Redis is an in-memory data store that can be used as a database, cache, or message broker. It supports various data structures like strings, lists, hashes, sets, and sorted sets. Redis stores the full dataset in memory for very fast access, and can persist data to disk using snapshots or append-only files. It is widely used for caching, session storage, real-time analytics, and more due to its high performance and flexibility in handling different data types.

1. Redis
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Redis is a NoSql in-memory data-store that supports
several canonical data-structures as persistent data
model. In its simplest use, Redis works as a key–value
store – allowing values to be set and retrieve by key.
However, values in Redis are typed. In addition to binary
String values (blobs), values can also represent data-
structure types, including: Lists, Hashes (dictionaries),
Sets, Sorted Sets, and HyperLogLog.
A distinguishing feature of Redis is that it loads the full
state of the data-store into memory, thus allowing for very
fast access. Redis uses two different techniques to
persist state – snapshoot files periodically written with the
full state kept in memory, and append-only files (AOF)
with the history of operations. Several techniques can be
used to mitigate the memory footprint of Redis when the
dataset grow beyond the physical memory limits – such
as expiring not so much used key-values, and partitioning
data across multiple Redis nodes connected as a cluster.
Redis also supports for replication of data for increased
fault-tolerance, using (semi-synchronous) master-slave
replication. High-available and automatic failover is
achieved by setting up a small cluster of dedicated
monitoring servers – known as sentinels.
Redis is most popular for its recognized high-
performance, and has a wide range if use cases – from
simple caching, to session state persistence, and as a
main database for applications where in-memory data
management is desirable.
Redis is very straightforward to install. In a typical Linux
installation or MacOS, it builds from the source code out-
of-the-box without requiring any configuration.
Downloading the source, extracting the tar ball, and
running make build tool should be enough. For
WindowsOS, there is a distribution managed by a
Microsoft team, that currently has an MSI installer that
setup Redis as a windows service.
To start Redis manually, both in Linux and WindowsOS,
use command redis-server. By default, Redis listen for
client connection on TCP/IP port 6379. Depending on the
OS, kernel version, and current kernel configuration,
Redis might suggest on startup some OS-level setting for
increased performance.
» Example: Installation Redis (Linux)
wget http://download.redis.io/releases/redis-3.2.0.tar.gz
tar -xzf redis-3.2.0.tar.gz
cd redis-3.2.0
make
make install
» Example: Installation Redis (Linux)
$ redis-server
[12536] 27 May 16:47:10.100 # Warning: no config file specified,
using the default config. In order to specify a config file use
redis-server /path/to/redis.conf
...
[12536] 27 May 16:47:10.124 # Server started, Redis version 3.0.501
[12536] 27 May 16:47:10.383 * DB loaded from disk: 0.258 seconds
[12536] 27 May 16:47:10.383 * The server is now ready to accept
connections on port 6379
Redis can be configured by starting the redis-server with
the path to a configuration file as argument. The
configuration file is a text file with a simple syntax – one
setting per line containing a property name followed by
one or more arguments.
» Example: Starting Redis with Configuration File
redis-server /path/redis.com
» Example: Redis Configuration File
port 5000
A more comprehensive review of the Redis configuration
and deployment options is covered in a companion
EInnovator refcard Redis Administration (#10).
Redis comes with a convenient command-line tool (CLI),
that can be used to execute arbitrary Redis commands.
The CLI support all the commands defined by Redis wire-
level protocol (also text-based).
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++QuickGuides»EInnovator.org
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Software Engineering School
Content
» Redis Install & Configuration
» Redis Common Operations
» Redis Data-Types:
» String, List, Hash, Set, ZSet
» HyperLogLog
» Redis Transactions
» LUA Scripting with Redis
Jorge Simão, Ph.D.
Redis
Data-Types & Operations
Redis Overview
Redis Installation
Configuring Redis
Redis CLI and Tools

2. © EInnovator.org
Redis
» Example: Starting Redis CLI Interactively
$ redis-cli
» Example: Execute Commands on Redis CLI
> SET mykey somevalue
> GET mykey
> KEYS *
» Example: Getting Help from Redis CLI
redis-cli --help
» Example: Execute Single Command w/ CLI
redis-cli keys "*"
Redis top-level data-model is a collection of key–value pairs,
with the key being a binary string and values one of several
predefined data-structure types (String, List, Hash, Set,
SortedSet, HyperLogLog, Geo). Redis support a wide-range
of operations to manipulate the key–value pairs. Most
operations are specific to each type of value, but some
operations are common to all types. Some operations are
additionally used to administer the Redis server, a sentinel, or
a cluster of servers.
Operations that operate on key–values are used either to
set/modify the value or to get the value, in part or in whole.
When a set/modify operation is applied and the key does not
exist yet, the key–value is created and the type is inferred from
the operation. If the key already exists and the current value is
of a type incompatible with the attempted operation an error
occurs.
The simplest value type is a String, where the values also
binary strings – like the keys. Command SET sets a String
value for a key; command GET gets the value. Commands that
are common to all types of value, include: EXISTS, to check if
a key exist (whatever the type); TYPE, to get the type of value
for a key; DEL, to delete an entry. Once a key is deleted it can
be set later with any other type.
» Example: Basic Operations on Keys
> SET user:1:login "2016-10-1T8:20"
OK
> EXISTS user:1:login
(integer) 1
> TYPE user:1:login
string
> DEL user:1:login
(integer) 1
Redis key-value entries can be automatically removed after a
certain amount of time. This is specially useful when Redis is
being used as a cache. Command (P)EXPIRE sets an
expiration interval in (milli-)seconds on a key, and
(P)EXPIREAT defines a the date-time when the expiration will
happen (Unix epoch time).
» Example: Set Expire Time on Key
> SET bid:101 "120,EUR"
> EXPIRE bid:101 10
> GET bid:101
"120,EUR"
> TTL bid:101
"7"
> GET bid:101
(nil)
It is also possible to a Redis server with multiple logical
independent databases within the same server, each identified
by an integer index. Command SELECT changes the current
database (default is 0). Command MOVE moves a key–value
to another database. The number of available databases is
defined in the property databases of the REDIS configuration
file.
Table below summarizes the Redis operations that are
common to all data-types.
Common Ops Description
KEYS Get value for key matching pattern
SCAN Scan keys matching pattern
EXISTS Check if key exists
TYPE Get type of key value
RENAME Rename key
RENAMENX Rename key (if target does not exists)
DEL Delete Key
RANDOMKEY Get random key
MOVE Move key across databases
SORT Sort values
EXPIRE, PEXPIRE Expire key in (mili-)seconds
EXPIREAT.
PEXPIREAT
Expire key at time (mili-)seconds (Unix epoch)
TTL, PTTL Get time before expiration in (milli-)seconds
PERSIST Cancel expiration time
MOVE Move key–value to another database
A common use for String values is to store serialized objects.
But it can also be used in several other ways, beyond full state
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Generic Redis Operations
Operations on Strings

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Redis
write/read with operations SET/GET (or MSET/MGET to
handle multiple keys at the same time). Method APPEND
concatenates a String value to the tail of an existing String
value (e.g. to append the text entry of a growing log). An
alternative approach to String appending, is to use the List
data-type (see below). Method STRLEN returns the current
length of the String. When values are numbers represented
as text, operations INCR(BY)(FLOAT) can be used to
atomically increment (or decrement) a value in an atomic way.
String values can also be used as bit arrays. Method
SETBIT/GETBIT sets/get the value of a bit with specified
index. Other operation exists also for bit inspection and
manipulation. Bit-wise logical operations are supported with
command BITOP. Since Redis 3.2, a String can also be used
as arrays of integers with operation BITFIELD.
» Example: Appending Log Entries to a Redis String
> APPEND log "[INFO] 2016-1-10: GET /n"
> APPEND log "[INFO] 2016-1-10: GET /refcard/redisn"
$ redis-cli GET log > locahost.log
Table below summarizes the Redis operations for Strings.
String Operation Description
GET Get value of entry by key
SET Set key-value entry
SETNX Set key-value (if key does not exit)
SETEX Set key-value (if key already exits)
GETSET Set value and return previous value
MGET Get values of multiple keys
MSET Set multiple key-value entries
MSETNX Set multiple key-value entries (if keys do not
exit)
SETEX,PSETEX Set value and expiration of key atomically
INCR, INCRBY Increment value (numeric)
INCRBYFLOAT Increment value (numeric)
APPEND Append string to tail
DECR, DECRBY Decrement value (numeric)
STRLEN Get string length
GETRANGE Get sub-string by start-end index
SETRANGE Set sub-string by start-end index
GETBIT Get bit
SETBIT Set bit
BITPOS Get position of first bit set to 1 or 0
BITCOUNT Count number of bit with value 1
BITOP Apply logical operation between two bit string
BITFIELD Get integer value at specified position in string
Lists are ordered collections of elements. They are useful as a
generic collection where the getting elements by order of
insertion is enough or where ordering might not be crucial (e.g.
when modelling entity associations). Redis support operations
to inspect & manipulate from both sides of a List – left and
right. Method LPUSH append an element to the left side the
list, while method RPUSH appends an element to the right side
of the List. The element values are always binary Strings –
i.e. Redis does not support nested data-structures. Method
LPOP/RPOP retrieves and removes the left/right most element
of the List (sometimes called head/tail, by convention).
Operation LRANGE retrieves a sub-set of a List by specifying
first and last index. Indexes start at 0. Negative indexes specify
positions counting from end/right of the List. Internally, a List
is represented as double-linked list plus two pointer for the
head and tail. So random access operations in a List (e.g.
Indexing a single element or pagination) are not very efficient
(O(N) complexity).
» Example: Pushing/Retreive Elements into/from List
> LPUSH user:101:msgs "Hi!"
> LPUSH user:101:msgs "What up!?" "Going out?!"
> LRANGE album:100:msgs 0 -1
Lists can also be used as work queues by using blocking
operations BLPOP and BRPOP. If the List is empty these
operations block the client, till some other client makes a
(R/L)PUSH on the List. Table below summarizes the Redis
operations for Lists.
List Operation Description (Default)
LPUSH Left push element in List
LPUSHX Left push element in List
LPOP Left pop element from List
LLEN Length of List
LSET Set/modify element in List index
LINDEX Get index of element
LRANGE Get sub-list in index range
LREM Remove element from List
LTRIM Truncate List to size
LINSERT Insert element in List
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Operations on Lists

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Redis
RPUSH Right push element in List
RPUSHX Right push element in List
RPOP Right pop element from List
BRPOP Pop element and block if empty
RPOPLPUSH Pop from list and push in another list
BRPOPLPUSH Pop from list and push in another list
BLPOP Pop element and block if empty
An Hash typed value is a key-value dictionary that supports
binary strings as entry value (i.e. no nesting is allowed, like in a
List). Hashes are useful to represent objects as a collection of
properties without requiring binary serialization or marshalling
to be done to encode the object (as would be the case for a
String value), or as a generic mapping data-structure.
Internally, a Redis Hash is implemented as a closed
HashTable which doubles the capacity when the fraction of
used entries exceeds a threshold. Redis also maintains two
tables (current and previous) to prevent copy delays on
insertion.
Keys inside Hashes are sometimes called fields. Fields can be
identified by name and get/set/modified individually or multiple
at the same time. Operation HMSET is used to set the value of
several Hash fields. Operation HGET get the value of a field,
and HGETALL the value of all fields.
» Example: Storing Entity Properties as Hash Fields
> HMSET user:1000 username adam birthyear 2001 sex male
» Example: Retrieve Entity Properties from Hash
> HKEYS user:1000
1) "username"
2) "birthyear"
3) "sex"
> HGET user:1000 username
"adam"
> HGETALL user:1000
1) "username"
2) "adam"
3) "birthyear"
4) "2001"
5) "sex"
6) "male"
Table below summarizes the Redis operations for Hashes.
Hash Operation Description
HKEYS Get keys of Hash entries
HVALS Get values of Hash entries
HSET Set key-value as Hash entry
HSETNX Set value in Hash entry (if not exits)
HMSET Set multiple key-value as Hash entries
HEXISTS Check if Hash entry exist for key
HLEN Get size of Hash (number of entries)
HDEL Delete entry from Hash with key
HGET Get value from Hash entry with key
HMGET Get values of multiples Hash entries
HGETALL Get all key-value in Hash
HINCRBY Increment value of hash entry (numbers)
HINCRBYFLOAT Increment value of hash entry (numbers)
HSTRLEN Length (in bytes) of hash entry
HSCAN Scan hash entries
A Set is an unordered collection of elements without repetition.
Internally, a Set is implemented similarly to an Hash with an
additional element count integer field for efficiency. Operation
SADD adds one or more elements (binary String values) to
the Set. Operation SISMEMBER checks for membership, and
operation SMEMBERS returns all elements. Redis also
supports algebraic operations between two sets, such as:
union (SUNION), interception (SINTER), and difference
(SDIFF). The result Set is returned, or stored as another Set if
using operations S{UNION|INTER|DIFF}STORE.
» Example: Adding/Retrieve Element to/from Set
> SADD users adam01 eva99 joe11 jack5 eva99
4
> SMEMBERS
1) "jack5"
2) "joe111"
3) "eva99"
4) "james007"
5) "adam01"
> SISMEMBER users eva99
(integer) 1
» Example: Set Algebraic Operations
> SADD user:eva99:friends adrian2 jack5 james007
> SADD user:adam01:friends joe111 james007
> SUNION user:eva99:friends user:adam01:friends
1) "james007"
2) "joe111"
3) "adrian2"
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Operations on Hashes
Operations on Sets

5. -
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Redis
4) "jack5"
Table below summarizes the Redis operations for Set.
Set Operation Description
SADD Add element to Set (if non existing)
SPOP Remove element from Set
SMOVE Move element from one Set to another
SREM Remove elements from Set (?)
SISMEMBER Check if element is member of Set
SMEMBERS Get elements in Set
SRANDMEMBER Get a random member from Set
SCARD Get cardinality of Set (number of elements)
SSCAN Scan elements in Set
SUNION Return union of two Sets
SUNIONSTORE Make union and store in named Set
SINTER Return interception of two Sets
SINTERSTORE Make interception and store in named Set
SDIFF Return difference of two Sets
SDIFFSTORE Make difference and store in named Set
A Sorted Set, or ZSet, is an ordered collection without
repetitions where the ordering is determined by a double
numeric value (the Z score) specified when the element is
added. ZSets as useful for a variety of purposes, such as
creating ordered indexes. Internally, a ZSet is represented as a
ordered skip list which guarantees O(log(N)) complexity for
most operations.
Operation ZRANGE retrieves the elements of the ZSet with a
rank order within a range, ordered by score. Operation
ZRANGEBYSCORE retrieves the elements of the ZSet with a
score within a range, ordered by score. Option WITHSCORES
retrieves the scores in addition to the values. For the special
case where all elements of the ZSet have the same score,
operation ZRANGEBYLEX retrieves the elements by
lexicographical (alphabetical) order.
» Example: Adding & Retrieving Elements on/from ZSet
> ZADD users:reputation 1346 adam1
> ZADD users:reputation 3452 eva33
...
ZRANGE users:reputation 0 -1
1)"eva33"
2)"adam1"
...
» Example: Retreiving Element Within Score Range
> ZRANGEBYSCORE users:reputation 2000 +INF WITHSCORES
1) "eva33"
2) "43452"
Table below summarizes the Redis operations for ZSet.
ZSet Operation Description
ZADD Add element (with score) to ZSet (if non
existing)
ZREM Remove element from ZSet
ZCARD Get cardinality (total number of elements)
ZCOUNT Get count of element with score in range
ZLEXCOUNT Get count of elements in lexicographical range
ZSCORE Get score of element
ZRANK Get rank (order) of element
ZREVRANK Get rank of element in reverse ordering
ZINCRBY Increments score of element by a value
ZSCAN Scan elements
ZRANGE Get elements with rank in a range
ZRANGEBYSCORE Get elements ordered lexicographically
ZRANGEBYLEX Get elements with score in a range
ZREVRANGE Get elements with rank in a range reversed
ZREVRANGEBYSCORE Get elements lexicographically reversed
ZREVRANGEBYLEX Get elements with score in a range reversed
ZREMRANGEBYRANK Remove elements with rank in a range
ZREMRANGEBYSCORE Remove elements ordered lexicographically
ZREMRANGEBYLEX Remove elements with score in a range
ZUNIONSTORE Compute&store the union of two ZSet
ZINTERSTORE Compute&store the interception of two ZSet
An HyperLogLog is a (probabilistic) data-structure, invented
by Philippe Flajolet, for counting the approximated number of
unique elements in the set without storing the actual elements.
Redis supports HyperLogLog due to its high-performance
constant-time approach for element counting. Command
PFADD add an element to a named HyperLogLog. And
command PFCOUNT returns the counting estimate.
» Example: Adding & Counting w/ HyperLogLog
> PFADD ips 192.168.255.251
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Operations on Sorted Sets
Operations on HyperLogLogs

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Redis
> PFADD ips 192.168.255.221
...
> PFCOUNT hll
(integer) 3218
Table below summarizes the Redis operations for
HyperLogLog.
HyperLogLog Operation Description
PFADD Add value to HyperLogLog
PFCOUNT (Estimate) Cardinality
PFMERGE Merge two HyperLogLog
In addition to data-structure storage&retrieval, Redis can also
be used as a simple messaging-middleware. Operation
SUBSCRIBE is used by a client that wants to subscribe to one
(or more) named channels. Operation PSUBSCRIBE is similar,
but a channel named pattern is specified. The client (thread)
making this operation should not send additional commands.
The sent messages are delivered as a stream through the
connection channel. To send messages to named channel
clients should issue command PUBLISH.
» Example: Subscribing to Channels
> SUBSCRIBE channel:1 channel:2
Reading messages... (press Ctrl-C to quit)
1) "subscribe"
2) "channel:1"
3) (integer) 1
1) "subscribe"
2) "channel:2"
3) (integer) 2
» Example: Subscribing Channels Matching Pattern
> PSUBSCRIBE channel:*
Reading messages... (press Ctrl-C to quit)
1) "psubscribe"
2) "channel:*"
3) (integer) 1
» Example: Publishing Message to Channel
> PUBLISH channel1 "Hello Redis!"
1) "message"
2) "channel1"
3) "Hello Redis!"
The message publishing/dissemination mechanism is also
used internally by Redis to notify clients of relevant events
(e.g. a key just expired, deleted, or modified). Clients can get
this notifications by subscribing to the relevant channel name
(e.g. deleting key xyz makes Redis send a notification
message to channels __keyspace@0__:xyz del, and
__keyevent@0__:del xyz).
Table below summarizes the Redis PubSub commands.
PubSub Command Description
SUBSCRIBE Subscribe to named channel(s)
PSUBSCRIBE Subscribe to matching channels
PUBLISH Publish/Send message to channel
UNSUBSCRIBE Unsubscribe to named channel(s)
PUNSUBSCRIBE Unsubscribe to matching channels
Redis supports a simple form of transactional semantics
where operations are buffered before execution until the
transaction completes. Command MULTI sets a client
connection in transactional mode. Command EXEC is used
mark the completion of the transaction with success, and
trigger the execution of buffered commands. All commands in a
completed transaction are guaranteed to be applied in an
isolated (no race conditions) and (quasi-)atomic way.
Command DISCARD is used to cancel all commands buffered
so far (i.e. they are nor executed). Redis does not support
transaction rollback in the traditional sense – if an operation
execution fails during the EXEC, the previous operation might
already have made some changes in some keys.
Redis also support a simple form of optimistic concurrency
control using command WATCH. Any (concurrent) modification
done in the specified key while the transaction is being
executed is detected and forces the EXEC to fail before the
first operation is applied.
» Example: Running Multiple Commans in Transaction
> MULTI
OK
> INCR foo
QUEUED
> INCR bar
QUEUED
> EXEC
1) (integer) 1
2) (integer) 1
Table below summarizes the Redis commands for transaction
management and concurrency control.
Tx Operation Description
MULTI Mark transaction start
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Redis Transactions
Redis PubSub (Messaging)

7. Redis
EXEC Mark transaction completion (commit)
DISCARD Mark transaction abort (discard)
WATCH Check for concurrent updates on key
Complex operations involving multiple/repetitive access to one
or more keys can be optimized by running them as Lua
Scripts inside the Redis server. Lua function redis.call() is
used to interface with Redis and invoke any desired
command. Command EVAL is used to evaluate a Lua script.
To avoid having to transfer a (possible long) script to the server
often, Redis maintains a script cache. A SHA digest is used to
unequivocally identify a script. Command SCRIPT LOAD is
used to load a script in the cache. Command EVALSHA
executes the script identified by a SHA digest.
» Example: Evaluating a Lua Script
> EVAL "return {KEYS[1],KEYS[2],ARGV[1]}" 2 key1 key2 abc
1) "key1"
2) "key2"
3) "abc"
» Example: Loading a Lua Script & Get SHA
SCRIPT LOAD "return redis.call('get','product:ids')"
> EVALSHA 6b1bf486c81ceb7edf3c093f4c48582e38c0e791 0
Table below summarizes the Redis commands for scripting.
Script Operation Description
EVAL Execute Script and return result
EVALSHA Execute cached Script by SHA
SCRIPT EXIST Check if Script with SHA exists
SCRIPT LOAD Load Scritpt from file
SCRIPT FLUSH Remove Script from cache
SCRIPT KILL Terminate script
• Redis Project home page: http://redis.io/
• Redis sample configuration file:
https://raw.githubusercontent.com/antirez/redis/3.2/redis.conf
About the Author
Jorge Simão is a software engineer and IT Trainer, with
two decades long experience on education delivery both in
academia and industry. Expert in a wide range of
computing topics, he his an author, trainer, and director
(Education & Consulting) at EInnovator. He holds a B.Sc.,
M.Sc., and Ph.D. in Computer Science and Engineering.
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