Object storage with hashes in Redis - Step-by-Step Execution

Concept Flow - Object storage with hashes

Start

↓

Create Hash Key

↓

Add Field-Value Pairs

↓

Store Hash in Redis

↓

Retrieve Hash by Key

↓

Access Specific Fields

↓

End

This flow shows how to create a hash key, add field-value pairs, store it in Redis, and retrieve specific fields from the hash.

Execution Sample

Redis

HSET user:1000 name "Alice" age "30" city "NY"
HGET user:1000 name
HGETALL user:1000

Store user data as a hash with fields name, age, city; then retrieve the name and all fields.

Execution Table

Step	Command	Action	Result
1	HSET user:1000 name "Alice" age "30" city "NY"	Create hash key 'user:1000' and set fields	(integer) 3
2	HGET user:1000 name	Retrieve 'name' field from hash 'user:1000'	"Alice"
3	HGETALL user:1000	Retrieve all fields and values from hash 'user:1000'	["name", "Alice", "age", "30", "city", "NY"]
4	HGET user:1000 country	Try to get non-existing field 'country'	(nil)

💡 No more commands; demonstration ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
user:1000 (hash)	empty	{"name":"Alice", "age":"30", "city":"NY"}	{"name":"Alice", "age":"30", "city":"NY"}	{"name":"Alice", "age":"30", "city":"NY"}	{"name":"Alice", "age":"30", "city":"NY"}
name field	none	set to "Alice"	"Alice"	"Alice"	"Alice"
country field	none	none	none	none	none (does not exist)

Key Moments - 2 Insights

Why does HGET return nil when asking for a field that does not exist?

Because in step 4 of the execution_table, the field 'country' was never set in the hash 'user:1000', so Redis returns nil to indicate no such field.

Does HSET overwrite existing fields or add new ones?

HSET adds new fields or overwrites existing ones. In step 1, all fields are set fresh. If you run HSET again with the same field, it updates the value.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result of the HGET command at step 2?

A "Alice"

B "30"

C nil

D ["name", "Alice"]

💡 Hint

Check the 'Result' column for step 2 in the execution_table.

At which step does the hash 'user:1000' get created and fields set?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Action' column to see when the hash key is created.

If you try to get a field that does not exist, what does Redis return according to the execution_table?

A An empty string

B nil

C An error message

D The whole hash

💡 Hint

See the result of step 4 in the execution_table.

Concept Snapshot

Object storage with hashes in Redis:
- Use HSET key field value [field value ...] to store multiple fields.
- Use HGET key field to get a single field's value.
- Use HGETALL key to get all fields and values.
- Non-existing fields return nil.
- Hashes store related data under one key for easy access.

Full Transcript

This visual execution shows how Redis stores objects using hashes. First, a hash key 'user:1000' is created with fields 'name', 'age', and 'city' using HSET. Then, HGET retrieves the 'name' field, returning 'Alice'. HGETALL returns all fields and values as a list. Trying to get a non-existing field 'country' returns nil. Variables track the hash content and field values step-by-step. Key moments clarify why nil is returned for missing fields and how HSET updates or adds fields. The quiz tests understanding of these steps and results.