---
title: "First-Class Failure"
date: 2014-07-22T00:00:00+00:00
draft: false
canonical_url: https://www.viget.com/articles/first-class-failure/
---

As a developer, nothing makes me more nervous than third-party
dependencies and things that can fail in unpredictable
ways[^1]. More often
than not, these two go hand-in-hand, taking our elegant, robust
applications and dragging them down to the lowest common denominator of
the services they depend upon. A recent internal project called for
slurping in and then reporting against data from
[Harvest](http://www.getharvest.com/), our time tracking service of
choice and a fickle beast on its very best days.

I knew that both components (`/(im|re)porting/`) were prone to failure.
How to handle that failure in a graceful way, so that our users see
something more meaningful than a 500 page, and our developers have a
fighting chance at tracking and fixing the problem? Here's the approach
we took.

## Step 1: Model the processes

Rather than importing the data or generating the report with procedural
code, create ActiveRecord models for them. In our case, the models are
`HarvestImport` and `Report`. When a user initiates a data import or a
report generation, save a new record to the database *immediately*,
before doing any work.

## Step 2: Give 'em status

These models have a `status` column. We default it to "queued," since we
offload most of the work to a series of [Resque](http://resquework.org/)
tasks, but you can use "pending" or somesuch if that's more your speed.
They also have an `error` field for reasons that will become apparent
shortly.

## Step 3: Define an interface

Into both of these models, we include the following module:

```ruby
module ProcessingStatus
  def mark_processing
    update_attributes(status: "processing")
  end

  def mark_successful
    update_attributes(status: "success", error: nil)
  end

  def mark_failure(error)
    update_attributes(status: "failed", error: error.to_s)
  end

  def process(cleanup = nil)
    mark_processing
    yield
    mark_successful
  rescue => ex
    mark_failure(ex)
  ensure
    cleanup.try(:call)
  end
end
```

Lines 2--12 should be self-explanatory: methods for setting the object's
status. The `mark_failure` method takes an exception object, which it
stores in the model's `error` field, and `mark_successful` clears said
error.

Line 14 (the `process` method) is where things get interesting. Calling
this method immediately marks the object "processing," and then yields
to the provided block. If the block executes without error, the object
is marked "success." If any[^2] exception is thrown, the object marked "failure" and the
error message is logged. Either way, if a `cleanup` lambda is provided,
we call it (courtesy of Ruby's
[`ensure`](http://ruby.activeventure.com/usersguide/rg/ensure.html)
keyword).

## Step 4: Wrap it up

Now we can wrap our nasty, fail-prone reporting code in a `process` call
for great justice.

```ruby
class ReportGenerator
  attr_accessor :report

  def generate_report
    report.process -> { File.delete(file_path) } do
      # do some fail-prone work
    end
  end

 # ...
end
```

The benefits are almost too numerous to count: 1) no 500 pages, 2)
meaningful feedback for users, and 3) super detailed diagnostic info for
developers -- better than something like
[Honeybadger](https://www.honeybadger.io/), which doesn't provide nearly
the same level of context. (`-> { File.delete(file_path) }` is just a
little bit of file cleanup that should happen regardless of outcome.)

***

I've always found it an exercise in futility to try to predict all the
ways a system can fail when integrating with an external dependency.
Being able to blanket rescue any exception and store it in a way that's
meaningful to users *and* developers has been hugely liberating and has
contributed to a seriously robust platform. This technique may not be
applicable in every case, but when it fits, [it's
good](https://www.youtube.com/watch?v=HNfciDzZTNM&t=1m40s).

[^1]: Well, [almost nothing](https://github.com/github/hubot/blob/master/src/scripts/google-images.coffee#L5).
[^2]: [Any descendent of `StandardError`](http://stackoverflow.com/a/10048406), in any event.
