Cloud Foundry Task with dotnet app

We’re investigating the CF run-task command from the cli. This is used to execute tasks that run to completion, like database migrations or print something to the console 😉

Hello console

We’ve created a new console project. The main function will print the first command-line argument. Everybody is familiar with “hello world“.

The key to getting this to work on Cloud Foundry is the manifest.yml:

applications:
- name: hellotaskworld
  path: bin/Debug/netcoreapp2.0/
  instances: 0
  memory: 256M
  no-route: true
  buildpacks: dotnet_core_buildpack

instances: 0 makes sure the container is not started. It would crash-and-burn since the application exits after it prints to the console. Cloud Foundry wants to have continuously running applications like a webapp.
no-route: true binds no route to the application. It cannot serve web requests.
path: bin/Debug/netcoreapp2.0/ together with buildpacks: dotnet_core_buildpack pushes the assembly and leaves the dotnet runtime in the container

After a CF PUSH we can run the application with the CLI:

cf run-task hellotaskworld "dotnet ./HelloTaskWorld.dll eric" --name hello-eric-task
cf tasks hellotaskworld
id   name             state      command
3    hello-eric-task  SUCCEEDED  dotnet ./HelloTaskWorld.dll eric

To get the console output we request the logs

cf logs hellotaskworld --recent
[CELL/0] OUT Creating container
[CELL/0] OUT Successfully created container
[APP/TASK/hello-eric-task/0] OUT Hello eric!
[APP/TASK/hello-eric-task/0] OUT Exit status 0
[CELL/0] OUT Stopping instance 1dc5766e-f3f4-4b0d-9332-e1e761db911d
[CELL/0] OUT Destroying container
[CELL/0] OUT Successfully destroyed container

Success! We’ve send our message to Cloud Foundry.

Hello web

What about web applications? We will be deploying our website to Cloud Foundry and want our hello eric (or database migration, file cleanup, backups, …) to be run from that application.

With dotnet core the starting point of a web application is again the Main function. We can check the command-line arguments for our custom tokens.
❗ warning ❗ Cloud Foundry uses command-line arguments to configure the application: ./websitename –server.urls http://0.0.0.0:1234. Make sure your argument name is different/unique.

public static void Main(string[] args) {
    if (args[0] != "--name") {
        BuildWebHost(args).Run();
    } else {
        // start task and print to console
        Console.WriteLine($"Hello {args[1]}");
    }
}

The manifest is very simple since the main use case is a web application.

applications:
- name: webwithtask
  instances: 1
  memory: 256M

After a CF PUSH we can run the application with the CLI. The location of the assembly we got from the logging of the PUSH which shows the command to run

type:            web
instances:       1/1
memory usage:    256M
start command:   cd ${DEPS_DIR}/0/dotnet_publish && ./WebWithTask --server.urls http://0.0.0.0:${PORT}
     state     since                  cpu    memory         disk           details
#0   running   2019-09-06T10:24:32Z   0.0%   156K of 256M   104.2M of 1G   

Now we can start the task by adding the –name parameter to get to the else branch in the main function.

cf run-task webwithtask "cd /home/vcap/deps/0/dotnet_publish && ./WebWithTask --name eric" --name hello-eric-task
cf tasks webwithtask
id   name             state      command
1    hello-eric-task  SUCCEEDED  cd /home/vcap/deps/0/dotnet_publish && ./WebWithTask --name eric

To get the console output we request the logs

cf logs webwithtask --recent
[CELL/0] OUT Creating container
[CELL/0] OUT Successfully created container
[APP/TASK/hello-eric-task/0] OUT Hello eric
[APP/TASK/hello-eric-task/0] OUT Exit status 0
[CELL/0] OUT Stopping instance 12af1740-dec5-4975-96a8-85713102045d
[CELL/0] OUT Destroying container
[CELL/0] OUT Successfully destroyed container

Success! We’ve send our message to Cloud Foundry.

References

https://docs.cloudfoundry.org/devguide/using-tasks.html
https://github.com/cloudfoundry/dotnet-core-buildpack/issues/163
https://dotnet-cookbook.cfapps.io/core/app-startup-tasks/

About erictummers

Working in a DevOps team is the best thing that happened to me. I like challenges and sharing the solutions with others. On my blog I’ll mostly post about my work, but expect an occasional home project, productivity tip and tooling review.
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