Tributary

Tributary is a helpful little plugin that pages your Ecto query for efficient processing without any concern on your part for state management. It does this by taking advantage of the Elixir Stream module which is a type of lazy enumerable. And it does this in a very efficient manner on the database side.

Using Tributary

Before using Tributary itself, it's important that you take a quick look at the table you want to query with Tributary. The table must have a unique btree index on a single field. In common database designs this is going to be a primary key that is an auto-incremented integer on the id column; however, Tributary knows how to deal with non-integer keys as long as the database knows how to deal with them, and you specify the initial key.

Tributary is addded in to your project by calling use Tributary inside your Ecto repository. For example, let's assume we are writing an application for a brokerage firm. You may set up the Repo like this:

defmodule Brokerage.Repo
  use Ecto.Repo, otp_app: brokerage
  use Tributary
end

Now, let's assume you have an Ecto model Brokerage.Client which has the residence country of each client listed in the country field. You need to send each US-based client a 1099-DIV. Let's create a mix task for it:

defmodule Mix.Tasks.Emails.SendUS1099DIV
  use Mix.Task
  alias Brokerage.{User, Repo}
  import Ecto.Query, only: [from: 2]

  def run(_)
    Repo.start_link

    from(u in User, where: u.country == "US")
	|> Repo.stream
	|> Stream.each(fn user ->
	  Brokerage.Emails.TaxUS1099DIV.send(user)
	end)
	|> Stream.run
  end
end

That's it.

How Tributary Works

Tributary is actually a very simple library. It relies on Stream.resource/3 to generate a Stream, abstracting away all the concerns about state management so that you can reliably process your data set as a single, continuous enumerable.

Tributary's key feature, though, is its reliance on indexes for paging: keyset pagination. Many libraries, regardless of language, will page the database using the SQL OFFSET keyword, which by its standardized definition will perform worse as the offset size grows because the database still has to acknowledge the rows it is not returning to the client. Instead, Tributary follows the "seek method," which adds a WHERE clause requesting only records higher than the last seen key it is paging by.

For more resources on keyset pagination and why it is so important, see the following posts:

• "Pagination Done the PostgreSQL Way" by Alexey Vasiliev
• "We need tool support for keyset pagination" by Markus Winand

Helpful Answers

Is Tributary a paging library?

In the simplest terms, yes, in that it will "page" through data. However, if you're looking for a library that will take a page number and a number of objects and find the approppriate set of objects from the database: no.

Tribtuary is designed for situations where you have to perform contextual or complex data operations over a large subset of records in a table. Instead of managing loops to page the data from the database and then creating internal loops to iterate over records for each page, Tributary provides you with a way to treat the entire data set you want to operate on as a single, large enumerable while it silently loads only the necessary data in the background.

Future Tasks

• Support using mutli-column indexes

Testing

To test Tributary, you will need to have Postgres installed and create a testing database. You'll also need a test environment secrets file, which you can create at config/test.secret.exs—this is already ignored by git.

The most basic form looks something like this:

use Mix.Config

config :tributary, Tributary.Repo,
  url: "postgres://localhost/tributary"

You may need to tweak the URL parameters. Or you can use the other provided Ecto Postgres adapter options.

You will need to run mix ecto.migrate with the environment variable MIX_ENV set to test:

MIX_ENV=test mix ecto.migrate

You should now be able to run mix test.

License

Tributary is issued under the MIT License. See the LICENSE.md file.