Archives for November, 2017

Reacting to promises

published on November 29, 2017.

I was working on something that includes the usage of ReactPHP promises. Given that I haven’t had the chance to take a closer look at it yet, I decided that this is the right time for it.

ReactPHP has several different components, with the end goal of providing a low-level library for event-driven programming in PHP. The one component I want to talk about today is the promise component, which is a Promises/A implementation for PHP.

What this promise library allows us is a nicer workflow with asynchronous code.

With promises, when we want to execute something asynchronously we defer the work that will be executed asynchronously. The Deferred unit of work will complete sometimes in the future, but we don’t know when. But it does promise that the work will be done, one way or the other.

The Promise is a sort of a placeholder for the result that will eventually be returned from our deferred work. This promise can then either be resolved or rejected by our deferred. When a promise is resolved successfully it has an associated value, and when it is rejected it has an associated reason for the rejection.

We use the then method on the promise to register handlers that will be called when the deferred is resolved or rejected.

To install the React/Promise component, run:

$ composer require react/promise

An example

Let’s say we have some code that does some asynchronous work. Checking the HTTP status code of a bunch of URLs, for example. We could create an invokable class that extends the Deferred:

FetchStatusCodes.php

<?php declare(strict_types=1);

use React\Promise\Deferred;

class FetchStatusCodes extends Deferred
{
    public function __invoke(array $urls)
    {
        $multiHandle = curl_multi_init();

        $handles = $this->getHandlesForUrls($urls, $multiHandle);

        $this->executeMultiHandle($multiHandle);

        $statusCodes = $this->getStatusCodes($handles);

        curl_multi_close($multiHandle);

        $successRate = $this->calculateSuccessRate($statusCodes);

        if ($successRate > 50) {
            $this->resolve($statusCodes);
        } else {
            $this->reject('Success rate too low: ' . $successRate);
        }
    }
}

I’ve left out here a bunch of code that deals with the actual fetching of the status codes, just to keep the “noise” down. The full example is available in this repository.

The important thing here is that we extend React\Promise\Deferred and that at the end we call the resolve() method to resolve this deferred if the success rate is over 50%, or that we call the reject() method if the success rate is below 50%.

The set up of the actual promise and its handlers would look something like this:

promise.php

<?php

$statusCodes = new FetchStatusCodes();
$promise = $statusCodes->promise();

$promise
    ->then(
        function($value) {
            var_dump($value);
        },
        function($reason) {
            echo $reason . PHP_EOL;
        }
    );

$urls = [
    'https://example.com/',
    'https://stackoverflow.com/',
    'https://www.google.com/',
    'https://www.google.com/no-such-url',
    'https://www.google.com:81'
];
$statusCodes($urls);

We create the FetchStatusCodes deferred object and get the promise. We setup the resolve and reject handler callbacks in the then method. They don’t do much for now:

  • the resolve handler dumps the value it got,
  • the reject handler prints out the reason of the rejection.

The output for a resolved promise would be something like this:

$ php promise.php
/home/robert/projects/react-promise-example/promise.php:32:
array(5) {
  'https://example.com/' => int(200)
  'https://stackoverflow.com/' => int(200)
  'https://www.google.rs/' => int(200)
  'https://www.google.com/no-such-url' => int(404)
  'https://www.google.com:81/' => int(0)
}

We’re not done yet!

The example above where we call the then method to set up our resolve/reject handlers, isn’t quite correct. Why?

When we call the then method it actually returns a new Promise. This feature of the Promises/A specification allows us to chain promises together.

On this second promise we can again set up our resolve/reject handlers calling the then method on it, same as we do for our first promise. The resolve handler of the second promise will be called with the return value of either the resolve or the reject handler of the first promise. The reject handler of the second promise will be called when either the resolve or the reject handler of the first promise throws an exception. And the then method of our second promise again returns a new, third promise.

Let’s see if an example makes it a bit more clearer:

promise.php

<?php

$statusCodes = new FetchStatusCodes();
$firstPromise = $statusCodes->promise();

$secondPromise = $firstPromise->then(
    function($statusCodes) {
        $successCodes = array_filter($statusCodes, function ($code) {
            if ($code >= 200 && $code < 300) {
                return true;
            }
            return false;
        });
        return $successCodes;
    },
    function($reason) {
        // handle rejected promise
        // gets called when Deferred gets reject-ed
    }
);

$thirdPromise = $secondPromise->then(
    function ($successCodes) {
        return json_encode($successCodes);
    },
    function ($reason) {
        // handle rejected promise
        // gets called when $firstPromise handlers throw an exception
    }
);

$urls = [
    'https://example.com/',
    'https://stackoverflow.com/',
    'https://www.google.com/',
    'https://www.google.com/no-such-url',
    'https://www.google.com:81'
];
$statusCodes($urls);

When our FetchStatusCodes deferred resolves, it will call the resolve handler of the $firstPromise. In that first resolve handler we get only the successful status codes and return them.

With this return from the resolve handler of the first promise, we “trigger” the resolve handler of the $secondPromise where we can, for example, json_encode our success codes. By returning this JSON string from the resolve handler of the second promise, we again “trigger” the resolve handler of the $thirdPromise, and so on.

Almost done!

When we call then, we make a new promise.

To actually be done with all the promises, we need to call the done method on the last promise in our chain. With done we stop making promises and use the result of our last promise:

promise.php

<?php
$thirdPromise->done(
    function ($jsonString) {
        echo $jsonString . PHP_EOL;
    },
    function ($reason) {
        // handle rejected promise
        // gets called when $secondPromise handlers throw an exception
    }
);

If we’d run the example now, we’d get something like this:

$ php promise.php | json_pp
{
   "https://example.com/" : 200,
   "https://www.google.rs/" : 200,
   "https://stackoverflow.com/" : 200
}

We additionally pipe the output of our example script to json_pp to pretty print the JSON string.

Now we’re done

ReactPHP promises have an ExtendedPromisesInterface that include additional shortcut and utility methods that are not part of the Promise/A specification. Their docs include some more examples, and Cees-Jan Kiewiet looks at examples using the react/dns component, among other things.

When we deal with asynchronous code in PHP, using ReactPHP promises gives us a way to deal with it in a much nicer, saner way.

Happy hackin’!

PHP FPM slow log

published on November 23, 2017.

The other day I was going through the configuration file for php-fpm, when I noticed a configuration directive I haven’t before: slowlog. I guess it’s been around for a while, I just never noticed it.

The php-fpm slow log is a pool configuration, meaning that we configure it in www.conf, and has two directives for it:

  • the slowlog, which is a path to a file where the slow requests will be logged,
  • and request_slowlog_timeout is a time unit after which PHP will dump a backtrace for that request in to the slow log file. We can configure it to be in seconds, minutes, hours, or days.

What’s in the box backtrace?

It has the date and time for when the slow request happened, the pool and PID for the php-fpm process. script_filename is the entry point to the request, and the backtrace includes a list of function calls up until the moment when the request_slowlog_timeout was hit.

[23-Nov-2017 15:28:21]  [pool www] pid 8992
script_filename = /var/www/example/web/app_dev.php
[0x00007efe32a14a40] sleep() /var/www/example/src/AppBundle/Controller/DefaultController.php:18
[0x00007efe32a149d0] indexAction() /var/www/example/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/HttpKernel.php:153
[0x00007efe32a14960] call_user_func_array() /var/www/example/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/HttpKernel.php:153
[0x00007efe32a14470] handleRaw() /var/www/example/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/HttpKernel.php:68
[0x00007efe32a14320] handle() /var/www/example/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/Kernel.php:169
[0x00007efe32a14250] handle() /var/www/example/web/app_dev.php:29

Even though it doesn’t reveal too much, together with other profiling tools, like Xdebug and kcachegrind, it can help us a great deal on finding and fixing performance problems in web applications.

Happy hackin’!

Visual sudo for shell scripts

published on November 06, 2017.

The other day I was putting together a small shell script to do some administrative tasks on my Fedora workstation.

Even though I spend most of my time in a terminal, I wanted to have this script available from “everywhere”, that is to have it available to run it as a keyboard shortcut.

The script requires sudo privileges, and up until now, I thought that the only way to get sudo was from the terminal.

But then I remembered that some applications, like firewall-config, ask for the sudo password via a pop-up window. Surely it’s available to whatever application needs it, right?

The answer is pkexec.

To quote the man pages:

pkexec allows an authorized user to execute program as another user. If program is not specified, the default shell will be run. If username is not specified, then the program will be executed as the administrative super user, root.

Looks like that this pkexec is a part of, or at least relates to, something called Polkit. I honestly don’t understand that part yet, and what does it really do. Need to learn more about it, but as this is the first time I came across it, the learning more about it thing might not happen soon.

To make the shell script ask for the sudo password through pkexec we add it to the she-bang line:

#!/usr/bin/pkexec /bin/bash

touch /some/path/requiring/permissions.txt

Now when we run this script either through the terminal, or through an application finder/launcher applet, or by invoking it with a keyboard shortcut, it’ll ask for the sudo password first with the pop-up window.

Happy hackin’!

Tags: sudo, shell, pkexec, gksudo, fedora.
Categories: Development, Software.

What implements an interface

published on November 02, 2017.

Creating and implementing interfaces in our code is important. It helps with swapping out components, eases testing, separates the what from the how.

But, it’s not enough just to slap an interface on a class and be done with it.

We also need to consider on what are we putting that interface on.

An example

Say, we’re creating a queuing system for an RSS feed reader. We can tell the queue to queue the feed URLs. Depending on our needs, we can use something like RabbitMq, or a database, to use as a queuing mechanism.

We haven’t decided on that yet, but either way, we start with an interface for this imaginary queue:

<?php declare(strict_types=1);

namespace Example\Infrastructure\Queue;

use Example\Domain\Rss\FeedUrl;

interface FeedUrlQueue
{
    public function add(FeedUrl $feedUrl);
}

By having this nice little interface, we can TDD the part of the code that will use an implementation of this interface.

After a while we decide we’ll go with a database queuing mechanism first, so we create an implementation for the FeedUrlQueue interface:

<?php declare(strict_types=1);

namespace Example\Infrastructure\Storage\Database;

use Example\Domain\Rss\FeedUrl;

class FeedUrlTable extends AbstractTable implements FeedUrlQueue
{
    public function add(FeedUrl $feedUrl)
    {
        $qb = $this->getQueryBuilder();

        $query = $qb->insert('feed_urls')
            ->values(
                [
                    'url' => '?',
                ]
            )
            ->setParameter(0, (string) $feedUrl);

        $query->execute();
    }
}

That’s nice! We have an interface, a concrete implementation, and the possibility to write new implementations and swap them out with existing ones with little effort.

Job well done.

Is it done, let alone well?

Sure it is, I repeat, we have an interface, a concrete implementation, and the possibility to write new implementations and swap them out with existing ones with little effort.

Something’s fishy

There’s three things that stand out for me here, telling me that something is not quite right with this code.

First, a class that represents a Table, also is a FeedUrlQueue. It really shouldn’t be two things at the same time. It either should be a queue, or a table, most certainly not both.

Second, a class whose only responsibility should be to store an URL into a database, no matter from where that URL comes from, is now limited to store feed URLs that come from the queue. OK, this may, or may not be, a legitimate limitation we decided on.

And third, it is also responsible to figure out how can it transform a FeedUrl domain object into a string that can be stored in the database. Does it have a __toString magic method, so we can cast it to a string? Or maybe it’s legacy code so it has one of those toString() method which we need to call? We don’t know without looking.

Killing three giants with one stone

A better, a correct way, would be to have something like a DatabaseFeedUrlQueue that implements the FeedUrlQueue, and uses the FeedUrlTable:

<?php declare(strict_types=1);

namespace Example\Infrastructure\Queue;

use Example\Domain\Rss\FeedUrl;

class DatabaseFeedUrlQueue implements FeedUrlQueue
{
    protected $table;

    public function __construct(FeedUrlTable $table)
    {
        $this->table = $table;
    }

    public function add(FeedUrl $feedUrl)
    {
        $payload = [
            'url' => (string) $feedUrl
        ];
        $this->table->save($payload);
    }
}

and the FeedUrlTable becomes something like this:

<?php declare(strict_types=1);

namespace Example\Infrastructure\Storage\Database;

class FeedUrlTable extends AbstractTable
{
    public function save(array $payload)
    {
        $qb = $this->getQueryBuilder();

        $query = $qb->insert('feed_urls')
            ->values(
                [
                    'url' => '?',
                ]
            )
            ->setParameter(0, $payload['url']);

        $query->execute();
    }
}

By refactoring the code like this, we pretty much fix all three problems at once:

  • a DatabaseFeedUrlQueue is a FeedUrlQueue, and the FeedUrlTable can stop being two things at once;
  • there’s a clearer separation of concerns, the DatabaseFeedUrlQueue is responsible to create the payload, and FeedUrlTable is responsible to store it;
  • the storage layer knows nothing about our domain objects and how to use them.

Yes, now we have one more class to maintain, but the overall maintainability, I believe, is reduced, as it is much clearer what each class does.

Happy hackin’!

Smarter tag search in Vim

published on November 01, 2017.

As part of my Vim setup for PHP development, I use the vim-php-namespace plugin to add use statements in my PHP code.

vim-php-namespace uses the tags file to find the class and the namespace it belongs to, and then adds it to the rest of the use statements.

It all works great, but there are times when it shows too much possibilities.

For example, when I want to import the namespace for the Transaction class, it finds the correct Transaction class, but it also finds functions called transaction in my codebase, and then gives me a choice what I want to import:

See? One class (kind c), and two functions (kind f).

I could exclude functions from being generated in tag files, but that’s not really an option because there are times when I need the functions tags.

I dove into the vim-php-namespace source code, determined to get rid of this “functionality”.

Turns out the plugin actually uses a Vim command, called ptjump, to search the tags file and show the preview window, so the user can pick out the correct tag in case there’s more than one.

Of course there’s an option for that

Then I started reading the help pages for tags in more detail, and after a while I found the answer: tagcase.

To quote the help file:

This option specifies how case is handled when searching the tags file.

And it has the following options:

  • followic Follow the ‘ignorecase’ option
  • followscs Follow the ‘smartcase’ and ‘ignorecase’ options
  • ignore Ignore case
  • match Match case
  • smart Ignore case unless an upper case letter is used

I’ve set it to smart and, well, now it does what I want it to do:

set tagcase=smart

It correctly finds only one match for the Transaction class and the plugin inserts the use statement for it. Yey!

Happy hackin’!

Tags: php, vim, tags, namespace, plugin.
Categories: Development, Software.
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