aws: cache zones list

[bpineau]

When it syncs AWS DNS with k8s cluster content (at --interval), external-dns submits two distinct Route53 API calls:

• to fetch available zones (eg. for tag based zones discovery, or when zones are created after exernal-dns started),
• to fetch relevant zones' resource records.

Each call taxes the Route53 APIs calls budget (5 API calls per second per AWS account/region hard limit), increasing the probability of being throttled. Changing synchronisation interval would mitigate those calls' impact, but at the cost of keeping stale records for a longer time.

For most practical uses cases, zones list aren't expected to change frequently. Even less so when external-dns is provided an explicit, static zones set (--zone-id-filter rather than --aws-zone-tags).

Using a zones list cache halves the number of Route53 read API calls.

Example

Route53 API calls before/after deploying that change at 08:50, on a single cluster:

Route53 calls returning http 400, after rolling change to several cluster at 10:44:

Logs entries containing "throttling", several clusters:

Checklist

• Update changelog in CHANGELOG.md, use section "Unreleased".

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[bpineau]

/assign @Raffo

[seanmalloy]

/kind feature

[njuettner]

I think this is helpful @bpineau, thank you for creating this PR 👍.
In case I don't want caching, any chance I can avoid that?

[bpineau]

Thanks @njuettner , --aws-zones-cache-duration=0s will now disable zones caching.

[njuettner]

One small thing @bpineau could you add one or two sentence for the docs. I'd like to have this somewhere so people understand how they can turn it off and what's the impact of this. I think this PR is pretty important for people who are dealing with throttling issues.

[tariq1890]

@bpineau Thank you for this PR. This feature would help us as well.

I also think we should default to disabling the cache because many users would be blindsided by this when they wonder why they may be getting state data. We should be careful about changing default behaviours IMO

[bpineau]

@tariq1890 ok, updated accordingly

[tariq1890]

@bpineau Looks like you need to rebase you PR. Sorry :(

[bpineau]

@njuettner would you mind taking an other view? thanks!

[Raffo]

@bpineau sorry for the changelog mess, please rebase once more.

[Raffo]

/approve
/lgtm

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This pull-request has been approved by: bpineau, Raffo

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