CS133 Lab 1: SimpleDB

Deadlines

• Part 0: Thursday, January 26: Read Section 1 Getting started and Exercise 0 (nothing to submit).
• Part 1: Thursday, February 2, 11:59 PM PT: Exercises 1-3
• Final: Thursday, February 9, 11:59 PM PT: Exercises 4-6

In the lab assignments in CS 133 you will write a basic database management system called SimpleDB. For this lab, you will focus on implementing the core modules required to access stored data on disk; in future labs, you will add support for various query processing operators, as well as transactions, locking, and concurrent queries.

SimpleDB is written in Java. We have provided you with a set of mostly unimplemented classes and interfaces. You will need to write the code for these classes. We will grade your code by running a set of unit and system tests written using JUnit.

Throughout the lab writeup you will see numbered exercises, offset in boxes, that describe where you should write code and which unit tests you should expect to pass once you are done. The due dates above correspond to these exercises. Note that there is no code to write for Exercise 0.

The remainder of this document describes the basic architecture of SimpleDB, and gives suggestions for coding, including the exercises you should complete. Section 3.2 discusses how to submit your code for both Part 1 and the Final version of the lab.

We strongly recommend that you start as early as possible on this lab. It requires you to write a fair amount of code!

Quick jump to exercises:

• Section 1 for Exercise 0
• Section 2.2 for Exercise 1
• Section 2.3 for Exercise 2
• Section 2.4 for Exercise 3
• Section 2.5 for Exercise 4 and 5
• Section 2.6 for Exercise 6

Jump to Submission instructions.

1. Getting started

Environment Setup: Java 1.8 and Ant

These instructions are written for a Unix-based platform (e.g., Linux, MacOS, etc.) Because the code is written in Java, it should work under Windows as well, though the directions in this document may need to be adjusted.

SimpleDB uses the Ant build tool to compile the code and run tests. Ant is similar to make, but the build file is written in XML and is somewhat better suited to Java code. Most modern Linux distributions include Ant.

You will need to use a programming environment that supports Java version 1.8 and has allows you to run Ant targets, typically on the command-line. Some ideas:

• Use Visual Studio Code to edit the Java files and its terminal window to run Ant (assuming both installed)
• Use Visual Studio Code to edit the Java files and run Ant tasks inside a Docker container (no need to install Ant). See our Docker guide for setting up a Docker container to run Ant tasks.
• Use the Eclipse IDE, which comes with a plugin for Ant. See our Eclipse guide for information on working with the Eclipse IDE.

Get the Starter Code

Download the code from http://www.cs.hmc.edu/~beth/courses/cs133/lab/cs133-lab1.tar.gz and untar it. For example, you can do this by issuing the following commands on the command line (the "$" is the command prompt):

$ wget http://www.cs.hmc.edu/~beth/courses/cs133/lab/cs133-lab1.tar.gz
$ tar xvzkf cs133-lab1.tar.gz
$ cd cs133-lab1

Running Unit Tests with Ant

To help you during development, we have provided a set of unit tests in addition to the end-to-end system tests discussed in Section 1.1. These are by no means comprehensive, and you should not rely on them exclusively to verify the correctness of your lab.

To run the unit tests use the test build target:

$ cd cs133-lab1
$ # run all unit tests
$ ant test
$ # run a specific unit test
$ ant runtest -Dtest=TupleTest

# build output...

test:
    [junit] Running simpledb.CatalogTest
    [junit] Testsuite: simpledb.CatalogTest
    [junit] Tests run: 6, Failures: 5, Errors: 1, Skipped: 0, Time elapsed: 0.079 sec
    [junit] Tests run: 6, Failures: 5, Errors: 1, Skipped: 0, Time elapsed: 0.079 sec

# ... stack traces and error reports ...

The output above indicates that two errors occurred during compilation; this is because the code we have given you doesn't yet work. As you complete parts of the lab, you will work towards passing additional unit tests. If you wish to write new unit tests as you code, they should be added to the test/simpledb directory.

For more details about how to use Ant, see the manual. The Running Ant section provides details about using the ant command. However, the quick reference table below should be sufficient for working on the labs.

  BUILD FAILED
  The following error occurred while executing this line:
  Test simpledb.TupleDescTest failed

1.1. Running end-to-end tests (unit tests in systemtest)

In addition to small unit tests, such as TupleDescTest, we have also provided a set of end-to-end tests, called system tests, for checking the functionality of the system you have built so far. These tests are structured as JUnit tests that live in the test/simpledb/systemtest directory. To run all the system tests, use the systemtest build target:

$ ant systemtest

# ... build output ...

    [junit] Testcase: testSmall took 0.017 sec
[junit] Caused an ERROR
    [junit] expected to find the following tuples:
[junit] 19128
    [junit] 
    [junit] java.lang.AssertionError: expected to find the following tuples:
[junit] 19128
    [junit] 
[junit] at simpledb.systemtest.SystemTestUtil.matchTuples(SystemTestUtil.java:122)
[junit] at simpledb.systemtest.SystemTestUtil.matchTuples(SystemTestUtil.java:83)
[junit] at simpledb.systemtest.SystemTestUtil.matchTuples(SystemTestUtil.java:75)
[junit] at simpledb.systemtest.ScanTest.validateScan(ScanTest.java:30)
[junit] at simpledb.systemtest.ScanTest.testSmall(ScanTest.java:40)

# ... more error messages ...

This indicates that this test failed, showing the stack trace where the error was detected. To debug, start by reading the source code where the error occurred. When the tests pass, you will see something like the following:

$ ant systemtest

# ... build output ...

    [junit] Testsuite: simpledb.systemtest.ScanTest
    [junit] Tests run: 3, Failures: 0, Errors: 0, Time elapsed: 7.278 sec
    [junit] Tests run: 3, Failures: 0, Errors: 0, Time elapsed: 7.278 sec
    [junit] 
    [junit] Testcase: testSmall took 0.937 sec
    [junit] Testcase: testLarge took 5.276 sec
    [junit] Testcase: testRandom took 1.049 sec

BUILD SUCCESSFUL
Total time: 52 seconds

1.2 Creating dummy tables

$ java -jar dist/simpledb.jar convert file.txt N

int1,int2,...,intN
int1,int2,...,intN
int1,int2,...,intN
int1,int2,...,intN

...where each intN is a non-negative integer. Be sure file.txt ends in a newline.

Note: To run the Jar file from within Eclipse instead of on the command-line, see the directions in "Running a Jar file Using Eclipse" in our Eclipse guide.

To view the contents of a table, use the print command:

$ java -jar dist/simpledb.jar print file.dat N

where file.dat is the name of a table created with the convert command, and N is the number of columns in the file.

1.3. Implementation Notes

Before beginning to write code, we strongly encourage you to read through this entire document to get a feel for the high-level design of SimpleDB.

You will need to fill in any piece of code that is not implemented. It should be obvious from the comments within and above each Java method where we think you should write code. You may need to add private methods, instance variables, and/or helper classes. You may change APIs, but make sure our grading tests still run and make sure to mention, explain, and defend your decisions in your writeup.

In addition to the methods that you need to fill out for this lab, the class interfaces contain numerous methods that you need not implement until subsequent labs. These will either be indicated per class:

// Not necessary for lab1.
public class Insert implements DbIterator {

public boolean deleteTuple(Tuple t) throws DbException {
    // some code goes here
    // not necessary for lab1
    return false;
}

Section 2 below walks you through the implementation steps for this lab and the unit tests corresponding to each one in more detail.

Transactions, locking, and recovery

2. SimpleDB Architecture and Implementation Guide

• Classes that represent fields, tuples, and tuple schemas;
• Classes that apply predicates and conditions to tuples;
• One or more access methods (e.g., heap files) that store relations on disk and provide a way to iterate through tuples of those relations;
• A collection of operator classes (e.g., select, join, insert, delete, etc.) that process tuples;
• A buffer pool that caches active tuples and pages in memory and handles concurrency control and transactions (neither of which you need to worry about for this lab); and,
• A catalog that stores information about available tables and their schemas.

SimpleDB does not include many things that you may think of as being a part of a "database." In particular, SimpleDB does not have:

• (In this lab), a SQL front end or parser that allows you to type queries directly into SimpleDB. Instead, queries are built up by chaining a set of operators together into a hand-built query plan (see Section 2.7). We will provide a simple parser for use in later labs.
• Views.
• Data types except integers and fixed length strings.
• (In this lab) Query optimizer.
• Indexes.

In the rest of this Section, we describe each of the main components of SimpleDB that you will need to implement in this lab. You should use the exercises in this discussion to guide your implementation. This document is by no means a complete specification for SimpleDB; you will need to make decisions about how to design and implement various parts of the system. Note that for Lab 1 you do not need to implement any operators (e.g., select, join, project) except sequential scan. You will add support for additional operators in future labs.

2.1. The Database Class

2.2. Fields and Tuples

Tuples in SimpleDB are quite basic. They consist of a collection of Field objects, one per field in the Tuple. Field is an interface that different data types (e.g., integer, string) implement; see IntField and StringField. Tuple objects are created by the underlying access methods (e.g., heap files, or B-trees), as described in the next section. Tuples also have a type (or schema), called a tuple descriptor, represented by a TupleDesc object. This TupleDesc object consists of a collection of Type objects, one per field in the tuple, each of which describes the type of the corresponding field. If a tuple is stored on disk, it will have a RecordId that identifies where in a file the tuple is located.

2.3. Catalog

The global catalog is a single instance of Catalog that is allocated for the entire SimpleDB process. The global catalog can be retrieved via the method Database.getCatalog(), and the same goes for the global buffer pool (using Database.getBufferPool()).

2.4. BufferPool

The Buffer Pool (class BufferPool in SimpleDB) is responsible for caching pages in memory that have been recently read from disk. All operators read and write pages from various files on disk through the buffer pool. The Buffer Pool consists of a fixed number of pages, defined by the numPages parameter to the BufferPool constructor. In later labs, you will implement an eviction policy. For this lab, you only need to implement the constructor and the BufferPool.getPage() method used by the Sequential Scan (SeqScan) operator. The BufferPool should store up to numPages pages. For this lab, if more than numPages requests are made for different pages, then instead of implementing an eviction policy, you will throw a DbException. In a future lab you will be required to implement an eviction policy.

Recall that the Database class provides a static method, Database.getBufferPool(), that returns a reference to the single BufferPool instance for the entire SimpleDB process.

2.5. HeapFile access method

A HeapFile object provides access to a set of pages stored on disk, each of which consists of a fixed number of bytes for storing tuples, (defined by the constant BufferPool.PAGE_SIZE), including a header. In SimpleDB, there is one HeapFile object for each table in the database.

Each page in a HeapFile is arranged as a set of slots, each of which can hold one tuple (tuples for a given table in SimpleDB are all of the same size). In addition to these slots, each page has a header that consists of a bitmap with one bit per tuple slot. If the bit corresponding to a particular tuple is 1, it indicates that the tuple is valid; if it is 0, the tuple is invalid (e.g., has been deleted or was never initialized). Recall that a tuple's RecordId indicates which page (of the file) and which slot on that page the tuple is located. Pages of HeapFile objects are of type HeapPage, which implements the Page interface. Pages are stored in the Buffer Pool but are read and written to/from disk using the HeapFile class.

SimpleDB stores the pages in heap files on disk in more or less the same format they are stored in memory. Each file consists of page data arranged consecutively on disk. Each page consists of one or more bytes representing the header, followed by the page size bytes of actual page content. Each tuple requires tuple size * 8 bits for its content and 1 bit for the header. Thus, the number of tuples that can fit in a single page is:

tuples per page = floor((page size * 8) / (tuple size * 8 + 1))

Where tuple size is the size of a tuple in the page in bytes. The idea here is that each tuple requires one additional bit of storage in the header. We compute the number of bits in a page (by mulitplying page size by 8), and divide this quantity by the number of bits in a tuple (including this extra header bit) to get the number of tuples per page. The floor operation rounds down to the nearest integer number of tuples (we don't want to store partial tuples on a page!)

Once we know the number of tuples per page, the number of bytes required to store the header is simply:

headerBytes = ceiling(tupsPerPage/8)

The ceiling operation rounds up to the nearest integer number of bytes (we never store less than a full byte of header information.)

The low (least significant) bits of each byte represents the status of the slots that are earlier in the file. Hence, the lowest bit of the first byte represents whether or not the first slot in the page is in use. Also, note that the high-order bits of the last byte may not correspond to a slot that is actually in the file, since the number of slots may not be a multiple of 8. Also note that all Java virtual machines are big-endian.

Now that you have implemented HeapPage, you will write methods for HeapFile in this lab to calculate the number of pages in a file and to read a page from the file. HeapFile is given the filename, represented as an instance of a java.io.File, that it will read from. After implementing HeapFile, you will then be able to fetch tuples from a file stored on disk!

2.6. Operators

Operators are connected together into a query execution plan, or simply "plan", by passing operators as input into other operators via their constructors, i.e., by 'chaining them together', forming a tree of operators. Access method operators at the leaves of the plan are responsible for reading data from the disk (and hence do not have any operators below them).

At the top of the query plan tree, the program interacting with SimpleDB simply calls getNext on the root operator; this operator then calls getNext on its children, and so on, until these leaf operators are called. They fetch tuples from disk and pass them up the tree (as return arguments to getNext); tuples propagate up the plan in this way until they are output at the root or combined or rejected by another operator in the plan.

For this lab, you will only need to implement one SimpleDB operator, the sequential scan (SeqScan); this operator is one of the access methods at the leaves of a query plan.

You will add other operators in subsequent labs.

2.7. A simple query

The purpose of this section is to illustrate how these various components are connected together to process a simple query. Suppose you have a data file, "some_data_file.txt", with the following contents:

1,1,1
2,2,2 
3,4,4

You can convert this into a binary file that SimpleDB can query as follows:

java -jar dist/simpledb.jar convert some_data_file.txt 3

Here, the argument "3" tells convert that the input has 3 columns.

The following code implements a simple selection query over this file. This code is equivalent to the SQL statement SELECT * FROM some_data_file.

package simpledb;
import java.io.*;

public class test {

    public static void main(String[] argv) {

        // construct a 3-column table schema
        Type types[] = new Type[]{ Type.INT_TYPE, Type.INT_TYPE, Type.INT_TYPE };
        String names[] = new String[]{ "field0", "field1", "field2" };
        TupleDesc descriptor = new TupleDesc(types, names);
        
        // create the table, associate it with some_data_file.dat
        // and tell the catalog about the schema of this table.
        HeapFile table1 = new HeapFile(new File("some_data_file.dat"), descriptor);
        Database.getCatalog().addTable(table1, "test");
        
        // construct the query: we use a simple SeqScan, which spoonfeeds
        // tuples via its iterator.
        TransactionId tid = new TransactionId();
        SeqScan f = new SeqScan(tid, table1.getId());
        
        try {
            // and run it
            f.open();
            while (f.hasNext()) {
                Tuple tup = f.next();
                System.out.println(tup);
            }
            f.close();
            Database.getBufferPool().transactionComplete(tid);
        } catch (Exception e) {
            System.out.println ("Exception : " + e);
        }
    }

}

The table we create has three integer fields. To express this, we create a TupleDesc object and pass it an array of Type objects, and optionally an array of String field names. Once we have created this TupleDesc, we initialize a HeapFile object representing the table stored in some_data_file.dat. Once we have created the table, we add it to the catalog. If this were a database server that was already running, we would have this catalog information loaded. We need to load it explicitly to make this code self-contained.

Once we have finished initializing the database system, we create a query plan. Our plan consists only of the SeqScan operator that scans the tuples from disk. In general, these operators are instantiated with references to the appropriate table (in the case of SeqScan) or child operator (in the case of e.g. Filter). The test program then repeatedly calls hasNext and next on the SeqScan operator. As tuples are output from the SeqScan, they are printed out on the command line.

ant
java -classpath dist/simpledb.jar simpledb.test

3. Submission and Grading Details

• Describe any design decisions you made. These may be minimal for Lab 1.
• Discuss and justify any changes you made to the API.
• Describe any missing or incomplete elements of your code.
• Describe how long you (and your partner) spent on the lab, and whether there was anything you found particularly difficult or confusing.

3.1. Collaboration

3.2. Submitting your assignment

You will submit a tarball of your code on Gradescope for intermediate deadlines and for your final version. You only need to include your writeup for the final version.

Generating Tarball

You can generate the tarball by using the ant handin target. This will create a file called cs133-lab.tar.gz that you can submit. You can rename the tarball file if you want, but the filename must end in tar.gz. The autograder won't be able to handle it if you package your code any other way!

Submitting on Gradescope

Click Lab 1 on your Gradescope dashboard. For deadlines besides the final version, you only need to upload cs133-lab.tar.gz.
For the final version: click Lab 1 and then click the "Resubmit" button on the bottom of the page ; upload both cs133-lab.tar.gz and writeup.txt containing your writeup.

If you worked with a partner, be sure to enter them as a group member on Gradescope after uploading your files.

3.3 Grading

Your grade for the lab will be based on the final version after all exercises are complete.

75% of your grade will be based on whether or not your code passes the test suite. Before handing in your code, you should make sure it produces no errors (passes all of the tests) from both ant test and ant systemtest.

Important: before testing, we will replace your build.xml and the entire contents of the test directory with our version of these files. This means you cannot change the format of .dat files! You should also be careful changing our APIs. You should test that your code compiles the unmodified tests. In other words, we will untar your tarball, replace the files mentioned above, compile it, and then grade it. It will look roughly like this:

$ tar xvzf cs133-lab.tar.gz
[replace build.xml and test]
$ ant test
$ ant systemtest

If any of these commands fail, we'll be unhappy, and, therefore, so will your grade.

An additional 25% of your grade will be based on the quality of your writeup, our subjective evaluation of your code, and on-time submission for the intermediate deadlines.

ENJOY!!

Acknowledgements