What is a key-value store

How it works

Redis stands for Remote Dictionary Server. Each item in Redis has a unique key; values are strings, hashes, lists, sets, sorted sets, streams and more. The same few commands cover most uses: set a value, get it, delete it, make it expire, and add one to a counter.

> SET session:42 "alice" EX 3600
OK
> GET session:42
"alice"
> INCR visits
(integer) 1

SET key value EX seconds sets a value that expires after the given number of seconds. INCR adds one to the number stored at a key and starts from 0 if the key does not exist. That makes it a good fit for sessions, counters, rate limits and caches, Redis itself lists caching, queuing and event processing among its uses.

A key-value store lives in a separate process. Many programs can read and write the same keys, and the data survives a restart of any one of them, if the store is set up to keep it.

Does it keep the data?

That depends on the setting. Redis works in memory first, and you choose how much of it goes to disk: RDB takes point-in-time snapshots at intervals, AOF logs every write, and you can turn persistence off, which is sometimes done for caches.

Redis suggests using both RDB and AOF for a degree of data safety comparable to PostgreSQL. Redis says to back up your database: disks break and cloud instances disappear, so copy the snapshot file somewhere else too.

Key-value, database and file storage

Files go to S3 object storage. Records you search and join go to a database such as managed Postgres. The key-value store sits between your app and those two, for the small data you read all the time.

Do you need it?

Two ways to do it on your computer

# 1. A key-value table in Postgres
create table kv (key text primary key, value jsonb not null);

insert into kv (key, value) values ('user:42', '{"plan":"plus"}')
  on conflict (key) do update set value = excluded.value;

select value from kv where key = 'user:42';

# 2. Redis in Docker, saving a snapshot every 60 seconds if anything changed
$ docker run --name some-redis -d -p 127.0.0.1:6379:6379 redis redis-server --save 60 1

INSERT ... ON CONFLICT (key) DO UPDATE SET value = EXCLUDED.value inserts a new row or updates the existing one. The PostgreSQL docs say most applications should store JSON data as jsonb, which can be indexed. Managed databases on Uno run Postgres only, so for Redis you run the container yourself.

The computer sleeps when idle, and memory and running programs are saved while it sleeps, so a running Redis is still there after the wake. Keep the snapshot setting anyway: it protects you from a crash or a deleted computer.

Questions