Speed up FindByPath.find 1000x on K8s/RDS environments

[aldavidson]

Trello card

Benchmarking under sustained load on staging after deploying content-store-proxy PR #48 showed that we had optimised to the point where the bottleneck was now the PostgreSQL content-store itself (see secondary_response_time in this Kibana screenshot)

Some more benchmarking in a console on the staging pods showed an unexpected slow point in FindByPath.find() - the if matches.any? call.

On the original Mongoid (main) application, this line is operating on an array, so any? is vanishingly fast.
However, in ActiveRecord, this is operating on an ActiveRecord::Relation, and it is much slower. Experimentation showed that the fastest functionally-equivalent call is not even .exists? but count > 0

The difference is hardly noticeable on local developer laptops, but huge on staging - where the database is remote (RDS) and the container is resource-constrained in a highly-contended cluster.

Before:

> Benchmark.measure{ 10.times{ FindByPath.new(ContentItem).find('/government/policies/improving-local-transport/supporting-pages/increasing-the-use-of-buses') } }
=> #<Benchmark::Tms:0x00007f9a4f4270e8 @cstime=0.0, @cutime=0.0, @label="", @real=23.315657227300107, @stime=0.0032899999999999874, @total=0.03041800000000003, @utime=0.02712800000000004>

After:

> Benchmark.measure{ 10.times{ FindByPath.new(ContentItem).find('/government/policies/improving-local-transport/supporting-pages/increasing-the-use-of-buses') } }
=> #<Benchmark::Tms:0x00007f9a50b67528 @cstime=0.0, @cutime=0.0, @label="", @real=0.02926267310976982, @stime=0.0, @total=0.009113000000000149, @utime=0.009113000000000149>

So, roughly 1000x faster

This application is owned by the publishing platform team. Please let us know in #govuk-publishing-platform when you raise any PRs.

⚠️ This repo is Continuously Deployed: make sure you follow the guidance ⚠️

Follow these steps if you are doing a Rails upgrade.

[sengi]

Nice!