Error handling

[chandu chanti]

Hi team,

We have drafted a proposal for  "Using default values in D-Bus properties
to indicate errors"  as communicated in previous mail.

Issue with Existing Implementation:
Error handling is not implemented in the existing bmcweb source. The values
fetched from D-Bus are used directly in the Redfish response, except for
some properties. There was no way to represent error for a given data type
on Redfish.

New Implementation Proposal:
We plan to handle errors from backend D-Bus services in bmcweb using a
custom data type class. We will implement a helper class to create a new
data type called mydatatype_t. An object of this class will be created with
the D-Bus property value as input. This object will be assigned to the JSON
response using the assignment operator. This assignment operation will
invoke the to_json function with the mydatatype_t object, where D-Bus
values will be validated based on their data types.

Data Types and Default Values:
The following values will be set on the D-Bus property in case of an error,
based on the data type. In Redfish, we will validate these properties. If
the D-Bus property value matches the error value, we will update the
property value as null in the Redfish JSON response; otherwise, the
corresponding value on D-Bus will be used.

*D-Bus*

*data type*

*Value of the D-Bus Property incase of Error *

*Property value as seen on Redfish *

INT16

INT_MAX

null

UINT16

UINT_MAX

null

INT32

LONG_MAX

null

UINT32

ULONG_MAX

null

INT64

LLONG_MAX

null

UINT64

ULLONG_MAX

null

DOUBLE

std::numeric_limits <http://en.cppreference.com/w/cpp/types/numeric_limits>
<T>::quiet_NaN()

null

STRING

BE_ERROR-XXX

null

BOOL

uint8_t
{0–false , 1–true,
  others- error}

null

In case of error conditions Redfish will report "null" for a property
irrespective  of the data type.

For error validation, we will implement a custom class, which will
facilitate error validation for D-Bus values. In instances of errors, a
null value will be allocated to the JSON response. Example for the uint16_t
data type.

[image: image.png]

[image: image.png]

For more implementation details please refer the following gerrit link, we
implemented it for redfish URI /redfish/v1/Systems/<str>/Memory/<str>/.
https://gerrit.openbmc.org/c/openbmc/bmcweb/+/66368

Your feedback and insights are greatly appreciated.

Thanks,
Chandrasekhar T.

On Fri, Aug 4, 2023 at 12:37 PM Shakeeb Pasha <shakeebbk at gmail.com> wrote:

> Thanks Joseph for the response.
> >> I didn't see anyone else answer this.  Is your question about how to
> >> architect or design your D-Bus interfaces?  Is the reference the BMCWeb
> >> merely to provide context as a consumer of these D-Bus services?  I
> >> assume so.
> Question is two pronged, one we wanted to know the best way to
> propagate error from dbus to bmcweb.
> Options we tried:
> 1. Have handlers of each property throw an exception if there was an
> error. But this did not work well with current bmcweb implementation(get
> all method call), as it would lead to the entire resource not getting
> populated even if we have one property failing.
> 2. Implement one additional associated rc property - and do the error
> handling bmcweb by looking at that, but this doubled up the number of
> properties on dbus.
> 3. Use "default values" per property data type, but this would fail for
> extreme cases like if the property is u32, and if the default value used
> was INT_MAX, then it breaks when the property could take value 0xFFFFFFFF
> as valid value.
> So the query here was to check if any preference from above options or
> hear any new approaches as well.
>
> The second query was about how to represent this error(per property error)
> on redfish, our proposal for now is to return "NULL" for any property that
> might be failing at the backend. Any thoughts on this approach?
>
> >> I don't have any special insights.  Are you looking to follow a design
> >> pattern?  Are you looking for direction from the BMCWeb maintainers?
>
> Yes, we are working on a design pattern proposal and will publish it here
> for review.
> It would be great if we could get some directions/inputs from the
> maintainers as well.
>
> Thanks,
> Shakeeb
>
> On Thu, Aug 3, 2023 at 10:33 PM Joseph Reynolds <jrey at linux.ibm.com>
> wrote:
>
>> On 7/20/23 2:04 AM, chandu chanti wrote:
>> > Hi Team, We are planning to handle errors from backend services in bmc
>> > web. We are considering the following approaches to implement it, but
>> > we are facing some issues. please provide your inputs. 1 . Using
>> > exceptions in our backend D-Bus service
>> > ZjQcmQRYFpfptBannerStart
>> > This Message Is From an Untrusted Sender
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>> >
>> > Hi Team,
>> >
>> > We are planning to handle errors from backend services in bmc web. We
>> > are considering the following approaches to implement it, but we are
>> > facing some issues. please provide your inputs.
>> >
>>
>> I didn't see anyone else answer this.  Is your question about how to
>> architect or design your D-Bus interfaces?  Is the reference the BMCWeb
>> merely to provide context as a consumer of these D-Bus services?  I
>> assume so.
>>
>> I don't have any special insights.  Are you looking to follow a design
>> pattern?  Are you looking for direction from the BMCWeb maintainers?
>>
>> Joseph
>>
>> > 1 . Using exceptions in our backend D-Bus service by throwing
>> > exceptions in the D-Bus property get handlers. It works fine for the
>> > Get property method call. However, when using the Get All method call,
>> > if one property fails, an error is returned without checking the other
>> > properties. Is there a way to implement exceptions in GetAll so that
>> > even if one property fails, we can still fetch the remaining properties.
>> >
>> > 2. Using default values in D-Bus properties to indicate errors. Is
>> > there a reference implementation available for setting default values
>> > for string and integer data types in bmc web, similar to the
>> > implementation of NaN (default value) for the double data type in
>> > cable.hpp.
>> >
>> > 3. Implement associated return code per property on dbus, but this
>> > would be very inefficient in terms of double the properties on dbus,
>> > something we would like to avoid
>> >
>> > Thanks
>> > Chandrasekhar T.
>> >
>>
>>
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