How do Redis lists work and how are they used for queues?

Question

Accepted Answer

Redis **lists** are ordered collections of strings (implemented as linked lists) with fast push/pop at both ends. They're commonly used to implement **queues** and **stacks** — a simple, popular way to use Redis for job processing and ordered data.

## List operations

```bash
LPUSH mylist "a"        # push to the LEFT (head)
RPUSH mylist "b"        # push to the RIGHT (tail)
LPOP mylist             # pop from the LEFT
RPOP mylist             # pop from the RIGHT
LRANGE mylist 0 -1      # get a range (0 to -1 = all elements)
LLEN mylist             # length
LINDEX mylist 0         # element at an index
```

Push/pop at either end are fast (O(1)). `LRANGE` reads ranges. These operations make lists ideal for ordered, queue-like data.

## Implementing a queue (FIFO)

```bash
# Producer adds jobs; consumer processes them in order (FIFO)
LPUSH job_queue "job1"    # producer pushes to the left
LPUSH job_queue "job2"
RPOP job_queue            # consumer pops from the right => "job1" (first in, first out)
# push left + pop right = FIFO queue
```

A **queue** (first-in-first-out): producers `LPUSH` (add to one end), consumers `RPOP` (remove from the other) — the classic Redis job queue pattern.

## Blocking pops (efficient consumers)

```bash
# BRPOP blocks (waits) until an element is available — no busy polling
BRPOP job_queue 5     # wait up to 5 seconds for a job (0 = wait forever)
# → the consumer efficiently waits for work instead of repeatedly polling
```

**Blocking pops** (`BLPOP`/`BRPOP`) let a consumer wait efficiently for new items instead of constantly polling — important for responsive, efficient queue workers.

## Stack and other patterns

```bash
# STACK (LIFO): push and pop from the SAME end
LPUSH stack "a"; LPOP stack    # last in, first out

# capped list (e.g. keep the last 100 items — recent activity feed)
LPUSH recent "event"; LTRIM recent 0 99   # keep only indices 0-99
```

## Why it matters

Redis lists and their use for queues are commonly-applied, practical Redis knowledge, so understanding them is valuable for many real use cases.

Lists (ordered string collections with **fast O(1) push/pop at both ends**) are versatile, but their most important application is implementing **queues** — using `LPUSH`/`RPOP` (or the reverse) to build a **FIFO job queue** where producers add work and consumers process it in order, a simple and extremely common pattern for background job processing, task distribution, and decoupling producers from consumers.

This makes Redis a popular lightweight message/job queue without needing a dedicated queue system for simpler needs.

Understanding **blocking pops** (`BLPOP`/`BRPOP`) is particularly important for efficient queue workers — they let consumers wait for new items rather than wastefully polling in a loop, making workers responsive and efficient (a key practical technique for real queue implementations).

Knowing other patterns — **stacks** (LIFO, pushing/popping the same end) and **capped lists** (using `LTRIM` to keep only recent N items, ideal for activity feeds and recent-items lists) — extends lists' usefulness.

Since job queues and ordered data are common needs, and since Redis lists provide a simple, fast, popular way to implement queues (with blocking pops for efficient consumers), stacks, and recent-items feeds, understanding Redis lists and the queue pattern is valuable, frequently-applied knowledge for using Redis in real applications, one of the most common practical uses of Redis beyond caching and a building block for background processing and decoupled architectures.