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Writing an external provider

External provider enables you to write a fully functional provider, using any scripting or programming language. Garm will call your executable to manage the lifecycle of the instances hosting the runners. This document describes the API that an executable needs to implement to be usable by garm.

Environment variables

When garm calls your executable, a number of environment variables are set, depending on the operation. There are three environment variables that will always be set regardless of operation. Those variables are:

  • GARM_COMMAND
  • GARM_PROVIDER_CONFIG_FILE
  • GARM_CONTROLLER_ID

The following are variables that are specific to some operations:

  • GARM_POOL_ID
  • GARM_INSTANCE_ID

The GARM_COMMAND variable

The GARM_COMMAND environment variable will be set to one of the operations defined in the interface. When your executable is called, you'll need to inspect this variable to know which operation you need to execute.

The GARM_PROVIDER_CONFIG_FILE variable

The GARM_PROVIDER_CONFIG_FILE variable will contain a path on disk to a file that can contain whatever configuration your executable needs. For example, in the case of the OpenStack external provider, this file is a toml which contains provider specific configuration options. The provider author decides what this file needs to contain for the provider to function properly.

GARM does not read this file in any way. It is simply passed to the executable via the environment variable.

The OpenStack provider mentioned above is written in Go, but it doesn't need to be. For example, if your provider is written in BASH, handling the config file could look something like this:

if [ -f "${GARM_PROVIDER_CONFIG_FILE}" ];then
    source "${GARM_PROVIDER_CONFIG_FILE}"
fi

Which would make the contents of that config available to you. Then you could implement the needed operations:

case "${GARM_COMMAND}" in
    "CreateInstance")
        # Run the create instance code
        ;;
    "DeleteInstance")
        # Run the delete instance code
        ;;
    # .... the rest of the operations detailed in next sections ....
    *)
        # handle unknown command
        echo "unknown command ${GARM_COMMAND}"
        exit 1
        ;;
esac

The GARM_CONTROLLER_ID variable

The GARM_CONTROLLER_ID variable is set for all operations.

When garm first starts up, it generates a unique ID that identifies it as an instance. This ID is passed to the provider and should always be used to tag resources in whichever cloud you write your provider for. This ensures that if you have multiple garm installations, one particular deployment of garm will never touch any resources it did not create.

In most clouds you can attach tags to resources. You can use the controller ID as one of the tags during the CreateInstance operation.

The GARM_POOL_ID variable

The GARM_POOL_ID environment variable is a UUID4 describing the pool in which a runner is created. This variable is set in two operations:

  • CreateInstance
  • ListInstances

As with the GARM_CONTROLLER_ID, this ID must also be attached as a tag or whichever mechanism your target cloud supports, to identify the pool to which the resources (in most cases the VMs) belong to.

The GARM_INSTANCE_ID variable

The GARM_INSTANCE_ID environment variable is used in four operations:

  • GetInstance
  • DeleteInstance
  • Start
  • Stop

It contains the provider_id of the instance. The provider_id is a unique identifier, specific to the IaaS in which the compute resource was created. In OpenStack, it's an UUID4, while in LXD, it's the virtual machine's name.

We need this ID whenever we need to execute an operation that targets one specific runner.

Operations

The operations that a provider must implement are described in the Provider interface available here. The external provider implements this interface, and delegates each operation to your external executable. These operations are:

  • CreateInstance
  • DeleteInstance
  • GetInstance
  • ListInstances
  • RemoveAllInstances
  • Stop
  • Start

CreateInstance

The CreateInstance command has the most moving parts. The ideal external provider is one that will create all required resources for a fully functional instance, will start the instance. Waiting for the instance to start is not necessary. If the instance can reach the callback_url configured in garm, it will update it's own status when it starts running the userdata script.

But aside from creating resources, the ideal external provider is also idempotent, and will clean up after itself in case of failure. If for any reason the executable will fail to create the instance, any dependency that it has created up to the point of failure, should be cleaned up before returning an error code.

At the very least, it must be able to clean up those resources, if it is called with the DeleteInstance command by garm. Garm will retry creating a failed instance. Before it tries again, it will attempt to run a DeleteInstance using the provider_id returned by your executable.

If your executable failed before a provider_id could be supplied, garm will send the name of the instance as a GARM_INSTANCE_ID environment variable.

Your external provider will need to be able to handle both. The instance name generated by garm will be unique, so it's fairly safe to use when deleting instances.

CreateInstance inputs

The CreateInstance command is the only command that needs to handle standard input. Garm will send the runner bootstrap information in stdin. The environment variables set for this command are:

  • GARM_PROVIDER_CONFIG_FILE - Config file specific to your executable
  • GARM_COMMAND - the command we need to run
  • GARM_CONTROLLER_ID - The unique ID of the garm installation
  • GARM_POOL_ID - The unique ID of the pool this node is a part of

The information sent in via standard input is a json serialized instance of the BootstrapInstance structure

Here is a sample of that:

  {
    "name": "garm-ny9HeeQYw2rl",
    "tools": [
      {
        "os": "osx",
        "architecture": "x64",
        "download_url": "https://github.com/actions/runner/releases/download/v2.299.1/actions-runner-osx-x64-2.299.1.tar.gz",
        "filename": "actions-runner-osx-x64-2.299.1.tar.gz",
        "sha256_checksum": "b0128120f2bc48e5f24df513d77d1457ae845a692f60acf3feba63b8d01a8fdc"
      },
      {
        "os": "linux",
        "architecture": "x64",
        "download_url": "https://github.com/actions/runner/releases/download/v2.299.1/actions-runner-linux-x64-2.299.1.tar.gz",
        "filename": "actions-runner-linux-x64-2.299.1.tar.gz",
        "sha256_checksum": "147c14700c6cb997421b9a239c012197f11ea9854cd901ee88ead6fe73a72c74"
      },
      {
        "os": "win",
        "architecture": "x64",
        "download_url": "https://github.com/actions/runner/releases/download/v2.299.1/actions-runner-win-x64-2.299.1.zip",
        "filename": "actions-runner-win-x64-2.299.1.zip",
        "sha256_checksum": "f7940b16451d6352c38066005f3ee6688b53971fcc20e4726c7907b32bfdf539"
      },
      {
        "os": "linux",
        "architecture": "arm",
        "download_url": "https://github.com/actions/runner/releases/download/v2.299.1/actions-runner-linux-arm-2.299.1.tar.gz",
        "filename": "actions-runner-linux-arm-2.299.1.tar.gz",
        "sha256_checksum": "a4d66a766ff3b9e07e3e068a1d88b04e51c27c9b94ae961717e0a5f9ada998e6"
      },
      {
        "os": "linux",
        "architecture": "arm64",
        "download_url": "https://github.com/actions/runner/releases/download/v2.299.1/actions-runner-linux-arm64-2.299.1.tar.gz",
        "filename": "actions-runner-linux-arm64-2.299.1.tar.gz",
        "sha256_checksum": "debe1cc9656963000a4fbdbb004f475ace5b84360ace2f7a191c1ccca6a16c00"
      },
      {
        "os": "osx",
        "architecture": "arm64",
        "download_url": "https://github.com/actions/runner/releases/download/v2.299.1/actions-runner-osx-arm64-2.299.1.tar.gz",
        "filename": "actions-runner-osx-arm64-2.299.1.tar.gz",
        "sha256_checksum": "f73849b9a78459d2e08b9d3d2f60464a55920de120e228b0645b01abe68d9072"
      },
      {
        "os": "win",
        "architecture": "arm64",
        "download_url": "https://github.com/actions/runner/releases/download/v2.299.1/actions-runner-win-arm64-2.299.1.zip",
        "filename": "actions-runner-win-arm64-2.299.1.zip",
        "sha256_checksum": "d1a9d8209f03589c8dc05ee17ae8d194756377773a4010683348cdd6eefa2da7"
      }
    ],
    "repo_url": "https://github.com/gabriel-samfira/scripts",
    "callback-url": "https://garm.example.com/api/v1/callbacks",
    "metadata-url": "https://garm.example.com/api/v1/metadata",
    "instance-token": "super secret JWT token",
    "extra_specs": {
      "my_custom_config": "some_value"
    },
    "ca-cert-bundle": null,
    "github-runner-group": "my_group",
    "os_type": "linux",
    "arch": "amd64",
    "flavor": "m1.small",
    "image": "8ed8a690-69b6-49eb-982f-dcb466895e2d",
    "labels": [
      "ubuntu",
      "openstack",
      "runner-controller-id:f9286791-1589-4f39-a106-5b68c2a18af4",
      "runner-pool-id:9dcf590a-1192-4a9c-b3e4-e0902974c2c0"
    ],
    "pool_id": "9dcf590a-1192-4a9c-b3e4-e0902974c2c0"
  }

In your executable you can read in this blob, by using something like this:

  # Test if the stdin file descriptor is opened
  if [ ! -t 0 ]
  then
      # Read in the information from standard in
      INPUT=$(cat -)
  fi

Then you can easily parse it. If you're using bash, you can use the amazing jq json processor. Other programming languages have suitable libraries that can handle json.

You will have to parse the bootstrap params, verify that the requested image exists, gather operating system information, CPU architecture information and using that information, you will need to select the appropriate tools for the arch/OS combination you are deploying.

Refer to the OpenStack or Azure providers available in the providers.d folder. Of particular interest are the cloudconfig folders, where the instance user data templates are stored. These templates are used to generate the needed automation for the instances to download the github runner agent, send back status updates (including the final github runner agent ID), and download the github runner registration token from garm.

Examples of external providers written in Go can be found at the following locations:

CreateInstance outputs

On success, your executable is expected to print to standard output a json that can be deserialized into an Instance{} structure defined here.

Not all fields are expected to be populated by the provider. The ones that should be set are:

  {
    "provider_id": "88818ff3-1fca-4cb5-9b37-84bfc3511ea6",
    "name": "garm-ny9HeeQYw2rl",
    "os_type": "linux",
    "os_name": "ubuntu",
    "os_version": "20.04",
    "os_arch": "x86_64",
    "status": "running",
    "pool_id": "41c4a43a-acee-493a-965b-cf340b2c775d",
    "provider_fault": ""
  }

In case of error, garm expects at the very least to see a non-zero exit code. If possible, your executable should return as much information as possible via the above json, with the status field set to error and the provider_fault set to a meaningful error message describing what has happened. That information will be visible when doing a:

  garm-cli runner show <runner name>

DeleteInstance

The DeleteInstance command will permanently remove an instance from the cloud provider.

The environment variables set for this command are:

  • GARM_COMMAND
  • GARM_CONTROLLER_ID
  • GARM_INSTANCE_ID
  • GARM_PROVIDER_CONFIG_FILE

This command is not expected to output anything. On success it should simply exit 0.

If the target instance does not exist in the provider, this command is expected to be a no-op.

GetInstance

The GetInstance command will return details about the instance, as seen by the provider.

The environment variables set for this command are:

  • GARM_COMMAND
  • GARM_CONTROLLER_ID
  • GARM_INSTANCE_ID
  • GARM_PROVIDER_CONFIG_FILE

On success, this command is expected to return a valid json that can be deserialized into an Instance{} structure (see CreateInstance). If possible, IP addresses allocated to the VM should be returned in addition to the sample json printed above.

On failure, this command is expected to return a non-zero exit code.

ListInstances

The ListInstances command will print to standard output, a json that is deserializable into an array of Instance{}.

The environment variables set for this command are:

  • GARM_COMMAND
  • GARM_CONTROLLER_ID
  • GARM_PROVIDER_CONFIG_FILE
  • GARM_POOL_ID

This command must list all instances that have been tagged with the value in GARM_POOL_ID.

On success, a json is expected on standard output.

On failure, a non-zero exit code is expected.

RemoveAllInstances

The RemoveAllInstances operation will remove all resources created in a cloud that have been tagged with the GARM_CONTROLLER_ID. External providers should tag all resources they create with the garm controller ID. That tag can then be used to identify all resources when attempting to delete all instances.

The environment variables set for this command are:

  • GARM_COMMAND
  • GARM_PROVIDER_CONFIG_FILE
  • GARM_CONTROLLER_ID

On success, no output is expected.

On failure, a non-zero exit code is expected.

Note: This command is currently not used by garm.

Start

The Start operation will start the virtual machine in the selected cloud.

The environment variables set for this command are:

  • GARM_COMMAND
  • GARM_CONTROLLER_ID
  • GARM_PROVIDER_CONFIG_FILE
  • GARM_INSTANCE_ID

On success, no output is expected.

On failure, a non-zero exit code is expected.

Stop

NOTE: This operation is currently not use by garm, but should be implemented.

The Stop operation will stop the virtual machine in the selected cloud.

Available environment variables:

  • GARM_COMMAND
  • GARM_CONTROLLER_ID
  • GARM_PROVIDER_CONFIG_FILE
  • GARM_INSTANCE_ID

On success, no output is expected.

On failure, a non-zero exit code is expected.