", but rewrite would cause it to be set), fixed the rewrite.
- TLS config in server.h was uninitialized (if no tls config args were provided)
Adding test for sanity and coverage
- add capability for each config to have a callback to check if value is valid and return error string
will enable converting many of the remaining custom configs into generic ones (reducing the x4 repetition for set,get,config,rewrite)
- add capability for each config to to run some update code after config is changed (only for CONFIG SET)
will also enable converting many of the remaining custom configs into generic ones
- add capability to move default values from server.h and server.c to config.c
will reduce many excess lines in server.h and server.c (plus, no need to rebuild the entire code base when a default change 8-))
other behavior changes:
- fix bug in bool config get (always returning 'yes')
- fix a bug in modifying jemalloc-bg-thread at runtime (didn't call set_jemalloc_bg_thread, due to bad merge conflict resolution (my fault))
- side effect when a failed attempt to enable activedefrag at runtime, we now respond with -ERR and not with -DISABLED
Random command like SPOP with count is replicated as
some SREM operations, and store them in also_propagate
array to propagate after the call, but this would break
atomicity.
To keep the command's atomicity, wrap also_propagate
array with MULTI/EXEC.
Is it sufficient... ? -- Yes it is. In standalone mode, we say READY=1
at the comment point; however in replicated mode, we delay sending
READY=1 until the replication sync completes.
