^^ s/immutability/idempotency
On Thu, Jan 5, 2017 at 12:39 PM, Casey Marshall <
casey.marsh...@canonical.com> wrote:
> On Thu, Jan 5, 2017 at 3:33 AM, Adam Collard <adam.coll...@canonical.com>
> wrote:
>
>> Hi,
>>
>> The automatic hook retries[0] that landed as part of 2.0 (are documented
>> as) run indefinitely[1] - this causes problems as an API user:
>>
>> Imagine you are driving Juju using the API, and when you perform an
>> operation (e.g. set the configuration of a service, or reboot the unit, or
>> add a relation..) - you want to show the status of that operation.
>>
>> Prior to the automatic retries, you simply perform your operation, and
>> watch the delta streams for the corresponding change to the unit - the
>> success or otherwise of the operation is reflected in the unit
>> agent-status/workload-status pair.
>>
>> Now, with retries, if you see a unit in the error state, you can't
>> accurately reflect the status of the operation, since the unit will
>> undoubtedly retry the hook again. Maybe it succeeds, maybe it fails again.
>> How can one say after receiving the first delta of a unit error if the
>> operation succeeded or failed?
>>
>> With no visibility up front on the retry strategy that Juju will perform
>> (e.g. something representing the exponential backoff and a fixed number of
>> retries before Juju admits defeat) it is impossible to say at any point in
>> the delta stream what the result of a failed-at-least-once operation is.
>>
>
> I think the retry strategy is great -- it leverages the immutability we
> expect hooks to provide, to deliver a robust result over unreliable
> substrates -- and all substrates are unreliable where there's
> internetworking involved!
>
> However I see your point about the retry strategy muddling status. I've
> noticed this sometimes when watching openstack or k8s bundles "shake out"
> the errors as they come up. I don't think this is always a charm quality
> issue, it's maybe because we're trying to show two different things with
> status?
>
>
>> What if Juju made a clearer distinction between result-state ("what I'm
> doing most recently or last attempted to do") vs. goal-state ("what I'm
> trying to get done") in the status? Would that help?
>
>
>> Can retries be limited to a small number, with a backoff algorithm
>> explicitly documented and stuck to by Juju, with the retry attempt number
>> included in the delta stream?
>>
>> Thanks,
>>
>> Adam
>>
>> [0] https://jujucharms.com/docs/2.0/reference-release-notes
>> [1] https://jujucharms.com/docs/2.0/models-config#retrying-failed-hooks
>>
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>
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