Now both master and replicas keep track of the last replication offset
that contains meaningful data (ignoring the tailing pings), and both
trim that tail from the replication backlog, and the offset with which
they try to use for psync.
the implication is that if someone missed some pings, or even have
excessive pings that the promoted replica has, it'll still be able to
psync (avoid full sync).
the downside (which was already committed) is that replicas running old
code may fail to psync, since the promoted replica trims pings form it's
backlog.
This commit adds a test that reproduces several cases of promotions and
demotions with stale and non-stale pings
Background:
The mearningful offset on the master was added recently to solve a problem were
the master is left all alone, injecting PINGs into it's backlog when no one is
listening and then gets demoted and tries to replicate from a replica that didn't
have any of the PINGs (or at least not the last ones).
however, consider this case:
master A has two replicas (B and C) replicating directly from it.
there's no traffic at all, and also no network issues, just many pings in the
tail of the backlog. now B gets promoted, A becomes a replica of B, and C
remains a replica of A. when A gets demoted, it trims the pings from its
backlog, and successfully replicate from B. however, C is still aware of
these PINGs, when it'll disconnect and re-connect to A, it'll ask for something
that's not in the backlog anymore (since A trimmed the tail of it's backlog),
and be forced to do a full sync (something it didn't have to do before the
meaningful offset fix).
Besides that, the psync2 test was always failing randomly here and there, it
turns out the reason were PINGs. Investigating it shows the following scenario:
cycle 1: redis #1 is master, and all the rest are direct replicas of #1
cycle 2: redis #2 is promoted to master, #1 is a replica of #2 and #3 is replica of #1
now we see that when #1 is demoted it prints:
17339:S 21 Apr 2020 11:16:38.523 * Using the meaningful offset 3929963 instead of 3929977 to exclude the final PINGs (14 bytes difference)
17339:S 21 Apr 2020 11:16:39.391 * Trying a partial resynchronization (request e2b3f8817735fdfe5fa4626766daa938b61419e5:3929964).
17339:S 21 Apr 2020 11:16:39.392 * Successful partial resynchronization with master.
and when #3 connects to the demoted #2, #2 says:
17339:S 21 Apr 2020 11:16:40.084 * Partial resynchronization not accepted: Requested offset for secondary ID was 3929978, but I can reply up to 3929964
so the issue here is that the meaningful offset feature saved the day for the
demoted master (since it needs to sync from a replica that didn't get the last
ping), but it didn't help one of the other replicas which did get the last ping.
When deffered reply is added the previous reply node cannot be used so
all the extra space we allocated in it is wasted. in case someone uses
deffered replies in a loop, each time adding a small reply, each of
these reply nodes (the small string reply) would have consumed a 16k
block.
now when we add anther diferred reply node, we trim the unused portion
of the previous reply block.
see #7123
Related to #5145.
Design note: clients may change type when they turn into replicas or are
moved into the Pub/Sub category and so forth. Moreover the recomputation
of the bytes used is problematic for obvious reasons: it changes
continuously, so as a conservative way to avoid accumulating errors,
each client remembers the contribution it gave to the sum, and removes
it when it is freed or before updating it with the new memory usage.
Makse sure call() doesn't wrap replicated commands with
a redundant MULTI/EXEC
Other, unrelated changes:
1. Formatting compiler warning in INFO CLIENTS
2. Use CLIENT_ID_AOF instead of UINT64_MAX
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
This bug is from the first version of Redis. Probably the problem here
is that before we used an SDS split function that created empty strings
for additional spaces, like in "SET foo bar".
AFAIK later we replaced it with the curretn sdssplitarg() API that has
no such a problem. As a result, we introduced a bug, where it is no
longer possible to do something like:
SET foo ""
Using the inline protocol. Now it is fixed.
looks like each platform implements long double differently (different bit count)
so we can't save them as binary, and we also want to avoid creating a new RDB
format version, so we save these are hex strings using "%La".
This commit includes a change in the arguments of ld2string to support this.
as well as tests for coverage and short reads.
coded by @guybe7
* replication hooks: role change, master link status, replica online/offline
* persistence hooks: saving, loading, loading progress
* misc hooks: cron loop, shutdown, module loaded/unloaded
* change the way hooks test work, and add tests for all of the above
startLoading() now gets flag indicating what is loaded.
stopLoading() now gets an indication of success or failure.
adding startSaving() and stopSaving() with similar args and role.
misc:
- handle SSL_has_pending by iterating though these in beforeSleep, and setting timeout of 0 to aeProcessEvents
- fix issue with epoll signaling EPOLLHUP and EPOLLERR only to the write handlers. (needed to detect the rdb pipe was closed)
- add key-load-delay config for testing
- trim connShutdown which is no longer needed
- rioFdsetWrite -> rioFdWrite - simplified since there's no longer need to write to multiple FDs
- don't detect rdb child exited (don't call wait3) until we detect the pipe is closed
- Cleanup bad optimization from rio.c, add another one
