I recently ran into an issue with Literate Minuteman that reminded me of the value of keeping your Rails background jobs as small as you can.

In Minuteman, looking up the state of a book at a particular library is a relatively slow operation, and so is done periodically outside the HTTP request/response, using Resque. For each book we want to update, we enqueue an UpdateBook worker with the book’s id, which fetches the book and then delegates to Book’s sync_copies method.

# app/workers/update_book.rb
def self.perform(book_id)
  book = Book.find(book_id)
  book.sync_copies
end

The book’s sync_copies method then loops through all the available LibrarySystems and asks the system in question for the current status of that book’s copies:

# app/models/book.rb
def sync_copies
  LibrarySystem.all.each do |system|
    copies = system.find(title, author)
    # book updating work happens here
  end
end

The problem with this design is that each LibrarySystem’s lookup is run in serial as a part of the same job. If an exception is raised by any system’s find call, subsequent system book lookups won’t happen at all. That’s what I found was happening this weekend-a change in the Boston Public Library’s site had caused the very first system lookup to fail, and now no books were being updated at all!

To fix this, I split the jobs out into more specific tasks-looking up the status of a book in a particular library system, instead of the status of a book in all library systems. UpdateBook simply took the library system id as a parameter and passed it to a Book#sync_copies method that just looked up copies in that system:

# app/workers/update_book.rb
def self.perform(book_id, library_system_id)
  book = Book.find(book_id)
  book.sync_copies(LibrarySystem.find(library_system_id))
end

# app/models/book.rb
def sync_copies(library_system)
  copies = system.find(title, author)
  # book updating work happens here
end

Now, the BPL system lookups were still failing, but the other system lookups could run independently and I was free to fix the BPL lookup system in isolation.

I’ve simplified the code in these examples slightly to make my point more clear; you can check out the full changes here.