Today I Learned

A Hashrocket project

What happens on Ecto.Repo.rollback?

The rollback function interupts the execution of the function that you pass to transaction, but what is the mechanism for interupting execution?

When you call rollback you are essentially calling this code:

throw({DBConnection, conn_ref, reason})

throw is no good without a catch and you can find the try ... catch in the DBConnection module as well. It can be boiled down to this:

try do
  # call function passed to Ecto.Repo.transaction
  :throw, {__MODULE__, ^conn_ref, reason} ->
    # send rollback request to database and return with {:error, reason}
    return {:error, reason}
  kind, reason ->
    # send rollback request to database and raise with reason
    :erlang.raise(kind, reason, stack)

This code exploits the ability of catch to both handle a throw and handle a raise.

Looking for help? Elixir is quickly gaining momentum for web applications that need concurrency, performance, and the ability to connect to many different clients. The developers at Hashrocket are learning along with the rest of the development community that Elixir and Phoenix are viable Rails alternatives for the right application. Check out the source code for Today I Learned, written in Elixir, and contact us if you need help with your Elixir project.