Refactor Xapi_event #6306
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Refactor Xapi_event #6306
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In Xapi_event, events are accumulated by folding over the set of tables associated with a subscriber's subscription record. These events are mostly accumulated as lists within a tuple. There is no analogue of functional record update for tuples in OCaml. This means that the separate accumulations have to cite values they will not update. By introducing records, we can only cite the fields we actually change. Signed-off-by: Colin James <[email protected]>
In event accumulation for event.from, the code uses a mutable variable to thread a value through event accumulation. However, this value itself is accumulated in the fold: it gets larger for each matching database event that matches a subscription. To avoid the complexity in effectively having a global, mutable variable, we drop it and make it more evident when it changes: it is changed when no events are accumulated (by grab_range). In the case that no events are accumulated, but the deadline hasn't been reached, the code tries to collect events again. It is during a retry that the last_generation needs to be bumped, as it defines the starting point by which to query the database for recent and deleted objects. In short, if no suitable events were gleaned from matching database object records since a given point, there's no point starting from there again. Signed-off-by: Colin James <[email protected]>
In order to provide event records to subscribers, we must convert the accumulated events of the form (table, objref, time) into event records. The process of doing this is simple for objects in the database. The only difference is that deletion events do not provide a snapshot (as the object has been deleted). To avoid repeating ourselves, we define an "events_of" function that accumulates event records. The function takes an argument that specifies whether an attempt to provide a snapshot should be performed. The reification of events associated with messages - which are not stored in the database - is untouched. This relies on a callback instated elsewhere. Signed-off-by: Colin James <[email protected]>
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Opening as draft until XenRT is stable enough for me to submit a BVT+BST suite. It goes without saying, but it's important that event-related code does not break. |
lindig
approved these changes
Feb 14, 2025
| let prepend_recent obj stat _ ({creates; mods; last; _} as entries) = | ||
| let Stat.{created; modified; deleted} = stat in | ||
| if Subscription.object_matches subs table obj then | ||
| let last = max last (max modified deleted) in |
| ([], [], [], !last_generation) | ||
| tables | ||
| ) | ||
| let events = |
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Above the type is called acc - where I think events is better.
contificate
commented
Feb 14, 2025
Further changes to turn tuples into records. Also partially uncurries `collect_events` to make its intended use as a fold more apparent. Signed-off-by: Colin James <[email protected]>
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BVT+BST: 212829. All green so far, with a few still running. Done some manual testing within XenCenter and XO-Lite, which rely entirely on the event API for their operation. These changes are refactoring, they should introduce no semantic change. |
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A code clarification effort, consisting of:
last_generation: this mutable variable is updated in a few places and complicates the code. Instead of relying it on being in scope (and mutated in other places), we explicitly pass it in and then thread the update to its value through the retry loop.(table, obj, time)entries into event records, defining the accumulation ofadd,mod, anddelevents in terms of it: message events remain special cased because their contents are not in the database. This avoids duplicated code.