The original RDB serialization format was not parsable without the
module loaded, becuase the structure was managed only by the module
itself. Moreover RDB is a streaming protocol in the sense that it is
both produce di an append-only fashion, and is also sometimes directly
sent to the socket (in the case of diskless replication).
The fact that modules values cannot be parsed without the relevant
module loaded is a problem in many ways: RDB checking tools must have
loaded modules even for doing things not involving the value at all,
like splitting an RDB into N RDBs by key or alike, or just checking the
RDB for sanity.
In theory module values could be just a blob of data with a prefixed
length in order for us to be able to skip it. However prefixing the values
with a length would mean one of the following:
1. To be able to write some data at a previous offset. This breaks
stremaing.
2. To bufferize values before outputting them. This breaks performances.
3. To have some chunked RDB output format. This breaks simplicity.
Moreover, the above solution, still makes module values a totally opaque
matter, with the fowllowing problems:
1. The RDB check tool can just skip the value without being able to at
least check the general structure. For datasets composed mostly of
modules values this means to just check the outer level of the RDB not
actually doing any checko on most of the data itself.
2. It is not possible to do any recovering or processing of data for which a
module no longer exists in the future, or is unknown.
So this commit implements a different solution. The modules RDB
serialization API is composed if well defined calls to store integers,
floats, doubles or strings. After this commit, the parts generated by
the module API have a one-byte prefix for each of the above emitted
parts, and there is a final EOF byte as well. So even if we don't know
exactly how to interpret a module value, we can always parse it at an
high level, check the overall structure, understand the types used to
store the information, and easily skip the whole value.
The change is backward compatible: older RDB files can be still loaded
since the new encoding has a new RDB type: MODULE_2 (of value 7).
The commit also implements the ability to check RDB files for sanity
taking advantage of the new feature.
BACKGROUND AND USE CASEj
Redis slaves are normally write only, however the supprot a "writable"
mode which is very handy when scaling reads on slaves, that actually
need write operations in order to access data. For instance imagine
having slaves replicating certain Sets keys from the master. When
accessing the data on the slave, we want to peform intersections between
such Sets values. However we don't want to intersect each time: to cache
the intersection for some time often is a good idea.
To do so, it is possible to setup a slave as a writable slave, and
perform the intersection on the slave side, perhaps setting a TTL on the
resulting key so that it will expire after some time.
THE BUG
Problem: in order to have a consistent replication, expiring of keys in
Redis replication is up to the master, that synthesize DEL operations to
send in the replication stream. However slaves logically expire keys
by hiding them from read attempts from clients so that if the master did
not promptly sent a DEL, the client still see logically expired keys
as non existing.
Because slaves don't actively expire keys by actually evicting them but
just masking from the POV of read operations, if a key is created in a
writable slave, and an expire is set, the key will be leaked forever:
1. No DEL will be received from the master, which does not know about
such a key at all.
2. No eviction will be performed by the slave, since it needs to disable
eviction because it's up to masters, otherwise consistency of data is
lost.
THE FIX
In order to fix the problem, the slave should be able to tag keys that
were created in the slave side and have an expire set in some way.
My solution involved using an unique additional dictionary created by
the writable slave only if needed. The dictionary is obviously keyed by
the key name that we need to track: all the keys that are set with an
expire directly by a client writing to the slave are tracked.
The value in the dictionary is a bitmap of all the DBs where such a key
name need to be tracked, so that we can use a single dictionary to track
keys in all the DBs used by the slave (actually this limits the solution
to the first 64 DBs, but the default with Redis is to use 16 DBs).
This solution allows to pay both a small complexity and CPU penalty,
which is zero when the feature is not used, actually. The slave-side
eviction is encapsulated in code which is not coupled with the rest of
the Redis core, if not for the hook to track the keys.
TODO
I'm doing the first smoke tests to see if the feature works as expected:
so far so good. Unit tests should be added before merging into the
4.0 branch.
This means that stopping a slave and restarting it will still make it
able to PSYNC with the master. Moreover the master itself will retain
its ID/offset, in case it gets turned into a slave, or if a slave will
try to PSYNC with it with an exactly updated offset (otherwise there is
no backlog).
This change was possible thanks to PSYNC v2 that makes saving the current
replication state much simpler.
The gist of the changes is that now, partial resynchronizations between
slaves and masters (without the need of a full resync with RDB transfer
and so forth), work in a number of cases when it was impossible
in the past. For instance:
1. When a slave is promoted to mastrer, the slaves of the old master can
partially resynchronize with the new master.
2. Chained slalves (slaves of slaves) can be moved to replicate to other
slaves or the master itsef, without requiring a full resync.
3. The master itself, after being turned into a slave, is able to
partially resynchronize with the new master, when it joins replication
again.
In order to obtain this, the following main changes were operated:
* Slaves also take a replication backlog, not just masters.
* Same stream replication for all the slaves and sub slaves. The
replication stream is identical from the top level master to its slaves
and is also the same from the slaves to their sub-slaves and so forth.
This means that if a slave is later promoted to master, it has the
same replication backlong, and can partially resynchronize with its
slaves (that were previously slaves of the old master).
* A given replication history is no longer identified by the `runid` of
a Redis node. There is instead a `replication ID` which changes every
time the instance has a new history no longer coherent with the past
one. So, for example, slaves publish the same replication history of
their master, however when they are turned into masters, they publish
a new replication ID, but still remember the old ID, so that they are
able to partially resynchronize with slaves of the old master (up to a
given offset).
* The replication protocol was slightly modified so that a new extended
+CONTINUE reply from the master is able to inform the slave of a
replication ID change.
* REPLCONF CAPA is used in order to notify masters that a slave is able
to understand the new +CONTINUE reply.
* The RDB file was extended with an auxiliary field that is able to
select a given DB after loading in the slave, so that the slave can
continue receiving the replication stream from the point it was
disconnected without requiring the master to insert "SELECT" statements.
This is useful in order to guarantee the "same stream" property, because
the slave must be able to accumulate an identical backlog.
* Slave pings to sub-slaves are now sent in a special form, when the
top-level master is disconnected, in order to don't interfer with the
replication stream. We just use out of band "\n" bytes as in other parts
of the Redis protocol.
An old design document is available here:
https://gist.github.com/antirez/ae068f95c0d084891305
However the implementation is not identical to the description because
during the work to implement it, different changes were needed in order
to make things working well.
It was noted by @dvirsky that it is not possible to use string functions
when writing the AOF file. This sometimes is critical since the command
rewriting may need to be built in the context of the AOF callback, and
without access to the context, and the limited types that the AOF
production functions will accept, this can be an issue.
Moreover there are other needs that we can't anticipate regarding the
ability to use Redis Modules APIs using the context in order to build
representations to emit AOF / RDB.
Because of this a new API was added that allows the user to get a
temporary context from the IO context. The context is auto released
if obtained when the RDB / AOF callback returns.
Calling multiple time the function to get the context, always returns
the same one, since it is invalid to have more than a single context.
Technically as soon as Redis 64 bit gets proper support for loading
collections and/or DBs with more than 2^32 elements, the 32 bit version
should be modified in order to check if what we read from rdbLoadLen()
overflows. This would only apply to huge RDB files created with a 64 bit
instance and later loaded into a 32 bit instance.
This patch, written in collaboration with Oran Agra (@oranagra) is a companion
to 780a8b1. Together the two patches should avoid that the AOF and RDB saving
processes can be spawned at the same time. Previously conditions that
could lead to two saving processes at the same time were:
1. When AOF is enabled via CONFIG SET and an RDB saving process is
already active.
2. When the SYNC command decides to start an RDB saving process ASAP in
order to serve a new slave that cannot partially resynchronize (but
only if we have a disk target for replication, for diskless
replication there is not such a problem).
Condition "1" is not very severe but "2" can happen often and is
definitely good at degrading Redis performances in an unexpected way.
The two commits have the effect of always spawning RDB savings for
replication in replicationCron() instead of attempting to start an RDB
save synchronously. Moreover when a BGSAVE or AOF rewrite must be
performed, they are instead just postponed using flags that will try to
perform such operations ASAP.
Finally the BGSAVE command was modified in order to accept a SCHEDULE
option so that if an AOF rewrite is in progress, when this option is
given, the command no longer returns an error, but instead schedules an
RDB rewrite operation for when it will be possible to start it.
As Oran Agra suggested, in startBgsaveForReplication() when the BGSAVE
attempt returns an error, we scan the list of slaves in order to remove
them since there is no way to serve them currently.
However we check for the replication state BGSAVE_START, which was
modified by rdbSaveToSlaveSockets() before forking(). So when fork fails
the state of slaves remain BGSAVE_END and no cleanup is performed.
This commit fixes the problem by making rdbSaveToSlavesSockets() able to
undo the state change on fork failure.
In previous commits we moved the FULLRESYNC to the moment we start the
BGSAVE, so that the offset we provide is the right one. However this
also means that we need to re-emit the SELECT statement every time a new
slave starts to accumulate the changes.
To obtian this effect in a more clean way, the function that sends the
FULLRESYNC reply was overloaded with a more important role of also doing
this and chanigng the slave state. So it was renamed to
replicationSetupSlaveForFullResync() to better reflect what it does now.
This commit attempts to fix a bug involving PSYNC and diskless
replication (currently experimental) found by Yuval Inbar from Redis Labs
and that was later found to have even more far reaching effects (the bug also
exists when diskstore is off).
The gist of the bug is that, a Redis master replies with +FULLRESYNC to
a PSYNC attempt that fails and requires a full resynchronization.
However, the baseline offset sent along with FULLRESYNC was always the
current master replication offset. This is not ok, because there are
many reasosn that may delay the RDB file creation. And... guess what,
the master offset we communicate must be the one of the time the RDB
was created. So for example:
1) When the BGSAVE for replication is delayed since there is one
already but is not good for replication.
2) When the BGSAVE is not needed as we attach one currently ongoing.
3) When because of diskless replication the BGSAVE is delayed.
In all the above cases the PSYNC reply is wrong and the slave may
reconnect later claiming to need a wrong offset: this may cause
data curruption later.
Previouly if we loaded a corrupt RDB, Redis printed an error report
with a big "REPORT ON GITHUB" message at the bottom. But, we know
RDB load failures are corrupt data, not corrupt code.
Now when RDB failure is detected (duplicate keys or unknown data
types in the file), we run check-rdb against the RDB then exit. The
automatic check-rdb hopefully gives the user instant feedback
about what is wrong instead of providing a mysterious stack
trace.
It's possible large objects could be larger than 'int', so let's
upgrade all size counters to ssize_t.
This also fixes rdbSaveObject serialized bytes calculation.
Since entire serializations of data structures can be large,
so we don't want to limit their calculated size to a 32 bit signed max.
This commit increases object size calculation and
cascades the change back up to serializedlength printing.
Before:
127.0.0.1:6379> debug object hihihi
... encoding:quicklist serializedlength:-2147483559 ...
After:
127.0.0.1:6379> debug object hihihi
... encoding:quicklist serializedlength:2147483737 ...
