Data Collection and Web Crawling

Overview Data intensive applications are likely to powered by some databases. How do you get the data in your database? – Your private secret data source – Public data from Internet In this tutorial, we will introduce how to collect data from Internet. – Use APIs – Web Crawlers

Collecting data from Internet: Use APIs The easiest way to get data from the Internet. Steps: – 1. Make sure the data source provide APIs for data collection. – 2. Obtain API key or other forms of authorization. – 3. Read documentation – 4. Coding

Collecting data from Internet: Use APIs Example: Twitter Search API 1. Make sure the data source provide APIs for data collection. – “Search API is focused on relevance and not completeness” – “Requests to the Search API, hosted on search.twitter.com, do not count towards the REST API limit. However, all requests coming from an IP address are applied to a Search Rate Limit. The Search Rate Limit isn't made public to discourage unnecessary search usage and abuse, but it is higher than the REST Rate Limit. We feel the Search Rate Limit is both liberal and sufficient for most applications and know that many application vendors have found it suitable for their needs.”

Collecting data from Internet: Use APIs 2. Obtain API key or other forms of authorization. – Read through devtwittercom and get them devtwittercom 3. Read documentation Found a Java implementation of Twitter API and read some documentation files and sample codes at

Collecting data from Internet: Use APIs 4. Coding Code based on the documentation and code samples. Refer to our sample code (DataCollection/TweetsCollector.java)

Collecting data from Internet: Web Crawlers However, other providers hosting the data you are interested in may not provide API for you. – Example case: You want all movies’ information from IMDB, but IMDB doesn’t provide API for programmers. – e.g. You want all the movie information found at a starting page You need to develop your own crawler. Prerequisite: HTTP Client and Regular Expression

Collecting data from Internet: Web Crawlers After browsing the website, you find out that each movie’s information can be found at where *****=movie id Pseudo Code: extract the movie ids from the starting page for each id in {ids} access store page content in dhttp:// obtain movie’s title t, year y, storyline s store (id, t,y,s) in database

Collecting data from Internet: Web Crawlers Selected Useful Java methods: Read html files: Regex that finds specific patterns in a text: Wait for several seconds to reduce the risks of being detected and banned URLConnection conn = new URL(String url).openConnection().getInputStream(); //Returns an InputStream object that contains the source html content for url. Matcher m=Pattern.compile(Stirng regex).matcher(String source_text); while (m.find()){String result=m.group(i)}; //From in source_text, find string(s) that matches the pattern specified by regex; //Then store the i th parenthesis group in regex. Thread.sleep((long) (1000*Math.random()*k)); //wait for 0~k seconds.

Regular Expression Regex - An advanced search. – “Normal search” only deals with finding fixed character sequences. – Regex can handle various patterns. An interactive tutorial: – A place to quickly test a written regex against a source text: –

Regular Expression The most useful ones for web crawlers: (.*?) match everything surrounded by

Example Stars: Ben Ziegler, Glenna Hill, Jason Woolfolk | See full cast and crew » html content:

Example Match the three names surrounded by tags – (.*?)

Example Convert this regex into Java expression: – we use \\d instead of \d in order to escape the escape character “\”.\\d – () controls the group to be extracted. – Feel the difference: What if we use (.*) instead of (.*?) ? Matcher m=Pattern.compile("(?mis) (.*?) ").matcher(html_content); while (m.find()){System.out.println(“name: ”+m.group(1)); Matcher m=Pattern.compile("(?mis) (.*?) ").matcher(html_content); while (m.find()){System.out.println(“name: ”+m.group(2));

Collecting data from Internet: Web Crawlers A complete sample code is provided in DataCollection/MovieSpider.java

Summary ProsCons Third party APIsConvenient, easy to use safe, won’t be blocked Fast Need to manage API keys Inflexible Limit on access Your own web crawlersVery flexible. Theoretically, you can collect anything you find. A lot of coding May be blocked