Redis Lists

7. Redis Lists

What is a Redis List?

A Redis List is an ordered collection of strings. You can add elements to the left (head) or right (tail) of the list. It is essentially a doubly linked list internally.

Analogy: A ticket queue at a counter. New people join at the back (right), and the counter serves from the front (left).

Queue vs Stack Patterns

flowchart LR
    subgraph Queue[Queue — FIFO
      First In, First Out]
        Q1["RPUSH task1..task3"] --> Q2["✅ queue: [task1, task2, task3]"]
        Q2 --> Q3["LPOP → task1"]
        Q3 --> Q4["Remaining: [task2, task3]"]
    end

    subgraph Stack[Stack — LIFO
      Last In, First Out]
        S1["LPUSH action1..action3"] --> S2["✅ stack: [action3, action2, action1]"]
        S2 --> S3["LPOP → action3"]
        S3 --> S4["Remaining: [action2, action1]"]
    end

    style Queue fill:#7c3aed,color:#fff
    style Stack fill:#3b82f6,color:#fff
    style Q3 fill:#059669,color:#fff
    style S3 fill:#f59e0b,color:#fff

Core Commands

# Push to the RIGHT end (tail) — queue-like
RPUSH tasks "send-email" "process-payment" "generate-report"

# Push to the LEFT end (head)
LPUSH tasks "urgent-task"

# List all items (0 = first, -1 = last/end)
LRANGE tasks 0 -1
# Output:
# 1) "urgent-task"
# 2) "send-email"
# 3) "process-payment"
# 4) "generate-report"

# Get length of list
LLEN tasks
# Output: (integer) 4

# Pop from the LEFT (dequeue)
LPOP tasks
# Output: "urgent-task"

# Pop from the RIGHT (stack pop)
RPOP tasks
# Output: "generate-report"

# Peek at specific index (0-based)
LINDEX tasks 0
# Output: "send-email"

Queue vs Stack Implementation

Pattern	Push	Pop	Use Case
Queue (FIFO)	`RPUSH`	`LPOP`	Task queue, job processing
Stack (LIFO)	`LPUSH`	`LPOP`	Browser history, undo operations

Real-World Examples

Example 1: Notification Queue

# Producer: add notifications
RPUSH notifications:user:42 "You have a new message"
RPUSH notifications:user:42 "Your order has shipped"

# Consumer: process notifications one by one
LPOP notifications:user:42
# Output: "You have a new message"

Example 2: Recent Activity Feed

# Add recent actions (trim to last 10 items)
LPUSH feed:user:42 "Liked post #101"
LTRIM feed:user:42 0 9   # Keep only last 10

# Get activity feed
LRANGE feed:user:42 0 -1

Example 3: Background Job Queue

# Worker 1 adds a job
RPUSH job:queue '{"type":"email","to":"user@example.com"}'

# Worker 2 picks up the job (blocks if empty — BLPOP)
BLPOP job:queue 30
# Blocks for up to 30 seconds waiting for a new job