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redis replication
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The SLAVEOF command can change the replication settings of a slave on the fly. If a Redis server is already acting as slave, the command SLAVEOF NO ONE will turn off the replication, turning the Redis server into a MASTER. In the proper form SLAVEOF hostname port will make the server a slave of another server listening at the specified hostname and port.
If a server is already a slave of some master, SLAVEOF hostname port will stop the replication against the old server and start the synchronization against the new one, discarding the old dataset.
The form SLAVEOF NO ONE will stop replication, turning the server into a MASTER, but will not discard the replication. So, if the old master stops working, it is possible to turn the slave into a master and set the application to use this new master in read/write. Later when the other Redis server is fixed, it can be reconfigured to work as a slave.
slaveof no one 命令将停止同步,会将从库设定为主库,但不会抛弃复制的内容,如果旧的主库停机了,将从库变为主库成为可能,应用可以使用这个新的主库进行读写操作,当另一个redis 设定好之后,这个新主库可以重新被设定为从库.
Return value Simple string reply
A note about slavery: it's unfortunate that originally the master-slave terminology was picked for databases. When Redis was designed the existing terminology was used without much analysis of alternatives, however a SLAVEOF NO ONE command was added as a freedom message. Instead of changing the terminology, that would require breaking backward compatible in the API and INFO output, we want to use this page to remember you about slavery, a crime against humanity now but something that was perpetuated throughout the whole human history. If slavery is not wrong, nothing is wrong. -- Abraham Lincoln
Redis replication is a very simple to use and configure master-slave replication that allows slave Redis servers to be exact copies of master servers. The following are some very important facts about Redis replication:
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Redis uses asynchronous replication. Starting with Redis 2.8, however, slaves will periodically acknowledge the amount of data processed from the replication stream.(slave 会周期性通过复制流获取master 的一系列的数据)
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A master can have multiple slaves.
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Slaves are able to accept connections from other slaves. Aside from connecting a number of slaves to the same master, slaves can also be connected to other slaves in a graph-like structure.
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Redis replication is non-blocking on the master side. This means that the master will continue to handle queries when one or more slaves perform the initial synchronization.
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Replication is also non-blocking on the slave side. While the slave is performing the initial synchronization, it can handle queries using the old version of the dataset, assuming you configured Redis to do so in redis.conf. Otherwise, you can configure Redis slaves to return an error to clients if the replication stream is down. However, after the initial sync, the old dataset must be deleted and the new one must be loaded. The slave will block incoming connections during this brief window.
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Replication can be used both for scalability, in order to have multiple slaves for read-only queries (for example, heavy
SORToperations can be offloaded to slaves), or simply for data redundancy. -
It is possible to use replication to avoid the cost of having the master write the full dataset to disk: just configure your master
redis.confto avoid saving (just comment all the "save" directives), then connect a slave configured to save from time to time. However in this setup make sure masters don't restart automatically (please read the next section for more information).
In setups where Redis replication is used, it is strongly advised to have persistence turned on in the master, or when this is not possible, for example because of latency concerns, instances should be configured to avoid restarting automatically.
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 slaves:
- We have a setup with node A acting as master, with persistence turned down, and nodes B and C replicating from node A.
- A crashes, however it has some auto-restart system, that restarts the process. However since persistence is turned off, the node restarts with an empty data set.
- Nodes B and C will replicate from A, which is empty, so they'll effectively destroy their copy of the data.
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 don't 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.
If you set up a slave, upon connection it sends a SYNC command. It doesn't matter if it's the first time it has connected or if it's a reconnection.
The master then starts background saving, and starts to buffer all new commands received that will modify the dataset. When the background saving is complete, the master transfers the database file to the slave, which saves it on disk, and then loads it into memory. The master will then send to the slave all buffered commands. 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.
Slaves are able to automatically reconnect when the master <-> slave link goes down for some reason. If the master receives multiple concurrent slave synchronization requests, it performs a single background save in order to serve all of them.
When a master and a slave reconnects after the link went down, a full resync is always performed. However, starting with Redis 2.8, a partial resynchronization is also possible.
Starting with Redis 2.8, master and slave are usually able to continue the replication process without requiring a full resynchronization after the replication link went down.
This works by creating an in-memory backlog of the replication stream on the master side. The master and all the slaves agree on a replication offset and a master run id, so when the link goes down, the slave will reconnect and ask the master to continue the replication. Assuming the master run id is still the same, and that the offset specified is available in the replication backlog, replication will resume from the point where it left off. If either of these conditions are unmet, a full resynchronization is performed (which is the normal pre-2.8 behavior). As the run id of the connected master is not persisted to disk, a full resynchronization is needed when the slave restarts.
The new partial resynchronization feature uses the PSYNC command internally,
while the old implementation uses the SYNC command. Note that a Redis 2.8
slave is able to detect if the server it is talking with does not support
PSYNC, and will use SYNC instead.
Normally a full resynchronization requires to create an RDB file on disk, then reload the same RDB from disk in order to feed the slaves with the data.
With slow disks this can be a very stressing operation for the master. Redis version 2.8.18 will be the first version to have experimental support for diskless replication. In this setup the child process directly sends the RDB over the wire to slaves, without using the disk as intermediate storage.
The feature is currently considered experimental.
To configure replication is trivial: just add the following line to the slave configuration file:
slaveof 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 SLAVEOF command and the
master host will start a sync with the slave.
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 repl-diskless-sync configuration
parameter. The delay to start the transfer in order to wait more slaves to
arrive after the first one, is controlled by the repl-diskless-sync-delay
parameter. Please refer to the example redis.conf file in the Redis distribution
for more details.
Since Redis 2.6, slaves support a read-only mode that is enabled by default.
This behavior is controlled by the slave-read-only option in the redis.conf file, and can be enabled and disabled at runtime using CONFIG SET.
Read-only slaves will reject all write commands, so that it is not possible to write to a slave because of a mistake. This does not mean that the feature is intended to expose a slave instance to the internet or more generally to a network where untrusted clients exist, because administrative commands like DEBUG or CONFIG are still enabled. However, security of read-only instances can be improved by disabling commands in redis.conf using the rename-command directive.
You may wonder why it is possible to revert the read-only setting and have slave instances that can be target of write operations. While those writes will be discarded if the slave and the master resynchronize or if the slave is restarted, there are a few legitimate use case for storing ephemeral data in writable slaves. However in the future it is possible that this feature will be dropped.
If your master has a password via requirepass, it's trivial to configure the
slave 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:
masterauth <password>
Starting with Redis 2.8, it is possible to configure a Redis master to accept write queries only if at least N slaves are currently connected to the master.
However, because Redis uses asynchronous replication it is not possible to ensure the slave actually received a given write, so there is always a window for data loss.
This is how the feature works:
- Redis slaves ping the master every second, acknowledging the amount of replication stream processed.
- Redis masters will remember the last time it received a ping from every slave.
- The user can configure a minimum number of slaves that have a lag not greater than a maximum number of seconds.
If there are at least N slaves, with a lag less than M seconds, then the write will be accepted.
You may think at it as a relaxed version of the "C" in the CAP theorem, 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.
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:
- min-slaves-to-write
<number of slaves> - min-slaves-max-lag
<number of seconds>
For more information, please check the example redis.conf file shipped with the
Redis source distribution.