Config changes invalidating cache. ATG Content Administration Programming Guide.



Config changes invalidating cache

Config changes invalidating cache

I believe there's a second factor at play. I duplicated this yesterday, unintentionally, while working on something else. Distribution configuration changes take time to propagate. They can apply very quickly, or at least give that impression, but they are not complete until they show to be complete.

Invalidation requests that are issued while a configuration change is pending will not necessarily coincide with the application of the configuration change. Consider the following sequence of events: Edge node receives invalidation, invalidates cache. Request for invalidated object arrives, edge node does origin fetch, caches object. Edge node receives configuration change, applies change.

Edge node serves old object until cache expiration or another invalidation is created. The invalidation was created after the configuration change, but was applied to some nodes before the configuration change. The timing of the two events is independent. Until the distribution transitions back to the "Deployed" state, there is no guarantee that an invalidation will not be applied before the configuration change is applied, even if the configuration change is created first.

To do otherwise would either slow down the processing of invalidations, or require time travel. The fact that an invalidation is still active does not mean that CloudFront is continuing to remove that object if it has been cached again. It only means that the entire system hasn't yet received the invalidation request. In places where it's already arrived, it's already been executed. If you want to be aggressive, you could submit multiple invalidations, starting another one when each prior one finishes.

Proof of this lies in observing the "Age" response header, which provides the number of seconds that this specific object has been in the CloudFront cache. If the number is lower than the amount of time ago that you started the invalidation, then this is an object that was cached after the invalidation had already been processed by that specific edge node, but before the configuration change was applied there.

I have a theory about invalidations that I have not thoroughly tested, but I suspect that the arrival time at a particular edge of the invalidation request is less significant than the timestamp on the invalidation.

My suspicion is that objects are evicted from the cache not based on whether they are in the cache at the moment of the arrival of the invalidation at the edge, but rather based on whether they are in the cache at the moment of the arrival and were cached before the timestamp on the invalidation request.

My suspicion is that objects cached after an invalidation is first created are not impacted by that invalidation if it happens to arrive seconds later, while objects cached prior to the timestamp of the invalidation would be purged.

This is consistent with my mental model of the likely design of some of the internal workings It is, however, consistent with anecdotal observations. CloudFront cache invalidation not working as expected Posted on: Sep 14, But to inform you, we didn't update distribution configurations anytime near to cache invalidation. So, don't think case 1 is happening with us. Regarding case 2, this would get resolved with consecutive invalidation requests.

We had tried that as well, even that didn't work for us. But I still don't understand, why this would be inconsistent?

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Config changes invalidating cache

I believe there's a second factor at play. I duplicated this yesterday, unintentionally, while working on something else. Distribution configuration changes take time to propagate. They can apply very quickly, or at least give that impression, but they are not complete until they show to be complete.

Invalidation requests that are issued while a configuration change is pending will not necessarily coincide with the application of the configuration change. Consider the following sequence of events: Edge node receives invalidation, invalidates cache. Request for invalidated object arrives, edge node does origin fetch, caches object.

Edge node receives configuration change, applies change. Edge node serves old object until cache expiration or another invalidation is created. The invalidation was created after the configuration change, but was applied to some nodes before the configuration change.

The timing of the two events is independent. Until the distribution transitions back to the "Deployed" state, there is no guarantee that an invalidation will not be applied before the configuration change is applied, even if the configuration change is created first. To do otherwise would either slow down the processing of invalidations, or require time travel. The fact that an invalidation is still active does not mean that CloudFront is continuing to remove that object if it has been cached again.

It only means that the entire system hasn't yet received the invalidation request. In places where it's already arrived, it's already been executed. If you want to be aggressive, you could submit multiple invalidations, starting another one when each prior one finishes.

Proof of this lies in observing the "Age" response header, which provides the number of seconds that this specific object has been in the CloudFront cache. If the number is lower than the amount of time ago that you started the invalidation, then this is an object that was cached after the invalidation had already been processed by that specific edge node, but before the configuration change was applied there.

I have a theory about invalidations that I have not thoroughly tested, but I suspect that the arrival time at a particular edge of the invalidation request is less significant than the timestamp on the invalidation.

My suspicion is that objects are evicted from the cache not based on whether they are in the cache at the moment of the arrival of the invalidation at the edge, but rather based on whether they are in the cache at the moment of the arrival and were cached before the timestamp on the invalidation request.

My suspicion is that objects cached after an invalidation is first created are not impacted by that invalidation if it happens to arrive seconds later, while objects cached prior to the timestamp of the invalidation would be purged.

This is consistent with my mental model of the likely design of some of the internal workings It is, however, consistent with anecdotal observations. CloudFront cache invalidation not working as expected Posted on: Sep 14, But to inform you, we didn't update distribution configurations anytime near to cache invalidation.

So, don't think case 1 is happening with us. Regarding case 2, this would get resolved with consecutive invalidation requests. We had tried that as well, even that didn't work for us. But I still don't understand, why this would be inconsistent?

Config changes invalidating cache

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