Recover Data from Production Backups with ActiveRecord

Bernerd Schaefer

What follows is a story about things going wrong with production data. But don’t worry, this one has a happy ending!

We’ve been working on a product which tracks interesting information about companies, pulling data from a number of different sources. Users can interact with these company records by leaving notes, following them, and updating their financial data.

The data model, then, looks like this:

class Company < ActiveRecord::Base
  has_many :financials
  has_many :followed_companies
  has_many :followers, through: :followed_companies, source: :user
  has_many :notes
end

class Financial < ActiveRecord::Base
  belongs_to :company
end

class FollowedCompany < ActiveRecord::Base
  belongs_to :company
  belongs_to :user
end

class Note < ActiveRecord::Base
  belongs_to :company
end

Note and FollowedCompany records are created based on user interactions, while Company and Financial records are created by automated systems, though users can edit them after they’re created.

One of the tricky things in this system is that we need to combine multiple data sources to get the information we need for a company and its financials – but the sources often don’t agree on details. For example, one source might claim a company has 25 employees, while another claims it has 50. Sometimes, they might not even agree on the company’s name, like “Frozzle” and “Frozzle, Inc.”

In the case of employee count and similar data disagreements, we were able to pick which source we trust more to accurately report that data, and default to their value. When sources disagree about things like a company’s name, though, we can end up with duplicate companies.

That’s why we started building a system to merge duplicate companies together.

When two companies are found to be duplicates we merge them with this process:

  1. Call the older of the companies the canonical company, and the newer the duplicate.

  2. Assign all of the duplicate company’s associations to the canonical one:

    duplicate.financials.update_all(company: canonical)
duplicate.followed_companies.update_all(company: canonical)
duplicate.notes.update_all(company: canonical)

  3. Mark the duplicate as merged by adding a notice to their name and recording which company they were merged into:

    duplicate.update(
  name: "#{duplicate.name} (merged)",
  merged_to: canonical,
)

But how do we know which companies to merge?

We’ve gone through a few iterations of this. Our first version only merged companies with identical names. Later versions worked similarly, but also ignored capitalization, whitespace, and punctuation.

But those versions really only dealt with the most obvious cases. But we still had a lot of companies like “Frozzle” and “Frozzle, Inc.”, or “Quibbles LLC” and “Quibbles LTD”.

So we put together another version to handle cases like that, and rolled it out. We took a database backup, and then ran the deduplication process. It was able to find around 1000 duplicate companies, and we confirmed that “Frozzle, Inc” was merged away, so everything looked good, and we moved on to other features.

Then we started seeing some strange production errors and bug reports over the next few days. About a week and a half after the rollout, we discovered what was going on: the new duplicate detector had incorrectly flagged a number of unrelated companies as duplicates.

Note that above we stored what company a duplicate was merged into, but did not do the same for its associations. That meant we couldn’t use the database directly to restore things to normal. We also weren’t logging the sequence of operations being done, which we could have used to reverse the process.

We were in a tricky spot. We had production data problems that we’d need to roll back, but users had been interacting with the system for almost two weeks, and we couldn’t undo all of their work!

We reverted our new duplicate detector, and started brainstorming solutions. The most promising solution seemed to be first to restore the pre-rollout backup to a separate database, and then figure out what associations the incorrectly merged companies had before, so that they could be re-assigned. In pseudocode, we wanted to do something like:

Company.incorrectly_merged.each do |company|
  company.backup_financials.each do |financial|
    Financial.where(id: financial.id).update!(comapny: company)
  end

  # repeat for followed companies and notes

  company.update!(
    name: company.name.without_merge_notice,
    merged_to: nil,
  )
end

We originally thought we’d do this in a series of steps, by writing some scripts which connected to the backup database and exported data as a CSV, and then other scripts which would use the CSV to update production data.

But in the end, we were actually able to do it with a single script, leveraging ActiveRecord tools to write something very similar to the pseudocode above.

Restore the backup locally

The project was deployed on Heroku and using Heroku’s PGBackups feature for both scheduled (daily) and manual backups.

We had already captured a backup of the production database before rolling out the faulty process with heroku pg:backups capture --app app-production. That backup was assigned an identifier by Heroku, in this case b046.

To restore that backup locally, then, we used:

curl -o backup.dump heroku pg:backups public-url b046
createdb backup
pg_restore -d backup backup.dump

With a complete copy of production data available locally, we were able to start scripting the recovery process.

Configure a named ActiveRecord connection

The first step was to add some new entries to config/database.yml:

backup:
  <<: *default
  database: backup

remote_production:
  <<: *deploy
  url: postgres://.../production

Define new models prefixed with Backup 

ActiveRecord::Base.establish_connection :remote_production

class BackupFinancial < ActiveRecord::Base
  establish_connection :backup
  self.table_name = "financials"
end

class BackupFollowedCompany < ActiveRecord::Base
  establish_connection :backup
  self.table_name = "followed_companies"
end

class BackupNote < ActiveRecord::Base
  establish_connection :backup
  self.table_name = "notes"
end

This will allow us to talk to both the production database and the backup at the same time. The normal models (Note, Financial) will point to production, and the models in the script (BackupNote) will point to the backup database.

Write the script, with print debugging

We’ll need some helper functions, starting with one to clean up the names of merged companies:

def strip_merge_notice(name)
  merge_notice = / \(merged\)\Z/

  if name =~ merge_notice
    name.sub(merge_notice, "")
  else
    raise "Expected #{name.inspect} to match #{merge_notice.inspect}"
  end
end

Then a function for printing structured log information, which we can use to debug or reverse this script if something goes wrong:

# log_info(company: 1, name: "bar", ids: [2, 3])
#   => company=1 name="bar" ids=[2,3]
def log_info(attributes)
  line = attributes.
    map { |key, value| [key, value.inspect].join("=") }.
    join(" ")

  puts line
end

We’ll also have a helper to restore a company’s association:

def restore_merged_association(company_id, backup_model, real_model)
  backup_ids = backup_model.
    where(company_id: company_id).
    pluck(:id)

  log_info(
    company: company_id,
    real_model.table_name => backup_ids.join(","),
  )

  real_model.
    where(id: backup_ids).
    update_all(company_id: company_id)
end

Run script in transaction

And finally, we can put it all together:

recently_merged_companies = Company.
  where.not(merged_to_id: nil).
  where("updated_at >= ?", Date.parse("2016-03-08"))

ActiveRecord::Base.transaction do
  recently_merged_companies.each do |company|
    new_name = strip_merge_notice(company.name)

    log_info(
      company: company.id,
      name: new_name,
      previous_name: company.name,
      previous_merged_to: company.merged_to_id,
    )
    company.update!(name: new_name, merged_to: nil)

    restore_merged_association company.id, BackupFinancial, Financial
    restore_merged_association company.id, BackupFollowedCompany, FollowedCompany
    restore_merged_association company.id, BackupNote, Note
  end
end

At this point, the script was ready for a few rounds of local iteration, before we finally ran it against production with rails runner unmerge.rb. It successfully rolled back the important associations that had been lost, without losing any information that our users had entered in the mean time!

What did we learn?

Once the dust had settled, we had a bit of time to reflect on what went wrong and how to improve going forward:

  • Backup before changing production data

    We were already doing this, but it is important to restate. We couldn’t have recovered from this if we hadn’t taken a backup before we rolled out our changes.

  • Log structured information from tasks

    It would have been much easier to recover if we had logged the operations in the deduplication process like we did in our recovery script. We likely wouldn’t have even needed the backups.

  • Consider the rollback strategy

    Our original script was optimistic about its correctness and didn’t account for the possibility of rollback. In retrospect, it may have made sense to build it with an eye towards being easy to undo.

We also learned some things along the way while building the recovery script itself:

  • You can have multiple models communicate with the same table by giving them unique names (like BackupNote above) and using self.table_name= to select the table.

  • You can connect to multiple databases at the same time with ActiveRecord! It’s particularly interesting that different models can be attached to different databases using establish_connection.

  • You can run a script like the recovery script multiple times and still check its effects by using transaction rollbacks and pry:

    ActiveRecord::Base.transaction do
  # do work...

  binding.pry
  raise ActiveRecord::Rollback
end