Almost every PHP developer has ever parsed some data from the Web. Often we need some data, which is available only on some website and we want to pull this data and save it somewhere. It looks like we open a browser, walk through the links and copy data that we need. But the same thing can be automated via script. In this tutorial, I will show you the way how you can increase the speed of you parser making requests asynchronously.
We are going to create a simple web scraper for parsing movie information from IMDB movie page:
Here is an example of the Venom movie page. We are going to request this page to get:
- release date
IMDB doesn’t provide any public API, so if we need this kind of information we have to scrap it from the site.
Why should we use ReactPHP and make requests asynchronously? The short answer is speed. Let’s say that we want to parse all movies from the Coming Soon page: 12 pages, a page for each month of the upcoming year. Each page has approximately 20 movies. So in common, we are going to make 240 requests. Making these requests one after another can take some time…
And now imagine that we can run these requests concurrently. In this way, the scraper is going to be significantly fast. Let’s try it.
Before we start writing the scraper we need to download the required dependencies via composer.
For traversing the DOM I’m going to use Symfony DomCrawler Component:
CSS-selector for DomCrawler allows to use jQuery-like selectors to traverse:
Now, we can start coding. This is our start:
We create an instance of the event loop and HTTP client. Next step is making requests.
Public interface of the client’s main
Clue\React\Buzz\Browser class is very straightforward. It has a set of methods named after HTTP verbs:
put() and so on. Each method returns a promise. In our case to request a page we can use
get($url, $headers = ) method:
The code above simply outputs the requested page on the screen. When a response is received the promise fulfills with an instance of
Psr\Http\Message\ResponseInterface. So, we can handle the response inside a callback and then return processed data as a resolution value from the promise.
Unlike ReactPHP HTTPClient,
clue/buzz-reactbuffers the response and fulfills the promise once the whole response is received. Actually, it is a default behavior and you can change it if you need streaming responses.
So, as you can see, the whole process of scraping is very simple:
- Make a request and receive the promise.
- Add fulfillment handler to the promise.
- Inside the handler traverse the response and parse the required data.
- If needed repeat from step 1.
The page that we need doesn’t require any authorization. If we look at the source of the page, we can see that all data that we need is already available in HTML. The task is very simple: no authorization, form submissions or AJAX-calls. Sometimes analysis of the target site takes several times more time than writing the scraper, but not this time.
After we have received the response we are ready to start traversing the DOM. And here Symfony DomCrawler comes into play. To start extracting information we need to create an instance of the
Crawler. Its constructor accepts HTML string:
Inside the fulfillment handler, we create an instance of the
Crawler and pass it a response cast to a string. Now, we can start using jQuery-like selectors to extract the required data from HTML.
The title can be taken from the
filter() is used to find an element in the DOM. Then we extract text from this element. This line in jQuery looks very similar:
Genres And Description
Genres are received as text contents of the appropriate links.
extract() is used to extract attribute and/or node values from the list of nodes. Here in
->extract(['_text']) statement special attribute
_text represents a node value. The description is also taken as a text value from the appropriate tag
Things become a little tricky with a release date:
As you can see it is inside
<div> tag, but we cannot simply extract the text from it. In this case, the release date will be
Release Date: 16 February 2018 (USA) See more ». And this is not what we need. Before extracting the text from this DOM element we need to remove all tags inside of it:
Here we select all
<div> tags from the Details section. Then, we loop through them and remove all child tags. This code makes our
<div>s free from all inner tags. To get a release date we select the fourth (at index
3) element and grab its text (now free from other tags).
The last step is to collect all this data into an array and resolve the promise with it:
Collect The Data And Continue Synchronously
Now, its time to put all pieces together. The request logic can be extracted into a function (or class), so we could provide different URLs to it. Let’s extract
It accepts an instance of the
Browser as a constructor dependency. The public interface is very simple and consists of two methods:
parse(array $urls)) and
getMovieData(). The first one does the job: runs the requests and traverses the DOM. And the seconds one is just to receive the results when the job is done.
Now, we can try it in action. Let’s try to asynchronously parse two movies:
In the snippet above we create a parser and provide an array of two URLs for scraping. Then we run an event loop. It runs until it has something to do (until our requests are done and we have scrapped everything we need). As a result instead of waiting for all requests in total, we wait for the slowest one. The output will be the following:
You can continue with these results as you like: store them to different files or save into a database. In this tutorial, the main idea was to show to make asynchronous requests and parse responses.
Our parser can be also improved by adding some timeout. What if the slowest request becomes too slow? Instead of waiting for it, we can provide a timeout and cancel all slow requests. To implement request cancellation we will use event loop timers. The idea is the following:
- Get the request promise.
- Create a timer.
- When the timer is out cancel the promise.
Now, we need an instance of the event loop inside our
Parser. Let’s provide it via constructor:
Then we can improve
parse() method and add optional parameter
If there is no provided
$timeout we use default
5 seconds. When the timer is out it tries to cancel the provided promise. In this case, all requests that last longer than 5 seconds will be cancelled. If the promise is already settled (the request is done) method
cancel() has no effect.
For example, if we don’t want to wait longer than 3 seconds the client code is the following:
A Note on Web Scraping: some sites don’t like being scrapped. Often scraping data for personal use is generally OK. But you should always scrap nicely. Try to avoid making hundreds of concurrent requests from one IP. The site may don’t like it and may block your scraper. To avoid this and improve your scraper read the next article about throttling requests.
You can find examples from this article on GitHub.
This article is a part of the ReactPHP Series.
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