Finding mongoDB records in batches (using mongoid

Question

Using rails 3 and mongoDB with the mongoid adapter, how can I batch finds to the mongo DB? I need to grab all the records in a particular mongo DB collection and index them in solr (initial index of data for searching).

The problem I'm having is that doing Model.all grabs all the records and stores them into memory. Then when I process over them and index in solr, my memory gets eaten up and the process dies.

What I'm trying to do is batch the find in mongo so that I can iterate over 1,000 records at a time, pass them to solr to index, and then process the next 1,000, etc...

The code I currently have does this:

Model.all.each do |r|
  Sunspot.index(r)
end

For a collection that has about 1.5 million records, this eats up 8+ GB of memory and kills the process. In ActiveRecord, there is a find_in_batches method that allows me to chunk up the queries into manageable batches that keeps the memory from getting out of control. However, I can't seem to find anything like this for mongoDB/mongoid.

I would LIKE to be able to do something like this:

Model.all.in_batches_of(1000) do |batch|
  Sunpot.index(batch)
end

That would alleviate my memory problems and query difficulties by only doing a manageable problem set each time. The documentation is sparse, however, on doing batch finds in mongoDB. I see lots of documentation on doing batch inserts but not batch finds.

Answer 1

The following will work for you , just try it

Model.all.in_groups_of(1000, false) do |r|
  Sunspot.index! r
end

Answer 2

I am not sure about the batch processing, but you can do this way

current_page = 0
item_count = Model.count
while item_count > 0
  Model.all.skip(current_page * 1000).limit(1000).each do |item|
    Sunpot.index(item)
  end
  item_count-=1000
  current_page+=1
end

But if you are looking for a perfect long time solution i wouldn't recommend this. Let me explain how i handled the same scenario in my app. Instead of doing batch jobs,

• i have created a resque job which updates the solr index

  class SolrUpdator
   @queue = :solr_updator
  
   def self.perform(item_id)
     item = Model.find(item_id)
     #i have used RSolr, u can change the below code to handle sunspot
     solr = RSolr.connect :url => Rails.application.config.solr_path
     js = JSON.parse(item.to_json)
     solr.add js         
   end
  

  end

• After adding the item, i just put an entry to the resque queue

  Resque.enqueue(SolrUpdator, item.id.to_s)
  

• Thats all, start the resque and it will take care of everything

Answer 3

It is faster to send batches to sunspot as well. This is how I do it:

records = []
Model.batch_size(1000).no_timeout.only(:your_text_field, :_id).all.each do |r|
  records << r
  if records.size > 1000
    Sunspot.index! records
    records.clear
  end
end
Sunspot.index! records

no_timeout: prevents the cursor to disconnect (after 10 min, by default)

only: selects only the id and the fields, which are actually indexed

batch_size: fetch 1000 entries instead of 100

Answer 4

As @RyanMcGeary said, you don't need to worry about batching the query. However, indexing objects one at a time is much much slower than batching them.

Model.all.to_a.in_groups_of(1000, false) do |records|
  Sunspot.index! records
end

Answer 5

If you are iterating over a collection where each record requires a lot of processing (i.e querying an external API for each item) it is possible for the cursor to timeout. In this case you need to perform multiple queries in order to not leave the cursor open.

require 'mongoid'

module Mongoid
  class Criteria
    def in_batches_of(count = 100)
      Enumerator.new do |y|
        total = 0

        loop do
          batch = 0

          self.limit(count).skip(total).each do |item|
            total += 1
            batch += 1
            y << item
          end

          break if batch == 0
        end
      end
    end
  end
end

Here is a helper method you can use to add the batching functionality. It can be used like so:

Post.all.order_by(:id => 1).in_batches_of(7).each_with_index do |post, index|
  # call external slow API
end

Just make sure you ALWAYS have an order_by on your query. Otherwise the paging might not do what you want it to. Also I would stick with batches of 100 or less. As said in the accepted answer Mongoid queries in batches of 100 so you never want to leave the cursor open while doing the processing.

Answer 6

With Mongoid, you don't need to manually batch the query.

In Mongoid, Model.all returns a Mongoid::Criteria instance. Upon calling #each on this Criteria, a Mongo driver cursor is instantiated and used to iterate over the records. This underlying Mongo driver cursor already batches all records. By default the batch_size is 100.

For more information on this topic, read this comment from the Mongoid author and maintainer.

In summary, you can just do this:

Model.all.each do |r|
  Sunspot.index(r)
end