Redis supports two models of distributed computing. Redis's master-slave replication is simple to use with horizontal scaling across multiple slaves connecting asynchronous with a master Redis instance. In addition, Redis is well suited to supporting the Publish/Subscribe model where a publisher application sends messages to one or more channels that subscriber applications monitor and then react to the messages in the channel.
Redis's Master-Slave replication starts with at least two Redis server instances; a master and one or more slave instances. All Redis server instances store a copy of data. The difference between the two is that the slaves are usually read-only while the master instance accepts reads and writes.
Open four command-line windows to run a Redis master, slave, and redis-cli instances
$ cd introduction-to-redis/replication-and-pub-sub/ $ ~/redis-3.0.2/src/redis-server --dbfilename master.rdb --port 6379
$ cd introduction-to-redis/replication-and-pub-sub/ $ ~/redis-3.0.2/src/redis-server --dbfilename slave.rdb --port 6380
$ ~/redis-3.0.2/src/redis-cli -p 6379 127.0.0.1:6379> DBSIZE (integer) 0 127.0.0.1:6379> exit $ $ ~/redis-3.0.2/src/redis-cli -p 6380 127.0.0.1:6380> DBSIZE (integer) 0 127.0.0.1:6380> SLAVEOF localhost 6379 OK 127.0.0.1:6380> exit $ $ ~/redis-3.0.3/src/redis-cli -p 6379 127.0.0.1:6379> HMSET Book:1 name "Infinite Jest" author "David Foster Wallace" OK 127.0.0.1:6379> exit $ $ ~/redis-3.0.2/src/redis-cli -p 6380 127.0.0.1:6380> KEYS * 127.0.0.1:6380> KEYS * 1) "Book:1" 127.0.0.1:6380> HGETALL Book:1 1) "name" 2) "Infinite Jest" 3) "author" 4) "David Foster Wallace" 127.0.0.1:6380> HSET Book:1 copyrightYear 1996 (error) READONLY You can't write against a read only slave.
If persistence is turned off on the master A and it has two slaves B, C the following problem can occur:
Replication is non-blocking on both master and slave instances; a master can continue to respond to queries while slaves are synchronizing, a slave can be configured to respond to queries with old data while synchronizing with the master
If the link between the master and slave goes down, the old behavior of Redis is to do a full synchronization with the slave when the connection is re-established. As of Redis 2.8 experimental support was added with PSYNC command that will only do a partial synchronization with MASTER based on if a time threshold has been exceeded or not. If time has been exceeded, a full SYNC is triggered between the master and slave.
An experimental feature that sends the full RDB file to the slave without forking the master and using the disk as an intermediate storage.
In this exercise, we will use redis-cli as a publisher and use the a Python command line to run a consumer application using PUBLISH and SUBSCRIBE commands. The Python client redis-py has its own Publish/Subscribe class with its own method calls for responding to messages published to a channel.
>>> import redis >>> local_redis = redis.StrictRedis() >>> circ_consumer = local_redis.pubsub() >>> circ_consumer.subscribe("checkouts", "checkins") >>> circ_consumer.get_message()
127.0.0.1:6379> publish checkouts Book:3 (integer) 1 127.0.0.1:6379> publish checkouts Book:1 (integer) 1 127.0.0.1:6379> publish checkins Book:3 (integer) 1 127.0.0.1:6379> publish checkins Book:3 (integer) 1
Redis Pub/Sub messages are defined as 3 or 4 part RESP (REdis Serialization Protocol) array. RESP is the communication protocol that Redis client communicate with Redis server and is used in other projects beside Redis. We see the raw RESP protocol if we use telnet and connect directly to Redis.
First, we will connect to a single Redis instance running at port 6379 with telnet.
$ telnet localhost 6379 Trying 127.0.0.1... Connected to localhost. Escape character is '^]'.
Now, we will subscribe to a chat_room channeland from a
redis-clisession, we will post a message to the channel.
SUBSCRIBE chat_room *3 $9 subscribe $9 chat_room :1 *3 $7 message $9 chat_room $18 Anybody out there?
To subscribe to more than one channel, you can use the PSUBSCRIBE that take a pattern to match to existing channels.
PSUBSCRIBE chat_* *3 $10 psubscribe $6 chat_* :1 *4 $8 pmessage $6 chat_* $9 chat_room $18 What is your name?
To unsubscribe to multiple channels at once, you can use the PUNSUBSCRIBE
PUNSUBSCRIBE chat* *3 $12 punsubscribe $5 chat* :1