Docker Compose — Volume Syntax

1 minute read

Compose supports two syntax forms for volume mounts: short form and long form. This page explains both and when to use each.

Short Form

The compact, most-used syntax:

volumes:
  # Named volume
  - pgdata:/var/lib/postgresql/data

  # Bind mount
  - ./src:/app/src

  # Bind mount (read-only)
  - ./config:/etc/myapp/config:ro

  # Anonymous volume
  - /app/node_modules

Format: [SOURCE:]TARGET[:OPTIONS]

Part Required Example
SOURCE Optional pgdata, ./src, /home/user/data
TARGET Yes /var/lib/postgresql/data
OPTIONS Optional ro, rw

Long Form

The verbose, explicit syntax:

volumes:
  # Named volume
  - type: volume
    source: pgdata
    target: /var/lib/postgresql/data

  # Bind mount
  - type: bind
    source: ./src
    target: /app/src

  # Bind mount (read-only)
  - type: bind
    source: ./config
    target: /etc/myapp/config
    read_only: true

  # tmpfs mount (in-memory)
  - type: tmpfs
    target: /tmp
    tmpfs:
      size: 100000000    # 100MB

Long Form Fields

Field Values Purpose
type volume, bind, tmpfs Mount type
source Volume name or host path Data source
target Container path Mount point inside container
read_only true, false Prevent writes
bind.propagation rprivate, shared, etc. Mount propagation
tmpfs.size Bytes Memory limit for tmpfs

Short vs Long: When to Use

Use Short When Use Long When
Simple volume mounts Need tmpfs mounts
Most everyday use Need mount propagation
Quick and readable Need explicit clarity
Team is comfortable Onboarding new developers

Most teams use short form for 90% of cases:

# Short form — most teams use this
services:
  db:
    image: postgres:16
    volumes:
      - pgdata:/var/lib/postgresql/data
      - ./init.sql:/docker-entrypoint-initdb.d/init.sql:ro

tmpfs Mounts

tmpfs mounts store data in memory only — useful for temporary files and secrets:

services:
  app:
    image: myapp
    volumes:
      - type: tmpfs
        target: /tmp
        tmpfs:
          size: 52428800    # 50MB

tmpfs data is lost when the container stops. Use for:

  • Temporary files
  • Session data
  • In-memory caches

Summary

Syntax Example Use Case
Short: named volume pgdata:/data Database storage
Short: bind mount ./src:/app/src Development code
Short: read-only ./config:/etc/config:ro Config files
Short: anonymous /app/node_modules Exclude host dirs
Long: volume type: volume, source: pgdata Explicit clarity
Long: bind type: bind, source: ./src Mount options needed
Long: tmpfs type: tmpfs, target: /tmp In-memory temp storage

Quick Reference

Item Detail
Short named pgdata:/var/lib/data
Short bind ./src:/app/src
Short read-only ./conf:/etc/conf:ro
Short anonymous /app/node_modules
Long form type: volume/bind/tmpfs
tmpfs In-memory only, lost on stop

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