data-model.dot

digraph {
  edge [weight = 1]

  // Special node.
  xo [color = "#064d99"]

  // Main hierarchy.
  {
    edge [weight = 2] // Force the main hierarchy to be straight.
    node [pin = true]

    host -> PIF
    host -> SR
    host -> task
    host -> VM

    pool -> host
    pool -> SR
    pool -> VM

    SR -> VDI

    VM -> VIF

    xo -> pool
  }

  // Link objects.
  {
    edge [color = "#4e9a06"]
    node [color = "#4e9a06"]

    VBD -> VDI
    VBD -> VM

    network -> PIF
    network -> VIF

    PBD -> host
    PBD -> SR
  }

  // Main objects have messages.
  {
    edge [color = "#e9b96e"]
    node [color = "#e9b96e"]

    {
      host
      pool
      SR
      VM
    } -> message
  }

  // Future links.
  {
    edge [color = red, style = dotted]

    xo -> host
  }
}