Hi all,
I have read RFC7950 and RFC8342 but I have still some questions for
clarification about how to deal with default values in YANG.
Let's consider a config true leaf for which a YANG default statement has been
defined in the YANG module.
If the client does not configure that leaf in the running datastore, the leaf
is not present (cannot be read) in the running datastore.
The default value for that leaf is assigned by server in the operational
datastore as part of the applied configuration (assuming that other
configurations do not override it).
If that leaf exists in the operational datastore, the server shall return the
default value if that leaf is read.
Is our understanding correct?
Is it possible for a server not to use that leaf and not to return any value if
that leaf is read?
According to this text from RFC8342, it seems that this is possible:
Requests to retrieve nodes from <operational> always return the value
in use if the node exists, regardless of any default value specified
in the YANG module. If no value is returned for a given node, then
this implies that the node is not used by the device.
However, according to this text from RFC7950, it seems that this is not
possible:
When the default value is in use, the server MUST operationally
behave as if the leaf was present in the data tree with the default
value as its value.
There are case where the default value to assign to a leaf depends on the
configuration of other leaves. In this case, the default value cannot be
defined using a YANG default statement.
In this case, is it possible in this case to define the default value in the
description (maybe referencing some other standard specification)?
Would the behavior of the system be exactly the same as in the cases where the
default value is assigned using a YANG default statement?
Would it be possible in this case to define a default value for a leaf defined
in a base model which depends on an attribute defined in another model which
augments the base model?
In this case, the description of the leaf in the base model cannot be changed
in the augmented model. Should the default value for the leaf defined in the
base model be described in the description for the leaf defined in the
augmenting module?
For example, would this YANG be valid/correct to define a default value 10 for
the leaf foo when the leaf bar exist?
module example-base {
container example {
leaf foo {
type uint8;
}
}
}
module example-augment {
import example {
prefix ex;
}
augment "ex:example" {
leaf bar {
type empty;
description
"When present, the default value for foo is 10.";
}
}
}
Thanks for your help
Italo
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