How to use Redis Hashes

A Redis hash is a data type that stores multiple field-value pairs under a single key. If you think of a Redis string as a variable, a hash is more like a JavaScript object: one key, many named properties.

So far with Lists and Sets we saw how to correlate a key with a value, or a group of values.

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

For example, a person has a name and an age.

We can create a person:1 hash:

HMSET person:1 name "Flavio" age 37

The person:1 key naming is a common Redis convention. The prefix tells you what kind of item it is, and the number identifies the specific one. person:2 would be another person.

To get all the properties of a user, use HGETALL:

HGETALL person:1

If you only need one field, HGET is cheaper than fetching everything:

HGET person:1 name

This returns "Flavio".

You can update a hash property using HSET:

HSET person:1 age 38

Since Redis 4.0, HSET also accepts multiple field-value pairs, so it does everything HMSET does. HMSET still works, but it’s considered deprecated. New code can use HSET for both creating and updating:

HSET person:1 name "Flavio" age 38

You can increment a value stored in a hash using HINCRBY:

HINCRBY person:1 age 2

Now age is 40. Pass a negative number to decrement.

Other useful hash commands

HDEL removes a field from the hash:

HDEL person:1 age

HEXISTS tells you if a field exists, returning 1 or 0:

HEXISTS person:1 name

HKEYS lists all the field names, and HVALS lists all the values.

One thing to watch out for

Every value in a hash is stored as a string, even if it looks like a number. HINCRBY works because Redis parses the string as an integer before incrementing.

If the field holds something that’s not an integer, like "thirty" or "38.5", the command fails with:

(error) ERR hash value is not an integer

The fix is to keep counters and other numeric fields strictly numeric, and use a separate field for any human-readable version. For floats there’s a dedicated command, HINCRBYFLOAT.

See all the hash commands here.