Hash Commands

Redis hashes are maps between string fields and string values. They’re ideal for representing objects and storing multiple related values under a single key.

HSET

Sets field-value pairs in a hash. Creates the key if it doesn’t exist.

Syntax

HSET key field value [field value ...]

key

required

The hash key. Created if it doesn’t exist.

field

string

required

The field name to set.

value

string

required

The value to set for the field.

count

integer

The number of fields that were added (not including updates to existing fields).

Time Complexity: O(1) for each field/value pair added, so O(N) for N pairs. History:

4.0.0: Accepts multiple field and value arguments.

Examples

redis> HSET myhash field1 "Hello"
(integer) 1
redis> HGET myhash field1
"Hello"
redis> HSET myhash field2 "World" field3 "!"
(integer) 2
redis> HGETALL myhash
1) "field1"
2) "Hello"
3) "field2"
4) "World"
5) "field3"
6) "!"

HGET

Returns the value of a field in a hash.

Syntax

HGET key field

key

required

The hash key.

field

string

required

The field name to retrieve.

value

string

The value associated with field, or nil if field doesn’t exist.

Time Complexity: O(1)

Examples

redis> HSET myhash field1 "foo"
(integer) 1
redis> HGET myhash field1
"foo"
redis> HGET myhash field2
(nil)

HGETALL

Returns all fields and values in a hash.

Syntax

HGETALL key

key

required

The hash key.

fields

array

Array of field-value pairs (field1, value1, field2, value2, …).

Time Complexity: O(N) where N is the size of the hash.

HGETALL loads all fields into memory. For large hashes, use HSCAN to iterate incrementally.

Examples

redis> HSET myhash field1 "Hello" field2 "World"
(integer) 2
redis> HGETALL myhash
1) "field1"
2) "Hello"
3) "field2"
4) "World"

HINCRBY

Increments the integer value of a hash field by the given number.

Syntax

HINCRBY key field increment

key

required

The hash key.

field

string

required

The field to increment.

increment

integer

required

The increment value (can be negative).

value

integer

The value of the field after incrementing.

Time Complexity: O(1)

Examples

redis> HSET myhash field 5
(integer) 1
redis> HINCRBY myhash field 1
(integer) 6
redis> HINCRBY myhash field -1
(integer) 5
redis> HINCRBY myhash field -10
(integer) -5

Additional Hash Commands

HMGET: Get multiple field values
HMSET: Set multiple fields (deprecated, use HSET)
HSETNX: Set field only if it doesn’t exist
HDEL: Delete one or more fields
HEXISTS: Check if field exists
HKEYS: Get all field names
HVALS: Get all values
HLEN: Get number of fields
HSTRLEN: Get length of field value
HINCRBYFLOAT: Increment by float
HSCAN: Iterate fields
HRANDFIELD: Get random field(s)

Use Cases

User Profile

Store user information:

redis> HSET user:1000 username "alice" email "[email protected]" age "30"
(integer) 3
redis> HGET user:1000 username
"alice"
redis> HGETALL user:1000
1) "username"
2) "alice"
3) "email"
4) "[email protected]"
5) "age"
6) "30"
redis> HINCRBY user:1000 age 1
(integer) 31

Session Store

Store session data:

redis> HSET session:abc123 user_id "1000" created "1640000000" last_seen "1640010000"
(integer) 3
redis> EXPIRE session:abc123 1800
(integer) 1
redis> HGET session:abc123 user_id
"1000"

Product Catalog

Store product details:

redis> HSET product:2000 name "Laptop" price "999.99" stock "50" category "electronics"
(integer) 4
redis> HMGET product:2000 name price
1) "Laptop"
2) "999.99"
redis> HINCRBY product:2000 stock -1
(integer) 49

Statistics Tracking

Track various counters:

redis> HINCRBY stats:website views 1
(integer) 1
redis> HINCRBY stats:website unique_visitors 1
(integer) 1
redis> HINCRBY stats:website page_errors 1
(integer) 1
redis> HGETALL stats:website
1) "views"
2) "1"
3) "unique_visitors"
4) "1"
5) "page_errors"
6) "1"

Rate Limiting by User

Track API calls per endpoint:

redis> HINCRBY rate:user:1000 "/api/users" 1
(integer) 1
redis> HINCRBY rate:user:1000 "/api/posts" 1
(integer) 1
redis> HGET rate:user:1000 "/api/users"
"1"
redis> EXPIRE rate:user:1000 60
(integer) 1

Best Practices

Performance Considerations

Field Access: HGET, HSET are O(1) - very fast
Full Hash: HGETALL is O(N) - use carefully on large hashes
Iteration: Use HSCAN for large hashes instead of HGETALL
Atomic Counters: HINCRBY is atomic and thread-safe

Memory Optimization

Small Hashes: Redis uses efficient ziplist encoding for small hashes
Field Count: Keep under 512 fields for optimal encoding
Value Size: Keep values under 64 bytes for ziplist encoding
Compression: Consider compressing large values before storing

Hash vs Separate Keys

Using Hash
Separate Keys

Store related data together:

redis> HSET user:1000 name "Alice" age "30"
(integer) 2
redis> HGET user:1000 name
"Alice"

Pros:

Memory efficient for small objects
Atomic operations on related data
Logical grouping

Cons:

Can’t set TTL per field
Less flexible for partial updates

Store fields as individual keys:

redis> SET user:1000:name "Alice"
OK
redis> SET user:1000:age "30"
OK
redis> GET user:1000:name
"Alice"

Pros:

Individual TTL per field
Better for selective access
Works with Redis Cluster key distribution

Cons:

More memory overhead
Multiple operations needed

Patterns

Object Storage

Store complete objects:

# Store article
redis> HSET article:1 title "Redis Guide" author "alice" views "0" likes "0"
(integer) 4

# Update counters
redis> HINCRBY article:1 views 1
(integer) 1
redis> HINCRBY article:1 likes 1
(integer) 1

Multi-Tenant Counters

Track metrics per tenant:

redis> HINCRBY metrics:tenant1 api_calls 1
(integer) 1
redis> HINCRBY metrics:tenant1 errors 0
(integer) 0
redis> HINCRBY metrics:tenant2 api_calls 1
(integer) 1
redis> HGETALL metrics:tenant1
1) "api_calls"
2) "1"
3) "errors"
4) "0"

Shopping Cart

Store cart items:

# Add items to cart
redis> HSET cart:user:1000 product:1 "2"
(integer) 1
redis> HSET cart:user:1000 product:2 "1"
(integer) 1

# Update quantity
redis> HINCRBY cart:user:1000 product:1 1
(integer) 3

# Get all items
redis> HGETALL cart:user:1000
1) "product:1"
2) "3"
3) "product:2"
4) "1"

# Remove item
redis> HDEL cart:user:1000 product:2
(integer) 1

Feature Flags

Store feature flags per user:

redis> HSET features:user:1000 dark_mode "true" beta_access "false"
(integer) 2
redis> HGET features:user:1000 dark_mode
"true"
redis> HEXISTS features:user:1000 admin_panel
(integer) 0

Configuration Storage

Store application config:

redis> HSET config:app max_connections "100" timeout "30" debug "false"
(integer) 3
redis> HMGET config:app max_connections timeout
1) "100"
2) "30"

Hash vs Other Data Structures

Feature	Hash	String (JSON)	Separate Keys
Memory	Efficient	More overhead	Most overhead
Field TTL	No	No	Yes
Atomic Ops	Yes (per field)	No	Yes (per key)
Field Access	O(1)	O(N) parse	O(1)
Use Case	Objects	Complex data	Distributed data

Advanced Patterns

Lazy Loading

Load hash fields on demand:

# Check if field exists
redis> HEXISTS user:1000 profile_data
(integer) 0

# Load from database and cache
redis> HSET user:1000 profile_data "{...json...}"
(integer) 1

# Next access is from cache
redis> HGET user:1000 profile_data
"{...json...}"

Aggregation

Aggregate metrics from multiple hashes:

# Daily stats
redis> HINCRBY stats:2024-03-03 views 100
(integer) 100
redis> HINCRBY stats:2024-03-03 clicks 25
(integer) 25

# Calculate CTR
redis> HMGET stats:2024-03-03 views clicks
1) "100"
2) "25"
# CTR = 25/100 = 0.25

Batch Updates

Update multiple fields atomically:

redis> HSET order:1000 status "processing" updated_at "1640000000" processor "worker:1"
(integer) 3

Sparse Data

Store only non-null values:

# Only set fields that have values
redis> HSET user:1000 username "alice"
(integer) 1
# Don't set middle_name if not provided
redis> HSET user:1000 last_name "smith"
(integer) 1

Hashes cannot be nested. Each field value must be a string. For complex nested structures, consider storing JSON strings or using multiple hashes.

Common Operations

Check and Set

# Set only if field doesn't exist
redis> HSETNX user:1000 email "[email protected]"
(integer) 1
redis> HSETNX user:1000 email "[email protected]"
(integer) 0

Bulk Retrieval

# Get multiple fields efficiently
redis> HMGET user:1000 username email age
1) "alice"
2) "[email protected]"
3) "30"

Field Counting

redis> HLEN user:1000
(integer) 3

Field Existence

redis> HEXISTS user:1000 username
(integer) 1
redis> HEXISTS user:1000 phone
(integer) 0

Commands

Modules API

Protocol

​HSET

​Syntax

​Examples

​HGET

​Syntax

​Examples

​HGETALL

​Syntax

​Examples

​HINCRBY

​Syntax

​Examples

​Related Commands

​Additional Hash Commands

​Use Cases

​User Profile

​Session Store

​Product Catalog

​Statistics Tracking

​Rate Limiting by User

​Best Practices

​Performance Considerations

​Memory Optimization

​Hash vs Separate Keys

​Patterns

​Object Storage

​Multi-Tenant Counters

​Shopping Cart

​Feature Flags

​Configuration Storage

​Hash vs Other Data Structures

​Advanced Patterns

​Lazy Loading

​Aggregation

​Batch Updates

​Sparse Data

​Common Operations

​Check and Set

​Bulk Retrieval

​Field Counting

​Field Existence

Build docs developers (and LLMs) love

HSET

Syntax

Examples

HGET

Syntax

Examples

HGETALL

Syntax

Examples

HINCRBY

Syntax

Examples

Related Commands

Additional Hash Commands

Use Cases

User Profile

Session Store

Product Catalog

Statistics Tracking

Rate Limiting by User

Best Practices

Performance Considerations

Memory Optimization

Hash vs Separate Keys

Patterns

Object Storage

Multi-Tenant Counters

Shopping Cart

Feature Flags

Configuration Storage

Hash vs Other Data Structures

Advanced Patterns

Lazy Loading

Aggregation

Batch Updates

Sparse Data

Common Operations

Check and Set

Bulk Retrieval

Field Counting

Field Existence