Define general behavior for object notifications

[alexvanin]

NeoFS Storage node should support notification extension: optional notification channel to produce events. This channel and event structure defined depending on extension implementation. API should define general behavior for object notifications (maybe in neofs-spec?) and well-known object attributes.

General behavior:

• Notification event consists of topic and message
• There is only object notifications for now
• Object notification message is always object address (format defined by notification extension)
• Object notification topic is defined in __NEOFS__TICK_TOPIC header (UTF-8 string)
• Node produces object notifications at epoch defined in __NEOFS__TICK_EPOCH header which is Base10 encoded number
• Node produces object notification if both __NEOFS__TICK_TOPIC and __NEOFS__TICK_EPOCH are set in object attributes

[carpawell]

@realloc, btw, should the attributes be separated?

what behavior is expected when there is only one of them presented?

[realloc]

If there is only TOPIC attribute set, it will have no effect and will never be taken into account.
If there is only TICK attribute set, it should send event to the default topic, as configured in Storage Node settings. The default notification settings may be different depending on the transport and are out of the scope of API description.

[carpawell]

If there is only TICK attribute set, it should send event to the default topic

what should a node do when there is no default topic in the config:

1. do not start at all;
2. start and try to fetch topic from attributes, if there is no such attr, skip it with a warning?

[realloc]

what should a node do when there is no default topic in the config:

That's incorrect configuration. Since each config parameter must have a default attribute (if it makes sense in each particular cease), such situation can't arise normally.

For node's default topic default value let's use it's node id.

[cthulhu-rider]

Moved to neofs-spec.