Redis Module #62

Question 1

What kind of database is Redis, generally speaking?

Answer 1

NoSQL Yay! And it uses a key/value store model to store data.

Question 2

Redis has a reputation for being very fast. What makes the software work so quickly?

Answer 2

Redis is fast because it is an in-memory database. In other words, instead of storing data on a hard drive it saves in memory(RAM). Must use quite a lot of RAM eh?

Question 3

How do you make sure that Redis starts automatically when the computer is rebooted?

Answer 3

brew services start redis

alternatively, maybe even better you can start it manually

redis-server /usr/local/etc/redis.conf

Question 4

Important detail: on which port does Redis listen?

Answer 4

Redis listens on port 6379

Question 5

What’s the basic syntax/structure to access a remote Redis server?

Answer 5

From the command line:

redis-cli -h -p -a

Question 6

Important detail: how do you drop into the Redis CLI?

Answer 6

Type: redis-cli
then you will know you’re in the redis-cli when you see the local host IP and port number.

tomphillips@Toms-MBP-2 Redis % redis-cli
127.0.0.1:6379>

Question 7

How can we store data into the Redis CLI?

Answer 7

Use the syntax SET

Example: SET name “Flavio”
In the above keyname = name value = Flavio

Question 8

How can we retrieve a value from the DB?

Answer 8

GET

Question 9

What if we’re not sure if a particular key exists?

Answer 9

EXISTS

If a 1 is returned, the key exists
0 means there was no match, key doesn’t exist

Question 10

How are keys deleted?

Answer 10

Use DEL name

Question 11

What if we want to set up a key only if it doesn’t yet exist?

Answer 11

SETNX name

Question 12

How can we see a list of keys?

Answer 12

You can see all keys by typing:
KEYS* (big list) or you can filter by this pattern
KEYS n* to find all keys that start with an “n”

Question 13

How can we create a temporary key that expires at a predefined span of time?

Answer 13

SETEX

Also, you can find the remaining time before a key expires by calling TTL

Question 14

How much data can a single key store?

Answer 14

512mb

Question 15

How is null represented in Redis and when might you expect to see this value?

Answer 15

Null is represented in Redis as nil and you’d see this when returning a key that has expired.

Question 16

How does Redis define a list?

Answer 16

A list is a set of key-values pairs linked to each other.

Question 17

How are lists created in Redis?

Answer 17

By using LPUSH (to add list items to the top of the list)
Both RPUSH and LPUSH accept multiple arguments for the value so you can add more than 1 list item at a time.

LPUSH for example,
LPUSH names “Flavio”

RPUSH keyname value (to add list items to the BOTTOM of the list.

Question 18

How can we increment or decrement keys?

Answer 18

#Increment or decrement key values 
INCR keyname
DECR keyname
#Increment or decrement by a set value
INCRBY keyname amount
DECRBY keyname amount

These commands are very well suited for high concurrent operations where many clients might interact with the same data, to ensure atomic transactions.

Question 19

How many items can a list hold in Redis?

Answer 19

4 Billion

Question 20

How can we remove multiple items from a list?

Answer 20

LTRIM listname 0 1

cuts the list to just 2 items, item at position 0 (the first) and item at position 1.

Question 21

What are the main differences between a set and a list in Redis?

Answer 21

1. Sets are not ordered (indexed)

2. Sets can only hold an item once (no duplicates)

Question 22

How do you add keys to a set?

Answer 22

SADD setname value
Example: 
SADD names "Flavio"
SADD names "Roger"
SADD names "Tony" "Mark" "Jane"

Question 23

How can we return the items in a set?

Answer 23

SMEMBERS setname
example:

SMEMBERS names

1) “Roger”
2) “Flavio”
3) “Syd”

Question 24

Suppose you wanted to find out if a value is in a set, what’s the command for that?

Answer 24

SISMEMBER setname value

Example:
SISMEMBER names “Flavio”

Question 25

What if you need to find the number of items in a set

Answer 25

SCARD setname

Question 26

Return a random member from a set

Answer 26

SRANDMEMBER setname

Question 27

Extract (and remove) an item from the set, casually ordered:

Answer 27

SPOP setnames

Question 28

How would you extract multiple items at once from a set?

Answer 28

SPOP setname 2

Question 29

Remove an item from a set by value

Answer 29

SREM setname “Flavio”

Question 30

What is a sorted set?

Answer 30

A sorted set associates a rank to each item in a set.

Question 31

How do you target sorted sets?

Answer 31

The same way you work with regular sets except you change the S to a Z example:

SADD -> ZADD
SPOP -> ZPO

ZADD names 1 “Flavio”
ZADD names 2 “Syd”
ZADD names 2 “Roger”

Question 32

We know we can set the score of an item in a sorted set but how can we do that\?

Answer 32

Run the ZRANK command example:

ZRANK names “Flavio”

Question 33

What does ZRANGE do?

Answer 33

It works the same as LRANGE except on sorted set

Question 34

What does WITHSCORES do?

Answer 34

It returns the scores of a set

Example: ZRANGE myset 0 -1 WITHSCORES

Question 35

If you need to increment the score of an item in a sorted set, how is that done?

Answer 35

Use ZINCRBY

The score value should be the string representation of a numeric value, and accepts double precision floating point numbers. It is possible to provide a negative value to decrement the score.

Example:
ZINCRBY myzset 2 “one”
#returns “3”

Question 36

Generally speaking, what are hashes used for?

Answer 36

Hashes let us associate more than one value to a single key, and they are perfect to store object-like items.

Question 37

How are hashes added to the database?

Answer 37

Add Hahses (Keys with multiple values) to the DB

HMSET hashname field1 “Value of Field1” field2 “Value of Field2”
HMSET myhash field1 “Hello” field2 “World”

Question 38

What if you need to return the value of a field in a HASH

Answer 38

Return the value of a field in a Hash

HGET hashname requested-field
HGET myhash field2

Question 39

How can we update a hash field/property?

Answer 39

HSET hashname property1 “propertyval1” property2 “propertyval2”

HSET person:1 age 38

Question 40

Hash values can be incremented, what does that look like?

Answer 40

HINCRBY hashname fieldname value
Example:
HSET myhash field 5
(integer) 1

HINCRBY myhash field 1
(integer) 6

Flavios example:
HINCRBY person:1 age 2

Question 41

How can we set up a publishing/subscription model?

Answer 41

SUBSCRIBE channel

PUBLISH channelname message