[feature] delayed_sidekiq strategy

[skcc321]

What does it do?

I added a new strategy - delayed_sidekiq.
It behaves the next way:

• the strategy has a latency option (for instance 10 seconds), margin & reindex_wrapper
• when you mutate active record objects very frequently the strategy will accumulate ids of mutated records during the latency window.
• that means that we are going to reindex all changes that happened during 10 seconds at once.
• such a latency gives us the opportunity to use "read replica" instead of "primary" DB (RoR multi DB connection)
• as a result, if you don't need strong consistency you can define an appropriate latency window and gain significant performance improvement (fewer calls to DB & replica usage)
• margin option is important to cover replication_lag

an example:

class CitiesIndex < Chewy::Index
  index_scope City
  
  strategy_config delayed_sidekiq: {
    latency: 10,  
    margin: 2,
    reindex_wrapper: ->(&reindex) { ActiveRecord::Base.connected_to(role: :reading) { reindex.call }} # fetch data from read replica instead of the primary node
  }
  
  field :name
  field :address
  ...
end

class City < ActiveRecord::Model
  update_index("cities") { self if persistently_changed? }
  ...
end

------ Request 1 -------
Chewy.strategy(:delayed_sidekiq) do
  City.create(name: "Amsterdam") # created city with id=1
end
------ Request 2 -------
Chewy.strategy(:delayed_sidekiq) do
  City.create(name: "Paris") # created city with id=2
end
------ Request 3 -------
Chewy.strategy(:delayed_sidekiq) do
  City.create(name: "Rome") # created city with id=3
end

those three parallel requests that happened during the "latency" window are going to schedule a single worker to call import with all accumulated ids 1, 2, 3

CityIndex.import([1, 2, 3]) # sql request will go to { role: :reading } due to definition in the index

Strategy option in #import method.

CityIndex.import([1, 2, 3], strategy: :delayed_sidekiq)

Accumulation of :update_fields option between chunks.

CityIndex.import([1, 2], update_fields: ["name"], strategy: :delayed_sidekiq) 
CityIndex.import([3], update_fields: ["address"], strategy: :delayed_sidekiq)

will cause

CityIndex.import([1, 2, 3], update_fields: ["name", "address"])

that can be very useful if you have some complex crutches in the index definition and you don't want to load the database with obviously useless SQL queries.

Configuration

• at CityIndex level

class CitiesIndex < Chewy::Index
  index_scope City
  
 strategy_config delayed_sidekiq: {
    latency: 10,  
    margin: 2,
    reindex_wrapper: ->(&reindex) { ActiveRecord::Base.connected_to(role: :reading) { reindex.call }} # fetch data from read replica instead of the primary node
 }

• at the initializer level

Chewy.settings = {
  strategy_config: {
    delayed_sidekiq: {
      reindex_wrapper: ->(&import) { ActiveRecord::Base.connected_to(role: :reading) { import.call } },
      margin: 2, # replication lag seconds
      latency: 10 # accumulation window seconds
    }
  }
}

• at chewy.yml level

strategy_config:
  delayed_sidekiq:
     latency: 10
     margin: 10
     # reindex_wrapper option is not supported in YAML config. Use either initializer either index file instead.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the changelog if the new code introduces user-observable changes. See changelog entry format for details.

[skcc321]

@konalegi could you please review this one please? (and trigger the CI)

[skcc321]

@konalegi one more time, please. Got failed CI due to no Redis service running.

[konalegi]

@skcc321 Thank you very much for the PR, I'll try to review this in a week or find someone who can do it.

[skcc321]

@konalegi one more time CI, please (I hope the last one) - fixed linters & ruby 2.x related failed tests.

[skcc321]

@konalegi sorry for bothering you. Any updates on that one?

[konalegi]

@skcc321, sorry, everyone is busy these days. I'll try to review this on Thursday/Friday, added my Todo list

[mrzasa]

Thanks for the contribution. I've added some early comments, I didn't have time to test it properly.

[skcc321]

@mrzasa thank you for the review. I pushed all suggested changes in a separate commit (just for easier review next time).
Could you take a look one more time and restart the CI, please?
I'll squash all commits when the review is done and there are no more comments.

[skcc321]

@konalegi sorry for bothering you, could you please trigger the CI again?

[skcc321]

@konalegi @mrzasa any updates?

[konalegi]

Sorry for making you wait for so long.
Looks good to me, I'll need to do some manual testing, if everything will be nice, I'll merge it.

[skcc321]

Thank you @konalegi. I'll work on your comments soon.

[skcc321]

@konalegi could you please trigger the CI and look through the latest changes?
Thank you

[konalegi]

LGTM, please address my last comment and rubocop offense.
Thanks for great feature!

[skcc321]

@konalegi done. Please take a look at the latest commit.

[skcc321]

@mrzasa could approve please (if you have no concerns) :)

[konalegi]

Since this PR doesn't modify any existing functionality and brings new feature going to be merge right away. Will be released on Monday

[konalegi]

Released