How preseeds work in MAAS

A preseed is what MAAS sends to the cloud-init process that starts up as part of node installation and commissioning. It is a specially formatted chunk of text. MAAS does not care what that formatting is, it just knows it is text and uses Tempita templates to render a final file based on some variables that are required depending on the context.

Preseed template structure

The preseed templates live in the source tree at contrib/preseeds_v2/ and the following files exist there:

commissioning  enlist_userdata  preseed_master
enlist         generic          preseed_xinstall

The file that is used as a template for the preseed depends on the state of the node that is being booted up.

State	Template used
Enlistment	enlist
Commissioning	commissioning
Installation	generic, plus one of: preseed_master or preseed_xinstall

The enlist_userdata is not a preseed but it is a template that’s used to send data to cloud-init when a new node is enlisted.

There are also User-provided preseeds, see below.

Installation preseed

The installation preseed is broken into the three files because the requirements are different depending on the installer being used. The Debian Installer uses preseed_master and the newer Curtin installer uses preseed_xinstall.

The base of both of these is generic, which defines some variables and then selects the right one depending on the installer being used. Note that Tempita’s inheritance mechanism is a little weird; the generic template inherits the right file but in effect this is in reverse, the inherited file becomes the new template and it can then reference the variables defined in generic.

User-provided preseeds

In addition to the standard preseed files, the base preseeds can be overridden by end users on a per-architecture, OS release and node name basis. The templates are looked up in the following order:

{prefix}_{node_architecture}_{node_subarchitecture}_{release}_{node_name}
{prefix}_{node_architecture}_{node_subarchitecture}_{release}
{prefix}_{node_architecture}_{node_subarchitecture}
{prefix}_{node_architecture}
{prefix}
'generic'

prefix is either empty or one of enlist or commissioning.

As you can see this mechanism is also used to calculate the base preseeds for all of installation, enlistment and commissioning. It allows end users to add, for example, a file named amd64_generic_saucy that would be used instead of the generic template at installation time.

Context variables

Most of the context variables comes from code in src/maasserver/preseed.py in the get_preseed_context() function. However there are many small functions in that file, this is the full call tree:

get_preseed()
  |
  render_preseed()
    |
    load_preseed_template()
    | |
    | get_preseed_filenames()
    | |
    | get_preseed_template()
    |
    get_preseed_context()
    |
    get_node_preseed_context()
    | |
    | compose_preseed() - (comes from src/maasserver/compose_preseed.py)
    |
    template.substitute(**context)