I just wanted an HTTP client.

I asked my Twitter community which HTTP client to use. Certainly not HTTParty, because cool people don't use HTTParty anymore. One takes these risks when one falls out of the loop with the latest and greatest tools.

"Use Faraday", they said. "It's the best", they said.

Everything went fine until I needed to follow a redirect. What happened from that point forward could form the basis of a solid one-act play. I'll spare you the horror and send you directly to the punch line.

connection.adapter Faraday.default_adapter

When Is A Default Not Really A Default?

When you use middleware with Faraday. Specifically when you use FaradayMiddleware::FollowRedirects to follow redirects with Faraday.

I tried to read the documentation. Honestly. I read Stack Overflow. I read the Faraday Middleware wiki. I even read the source. Nothing seemed to warn me about this "default adapter" business, and given the name "default", shouldn't it—you know—be the default?!

Remember: ask why, but never answer

I looked through Faraday Middleware's issues and found evidence that other people had successfully followed redirects with FaradayMiddleware::FollowRedirects, and so I concluded that I'd done something wrong. I looked for any issue that related to redirects for a clue. I ended up reading an issue that suggested the possibility of following redirects depending on more details in the original request: follow redirects for this path info, but not that path info, for example.

Something caught my eye.

Consider the following code slightly adapted from the readme:

connection = '' do |conn|
  conn.use FaradayMiddleware::FollowRedirects, limit: 5
  conn.adapter Faraday.default_adapter

Every request will follow redirects based on the configuration above.

Dimitrios Zorbas

What the hell is this default adapter business?! I wondered.

Then I tried it.

Then it worked.

Then I tweeted something.

Then my wife found some calming Dylan Moran comedy1 and I felt better.

Of course, now that I know what to look for, I find the exact issue that I've just run into—and it's TWO YEARS OLD as of the time I write these words.

In that issue, Mislav Marohnić helpfully points the problem out and suggests following an issue that no longer exists to receive notification when this curious behavior changes. Fortunately, I could easily track down the issue he means, as the project has since moved. He means this issue. I've started following it, and now so can you, in case you want to know when you can safely ignore the dreaded default_adapter.

What Does This All Mean?!

It means that if you have this:

def http_get(base, uri)
  Faraday.get(base + uri)

and you want to refactor it to add middleware, you absolutely positively must write this:

def http_get(base, uri) { | connection |
    # IMPORTANT Without this line, nothing will happen.
    connection.adapter Faraday.default_adapter

If you forget to specify the adapter, then you have not refactored. I hope this helps.

But Wait! There's More!

I really loved The Pragmatic Programmer, and will probably always remember its section on "programming by coincidence" or, as I like to call it, programming by accident. One brand of programming by accident involves moving statements around until they work. This relates to temporal coupling, the extent to which statements depend not just on each other, but on the sequence in which they occur. Some temporal coupling makes perfect sense:

list = []
list << "hello"
list.empty? => false

Rearrange these statements and they say something fundamentally different. This temporal coupling seems quite reasonable to me.

Accidental2 temporal coupling, however, creates problems for everyone, except possibly the programmer who created the accidental temporal coupling. Of course, I had to find out whether this code exhibits accidental temporal coupling. It does.

def http_get(base, uri) { | connection |
    # IMPORTANT Without this line AT THE END OF THIS BLOCK, nothing will happen.
    connection.adapter Faraday.default_adapter

It behooves me, the programmer, to introduce this:

def faraday_with_default_adapter(base, &block) { | connection |
    yield connection

    # IMPORTANT Without this line, nothing will happen.
    connection.adapter Faraday.default_adapter

which I can use like this:

def http_get(base, uri)
  faraday_with_default_adapter(base) { | connection |
    connection.use FaradayMiddleware::FollowRedirects, limit: 1

Thus I program deliberately.

It behooves me more, even, to contribute this back to Faraday. I'm tired. I'll do it later.

  1. You don't know Dylan Moran?! Start here.

  2. As in Fred Brooks and his distinction between essential complication and accidental complication in his classic essay, "No Silver Bullet". Read more about this distinction in "The Eternal Struggle Between Business and Programmers".


Design credit: Shashank Mehta