Scraping a website from a windows service

Question

Hi there.  I have a windows forms application that scrapes a website to retrieve some data.  I would like to implement the same functionality as a windows service.  The reason for this is to allow the program to run 24/7 without  having a user signed in. 

To that end, my current version of the program uses a web browser control (system.windows.forms.webbrowser) to navigate the pages, click the buttons, allow scripts to do their thing, etc.  I cannot figure out a way to do the same without the web browser control, but the web browser control cannot be instantiated in a windows service (because there is no user interface in a web service).

Does anyone have any brilliant ideas on how to get around this?

Thank you very much!

Answers

Hi Andy,

There is a tool which could let you manipulate anything you want on the website. This agile HTML parser builds a read/write DOM and supports plain XPATH or XSLT. It is a .NET code library that allows you to parse "out of the web" HTML files. The parser is very tolerant with "real world" malformed HTML. The object model is very similar to what proposes System.Xml, but for HTML documents (or streams). More information, please check:

http://htmlagilitypack.codeplex.com/

Have a nice day.

Best regards

All replies

You are not telling if you are using a .NET Express edition or not

You are not telling which Framework

You are not realy saying what data you are getting from the web site.

So

I made an example of service that work on any Studio edition (including the Express)

to install it, I supposed that you have at least the Framework2, so you will use something similar to:

    %SystemRoot%\Microsoft.NET\Framework\v2.0.50727\installutil /i C:\Test\MyWindowService\MyWindowService\bin\Release\MyWindowService.exe

In the example, I supposed that you are downloading some file from the site

You will need a reference to Windows.Form for the timer


Imports System.ServiceProcess

Imports System.Configuration.Install

Public Class WindowsService : Inherits ServiceBase

  Private Minute As Integer = 60000

  Private WithEvents Timer As New Timer With {.Interval = 30 * Minute, .Enabled = True}

  Public Sub New()

    Me.ServiceName = "MyService"

    Me.EventLog.Log = "Application"

    Me.CanHandlePowerEvent = True

    Me.CanHandleSessionChangeEvent = True

    Me.CanPauseAndContinue = True

    Me.CanShutdown = True

    Me.CanStop = True

  End Sub


  Private Sub Timer_Tick(ByVal sender As Object, ByVal e As System.EventArgs) Handles Timer.Tick

    If IO.File.Exists("C:\MyPath.Data") Then IO.File.Delete("C:\MyPath.Data")

    My.Computer.Network.DownloadFile("http://MyURL.com", "C:\MyPath.Data", "MyUserName", "MyPassword")

    'Do Something with the data downloaded

  End Sub

End Class

<Microsoft.VisualBasic.HideModuleName()> _

Module MainModule

  Public TheServiceName As String

  Public Sub main()

    Dim TheServiceApplication As New WindowsService

    TheServiceName = TheServiceApplication.ServiceName

    ServiceBase.Run(TheServiceApplication)

  End Sub

End Module

<System.ComponentModel.RunInstaller(True)> _

Public Class WindowsServiceInstaller : Inherits Installer

  Public Sub New()

    Dim serviceProcessInstaller As ServiceProcessInstaller = New ServiceProcessInstaller()

    Dim serviceInstaller As ServiceInstaller = New ServiceInstaller()

    serviceProcessInstaller.Account = ServiceAccount.LocalSystem

    serviceProcessInstaller.Username = Nothing

    serviceProcessInstaller.Password = Nothing

    serviceInstaller.DisplayName = "My Windows Service"

    serviceInstaller.StartType = ServiceStartMode.Automatic

    serviceInstaller.ServiceName = TheServiceName

    Me.Installers.Add(serviceProcessInstaller)

    Me.Installers.Add(serviceInstaller)

  End Sub

End Class



 Hello Andy,

Thanks for your post.

What do you want to scrape from the page? HttpWebRequest class ans WebClient class may be what you need. More information, please check:

The HttpWebRequest class provides support for the properties and methods defined in WebRequest and for additional properties and methods that enable the user to interact directly with servers using HTTP.

http://msdn.microsoft.com/en-us/library/system.net.httpwebrequest.aspx

The WebClient class provides common methods for sending data to or receiving data from any local, intranet, or Internet resource identified by a URI

http://msdn.microsoft.com/en-us/library/system.net.webclient.aspx

If you have any concenrs, please feel free to follow up.

Best regards



Hi Andy,

What about this problem on your side now? If you have any concerns, please feel free to follow up.

Have a nice day.

Best regards



Hi Andy,

When you come back, if you need further assistance about this issue, please feel free to let us know. We will continue to work with this issue.

Have a nice day.

Best regards



Thank you for the reply. Sorry it has taken me so long to respond.  I did not receive any notification that someone had replied!

I am using Visual Studio 2010 Ultimate Edition and the .NET framework 4.0.  Actually, I am upgrading some old code written in VB 6.0, but I can use the latest and greatest thats available.

The application uses a browser control to go to the page, fill in values, click on UI elements, read the HTML that returns, etc.  The purpose of the application is to collection useful information regularily/automatically.

I know how to create a web service, but using the web control in such a service is problematic because the web browser control was meant to be placed on a windows form.  I am not able to create a new instance of it in a project designated as a windows service.



Andy

Thank you for the reply. Sorry it has taken me so long to respond.  I did not receive any notification that someone had replied!

I thought a web request was for web services (retrieving information from them).  I am trying to retreive useful information from a website designed for interaction by a human, such as selecting items from lists and clicking buttons.   I currently use a web browser control to programmatically do what a person would do and get the pages back which in turn get parsed.

Andy



Hi Andy,

There is a tool which could let you manipulate anything you want on the website. This agile HTML parser builds a read/write DOM and supports plain XPATH or XSLT. It is a .NET code library that allows you to parse "out of the web" HTML files. The parser is very tolerant with "real world" malformed HTML. The object model is very similar to what proposes System.Xml, but for HTML documents (or streams). More information, please check:

http://htmlagilitypack.codeplex.com/

Have a nice day.

Best regards



Thanks for the suggestion.  I will go to that link and see if it will work.  I will update this post with what I find.

I am writing to check the status of the issue on your side. Would you mind letting us know the result of the suggestions? If you have any concerns, please feel free to follow up.

Have a nice day.



Best regards

Hi Liliane

Thank for the follow up reply.  I don't have an answer as of yet.  Implementing this is going to take time and I haven't been given the go-ahead by my boss to spend the time to pursue it.



Hi Andy,

Never minde. You could have a try when you feel free. If you have any further questions about this issue, please feel free to let us know. We will continue to work with you on this issue.

Have a nice day.

Best regards

Source: https://social.msdn.microsoft.com/Forums/vstudio/en-US/f5d565b1-236b-43c2-90c7-f5cc3b2c341b/scraping-a-website-from-a-windows-service

I Don’t Need No Stinking API: Web Scraping For Fun and Profit

If you’ve ever needed to pull data from a third party website, chances are you started by checking to see if they had an official API. But did you know that there’s a source of structured data that virtually every website on the internet supports automatically, by default?

scraper toolThat’s right, we’re talking about pulling our data straight out of HTML — otherwise known as web scraping. Here’s why web scraping is awesome:

Any content that can be viewed on a webpage can be scraped. Period.

If a website provides a way for a visitor’s browser to download content and render that content in a structured way, then almost by definition, that content can be accessed programmatically. In this article, I’ll show you how.

Over the past few years, I’ve scraped dozens of websites — from music blogs and fashion retailers to the USPTO and undocumented JSON endpoints I found by inspecting network traffic in my browser.

There are some tricks that site owners will use to thwart this type of access — which we’ll dive into later — but they almost all have simple work-arounds.

Why You Should Scrape

But first we’ll start with some great reasons why you should consider web scraping first, before you start looking for APIs or RSS feeds or other, more traditional forms of structured data.

Websites are More Important Than APIs

The biggest one is that site owners generally care way more about maintaining their public-facing visitor website than they do about their structured data feeds.

We’ve seen it very publicly with Twitter clamping down on their developer ecosystem, and I’ve seen it multiple times in my projects where APIs change or feeds move without warning.

Sometimes it’s deliberate, but most of the time these sorts of problems happen because no one at the organization really cares or maintains the structured data. If it goes offline or gets horribly mangled, no one really notices.

Whereas if the website goes down or is having issues, that’s a more of an in-your-face, drop-everything-until-this-is-fixed kind of problem, and gets dealt with quickly.

No Rate-Limiting

Another thing to think about is that the concept of rate-limiting is virtually non-existent for public websites.

Aside from the occasional captchas on sign up pages, most businesses generally don’t build a lot of defenses against automated access. I’ve scraped a single site for over 4 hours at a time and not seen any issues.

Unless you’re making concurrent requests, you probably won’t be viewed as a DDOS attack, you’ll just show up as a super-avid visitor in the logs, in case anyone’s looking.

Anonymous Access

There are also fewer ways for the website’s administrators to track your behavior, which can be useful if you want gather data more privately.

With APIs, you often have to register to get a key and then send along that key with every request. But with simple HTTP requests, you’re basically anonymous besides your IP address and cookies, which can be easily spoofed.

The Data’s Already in Your Face

Web scraping is also universally available, as I mentioned earlier. You don’t have to wait for a site to open up an API or even contact anyone at the organization. Just spend some time browsing the site until you find the data you need and figure out some basic access patterns — which we’ll talk about next.

Let’s Get to Scraping

So you’ve decided you want to dive in and start grabbing data like a true hacker. Awesome.

Just like reading API docs, it takes a bit of work up front to figure out how the data is structured and how you can access it. Unlike APIs however, there’s really no documentation so you have to be a little clever about it.

I’ll share some of the tips I’ve learned along the way.

Fetching the Data

So the first thing you’re going to need to do is fetch the data. You’ll need to start by finding your “endpoints” — the URL or URLs that return the data you need.

If you know you need your information organized in a certain way — or only need a specific subset of it — you can browse through the site using their navigation. Pay attention to the URLs and how they change as you click between sections and drill down into sub-sections.

The other option for getting started is to go straight to the site’s search functionality. Try typing in a few different terms and again, pay attention to the URL and how it changes depending on what you search for. You’ll probably see a GET parameter like q= that always changes based on you search term.

Try removing other unnecessary GET parameters from the URL, until you’re left with only the ones you need to load your data. Make sure that there’s always a beginning ? to start the query string and a & between each key/value pair.

Dealing with Pagination

At this point, you should be starting to see the data you want access to, but there’s usually some sort of pagination issue keeping you from seeing all of it at once. Most regular APIs do this as well, to keep single requests from slamming the database.

Usually, clicking to page 2 adds some sort of offset= parameter to the URL, which is usually either the page number or else the number of items displayed on the page. Try changing this to some really high number and see what response you get when you “fall off the end” of the data.

With this information, you can now iterate over every page of results, incrementing the offset parameter as necessary, until you hit that “end of data” condition.

The other thing you can try doing is changing the “Display X Per Page” which most pagination UIs now have. Again, look for a new GET parameter to be appended to the URL which indicates how many items are on the page.

Try setting this to some arbitrarily large number to see if the server will return all the information you need in a single request. Sometimes there’ll be some limits enforced server-side that you can’t get around by tampering with this, but it’s still worth a shot since it can cut down on the number of pages you must paginate through to get all the data you need.

AJAX Isn’t That Bad!

Sometimes people see web pages with URL fragments # and AJAX content loading and think a site can’t be scraped. On the contrary! If a site is using AJAX to load the data, that probably makes it even easier to pull the information you need.

The AJAX response is probably coming back in some nicely-structured way (probably JSON!) in order to be rendered on the page with Javscript.

All you have to do is pull up the network tab in Web Inspector or Firebug and look through the XHR requests for the ones that seem to be pulling in your data.

Once you find it, you can leave the crufty HTML behind and focus instead on this endpoint, which is essentially an undocumented API.

(Un)structured Data?

Now that you’ve figured out how to get the data you need from the server, the somewhat tricky part is getting the data you need out of the page’s markup.

Use CSS Hooks

In my experience, this is usually straightforward since most web designers litter the markup with tons of classes and ids to provide hooks for their CSS.

You can piggyback on these to jump to the parts of the markup that contain the data you need.

Just right click on a section of information you need and pull up the Web Inspector or Firebug to look at it. Zoom up and down through the DOM tree until you find the outermost <div> around the item you want.

This <div> should be the outer wrapper around a single item you want access to. It probably has some class attribute which you can use to easily pull out all of the other wrapper elements on the page. You can then iterate over these just as you would iterate over the items returned by an API response.

A note here though: the DOM tree that is presented by the inspector isn’t always the same as the DOM tree represented by the HTML sent back by the website. It’s possible that the DOM you see in the inspector has been modified by Javascript — or sometime even the browser, if it’s in quirks mode.

Once you find the right node in the DOM tree, you should always view the source of the page (“right click” > “View Source”) to make sure the elements you need are actually showing up in the raw HTML.

This issue has caused me a number of head-scratchers.

Get a Good HTML Parsing Library

It is probably a horrible idea to try parsing the HTML of the page as a long string (although there are times I’ve needed to fall back on that). Spend some time doing research for a good HTML parsing library in your language of choice.

Most of the code I write is in Python, and I love BeautifulSoup for its error handling and super-simple API. I also love its motto:

    You didn’t write that awful page. You’re just trying to get some data out of it. Beautiful Soup is here to help. :)

You’re going to have a bad time if you try to use an XML parser since most websites out there don’t actually validate as properly formed XML (sorry XHTML!) and will give you a ton of errors.

A good library will read in the HTML that you pull in using some HTTP library (hat tip to the Requests library if you’re writing Python) and turn it into an object that you can traverse and iterate over to your heart’s content, similar to a JSON object.

Some Traps To Know About

I should mention that some websites explicitly prohibit the use of automated scraping, so it’s a good idea to read your target site’s Terms of Use to see if you’re going to make anyone upset by scraping.

For two-thirds of the website I’ve scraped, the above steps are all you need. Just fire off a request to your “endpoint” and parse the returned data.

But sometimes, you’ll find that the response you get when scraping isn’t what you saw when you visited the site yourself.

When In Doubt, Spoof Headers

Some websites require that your User Agent string is set to something they allow, or you need to set certain cookies or other headers in order to get a proper response.

Depending on the HTTP library you’re using to make requests, this is usually pretty straightforward. I just browse the site in my web browser and then grab all of the headers that my browser is automatically sending. Then I put those in a dictionary and send them along with my request.

Note that this might mean grabbing some login or other session cookie, which might identify you and make your scraping less anonymous. It’s up to you how serious of a risk that is.

Content Behind A Login

Sometimes you might need to create an account and login to access the information you need. If you have a good HTTP library that handles logins and automatically sending session cookies (did I mention how awesome Requests is?), then you just need your scraper login before it gets to work.

Note that this obviously makes you totally non-anonymous to the third party website so all of your scraping behavior is probably pretty easy to trace back to you if anyone on their side cared to look.

Rate Limiting

I’ve never actually run into this issue myself, although I did have to plan for it one time. I was using a web service that had a strict rate limit that I knew I’d exceed fairly quickly.

Since the third party service conducted rate-limiting based on IP address (stated in their docs), my solution was to put the code that hit their service into some client-side Javascript, and then send the results back to my server from each of the clients.

This way, the requests would appear to come from thousands of different places, since each client would presumably have their own unique IP address, and none of them would individually be going over the rate limit.

Depending on your application, this could work for you.

Poorly Formed Markup

Sadly, this is the one condition that there really is no cure for. If the markup doesn’t come close to validating, then the site is not only keeping you out, but also serving a degraded browsing experience to all of their visitors.

It’s worth digging into your HTML parsing library to see if there’s any setting for error tolerance. Sometimes this can help.

If not, you can always try falling back on treating the entire HTML document as a long string and do all of your parsing as string splitting or — God forbid — a giant regex.

Source: https://blog.hartleybrody.com/web-scraping/

What are the ethics of web scraping?

Someone recently asked: "Is web scraping an ethical concept?" I believe that web scraping is absolutely an ethical concept. Web scraping (or screen scraping) is a mechanism to have a computer read a website. There is absolutely no technical difference between an automated computer viewing a website and a human-driven computer viewing a website. Furthermore, if done correctly, scraping can provide many benefits to all involved.

There are a bunch of great uses for web scraping. First, services like Instapaper, which allow saving content for reading on the go, use screen scraping to save a copy of the website to your phone. Second, services like Mint.com, an app which tells you where and how you are spending your money, uses screen scraping to access your bank's website (all with your permission). This is useful because banks do not provide many ways for programmers to access your financial data, even if you want them to. By getting access to your data, programmers can provide really interesting visualizations and insight into your spending habits, which can help you save money.

That said, web scraping can veer into unethical territory. This can take the form of reading websites much quicker than a human could, which can cause difficulty for the servers to handle it. This can cause degraded performance in the website. Malicious hackers use this tactic in what’s known as a "Denial of Service" attack.

Another aspect of unethical web scraping comes in what you do with that data. Some people will scrape the contents of a website and post it as their own, in effect stealing this content. This is a big no-no for the same reasons that taking someone else's book and putting your name on it is a bad idea. Intellectual property, copyright and trademark laws still apply on the internet and your legal recourse is much the same. People engaging in web scraping should make every effort to comply with the stated terms of service for a website. Even when in compliance with those terms, you should take special care in ensuring your activity doesn't affect other users of a website.

One of the downsides to screen scraping is it can be a brittle process. Minor changes to the backing website can often leave a scraper completely broken. Herein lies the mechanism for prevention: making changes to the structure of the code of your website can wreak havoc on a screen scraper's ability to extract information. Periodically making changes that are invisible to the user but affect the content of the code being returned is the most effective mechanism to thwart screen scrapers. That said, this is only a set-back. Authors of screen scrapers can always update them and, as there is no technical difference between a computer-backed browser and a human-backed browser, there's no way to 100% prevent access.

Going forward, I expect screen scraping to increase. One of the main reasons for screen scraping is that the underlying website doesn't have a way for programmers to get access to the data they want. As the number of programmers (and the need for programmers) increases over time, so too will the need for data sources. It is unreasonable to expect every company to dedicate the resources to build a programmer-friendly access point. Screen scraping puts the onus of data extraction on the programmer, not the company with the data, which can work out well for all involved.

Source: https://quickleft.com/blog/is-web-scraping-ethical/