Most applications I’ve worked on at some point required that ‘Export’ feature so people would be able to play with the data using the familiar Excel interface. I’m sharing some code here from a recent work that did the following:
Generate a CSV file for download with up to 100,000 rows in it. Since the contents of the file depends on some dynamic parameters, and the underlying data is changing all the time, the file must be generated live. Generating a large file takes time and the load balancer will drop the connection if it takes more than 1 minute. In fact, as a consumer I myself would be frustrated had it took even 1 minute to see something happening. This problem natually requires a streaming solution.
For a familiar example, let’s say we are downloading a CSV file containing transactions on an online store for the accounting folks. Lets say the URL is as follows:
So, this would download a file containing the transactions from January to April of 2013, where a CreditCard was used for a purchase over $400. Here goes the code example with inline comments describing interesting parts.
class Transaction
belongs_to :store
attr_accessible :time, :amount
def self.csv_header
#Using ruby's built-in CSV::Row class
#true - means its a header
CSV::Row.new([:time, :store, :amount], ['Time', 'Store', 'Amount'], true)
end
def to_csv_row
CSV::Row.new(title: title, store: store.name, amount: amount)
end
def self.find_in_batches(filters, batch_size, &block)
#find_each will batch the results instead of getting all in one go
where(filters).find_each(batch_size: batch_size) do |transaction|
yield transaction
end
end
end
Given this Transaction model, the controller can call the methods and set appropriate http headers to stream the rows as they are generated instead of waiting for the whole file to be generated. Here’s the example controller code:
class TransactionsController
def index
respond_to do |format|
format.csv render_csv
end
end
private
def render_csv
set_file_headers
set_streaming_headers
response.status = 200
#setting the body to an enumerator, rails will iterate this enumerator
self.response_body = csv_lines(filters)
end
def set_file_headers
file_name = "transactions.csv"
headers["Content-Type"] = "text/csv"
headers["Content-disposition"] = "attachment; filename=\"#{file_name}\""
end
def set_streaming_headers
#nginx doc: Setting this to "no" will allow unbuffered responses suitable for Comet and HTTP streaming applications
headers['X-Accel-Buffering'] = 'no'
headers["Cache-Control"] ||= "no-cache"
headers.delete("Content-Length")
end
def csv_lines
Enumerator.new do |y|
y << Transaction.csv_header.to_s
#ideally you'd validate the params, skipping here for brevity
Transaction.find_in_batches(params){ |transaction| y << transaction.to_csv_row.to_s }
end
end
end
As you see in this example, it’s pretty straight forward once you put the pieces together. These streaming headers work under most servers including Passenger, Unicorn, etc. but webrick doesn’t support streaming responses. It took me some time to figure out the headers and the enumerator thing, but since then it’s working beautifully for us. Hope it will help someone with a similar need.