Chapter 1 - The Basics
What makes Redis special? What types of problems does it solve? What should developers
watch out for when using it? Before we can answer any of these questions, we need to
understand what Redis is.
Redis is often described as an in-memory persistent key-value store. I don't think that's an
accurate description. Redis does hold all the data in memory (more on this in a bit), and it
does write that out to disk for persistence, but it's much more than a simple key-value store.
It's important to step beyond this misconception otherwise your perspective of Redis and the
problems it solves will be too narrow.
The reality is that Redis exposes five different data structures, only one of which is a typical
key-value structure. Understanding these five data structures, how they work, what methods
they expose and what you can model with them is the key to understanding Redis. First
though, let's wrap our heads around what it means to expose data structures.
If we were to apply this data structure concept to the relational world, we could say that
databases expose a single data structure - tables. Tables are both complex and flexible. There
isn't much you can't model, store or manipulate with tables. However, their generic nature
isn't without drawbacks. Specifically, not everything is as simple, or as fast, as it ought to be.
What if, rather than having a one-size-fits-all structure, we used more specialized structures?
There might be some things we can't do (or at least, can't do very well), but surely we'd gain
in simplicity and speed?
Using specific data structures for specific problems? Isn't that how we code? You don't use a
hashtable for every piece of data, nor do you use a scalar variable. To me, that defines Redis'
approach. If you are dealing with scalars, lists, hashes, or sets, why not store them as scalars,
lists, hashes and sets? Why should checking for the existence of a value be any more complex
than calling exists(key) or slower than O(1) (constant time lookup which won't slow down
regardless of how many items there are)?
The Building Blocks
Databases
Redis has the same basic concept of a database that you are already familiar with. A database
contains a set of data. The typical use-case for a database is to group all of an application's
data together and to keep it separate from another application's.
In Redis, databases are simply identified by a number with the default database being number
0. If you want to change to a different database you can do so via the select command. In
the command line interface, type select 1. Redis should reply with an OK message and your
prompt should change to something like redis 127.0.0.1:6379[1]>. If you want to switch
back to the default database, just enter select 0 in the command line interface..
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