At the base of Redis replication (excluding the high availability features provided as an additional layer by Redis Cluster or Redis Sentinel) there is a leader follower (master-replica) replication that is simple to use and configure. It allows replica Redis instances to be exact copies of master instances. The replica will automatically reconnect to the master every time the link breaks, and will attempt to be an exact copy of it regardless of what happens to the master.
This system works using three main mechanisms:
Redis uses by default asynchronous replication, which being low latency and high performance, is the natural replication mode for the vast majority of Redis use cases. However, Redis replicas asynchronously acknowledge the amount of data they received periodically with the master. So the master does not wait every time for a command to be processed by the replicas, however it knows, if needed, what replica already processed what command. This allows having optional synchronous replication.
Synchronous replication of certain data can be requested by the clients using
WAIT command. However
WAIT is only able to ensure there are the
specified number of acknowledged copies in the other Redis instances, it does not
turn a set of Redis instances into a CP system with strong consistency: acknowledged
writes can still be lost during a failover, depending on the exact configuration
of the Redis persistence. However with
WAIT the probability of losing a write
after a failure event is greatly reduced to certain hard to trigger failure
You can check the Redis Sentinel or Redis Cluster documentation for more information about high availability and failover. The rest of this document mainly describes the basic characteristics of Redis basic replication.
redis.confto avoid persisting to disk at all, then connect a replica configured to save from time to time, or with AOF enabled. However, this setup must be handled with care, since a restarting master will start with an empty dataset: if the replica tries to sync with it, the replica will be emptied as well.
In setups where Redis replication is used, it is strongly advised to have persistence turned on in the master and in the replicas. When this is not possible, for example because of latency concerns due to very slow disks, instances should be configured to avoid restarting automatically after a reboot.
To better understand why masters with persistence turned off configured to auto restart are dangerous, check the following failure mode where data is wiped from the master and all its replicas:
When Redis Sentinel is used for high availability, also turning off persistence on the master, together with auto restart of the process, is dangerous. For example the master can restart fast enough for Sentinel to not detect a failure, so that the failure mode described above happens.
Every time data safety is important, and replication is used with master configured without persistence, auto restart of instances should be disabled.
Every Redis master has a replication ID: it is a large pseudo random string that marks a given story of the dataset. Each master also takes an offset that increments for every byte of replication stream that it is produced to be sent to replicas, to update the state of the replicas with the new changes modifying the dataset. The replication offset is incremented even if no replica is actually connected, so basically every given pair of:
Replication ID, offset
Identifies an exact version of the dataset of a master.
When replicas connect to masters, they use the
PSYNC command to send
their old master replication ID and the offsets they processed so far. This way
the master can send just the incremental part needed. However if there is not
enough backlog in the master buffers, or if the replica is referring to an
history (replication ID) which is no longer known, than a full resynchronization
happens: in this case the replica will get a full copy of the dataset, from scratch.
This is how a full synchronization works in more details:
The master starts a background saving process to produce an RDB file. At the same time it starts to buffer all new write commands received from the clients. When the background saving is complete, the master transfers the database file to the replica, which saves it on disk, and then loads it into memory. The master will then send all buffered commands to the replica. This is done as a stream of commands and is in the same format of the Redis protocol itself.
You can try it yourself via telnet. Connect to the Redis port while the
server is doing some work and issue the
SYNC command. You'll see a bulk
transfer and then every command received by the master will be re-issued
in the telnet session. Actually
SYNC is an old protocol no longer used by
newer Redis instances, but is still there for backward compatibility: it does
not allow partial resynchronizations, so now
PSYNC is used instead.
As already said, replicas are able to automatically reconnect when the master-replica link goes down for some reason. If the master receives multiple concurrent replica synchronization requests, it performs a single background save in to serve all of them.
In the previous section we said that if two instances have the same replication ID and replication offset, they have exactly the same data. However it is useful to understand what exactly is the replication ID, and why instances have actually two replication IDs the main ID and the secondary ID.
A replication ID basically marks a given history of the data set. Every time an instance restarts from scratch as a master, or a replica is promoted to master, a new replication ID is generated for this instance. The replicas connected to a master will inherit its replication ID after the handshake. So two instances with the same ID are related by the fact that they hold the same data, but potentially at a different time. It is the offset that works as a logical time to understand, for a given history (replication ID) who holds the most updated data set.
For instance, if two instances A and B have the same replication ID, but one with offset 1000 and one with offset 1023, it means that the first lacks certain commands applied to the data set. It also means that A, by applying just a few commands, may reach exactly the same state of B.
The reason why Redis instances have two replication IDs is because of replicas that are promoted to masters. After a failover, the promoted replica requires to still remember what was its past replication ID, because such replication ID was the one of the former master. In this way, when other replicas will sync with the new master, they will try to perform a partial resynchronization using the old master replication ID. This will work as expected, because when the replica is promoted to master it sets its secondary ID to its main ID, remembering what was the offset when this ID switch happened. Later it will select a new random replication ID, because a new history begins. When handling the new replicas connecting, the master will match their IDs and offsets both with the current ID and the secondary ID (up to a given offset, for safety). In short this means that after a failover, replicas connecting to the newly promoted master don't have to perform a full sync.
In case you wonder why a replica promoted to master needs to change its replication ID after a failover: it is possible that the old master is still working as a master because of some network partition: retaining the same replication ID would violate the fact that the same ID and same offset of any two random instances mean they have the same data set.
Normally a full resynchronization requires creating an RDB file on disk, then reloading the same RDB from disk to feed the replicas with the data.
With slow disks this can be a very stressing operation for the master. Redis version 2.8.18 is the first version to have support for diskless replication. In this setup the child process directly sends the RDB over the wire to replicas, without using the disk as intermediate storage.
To configure basic Redis replication is trivial: just add the following line to the replica configuration file:
replicaof 192.168.1.1 6379
Of course you need to replace 192.168.1.1 6379 with your master IP address (or
hostname) and port. Alternatively, you can call the
REPLICAOF command and the
master host will start a sync with the replica.
There are also a few parameters for tuning the replication backlog taken
in memory by the master to perform the partial resynchronization. See the example
redis.conf shipped with the Redis distribution for more information.
Diskless replication can be enabled using the
parameter. The delay to start the transfer to wait for more replicas to
arrive after the first one is controlled by the
parameter. Please refer to the example
redis.conf file in the Redis distribution
for more details.
Since Redis 2.6, replicas support a read-only mode that is enabled by default.
This behavior is controlled by the
replica-read-only option in the redis.conf file, and can be enabled and disabled at runtime using
Read-only replicas will reject all write commands, so that it is not possible to write to a replica because of a mistake. This does not mean that the feature is intended to expose a replica instance to the internet or more generally to a network where untrusted clients exist, because administrative commands like
CONFIG are still enabled. The Security page describes how to secure a Redis instance.
You may wonder why it is possible to revert the read-only setting and have replica instances that can be targeted by write operations. The answer is that writable replicas exist only for historical reasons. Using writable replicas can result in inconsistency between the master and the replica, so it is not recommended to use writable replicas. To understand in which situations this can be a problem, we need to understand how replication works. Changes on the master is replicated by propagating regular Redis commands to the replica. When a key expires on the master, this is propagated as a DEL command. If a key which exists on the master but is deleted, expired or has a different type on the replica compared to the master will react differently to commands like DEL, INCR or RPOP propagated from the master than intended. The propagated command may fail on the replica or result in a different outcome. To minimize the risks (if you insist on using writable replicas) we suggest you follow these recommendations:
Don't write to keys in a writable replica that are also used on the master. (This can be hard to guarantee if you don't have control over all the clients that write to the master.)
Don't configure an instance as a writable replica as an intermediary step when upgrading a set of instances in a running system. In general, don't configure an instance as a writable replica if it can ever be promoted to a master if you want to guarantee data consistency.
Historically, there were some use cases that were consider legitimate for writable replicas. As of version 7.0, these use cases are now all obsolete and the same can be achieved by other means. For example:
Computing slow Set or Sorted set operations and storing the result in temporary local keys using commands like SUNIONSTORE and ZINTERSTORE. Instead, use commands that return the result without storing it, such as SUNION and ZINTER.
Using the SORT command (which is not considered a read-only command because of the optional STORE option and therefore cannot be used on a read-only replica). Instead, use SORT_RO, which is a read-only command.
Using EVAL and EVALSHA are also not considered read-only commands, because the Lua script may call write commands. Instead, use EVAL_RO and EVALSHA_RO where the Lua script can only call read-only commands.
While writes to a replica will be discarded if the replica and the master resync or if the replica is restarted, there is no guarantee that they will sync automatically.
Before version 4.0, writable replicas were incapable of expiring keys with a time to live set.
This means that if you use
EXPIRE or other commands that set a maximum TTL for a key, the key will leak, and while you may no longer see it while accessing it with read commands, you will see it in the count of keys and it will still use memory.
Redis 4.0 RC3 and greater versions are able to evict keys with TTL as masters do, with the exceptions of keys written in DB numbers greater than 63 (but by default Redis instances only have 16 databases).
Note though that even in versions greater than 4.0, using
EXPIRE on a key that could ever exists on the master can cause inconsistency between the replica and the master.
Also note that since Redis 4.0 replica writes are only local, and are not propagated to sub-replicas attached to the instance. Sub-replicas instead will always receive the replication stream identical to the one sent by the top-level master to the intermediate replicas. So for example in the following setup:
A ---> B ---> C
B is writable, C will not see
B writes and will instead have identical dataset as the master instance
If your master has a password via
requirepass, it's trivial to configure the
replica to use that password in all sync operations.
To do it on a running instance, use
redis-cli and type:
config set masterauth <password>
To set it permanently, add this to your config file:
Starting with Redis 2.8, you can configure a Redis master to accept write queries only if at least N replicas are currently connected to the master.
However, because Redis uses asynchronous replication it is not possible to ensure the replica actually received a given write, so there is always a window for data loss.
This is how the feature works:
If there are at least N replicas, with a lag less than M seconds, then the write will be accepted.
You may think of it as a best effort data safety mechanism, where consistency is not ensured for a given write, but at least the time window for data loss is restricted to a given number of seconds. In general bound data loss is better than unbound one.
If the conditions are not met, the master will instead reply with an error and the write will not be accepted.
There are two configuration parameters for this feature:
<number of replicas>
<number of seconds>
For more information, please check the example
redis.conf file shipped with the
Redis source distribution.
Redis expires allow keys to have a limited time to live (TTL). Such a feature depends on the ability of an instance to count the time, however Redis replicas correctly replicate keys with expires, even when such keys are altered using Lua scripts.
To implement such a feature Redis cannot rely on the ability of the master and replica to have syncd clocks, since this is a problem that cannot be solved and would result in race conditions and diverging data sets, so Redis uses three main techniques to make the replication of expired keys able to work:
DELcommand which is transmitted to all the replicas.
DELcommand in time. In to deal with that the replica uses its logical clock to report that a key does not exist only for read operations that don't violate the consistency of the data set (as new commands from the master will arrive). In this way replicas avoid reporting logically expired keys are still existing. In practical terms, an HTML fragments cache that uses replicas to scale will avoid returning items that are already older than the desired time to live.
Once a replica is promoted to a master it will start to expire keys independently, and will not require any help from its old master.
When Docker, or other types of containers using port forwarding, or Network Address Translation is used, Redis replication needs some extra care, especially when using Redis Sentinel or other systems where the master
ROLE commands output is scanned to discover replicas' addresses.
The problem is that the
ROLE command, and the replication section of
INFO output, when issued into a master instance, will show replicas
as having the IP address they use to connect to the master, which, in
environments using NAT may be different compared to the logical address of the
replica instance (the one that clients should use to connect to replicas).
Similarly the replicas will be listed with the listening port configured
redis.conf, that may be different from the forwarded port in case
the port is remapped.
To fix both issues, it is possible, since Redis 3.2.2, to force a replica to announce an arbitrary pair of IP and port to the master. The two configurations directives to use are:
replica-announce-ip 126.96.36.199 replica-announce-port 1234
And are documented in the example
redis.conf of recent Redis distributions.
There are two Redis commands that provide a lot of information on the current
replication parameters of master and replica instances. One is
INFO. If the
command is called with the
replication argument as
INFO replication only
information relevant to the replication are displayed. Another more
computer-friendly command is
ROLE, that provides the replication status of
masters and replicas together with their replication offsets, list of connected
replicas and so forth.
Since Redis 4.0, when an instance is promoted to master after a failover, it will be still able to perform a partial resynchronization with the replicas of the old master. To do so, the replica remembers the old replication ID and offset of its former master, so can provide part of the backlog to the connecting replicas even if they ask for the old replication ID.
However the new replication ID of the promoted replica will be different, since it constitutes a different history of the data set. For example, the master can return available and can continue accepting writes for some time, so using the same replication ID in the promoted replica would violate the rule that a replication ID and offset pair identifies only a single data set.
Moreover, replicas - when powered off gently and restarted - are able to store
RDB file the information needed to resync with their
master. This is useful in case of upgrades. When this is needed, it is better to
SHUTDOWN command in order to perform a
save & quit operation on the
It is not possible to partially sync a replica that restarted via the AOF file. However the instance may be turned to RDB persistence before shutting down it, than can be restarted, and finally AOF can be enabled again.
By default, a replica will ignore
maxmemory (unless it is promoted to master after a failover or manually).
It means that the eviction of keys will be handled by the master, sending the DEL commands to the replica as keys evict in the master side.
This behavior ensures that masters and replicas stay consistent, which is usually what you want. However, if your replica is writable, or you want the replica to have a different memory setting, and you are sure all the writes performed to the replica are idempotent, then you may change this default (but be sure to understand what you are doing).
Note that since the replica by default does not evict, it may end up using more memory than what is set via
maxmemory (since there are certain buffers that may be larger on the replica, or data structures may sometimes take more memory and so forth).
Make sure you monitor your replicas, and make sure they have enough memory to never hit a real out-of-memory condition before the master hits the configured
To change this behavior, you can allow a replica to not ignore the
maxmemory. The configuration directives to use is: