Human-readable user agents for JavaScript error reporting

Some time ago I published a few posts on logging, one on how to report JavaScript errors and another one which illustrates how to log on New Relic with Monolog.

In this post I’d like to go even further with JavaScript error logging and tell you how we – at Namshi – are trying to ease frontend debugging across multiple browsers.

User agents, the gotchas

With JavaScript, it’s pretty easy to detect the user agent from a client ( you just need to access navigator.userAgent), the problem is that user agents are one of the most incredible gotchas in web development.

For example, how would you guess that

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Mozilla/5.0
(Windows; U; Windows NT 6.1; WOW64; en-US; rv:2.0.4)
Gecko/20120718 AskTbAVR-IDW/3.12.5.17700 Firefox/14.0.1

represents Firefox 14 on Windows 7?

Let me tell you, you wouldn’t, that’s why you should convert user agents in a human-readable format for the people who are going to debug the frontend.

Converting user agents with remote calls

Luckily, UserAgentString is a service which allows you to query them whenever you need to retrieve useful and meaningful informations from a user agent string; combined with Guzzle, you can directly have meaningful JavaScript errors’ reports with a few lines of code.

For example, this is a simple class which retrieves the informations from the service:

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<?php

namespace Vendor\Service;

use Guzzle\Http\ClientInterface;

class UserAgentConverter
{
    const URL_USERAGENT_API = 'http://www.useragentstring.com/';
    const BROWSER_INFO      = '%s %s on %s';

    protected $client;

    /**
     * Instantiates the service and injects the HTTP client that will be used
     * to perform requests.
     * 
     * @param ClientInterface $client 
     */
    public function __construct(ClientInterface $client)
    {
        $this->client = $client;
    }

    /**
     * Retrieves a human-readable string identifying the $userAgent for error
     * reporting (ie Internet Explorer 8 on Windows 7).
     * 
     * @param string $userAgent
     * @return string|null
     */
    public function lookup($userAgent)
    {
        $request  = $this->client->post(self::URL_USERAGENT_API, null, sprintf('uas=%s&getJSON=all', $userAgent));
        $response = $request->send();

        if ($response->getStatusCode() === 200) {
            return json_decode($response->getBody(true), true);
        }

        return null;
    }
}

and you can use it like this:

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<?php

$ua         = 'Mozilla/5.0 (Windows; U; Windows NT 6.1; WOW64; en-US; rv:2.0.4) Gecko/20120718 AskTbAVR-IDW/3.12.5.17700 Firefox/14.0.1';
$uaService  = new Vendor\Service\UserAgentConverter(new Guzzle\Http\Client());
$userAgent  = $uaService->lookup($ua);

if ($userAgent) {
  // outputs "Firefox 14.0.1 on Windows 7"
  echo sprintf(
      "%s %d on %s",
      $userAgent['agent_name'],
      $userAgent['agent_version'],
      $userAgent['os_name']
  );
}

That’s pretty easy, even easier if you use the dependency injection container of Symfony2, where you can just do the instantiation in a config file:

container.yml
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services:
  http.client:
    class: "Guzzle\\Http\\Client"
  useragent.converter:
    class: "Vendor\\Service\\UserAgentConverter"
    arguments:
      client: @http.client
The one liner to get the user agent’s informations
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<?php

$container->get('useragent.converter')->lookup($ua);

Caching

At this point it becomes obvious that you should put a caching layer in front of the UserAgentConverter since you don’t want to always query a remote service to retrieve informations that you already have: something like Redis should perfectly do the job, as a cache – in this scenario – is essential, needs to be as fast as hell and you don’t need a SLA with it, so if the Redis server is down you are gracefully degradating: at the same time, Memcache can be a good candidate to substitute Redis, but remember that you will renounce to persistence, since you won’t be able to store informations on the disk as you would do with Redis.

The implementations is very trivial:

Adding a caching layer to our code
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<?php

$ua         = 'Mozilla/5.0 (Windows; U; Windows NT 6.1; WOW64; en-US; rv:2.0.4) Gecko/20120718 AskTbAVR-IDW/3.12.5.17700 Firefox/14.0.1';
$cache      = new CacheProvider();
$userAgent  = $cache->lookup($ua)

if (!$userAgent) {
  $uaService  = new Vendor\Service\UserAgentConverter(new Guzzle\Http\Client());
  $userAgent  = $uaService->lookup($ua);
}

if ($userAgent) {
  $cache->store($ua, $userAgent);

  // outputs "Firefox 14.0.1 on Windows 7"
  echo sprintf(
      "%s %d on %s",
      $userAgent['agent_name'],
      $userAgent['agent_version'],
      $userAgent['os_name']
  );
}
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