Resources¶
Resources are a mechanism that allows to constrain certain job to execute only on devices with appropriate hardware or software dependencies. This mechanism allows some types of jobs to publish resource objects to an abstract namespace and to a way to evaluate a resource program to determine if a job can be started.
Resources in Plainbox¶
The following chapters explain how resources actually work in Plainbox. Currently there is a subtle difference between this and the original Checkbox implementation.
Resource programs¶
Resource programs are multi-line statements that can be embedded in job definitions. By far, the most common use case is to check if a required package is installed, and thus, the job can use it as a part of a test. A check like this looks like this:
package.name == "fwts"
This resource program codifies that the job needs the fwts
package to run.
There is a companion job with the same name that interrogates the local package
database and publishes a set of resource objects. Each such object is a
collection of arbitrary key-value pairs. The package
job simply publishes
the name
and version
of each installed package but the mechanism is
generic and applies to all resources.
As stated, resource programs can be multi-line, a real world example of that is presented below:
device.category == 'CDROM'
optical_drive.cd == 'writable'
This example is much like the one above, referring to some resources, here
coming from jobs device
and optical_drive
. What is important to point
out is that, as a rule of a thumb, multi line programs have an implicit and
operator between each line. This program would only evaluate to True if there
is a writable CD-ROM available.
Each resource program is composed of resource expressions. Each line maps directly onto one expression so the example program above uses two resource expressions.
Resource expressions¶
Resource expressions are evaluated like normal python programs. They use all of the same syntax, semantics and behavior. None of the operators are overridden to do anything unexpected. The evaluator tries to follow the principle of least surprise but this is not always possible.
Resource expressions cannot execute arbitrary python code. In general almost everything is disallowed, except as noted below:
- Expressions can use any literals (strings, numbers, True, False, lists and tuples)
- Expressions can use boolean operators (
and
,or
,not
) - Expressions can use all comparison operators
- Expressions can use all binary and unary operators
- Expressions can use the set membership operator (
in
) - Expressions can use read-only attribute access
Anything else is rejected as an invalid resource expression.
In addition to that, each resource expression must use exactly one variable, which must be used like an object with attributes. The name of that variable must correspond to the name of the job that generates resources. Attempts to use more than one variable or to not use any variables are detected early and rejected as invalid resource expressions.
The name of the variable determines which resource group to use. It must match the name of the job that generates such resources.
In the examples elsewhere in this page the package
resources are generated
by the package
job. Plainbox uses this to know which resources to try but
also to implicitly to express dependencies so that the package
job does not
have to be explicitly selected and marked for execution prior to the job that
in fact depends on it. This is all done automatically.
Evaluation¶
Due to mandatory compatibility with existing Checkbox jobs there are some unexpected aspects of how evaluation is performed. Those are marked as unexpected below:
- First Plainbox looks at the resource program and splits it into lines. Each non-empty line is parsed and converted to a resource expression.
- unexpected Each resource expression is repeatedly evaluated, once for
each resource from the group determined by the variable name. All exceptions
are silently ignored and treated as if the iteration had evaluated to False.
The whole resource expression evaluates to
True
if any of the iterations evaluated toTrue
. In other words, there is an implicitany()
around each resource expression, iterating over all resources. - unexpected The resource program evaluates to
True
only if all resource expressions evaluated toTrue
. In other words, there is an implicitand
between each line.
Limitations¶
The design of resource programs has the following shortcomings. The list is non-exhaustive, it only contains issues that we came across found not to work in practice.
Exactly one variable per expression¶
Each resource expression must refer to exactly one variable. This is a side effect of the way the evaluator works. It basically bind one object (a particular resource) to that variable and evaluates the expression.
The expression parser / syntax analyzer identifies expressions with this problem early and rejects them with an appropriate error message. Here are some examples of hypothetical expressions that exhibit this problem.
“I want to have mplayer and an audio device so that I can play some sounds”:
device.category == "AUDIO" and package.name == "mplayer"
To work around this, split the expression to two separate expressions. The
evaluator will put an implicit and
between them and it will do exactly what
you intended:
device.category == "AUDIO"
package.name == "mplayer"
“I want to always run this test”:
True
To work around this, simply remove the requirement program entirely!
“I want to never run this test”:
False
To work around this remove this job from the selection. You may also use a special resource that produces one constant value, and check that it is equal to something different.
Exactly one resource bound to a variable at once¶
It’s not possible to refer to two different resources, from the same resource group, in one resource expression. In other terms, the variable always points to one object, it is not a collection of objects.
For example, let’s consider this program:
package.name == 'xorg' and package.name == 'procps'
Seemingly the intent was to ensure that both xorg
and procps
are
installed. The reason why this does not work is that at each iteration of the
the expression evaluator, the name package
refers to exactly one resource
object. In other words, that expression is equivalent to this one:
A == True and A == False
This type of error is not captured by our limited semantic analyzer. It will silently evaluate to False and inhibit the job from being stated.
To work around this, split the expression to two consecutive lines. As stated
in rule 3 in the list above, there is an implicit and
operator between all
expressions. A working example that expresses the same intent looks like this:
package.name == 'xorg'
package.name == 'procps'
Operator != is useless¶
This is strange at first but quickly becomes obvious once you recall rule 2
from the list above. That rule states that the expression is evaluated
repeatedly for each resource from a particular group and that any True
iteration marks the whole expression as True
).
Let’s look at a real-world example:
xinput.device_class == 'XITouchClass' and xinput.touch_mode != 'dependent'
So seemingly, the intent here was to have at least xinput
resource with a
device_class
attribute equal to XITouchClass
that has touch_mode
attribute equal to anything but dependent
.
Now let’s assume that we have exactly two resources in the xinput
group:
device_class: XITouchClass
touch_mode: dependant
device_class: XITouchClass
touch_mode: something else
Now, this expression will evaluate to True
, as the second resource fulfils
the requirements. Is this what the test designer had expected? That’s hard to
say. The problem here is that this expression can be understood as at least
one resource isn’t something or all resources weren’t something. Both
are equally valid desires and, depending on how the test is implemented, may or
many not work correctly in practice.
Currently there is no workaround. We are considering adding a new syntax that would allow to specify this explicitly. The proposal is documented below as “implicit any(), explicit all()”
Everything is a string¶
Resource programs are regular python programs evaluated in unusual ways but all of the variables that are exposed through the resource object are strings.
This has considerable impact on comparison, unless you are comparing to a string the comparison will always silently fail as python has dynamic but strict, not loose types (there is no implicit type conversion). To alleviate this problem several type names / conversion functions are allowed in requirement programs. Those are:
int
, to convert to integer numbersfloat
, to convert to floating point numbersbool
, to convert to a boolean context
Considered enhancements¶
We are currently considering one improvement to resource programs. This would allow us to introduce a fix that resolves some issues in a backwards compatible way. Technical aspects are not yet resolved as that extension would not be available in Checkbox until Checkbox can be built on top of Plainbox
Implicit any(), explicit all()¶
This proposal changes the way resource expressions are evaluated.
The implicit any()
implemented as a loop over all resources from the
resource group designated by variable name would be configurable.
A developer may choose to wrap the whole expression in the all()
function
to indicate that the expression inside all()
must evaluate to True
for
all iterations (all resources).
This would allow solving the case where a job can only run, for example, when a certain package is not installed. This could be expressed as:
all(package.name != 'ubuntu-desktop')