Difference between links and depends on in docker compose yml

Question

According to the Docker Compose s compose-file documentation    depends on - Express dependency between services  links - Link to containers in another service and also express dependency between services in the same way as depends on    I don t understand the purpose of linking to other containers so the difference between two options still seems quite difficult for me   It would be much easier if there is an example  but I can t find any   I noticed  when I link container B with container A then container B will be  pingable  inside container A s shell   I ran ping B inside container A s bash and got result like this  just for reference  image from the Internet

User · Answer

Update Sep 2016   This answer was intended for docker compose file v1  as shown by the sample compose file below   For v2  see the other answer by  Windsooon    Original answer    It is pretty clear in the documentation  depends on decides the dependency and the order of container creation and links not only does these  but also      Containers for the linked service will be reachable at a hostname identical to the alias  or the service name if no alias was specified    For example  assuming the following docker-compose yml file   web    image  example my web app latest   links      - db     - cache  db    image  postgres latest  cache    image  redis latest   With links  code inside web will be able to access the database using db 5432  assuming port 5432 is exposed in the db image   If depends on were used  this wouldn t be possible  but the startup order of the containers would be correct

User · Answer

The post needs an update after the links option is deprecated   Basically  links is no longer needed because its main purpose  making container reachable by another by adding environment variable  is included implicitly with network  When containers are placed in the same network  they are reachable by each other using their container name and other alias as host   For docker run  --link is also deprecated and should be replaced by a custom network    docker network create mynet docker run -d --net mynet --name container1 my image docker run -it --net mynet --name container1 another image   depends on expresses start order  and implicitly image pulling order   which was a good side effect of links

User · Answer

This answer is for docker-compose version 2 and it also works on version 3 You can still access the data when you use depends on  If you look at docker docs Docker Compose and Django  you still can access the database like this  version   2  services    db      image  postgres   web      build        command  python manage py runserver 0 0 0 0 8000     volumes        -    code     ports        -  quot 8000 8000 quot      depends on        - db  What is the difference between links and depends on  links  When you create a container for a database  for example  docker run -d --name test-mysql --env  quot MYSQL ROOT PASSWORD mypassword quot  -P mysql  docker inspect d54cf8a0fb98  grep HostPort  And you may find  quot HostPort quot    quot 32777 quot   This means you can connect the database from your localhost port 32777  3306 in container  but this port will change every time you restart or remove the container  So you can use links to make sure you will always connect to the database and don t have to know which port it is  web    links     - db  depends on  I found a nice blog from Giorgio Ferraris Docker-compose yml  from V1 to V2  When docker-compose executes V2 files  it will automatically build a network between all of the containers defined in the file  and every container will be immediately able to refer to the others just using the names defined in the docker-compose yml file   And  So we don   t need links anymore  links were used to start a network communication between our db container and our web-server container  but this is already done by docker-compose  Update depends on Express dependency between services  which has two effects   docker-compose up will start services in dependency order  In the following example  db and redis will be started before web  docker-compose up SERVICE will automatically include SERVICE   s dependencies  In the following example  docker-compose up web will also create and start db and redis   Simple example  version   2  services    web      build        depends on        - db       - redis   redis      image  redis   db      image  postgres   Note  depends on will not wait for db and redis to be    ready    before starting web - only until they have been started  If you need to wait for a service to be ready  see Controlling startup order for more on this problem and strategies for solving it

[docker] Difference between links and depends_on in docker_compose.yml

Examples related to docker

Examples related to docker-compose

Examples related to dockerfile