Resource Job Unit¶
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 also serve as a “generator” for template units.
Resource Jobs are jobs with a plugin set to resource:
Command that they run should print resource information in a predefined manner. This command may be considered a Resource Program
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
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
optical_drive. What is important to point
out is that, as a rule of a thumb, multi line programs have an implicit
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 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 (
Expressions can use all comparison operators
Expressions can use all binary and unary operators
Expressions can use the set membership operator (
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 at least 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. You can use
imports field (at a job definition level) to rename a resource job to
be compatible with the identifier syntax. It can also be used to refer to
resources from another namespace.
In the examples elsewhere in this page the
package resources are generated
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.
The job definition below generates a RTC resource:
if [ -e /sys/class/rtc ]
echo "state: supported"
echo "state: unsupported"
_description: Creates resource info for RTC
Next let’s define a Job that uses that resource.
rtc.state == 'supported'
package.name == 'util-linux'
cpuinfo.other != 'emulated by qemu'
command: hwclock -r
_summary: Test that RTC functions properly (if present)
Verify that the Real-time clock (RTC) device functions properly, if present.
Now the power-management/rtc job will only be run on systems where
/sys/class/rtc directory exists (which is true for systems supporting RTC)
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
Trueif any of the iterations evaluated to
True. In other words, there is an implicit
any()around each resource expression, iterating over all resources.
unexpected The resource program evaluates to
Trueonly if all resource expressions evaluated to
True. In other words, there is an implicit
andbetween each line.
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.
Joins are not optimized¶
Starting with plainbox 0.24, a resource expression can use more than one resource object (resource job) at the same time. This allows the use of joins as the whole expression is evaluated over the Cartesian product of all the resource records. This operation is not optimized, you can think of it as a JOIN that is performed on a database without any indices.
Let’s look at a practical example:
package.name == desired_package.name
Here, two resource jobs would run. The classic package resource (that
produces, typically, a great number of resource records, one for each package
installed on the system) and a hypothetical desired_package resource (for
this example let’s pretend that it is a simple constant resource that just
contains one object). Here, this operation is not any worse than before because
size(desired_package) * size(package) is not any larger. If, however,
desired_package was on the same order as package (approximately a thousand
resource objects). Then the computational cost of evaluating that expression
would be quadratic.
In general, the cost, assuming all resources have the same order, is exponential with the number of distinct resource jobs referenced by the expression.
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
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
iteration marks the whole expression as
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
attribute equal to anything but
Now let’s assume that we have exactly two resources in the
touch_mode: something else
Now, this expression will evaluate to
True, as the second resource fulfills
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 numbers
float, to convert to floating point numbers
bool, to convert to a boolean context