# [Redis](https://redis.io/) Redis is in the family of databases called **key-value stores**. The essence of a key-value store is the ability to store some data, called a value, inside a key. This data can later be retrieved only if we know the exact key used to store it. Often Redis it is called a *data structure* server because it has outer key-value shell, but each value can contain a complex data structure, such as a string, a list, a hashes, or ordered data structures called sorted sets as well as probabilistic data structures like *hyperloglog*. ## [Redis Commands](https://redis.io/commands) ### Server Startup ```bash redis-server # start the server redis-cli ``` ### [Key-Value Pairs](https://redis.io/commands#generic) ```sh SET [ EX ] # store a key-value pair, TTL optional GET # read a key content EXISTS # check if a key exists DEL # delete a key-value pair INCR # atomically increment a number stored at a given key INCRBY # increment the number contained inside a key by a specific amount DECR DECRBY # re-setting the key will make it permanent (TTL -1) EXPIRE # make the key expire after seconds TTL # see remaining seconds before expiry PEXPIRE # make the key expire after milli-seconds PTTL # see remaining milli-seconds before expiry PERSIST # make the key permanent ``` ### [Lists](https://redis.io/commands#list) A list is a series of ordered values. ```sh RPUSH ... # add one or more values to the end of the list LPUSH ... # add one or more values to the start of a list LLEN # number of items in the list LRANGE # return a subset of the list, end index included. Negative indexes count backwards from the end LPOP # remove and return the first item fro the list RPOP # remove and return the last item fro the list ``` ### [Sets](https://redis.io/commands#set) A set is similar to a list, except it does not have a specific order and each element may only appear once. ```sh SADD ... # add one or more values to the set (return 0 if values are already inside) SREM # remove the given member from the set, return 1 or 0 to signal if the member was actually there or not. SPOP # remove and return value from the set SISMEMBER # test if value is in the set SMEMBERS # lis of all set items SUINION ... # combine two or more sets and return the list of all elements. ``` ### [Sorted Sets](https://redis.io/commands#sorted_set) Sets are a very handy data type, but as they are unsorted they don't work well for a number of problems. This is why Redis 1.2 introduced Sorted Sets. A sorted set is similar to a regular set, but now each value has an associated score. This score is used to sort the elements in the set. ```sh ZADD # add a value with it's score ZRANGE # return a subset of the sortedSet ... ``` ### [Hashes](https://redis.io/commands#hash) Hashes are maps between string fields and string values, so they are the perfect data type to represent objects. ```sh HSET [ ... ] # set the string of a hash field HSETNX # set the value of a hash field, only if the field does not exist HEXISTS # determine if a hash field exists HLEN # get the number of fields in a hash HSTRLEN # get the length of the value of a hash field HGETALL # get all fields and values in a hash HGET # get data on a single field HKEYS # get all the fields in a hash HVALS # get all the values in a hash HDEL ... # delete one or more field hashes HMGET [ ...] # get the values of all the given hash fields HMSET [ ...] # set multiple hash fields to multiple values HINCRBY # increment the integer value of a hash field by the given number HINCRBYFLOAT # increment the float value of a hash field by the given amount HSCAN [MATCH ] [COUNT ] # incrementally iterate hash fields and associated values ```