CIS 455/555: Internet and Web Systems
Fall 2020
Homework 2: Web Crawling and Stream Processing
Milestone 1 due October 28, 2020, at 10:00pm ET
Milestone 2 due November 11, 2020, at 10:00pm ET
1. Background
In this assignment, you will explore several more Web technologies and continue to build components
useful towards your course project, by building a topic-specific Web crawler. A topic-specific crawler
looks for documents or data matching certain phrases or content in the header or body.
This assignment will entail, in Milestone 1:
● Expanding a dynamic Web application, which runs on the framework you built for Assignment 1
(or Spark Framework) and allows users to (1) create user identities, (2) topic-specific "channels"
defined by a set of XPath expressions, and (3) to display documents that match a channel;
● Implementing and expanding a persistent data store (using Oracle Berkeley DB) to hold retrieved
HTML/XML documents and channel definitions.
● Fleshing out a crawler that traverses the Web, looking for HTML and XML documents that match
one of the patterns.
In Milestone 2:
● Refactoring the crawler to fit into a stream processing system’s basic abstractions;
● Routing documents from the crawler through a stream engine for processing one at a time;
● Writing a pattern matcher that determines if an HTML or XML document matches one of a set of
patterns;
The resulting application will be very versatile, able to filter documents into categories with specified
keywords. Assignment 2 can build on your application server from Assignment 1. However, if you are not
confident that your web server is working well, please use Spark Framework (http://www.sparkjava.com/)
to test your application. (Using your own server will earn you +5% extra credit. You are allowed to make
fixes to it as necessary.)
2. Developing and running your code
You should fork, clone, and import the framework code for HW2 using the same process as for HW0 and
HW1 (fork from ssh://git@bitbucket.org/upenn-cis555/555-hw2.git to your own private repository, then
clone from your repository to your VirtualBox/Vagrant instance). You should, of course, regularly commit
code as you make changes so you can revert; and you should periodically push to your own repository on
bitbucket, in case your computer crashes.
Initially you will be using the Spark Framework for this assignment, but for extra credit you can run it using
your own HW1 framework (see Section 6).
Carefully read the entire assignment (both milestones) from front to back and make a list of the features
you need to implement. There are many important details that are easily overlooked! Spend at least some
time thinking about the design of your solution. What classes will you need? How many threads will there
be? What will their interfaces look like? Which data structures need synchronization? And so on.
We strongly recommend that you regularly check the discussions on Piazza for clarifications and solutions
to common problems.
3. Milestone 1: Crawler Manager, B+ Tree Storage, and Crawler
For the first milestone, your task is to crawl and store Web documents. These will ultimately be fed into
a pattern matching engine for Milestone 2.
3.1 Routes-Based Web Interface / Crawler Manager
In preparation for Milestone 2, we will have a Web interface for login. Your main class should be called
edu.upenn.cis455.crawler.WebInterface and it should register routes for various behaviors. We
have given you a partial implementation of the login handler so you can get started.
1. If the user makes a request for a page and is not logged in (has no Session), the server should output
the login form, login-form.html. This form should submit a POST message as described below
under “log in to existing account.”
2. If the user is logged in (has a session), requests to the root URL (or /index.html) should present a
simple login page showing “Welcome ” followed by the user’s username. (We will add more
functionality in Milestone 2.)
You will see in the provided code how Filters allow you to make decisions about whether the user’s request
should proceed, or the user should be redirected to the login form. So some of the above should already be
present.
Beyond the above, you should build additional routes to support the following functions:
● Create a new account. This should take a POST to URL localhost:45555/register with
two parameters, username and password. Upon success it should return an appropriate success
code with a link to the main page and its login screen. On failure, it should return an appropriate
error. ● Log into an existing account (multiple users should be able to log in at the same time). This
should take a POST to URL localhost:45555/login with two parameters, username and
password. Upon success it should create a new Session with the user’s info and return the user
to the main page, which should show the logged-in info as above. The Session should time out
after 5 minutes of inactivity.
● Log out of the account that is currently logged in, via /logout. Upon success it should redirect
the user to the login page.
Note that you may take advantage of Sessions and the various other capabilities of the Spark Framework
and/or the ones you developed in Homework 1 Milestone 2. Some of the above functionality is provided,
so please look carefully to understand what is and is not there.
3.2. Storage of Document and User Credentials in a B+ Tree
We will use Berkeley DB Java Edition (http://www.oracle.com/technetwork/database/berkeleydb/
overview/index.html), which may be downloaded freely from their website, to implement a disk-backed
data store. Berkeley DB is a popular embedded database, and it is relatively easy to use as a key-value
store; there is ample documentation and reference information available on its Web site, e.g., the Getting
Started Guide. Berkeley DB has several different ways of storing and retrieving data; perhaps the simplest
is through the Java Collections interface.
Your store will hold (at least):
● the usernames and encrypted passwords of registered users (see below),
● (in Milestone 2) information about user channels
● and the raw content of HTML or XML files retrieved from the Web, as well as the time the file
was last checked by the crawler.
If you use the Collections interface, you will create objects, representing your data, that extend
java.io.Serializable and store them in objects like StoredSortedMaps. User passwords should
instead be saved using SHA-256 hashing. No cleartext passwords should be saved.
The WebInterface program, when run from the command-line, should take as the first argument a path for
the BerkeleyDB data storage instance, and as a second argument, a path to your static HTML files. You
should create a data storage directory if it does not already exist.
3.3. Basic Web Crawler
Your web crawler will initially be a Java application that can be run in Eclipse by creating a Run
Configuration (as in HW0) with the goal “clean install exec:java@crawler”. From the command
line, you can also run mvn exec:java@crawler. In both cases, the crawler will take the following
command-line arguments (in this specific order, and the first three are required):
1. The URL of the Web page at which to start. Note that there are several ways to open the URL.
a. For plain HTTP URLs you will probably get the best performance by just opening a socket
to the port (we’ve provided the URLInfo class to help parse the pieces out). It is also
acceptable to use Java’s HTTPUrlConnection.
b. For HTTPS URLs you may want to use java.net.URL’s openConnection() method
and cast to javax.net.ssl.HttpsURLConnection. This in turn has input and output
streams as usual. Here you can keep relying on the sample code.
2. The directory containing the BerkeleyDB database environment that holds your store (this will
match the path the WebInterface takes). As above, the directory should be created if it does not
already exist. Your crawler should recursively follow links from the page it starts on.
3. The maximum size, in megabytes, of documents to be retrieved from a Web server
4. An optional argument indicating the number of files (HTML and XML) to retrieve before
exiting. This will be useful for testing!
The crawler is intended to be run periodically, either by hand or from an automated tool like the cron
command. It is, therefore, not necessary to build a connection from the Web interface to the crawler,
except that the two will share a common database. Note also that BerkeleyDB does not like to share
database instances across concurrent programs so it’s okay to assume only one runs at a time.
The crawler traverses links in HTML documents. You can extract these using a HTML parser, such as
JSoup (https://jsoup.org/, included with your Maven package), or simply by searching the HTML document
for occurrences of the pattern href="URL" and its subtle variations.
If a link points to another HTML document, it should be retrieved and scanned for links as well. The same
is true if it points to an XML or RSS document. Don't bother crawling images or trying to extract links
from XML files. All retrieved HTML and XML documents should be stored in the BerkeleyDB database
(so that the crawler does not have to retrieve them again if they do not change before the next crawl). The
crawler must be careful not to search the same page multiple times during a given crawl, and it should
exit when it has no more pages to crawl. You’ll need to understand what parts of functionality are provided
and where you need to supplement.
Redundant documents and cyclic crawls: Your crawler should compute an MD5 hash of every
document that is fetched, and store this in a “content seen” table in BerkeleyDB. If you crawl a
document with a matching hash during the same crawler run, you should not index it or traverse its
outgoing links.
When your crawler is processing a new HTML or XML page, it should print a short status report to the
Apache Log4J logger, using the “info” status level. At the very least, you should print:
"http://xyz.com/index.html: downloading" (if the page is actually downloaded) or "http://abc.com/def.html:
not modified" (if the page is not downloaded because it has not changed). Make sure you follow the above
format to comply with the autograder’s assumptions.
3.4. Politeness
Your crawler must be a considerate Web citizen. First, it must respect the robots.txt file, as described in
A Standard for Robot Exclusion (http://www.robotstxt.org/robotstxt.html). It must support the CrawlDelay directive (see http://en.wikipedia.org/wiki/Robots.txt) and "User-agent: *", but it need not support
wildcards in the Disallow: paths. Second, it must always send a HEAD request first to determine the type
and size of a file on a Web server. If the file has the type text/html or one of the XML MIME types:
● text/xml
● application/xml
● Any mime types that ends with +xml
and if the file is less than or equal to the specified maximum size, then the crawler should retrieve the file
and process it; otherwise it should ignore it and move on. For more details on XML media types, see RFC
3023 (http://www.ietf.org/rfc/rfc3023.txt). Your crawler should also not retrieve the file if it has not been
modified since the last time it was crawled, but it should still process unchanged files (i.e., match them
against XPaths and extract links from them) using the copy in its local database.
We have given you some “helper” classes in the crawler.utils package, which might be useful to store
information about URLs and robots.txt.
Certain web content, such as the papers in ACM's Digital Library, normally costs money to download but
is free from Penn's campus network. If your crawler accidentally downloads a lot of this content, this will
cause a lot of trouble. To prevent this, you must send the header User-Agent: cis455crawler in each
request.
3.5. Test cases
You must develop at least two JUnit tests for storage system and two more for the crawler.
You should next add a Route to enable retrieval of documents from the BDBstore, using the following
form:
localhost:45555/lookup?url=…
that takes a GET request with a URL as parameter url, and returns the stored document corresponding to
that URL. Think of this as the equivalent of Google’s cache. If the document was not crawled, your server
should return a 404 error. We will use this interface for testing.
3.6. Milestone 1 Submission
Submit a zip file on Canvas as before.
4. Milestone 2: Streaming Crawler and Matching Engine
The next milestone will consist of an evaluator for streams of document content. You will extend your
Milestone 1 project to run on a stream processing engine that enables multithreaded execution. You will
also extend your application and storage system to enable users to register “subscription channels” with the
system. Finally, you will build a stream processing component that checks documents against the various
“channels” and outputs results per user.
4.1 Rework the Crawler as a “Spout,” “Bolt,” and Shared Modules in
StormLite
Your Milestone 1 project had a simple execution model, in which you controlled the execution of the
crawler and presumably did this in a crawler loop. Now we want to break in into a smaller unit of work
that can be parallelized.
To do this, we’ll be using a CIS 455/555-custom emulation of the Apache Storm stream engine, which we
call StormLite (it should show up in your HW2 repo already). Please see the document on StormLite and
see TestWordCount (in the test directory) as an example of a multithreaded stream job. Storm has spouts
that produce data one result at a time, and bolts that process data one result at a time. As with our emulation
of the Spark Framework, you should be able to use examples of Apache Storm code to understand how
StormLite works.
You should refactor your Milestone 1 crawl task to run within StormLite, as follows. Note that StormLite
supports multiple worker threads but you can control the number of “executors” (and start with 1).
1. You will maintain (or update) your frontier/crawl queue of URLs from Milestone 1. However, you
want to place it in a “spout” (implement IRichSpout) so it sends URLs one at a time to the crawler
via the nextTuple interface.
2. You will maintain your BerkeleyDB storage system from Milestone 1. This will also be a shared
object, at least across some aspects of your Milestone 2 implementation. Again, you may want to
use a “singleton factory” pattern.
3. The crawler should be placed in a bolt – its execute method gets called once for each URL from
the crawler queue. The crawler should output documents one at a time to its output stream. See
the IRichBolt interface and the example code.
4. Now, in our suggested (but not mandatory) architecture, there should be two “downstream” bolts
that take documents. (It is perfectly possible to send an output stream to two destinations.)
5. Lower branch:
a. One boltshould have an execute method that takes a document, writes it to the BerkeleyDB
storage system, and outputs a stream of extracted URLs.
b. Next, there should be a bolt that filters URLs (using appropriate techniques and data
structures) and updates the shared frontier queue.
6. Upper branch:
a. A second (in parallel) bolt should take a document and parse it using JSoup or another
parser that takes into account element structure. It should send streams of
OccurrenceEvents. This bolt will traverse the entire DOM tree in the DOM, using a
standard tree traversal. You will send an OccurrenceEvent each time you traverse to an
element node from its parent (ElementOpen), each time you traverse to a text node (Text),
or each time you traverse back up to an element node from its children (ElementClose).
b. Finally, there should be a bolt that checks for matches to channels by using the streams
of events. When there is a match, it should update the BerkeleyDB store accordingly.
Based on the sample code in test.upenn.cis.stormlite, the StormLite document, and (in fact) the
documentation on Storm available from Stack Overflow and the Web, you should be able to assemble a
stream dataflow like the one illustrated above.
You can assume a single crawler queue bolt, but should look at how the fieldGrouping, allGrouping, and
shuffleGrouping specifiers allow you to specify how data gets distributed when there are multiple
“executors” such as multiple copies of the crawler, parser, etc.
4.2 Extended Routes-based Web Interface
For Milestone 2, you will also enhance the Web application to support the following functions for logged
in users.
4.2.1. Channels
Now that you have users and HTML or XML, we want to “connect” users with “interesting” content. To
do this, any logged in user will be able to create channels. A channel is a named pattern describing a class
of documents. An example of a channel definition would be
sports : /rss/channel/title[contains(text(),”sports”)]
and you can see an example of content that would match the channel at:
http://rss.nytimes.com/services/xml/rss/nyt/Sports.xml
Assume that channels and their names are global.
You should implement an interface to create a channel, as a GET call:
localhost:45555/create/{name}?xpath={xpath-pattern}
4.2.2. Updated Login Screen
As before, you should have a login/registration form at localhost:45555/register. Once a user is
logged in, you should have a “home page” at localhost:45555/. ● List all channels available on the system, and for each
● Include a link to the documents matching each channel, which triggers the application at
localhost:45555/show?channel={name}
Obviously, you will need to add some logic to the BerkeleyDB storage system to store user subscriptions
and to store which documents correspond to a channel (see Section 3.2 for how this will be populated).
How you implement most of the functionality of the Web interface is entirely up to you; we are just
constraining the URL interfaces. To make things consistent across assignments, we are specifying how the
channel must be displayed by the “show” request.
● For each channel, a
“Channel name: ” followed by the name of the channel; a and the string “, created by: ”
followed by the username of the user who created the channel.
● For each HTTP or XML (e.g., RSS) document that matched an XPath in the channel:
○ The string “Crawled on: ” followed by a date in the same format as 2019-10-
31T17:45:48, i.e. YYYY-MM-DDThh:mm:ss, where the T is a separator between the day
and the time.
○ The string “Location: ” followed by the URL of the document. ○ A
element with the contents of the document.
● If the channel does not exist, return a 404.
We expect this application to run on your application server from the HW1. If you did not complete the
HW1, or for some other reason do not want to continue to use the application server that you wrote, you
may continue to use Spark Java with no penalty.
4.3 Pattern Engine as a StormLite Bolt
You need to write a class edu.upenn.cis455.xpathengine.XPathEngineImpl that implements
edu.upenn.cis455.xpathengine.XPathEngine (included with the code in Bitbucket), and evaluates
a set of XPath expressions on the specified HTML or XML document. Both protocols can be handled
similarly, except that HTML, unlike XML, is case insensitive. Once you have tested that individually, you
will incorporate it into (call it from) a StormLite bolt. We will be focusing only on elements, sub-elements,
and text nodes.
The implementation object (instance of XPathEngineImpl) should be created via the
XPathEngineFactory. The setXPaths method gives the class a number of XPaths to evaluate. The
isValid(i) method should return false if the ith XPath was invalid, and true otherwise. You should
implement the evaluateEvent() method:
1. This takes an OccurrenceEvent, which will have a document ID and a “parse event.”
2. Given a set of registered XPaths, if the document associated with the event has satisfied (at any
point) the XPath, a bit corresponding to that XPath should be set.
3. Your XPathEngine will need to store state for each document, to monitor its progress. This state
should be initialized when you first encounter a document with an ElementOpen; it should be
updated each time you get an event, and it should remove the state once all elements in the
document have been closed.
To make things simpler, we are supporting a very limited subset of XPath, as specified by the following
grammar (modulo white space, which your engine should ignore):
XPath → (/ step)+
step → nodename ([ test ])*
test → text() = "..." → contains(text(), "...")
where nodename is a valid XML identifier, and "..." indicates a quoted string. This means that a query like
/db/record/name[text() = "Alice"] is valid. Recall that if two separate bracketed conditions are
imposed at the same step in a query, both must apply for a node to be in the answer set.
Below are some examples of valid XPaths that you need to support (not an exhaustive list):
/foo/bar/xyz
/xyz/abc[contains(text(),"someSubstring")]
/a/b/c[text()="theEntireText"]
/d/e/f/foo[text()="something"]/bar
/a/b/c[text() = "whiteSpacesShouldNotMatter"]
You should be able to think about these kinds of XPaths as regular expressions over open and close events.
The stream of OccurrenceEvents will be coming from a separate parser bolt (in our standard architecture;
you can diverge from this if you prefer). You will probably want to create a test bolt when developing the
XPath engine. The easiest HTML/XML parser to use in Java is probably a DOM (Document Object Model)
parser, e.g., the one from JSoup. Such a parser builds an in-memory data structure holding an entire HTML
or XML document. From there, it is easy to walk the tree and output events. You can also look into SAX
parsers.
Once your XPath engine works over individual documents, you’ll want to write a StormLite bolt whose
execute() method instantiates the XPath engine for a given input document (passed in as a tuple), looks up
all of the channels defined in the BerkeleyDB database, and for each document that matches an XPath for
a channel, records the document as a match to the channel. Subsequently, the Web application interface
will be able to show the documents as matches.
4.4 Unit Tests
In addition, you must implement at least 5 unit tests, using the JUnit package (see the section on Testing
below for helpful Web page references). JUnit provides automated test scaffolding for your code: you can
set up a set of basic objects for all of the test cases, then have the test cases run one-by-one.
Your JUnit test suite should instantiate any necessary objects with some test data (e.g., parse a given HTML
or XML document or build a DOM tree), then run a series of unit tests that validate your Web application
and your XPath matcher. In total you must have at least 5 unit tests (perhaps each designed to exercise some
particular functionality) and at least one must be for the Web application and one for the XPath evaluator.
4.5. Submitting Milestone 2
Your solution must meet the following requirements (please read carefully!):
1. You must implement the edu.upenn.cis455.xpathengine.XPathEngine interface.
2. Your XPath engine class must be must be created by the XPathFactory when the appropriate
static method is called.
3. Your submission must contain a) the entire source code, as well as any supplementary files needed
to build your solution, b) a working Maven pom.xml, and c) a README file that contains 1) your
full name and SEAS login name, 2) any extra credit claimed, 3) any special instructions for building
or running.
4. Your code must contain a reasonable amount of useful documentation.
Reminder: All the code you submit (other than the dependencies on the JSoup/JTidy/TagSoup parser, the
standard Java libraries, and any code we have provided) must have been written by you personally, and you
may not collaborate with anyone else on this assignment. Copying code from the web is considered
plagiarism.
5. Testing the Crawler
5.1. 'Sandbox'
We have implemented a small sandbox for you to test your code on. It runs on machines in Penn
Engineering, so it will be fast to access, and it will not contain any links out of itself. The start URL of the
sandbox is https://crawltest.cis.upenn.edu/. There should be adequate XML and HTML documents there
to test your XPath matching.
5.2. JUnit
In order to encourage modularization and test driven development, you will be required to code test cases
using the JUnit package (http://www.junit.org/) - a framework that allows you to write and run tests over
your modules. A single test case consists of a class of methods, each of which (usually) tests one of your
source classes and its methods. A test suite is a class that allows you to run all your test cases as a single
program. You can get more information here: http://www.onjava.com/pub/a/onjava/2004/02/04/juie.html.
For Milestone 1, you must include 5 test cases and for Milestone 2, a test suite consisting of these 5 and at
least 2 more for each of DOM Parser and Path Matcher (for a total of 5 new tests). If your test suite uses
any files (e.g., test inputs), please put them into your project folder and use a relative path, so your tests
will run correctly on the graders' machines.
6. Extra credit
There are several enhancements you can add to your assignment for extra credit. In all cases, if you
implement an improved component, you do not need to implement the simpler version described above;
however, your improved component must still pass our test suite for the basic version, and you will lose
points if it does not. A safer approach is to implement the basic version first and to extend it later; if you
choose to do this, and if you submit both versions, please document in your README file how to enable the
extra functionality.
6.1 BYOF - Bring your own framework (+5%)
Rather than using Spark Framework, get your Homework 2 working on your Homework 1 Milestone 2
framework. From your Terminal, after 555-hw2 is pulled to your local repository, also run “git clone”
(with your HW1 repo) to pull your Homework 1 Milestone 2 code. Then go into the appropriate
subdirectory for HW1 and run the following to put it into a local Maven repository:
mvn clean install
mvn deploy:deploy-file -Dfile=target/homework-1-1.0-SNAPSHOT.jar -
DpomFile=pom.xml -Durl=file:/vagrant/555-hw2/maven-repository/ -
DrepositoryId=maven-repository -DupdateReleaseInfo=true
Then modify your Homework 2 pom.xml to add:
edu.upenn.cis.cis455
1.0-SNAPSHOT
and of course, remove Spark Framework and change the imports from spark.Spark.* to the appropriate
imports for your HW1.
6.2. Crawler web interface (+10%)
For 10% extra credit, provide a Web interface for the crawler. An admin user (not all users) should be able
to start the crawler at a specific page, set crawler parameters, stop the crawler if it is running, and display
statistics about the crawler's execution, such as
● the number of HTML pages scanned for links,
● the number of XML documents retrieved,
● the amount of data downloaded,
● the number of servers visited,
● the number of XML documents that match each channel, and
● the servers with the most XML documents that match one of the channels.
This will entail using some sort of communication between processes, potentially through the storage
system or via an HTTP request.
6.3. User / channel subscriptions (+5%)
For 5% extra credit, allow users to choose which channels to subscribe to from a list of available channels.
Add a list of the channels to which they are subscribed, in addition to all channels. Add a “subscription”
interface at: localhost:45555/subscribe?channel={name}
Here, when a user logs in, you should only present content from the subscribed channels, as opposed to all
channels.
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