Assigned Host and Assigned Ports

• How does Bolt assign host to your services
• How does Bolt assign ports to your services

How does Bolt assign host to your services

• Whenever Bolt runs a service, it automatically inserts the key name ASSIGNED_HOST with the default host value of localhost into the .env.tpl file of that service. For example suppose you have a service named nextapp, and you run it using bolt service:up postgres -sr local, or bolt up then Bolt will automatically insert the following line into the .env.tpl file of the nextapp service:

  ASSIGNED_HOST=localhost

• Now in order to access host variable in .env.tpl of one service say postgres to another service say nextapp, you can use the following syntax in .env.tpl file of nextapp:

  PGHOST=%POSTGRES_ASSIGNED_HOST%

  Here %POSTGRES_ASSIGNED_HOST% will automatically gets replaced in .env file of the nextapp as per following rules:

  • If nextapp is running on local or vmlocal then %POSTGRES_ASSIGNED_HOST% will be replaced with localhost.
  • If nextapp is running on docker or vmdocker then %POSTGRES_ASSIGNED_HOST% will be replaced with host.docker.internal if your operating system is macOS or windows otherwise it will be replaced with localhost.

  So overall result will be something like this:

  # If service1 is running on local or vmlocal
  POSTGRESHOST=localhost
  
  # OR
  
  # If service1 is running on docker or vmdocker and current OS is macOS or windows
  POSTGRESHOST=host.docker.internal
  
  # OR
  
  # If service1 is running on docker or vmdocker and current OS is any other platform
  POSTGRESHOST=localhost

How does Bolt assign ports to your services

• At first bolt picks up the first port from service_discovery_offset and checks if it is already in use or not. If it is already in use then bolt will increment the port value by 1 and will retry the same process upto 5 times until it finds a port which is not in use otherwise it will stop the process and will throw an error.
• Once it gets a port to run the service it will insert the key named ASSIGNED_PORT with the port value into the .env.tpl file of that service. For example suppose you have a service named postgres and assume this is how its bolt.service.yaml files looks like:

  container_name: postgres
  stateless: true
  default_service_runner: docker
  service_discovery_offset:
    - 5432
  supported_service_runners:
    - docker
  service_runners:
    docker:
      envfile: .env
      build: ./run.Dockerfile
      ports:
        - ${ASSIGNED_PORT}:5432
      volumes:
        - ./services/postgres/init.db/:/docker-entrypoint-initdb.d/

  You can run it using bolt service:up postgres -sr local, or bolt up then Bolt will look for the free ports starting from 5432 and whatever port it will find free it will automatically insert that port's value into the .env.tpl file of the postgres service like this:

  # Here I am assuming that bolt found port 5432 is free to run the postgres service
  ASSIGNED_PORT=5432

  Now this port will be used to expose the postgres service from docker to your system.
• If a particular service requires more than one port to run, then you can add those ports in service_discovery_offset key in bolt.yaml file, for example minio requires two ports to run, first one is to run minio and second one is to run minio console, so its bolt.service.yaml file looks like this:

  container_name: minio
  stateless: true
  default_service_runner: docker
  service_discovery_offset:
    - 9000
    - 9001
  supported_service_runners:
    - docker
  service_runners:
    docker:
      envfile: .env
      build: ./run.Dockerfile
      ports:
        - ${ASSIGNED_PORT}:9000
        - ${ASSIGNED_PORT_1}:9001
      volumes:
        - ./services/minio/data/:/data/

  Now when you run this service using bolt service:up minio -sr docker, or bolt up then Bolt will look for the free ports starting from 9000 and 9001 and whatever ports it will find free it will automatically insert those ports' values into the .env.tpl file of the minio service like this:

  # Here I am assuming that bolt found port 9000 and 9001 are free to run the minio service
  ASSIGNED_PORT=9000
  ASSIGNED_PORT_1=9001

  Now these ports will be used to expose the minio service from docker to your system.
• Now in order to access port variable in .env of one service say postgres to another service say nextapp, you can use the following syntax in .env.tpl file of nextapp:

  PGPORT=%POSTGRES_ASSIGNED_PORT%

  Here %POSTGRES_ASSIGNED_PORT% will automatically gets replaced by the actual value of POSTGRES_ASSIGNED_PORT. So if postgres service's assigned port is 5432 then %POSTGRES_ASSIGNED_PORT% will be replaced with 5432, so actual value in .env file will be something like this:

  PGPORT=5432