25.8. Workflow

dr-provision implements a basic Workflow system to help automate the various tasks needed to provision and decommission systems. The workflow system is the results of several other components in dr-provision interacting. The rest of this section goes over those parts in more detail.

25.8.1. The Bits and Bobs

25.8.1.1. Tasks

The basic unit of work that dr-provision sequences to drive Machines through a workflow is a Task. Individual Tasks are executed against Machines by creating a Job for them.

Tasks contain individual Templates that are expanded for a Machine whenever a Job is created. Each of these individual Templates can expand to either a script to be executed (if the Path parameter is empty or not present), or a file to be placed on the filesystem at the location indicated by template-expanding the Path parameter.(if the Path parameter is not empty).

25.8.1.2. Jobs

A Job is used to track the execution history of Tasks against a specific Machine. A Job is created every time a Task is executed against a machine – Jobs keep track of their state of execution. The history of what has been executed (including all log output from scripts) is stored as a chain of Jobs, and the exit status of the Job determines what a machine agent will do next.

25.8.1.3. Stages

A Stage is used to provide a list of Tasks that should be run on a Machine along with the BootEnv the tasks should be run in.

25.8.1.4. Machine Agent

The Machine Agent is responsible for creating Jobs, writing out or executing any JobActions, streaming job Logs back to dr-provision for archival purposes, and updating the Job state based on the exit state of any Actions. The Machine Agent is also responsible for rebooting, powering off, and changing to a different Stage as indicated by the Job exit status or the change stage map. Unless directed to exit, the Machine Agent watches the event stream for the Machine it is running on and will execute new tasks as they come to be available.

25.8.1.5. Change Stage Map (DEPRECATED)

The change-stage/map parameter defines what stage to change to when you finish all the tasks in the current stage. The change stage map is a map whose keys correspond to the stage the machine is currently in and whose values indicate the next stage to transition to and what to have the runner do on the stage transition.

The change-stage/map mechanism has been replaced by the Workflow mechanism, but will be maintained for the forseeable future. You are encouraged to migrate to using Workflows.

25.8.1.6. Workflows

A Workflow is used to provide a list of Stages that a Machine should run through to get to a desired end state. When a Workflow is added to a Machine, it renders the BootEnv and Stage for the Machine read-only, replaces the task list on the Machine with one that will step through all the Stages, BootEnvs, and Tasks needed to drive the machine through the Workflow.

25.8.2. How They Work Together

25.8.2.1. Machine Agent (client side)

The Machine Agent runs on the Client and is responsible for executing tasks and rebooting the Machine as needed. It is structured as a finite state machine for increased reliability and auditability. The Machine Agent always starts in the AGENT_INIT state.

AGENT_INIT

Initializes the Agent with a fresh copy of the Machine data, marks the current Job for the machine as failed if it is created or running,and creates an event stream that recieves events for that Machine from dr-provision. If an error was recorded, the Agent prints it to stderr and then clears it out.

If an error occurrs during this, the agent will sleep for a bit and transition back to AGENT_INIT, otherwise it will transition to AGENT_WAIT_FOR_RUNNABLE.

AGENT_WAIT_FOR_RUNNABLE
Waits for the Machine to be both Available and Runnable. Once it is, the Agent transitions to AGENT_REBOOT if the machine changed BootEnv, AGENT_EXIT if the Agent recieved a termination signal, AGENT_INIT if there was an error waiting for the state change, and AGENT_RUN_TASK otherwise.
AGENT_RUN_TASK

Tries to create a new Job to run on the machine.

If there was an error creating the Job, transitions back to AGENT_INIT.

If there was no job created, the Agent transitions to AGENT_CHANGE_STATE if the Machine does not have a Workflow, and AGENT_WAIT_FOR_CHANGE_STAGE if it does.

If a Job was created, the Agent attempts to execute all the steps in the Task for which the Job was created, and updates the Job depending on the exit status of the steps.

If there was an error executing the Job, the agent will transition back to AGENT_INIT.

If the Job signalled that a reboot is needed, the Agent transitions to AGENT_REBOOT.

If the Job signalled that the system should be powered off, the Agent transitions to AGENT_POWEROFF.

If the Job signalled that the Agent should stop processing Jobs, the Agent transitions to AGENT_EXIT.

Otherwise, the Agent transitions to AGENT_WAIT_FOR_RUNNABLE.

AGENT_WAIT_FOR_STAGE_CHANGE

Waits for the Machine to be Available, and for any of the following fields on the Machine to change:

  • CurrentTask
  • Tasks
  • Runnable
  • BootEnv
  • Stage

Once those conditions are met, follows the same rules as AGENT_WAIT_FOR_RUNNABLE.

AGENT_CHANGE_STAGE

Checks the change-stage/map to determine what (and how) to transition to the next Stage when AGENT_RUN_TASK does not get a Job to run from dr-provision.

The Agent first tries to retrieve the change-stage/map Param for the Machine from dr-provision. If it fails due to connection issues, the Agent will transition to AGENT_INIT. If there is no change-stage map, the Agent uses an empty one.

If there is a key in the change-stage/map for the current Stage, the Agent saves the corresponding value as val for further processing.

If there is no next entry for the current Stage in the change-stage/map and the Machine is in a BootEnv that ends in -install, the Agent assumes that val is “local”, otherwise the Agent transitions to AGENT_WAIT_FOR_STAGE_CHANGE.

The Agent splits val into nextStage and targetState on the first ‘:’ character in val.

If targetState is empty, it is set according to the following rules:

  • If the BootEnv for nextStage is not empty or different from the current BootEnv, targetState is set to “Reboot”
  • Otherwise targetState is set to “Success”

The Agent changes the machine Stage to the value indicated by nextStage. If an error occurs during that process, the Agent transitions to AGENT_INIT.

If targetState is “Reboot”, the agent transitions to AGENT_REBOOT. if targetState is “Stop”, the agent transitions to AGENT_EXIT. If targetState is “Shutdown”, the agent transitions to AGENT_POWEROFF. If targetState is anything else, the agent transitions to AGENT_WAIT_FOR_RUNNABLE.

AGENT_EXIT
Exits the Agent.
AGENT_REBOOT
Reboots the system.
AGENT_POWEROFF
Cleanly shuts the system down.

25.8.2.1.1. Reboot! Using Agent State Changes in Scripts

These functions are implemented in the community content shared template accessed by adding {{ template “setup.tmpl” }} in your content.

By adding this library, you can call the functions `exit, exit_incomplete, exit_reboot, exit_shutdown, exit_stop, exit_incomplete_reboot, exit_incomplete_shutdown` to access the agent states.

Script authors can also force behaviors using specialized exit code in their routines. While exit 0 provides a regular clean exit, non-0 values provide enhanced functionality:

  • exit 16 stops the script
  • exit 32 triggers a shutdown
  • exit 64 triggers a reboot
  • exit 128 means task is incomplete
  • exit 192 means task is incomplete AND system should reboot
  • exit 160 means task is incomplete AND system should shutdown.

The codes are based on intpretation of bit position left as a trivial exercise to the reader until someone updates the documentation.

25.8.2.2. dr-provision (server side)

In dr-provision, the machine Agent relies on these API endpoints to perform its work:

  • GET from /api/v3/machines/<machine-uuid> to get a fresh copy of the Machine during AGENT_INIT.
  • PATCH to /api/v3/machines/<machine-uuid> to update the machine Stage and BootEnv during the AGENT_CHANGE_STAGE.
  • GET from /api/v3/machines/<machine-uuid>/params/change-stage/map to fetch the change-stage/map for the system during AGENT_CHANGE_STAGE.
  • POST to /api/v3/jobs to retrieve the next Job to run during AGENT_RUN_TASK.
  • PATCH to /api/v3/jobs/<job-uuid> to update Job status during AGENT_RUN_TASK and during AGENT_INIT.
  • PUT to /api/v3/jobs/<job-uuid>/log to update the job log during AGENT_RUN_TASK.
  • UPGRADE to /api/v3/ws to create the EventStream websocket that recieves Events for the Machine from dr-provision. Each Event contains a copy of the Machine state at the point in time that the event was created.

25.8.2.2.1. Retrieving the next Job

Out of all those endpoints, the one that does the most work is the POST /api/v3/jobs endpoint, which is responsible for figuring out what (if any) is the next Job that should be provided to the Machine Agent. It encapsulates the following logic:

  1. dr-provision recieves an incoming POST on /api/v3/jobs that contains a Job with just the Machine filled out.

    If the Machine does not exist, the endpoint returns an Unprocessable Entity HTTP status code.

    If the Machine is not Runnable and Available, the endpoint returns a Conflict status code.

    If the Machine has no more runnable Tasks (as indicated by CurrentTask being greater than or equal to the length of the Machine Tasks list), the endpoint returns a No Content status code, indicating to the Machine Agent that there are no more tasks to run.

  2. dr-provision retrieves the CurrentJob for the Machine. If the Machine does not have a CurrentJob, we create a fake one in the Failed state and use that as CurrentJob for the rest of this process.

  3. dr-provision tentatively sets nextTask to CurrentTask + 1.

  4. If the CurrentTask is set to -1 or points to a stage: or bootenv: entry in the machine Task list, we mark the CurrentTask as failed if it is not already failed or created.

  5. If CurrentTask is set to -1, we update it to 0 and set nextTask to 0.

  6. If CurrentTask points to a stage: or a bootenv: entry in the Tasks list, and the Machine is not already in the appropriate Stage or BootEnv, we skip the next step. Otherwise we skip past these entries in the Tasks list until we get to an entry that refers to a Task and update CurrentTask and nextTask to point to that entry.

  7. Depending on the State of the CurrentJob, we take one of the following actions:

    • “incomplete”: This indicates that CurrentJob did not fail, but it also did not finish. dr-provision returns CurrentJob unchanged, along with the Accepted status code.
    • “finished”: This indicates that the CurrentJob finished without error, and dr-provision should create a new Job for the next Task in the Tasks list. dr-provision sets CurrentTask to nextTask.
    • “failed”: This indicates that the CurrentJob failed. Since updating a Job to the failed state automatically makes the Machine not Runnable, something else has intervened to make the machine Runnable again. dr-provision will create a new Job for the current Task in the Tasks list.
  8. dr-provision creates a new Job for the Task in the Tasks list pointed to by CurrentTask. If CurrentTask points to a stage: or a bootenv: task entry, the new Job is created in the finished state, otherwise it is created in the created state. The Machine CurrentJob is updated with the UUID of the new Job. The new Job and the Machine are saved.

  9. If the the Entry in the Tasks list pointed to by CurrentTask starts with action:, then the rest of the entry is interpreted as either a plugin:action_name or as a action_name. dr-provision will try to invoke the requested action_name on the machine (optionally using the specified plugin). If the plugin invocation succeeds, the results of the invocation are saved in the log, the Job is set to finished, and returned along with the Created HTTP status coe. If the plugin invocation fails for any reason, the reason it failed is saved in the log along with any diagnostic output from the plugin, the job is set to failed, and nothing is returned along with the NoContent status code.

  10. If the new Job is in the created state, it is returned along with Created HTTP status code, otherwise nothing is returned along with the NoContent status code.

25.8.2.2.2. Changing the Workflow on a Machine

Changing a Workflow on the Machine has the following effects:

  • The Stages in the Workflow are expanded to create a new Tasks list. Each Stage gets expanded into a List as follows:

    • stage:<stageName>
    • bootenv:<bootEnvName> if the Stage specifies a non-empty BootEnv.
    • The Tasks list in the Stage

    The Tasks list on the Machine are replaced with the results of the above expansion.

  • The CurrentTask index is set directly to -1.

  • The Stage and BootEnv fields become read-only from the API. Instead, they will change in accordance with any stage: and bootenv: elements in the Task list resulting from expanding the Stages in the Workflow. Any Stage changes that happen during processing a Workflow do not affect the Tasks list or the CurrentTask index.

25.8.2.2.3. Removing a Workflow from a Machine

To remove a workflow from a Machine, set the Workflow field to the empty string. The Stage field on the Machine is set to none, the Tasks list is emptied, and the CurrentTask index is set back to -1.

25.8.2.2.4. Changing the Stage on a Machine

Changing a Stage on a Machine has the folowing effects when done via the API and the Machine does not have a Workflow:

  • The Tasks list on the Machine is replaced by the Tasks list on the Stage.
  • If the BootEnv field on the Stage is not empty, it replaces the BootEnv on the Machine.
  • The CurrentTask index is set to -1
  • If the Machine has a different BootEnv now, it is marked as not Runnable.

25.8.2.2.5. Resetting the CurrentTask index to -1

If the Machine does not have a Workflow, the CurrentTask index is simply set to -1. Otherwise. it is set to the most recent entry that would not occur in a different BootEnv from the machine’s current BootEnv.