Virtual Machine States and Transitions
=======================================

The following diagrams and tables show the required virtual machine (VM)
states and task states for various commands issued by the user.

Allowed State Transitions
--------------------------

.. graphviz::

  digraph states {
    graph [pad=".35", ranksep="0.65", nodesep="0.55", concentrate=true];
    node [fontsize=10 fontname="Monospace"];
    edge [arrowhead="normal", arrowsize="0.8"];
    label="All states are allowed to transition to DELETED and ERROR.";
    forcelabels=true;
    labelloc=bottom;
    labeljust=left;

    /* states */
    building [label="BUILDING"]
    active [label="ACTIVE"]
    paused [label="PAUSED"]
    suspended [label="SUSPENDED"]
    stopped [label="STOPPED"]
    rescued [label="RESCUED"]
    resized [label="RESIZED"]
    soft_deleted [label="SOFT_DELETED"]
    shelved [label="SHELVED"]
    shelved_offloaded [label="SHELVED_OFFLOADED"]
    deleted [label="DELETED", color="red"]
    error [label="ERROR", color="red"]

    /* transitions [action] */
    building -> active

    active -> active [headport=nw, tailport=ne]  // manual layout
    active -> soft_deleted [tailport=e]  // prevent arrowhead overlap
    active -> suspended
    active -> paused [tailport=w]  // prevent arrowhead overlap
    active -> stopped
    active -> shelved
    active -> shelved_offloaded
    active -> rescued
    active -> resized

    soft_deleted -> active [headport=e]  // prevent arrowhead overlap

    suspended -> active
    suspended -> shelved
    suspended -> shelved_offloaded

    paused -> active
    paused -> shelved
    paused -> shelved_offloaded

    stopped -> active
    stopped -> stopped [headport=nw, tailport=ne]  // manual layout
    stopped -> resized
    stopped -> rescued
    stopped -> shelved
    stopped -> shelved_offloaded

    resized -> active

    rescued -> active

    shelved -> shelved_offloaded
    shelved ->  active

    shelved_offloaded -> active
  }


Requirements for Commands
-------------------------

==================  ==================  ====================  ================
Command             Req'd VM States     Req'd Task States     Target State
==================  ==================  ====================  ================
pause               Active, Shutoff,    Resize Verify, unset  Paused
                    Rescued
unpause             Paused              N/A                   Active
suspend             Active, Shutoff     N/A                   Suspended
resume              Suspended           N/A                   Active
rescue              Active, Shutoff     Resize Verify, unset  Rescued
unrescue            Rescued             N/A                   Active
set admin password  Active              N/A                   Active
rebuild             Active, Shutoff     Resize Verify, unset  Active, Shutoff
force delete        Soft Deleted        N/A                   Deleted
restore             Soft Deleted        N/A                   Active
soft delete         Active, Shutoff,    N/A                   Soft Deleted
                    Error
delete              Active, Shutoff,    N/A                   Deleted
                    Building, Rescued,
                    Error
backup              Active, Shutoff     N/A                   Active, Shutoff
snapshot            Active, Shutoff     N/A                   Active, Shutoff
start               Shutoff, Stopped    N/A                   Active
stop                Active, Shutoff,    Resize Verify, unset  Stopped
                    Rescued
reboot              Active, Shutoff,    Resize Verify, unset  Active
                    Rescued
resize              Active, Shutoff     Resize Verify, unset  Resized
revert resize       Active, Shutoff     Resize Verify, unset  Active
confirm resize      Active, Shutoff     Resize Verify, unset  Active
==================  ==================  ====================  ================

VM states and Possible Commands
-------------------------------

============  =================================================================
VM State      Commands
============  =================================================================
Paused        unpause
Suspended     resume
Active        set admin password, suspend, pause, rescue, rebuild, soft delete,
              delete, backup, snapshot, stop, reboot, resize, revert resize,
              confirm resize
Shutoff       suspend, pause, rescue, rebuild, soft delete, delete, backup,
              start, snapshot, stop, reboot, resize, revert resize,
              confirm resize
Rescued       unrescue, pause
Stopped       rescue, delete, start
Soft Deleted  force delete, restore
Error         soft delete, delete
Building      delete
Rescued       delete, stop, reboot
============  =================================================================


Create Instance States
----------------------

The following diagram shows the sequence of VM states, task states, and
power states when a new VM instance is created.

.. image:: /_static/images/create-vm-states.svg
   :alt: Sequence of VM states, task states, and power states when a new
         VM instance is created.
