Minibase Assignment #2: Heap File Page

Assigned on Wednesday, April 17.
Due on Wednesday, May 1.

Introduction

In this assignment, you will implement the page structure for the Heap File layer. You will be given libraries for the lower layers (Buffer Manager and Disk Space Manager), and some driver routines to test the code.

Preliminary Work

Begin by reading the description of Heap Files in section 7.5.1 of the text book, and the description of page formats in section 7.6. A HeapFile is seen as a collection of records. Internally, records are stored on a collection of pages. The pages are HFPage objects.

You will be implementing the HFPage class: a slotted page class that stores variable length records. The rest of the HeapFile code will be given to you. Read the description in the text of how variable length records can be stored on a slotted page, and follow this page organization.

Getting Started

Create a directory for the assignment, and change to that directory. Copy the file /usr/local/mini_hwk/assign/HFPage/src/Makefile to your current directory. To obtain the rest of the files that you will need, type make setup. If you make the project, it should create an executable named hfpage. You will need to provide the definition of the HFPage class in the file hfpage.C.

Sample output of a correct implementation is available in sample_output.

Design Overview and Implementation Details

Take a look at the file hfpage.h. It contains the interfaces for the HFPage class. This class implements a "heap-file page" object. Note that the protected data members of the class are given to you. All you need to do is implement the public member functions. You should put all your code into the file hfpage.C.

A Note on the Slot Directory

In the description in the text, the slot directory is located at the end of the page, and grows toward the beginning. This has confused students in the past, since it means that negative offsets into the slot directory have to be used. The current definition of HFPage has the slot directory at the beginning of the page, after a few fixed member fields, and growing toward the end. This does mean, however, that you will need to write the code so the records themselves are placed beginning at the end of the page. Be very careful with your pointer arithmetic.

Also note that in order to add a record to a page, there has to be room for the record itself in the data area, and there also has to be room for a new slot for this record in the slot directory (unless there happens to be a pre-allocated slot that is empty).

Error Reporting in the Code

Please follow the Minibase Error Protocol, in the same fashion that you did for the first Minibase assignment. Note that this layer of the code is referred to in error reporting as HEAPFILE (like BUFMGR was in the last assignment).

Methods to be Implemented

void HFPage::init(PageId pageNo)
This member function is used to initialize a new heap file page with page number pageNo. It should set the following data members to reasonable defaults: nextPage, prevPage, slotCnt, curPage, usedPtr, freeSpace. You will find the definitions of these data members in hfpage.h. The nextPage and prevPage data members are used for keeping track of pages in a HeapFile.
A good default unknown value for a PageId is INVALID_PAGE, as defined in page.h. Note that usedPtr is an offset into the data array, not a pointer.
PageId HFPage::getNextPage()
This member function should return the page id stored in the nextPage data member.
PageId HFPage::getPrevPage()
This member function should return the page id stored in the prevPage data member.
void HFPage::setNextPage(PageId pageNo)
This member function sets the nextPage data member.
void HFPage::setPrevPage(PageId pageNo)
This member function sets the prevPage data member.
Status HFPage::insertRecord(char* recPtr, int reclen, RID& rid)
This member function should add a new record to the page. It returns OK if everything went OK, and DONE if sufficient space does not exist on the page for the new record. If it returns OK, it should set rid to be the RID of the new record (otherwise it can leave rid untouched.) Please note in the parameter list recPtr is a char pointer and RID& denotes pass by reference.
The Status enumeration type is defined in new_error.h. You may want to look that file over and handle errors in a more informative manner than suggested here.
The RID struct is defined as follows:
Struct RID {
PageID pageNo;
int slotNo;

int operator == (const RID rid) const
{ return (pageNo == rid.pageNo) && (slotNo == rid.slotNo); };

int operator != (const RID rid) const
{ return (pageNo != rid.pageNo) || (slotNo != rid.slotNo); };

friend ostream& operator << (ostream& out, const struct RID rid);
};
The pageNo identifies a physical page number (something that the buffer manager and the DB layers understand) in the file. The slotNo specifies an entry in the slot array on the page.
Status HFPage::deleteRecord(const RID& rid)
This member function deletes the record with the given RID from the page, compacting the hole created. Compacting the hole, in turn, requires that all the offsets (in the slot array) of all records after the hole be adjusted by the size of the hole, because you are moving these records to "fill" the hole. You should leave a "hole" in the slot array for the slot which pointed to the deleted record, if necessary, to make sure that the rids of the remaining records do not change. The slot array can be compacted only if the record corresponding to the last slot is being deleted. This function returns OK if everything goes OK, or FAIL otherwise.
Status HFPage::firstRecord(RID& firstRid)
This routine should set firstRid to be the rid of the "first" record on the page. The order in which you return records from a page is entirely up to you. If you find a first record, return OK, else return DONE.
Status HFPage::nextRecord(RID curRid, RID& nextRid)
Given a valid current RID, curRid, this member function stores the next RID on the page in the nextRid variable. Again, the order of your return records is up to you, but do make sure you return each record exactly once if someone continually calls nextRecord. Don't worry about changes to the page between successive calls (e.g. records inserted to or deleted from the page). If you find a next RID, return OK, else return DONE. In case of an error, return FAIL.
Status HFPage::getRecord(RID rid, char * recPtr, int& recLen)
Given a rid, this routine copies the associated record into the memory starting at address recPtr. You may assume that the memory pointed by recPtr has been allocated by the caller. RecLen is set to the number of bytes that the record occupies. If all goes well, return OK, else return FAIL.
Status HFPage::returnRecord(RID rid, char*& recPtr, int& recLen)
This routine is very similar to HFPage::getRecord, except in this case you do not copy the record into a caller-provided pointer, but instead you set the caller's recPtr to point directly to the record on the page. Again, return either OK or FAIL.
DONE is a special code for non-errors that are nonetheless not "OK": it generally means "finished" or "not found." FAIL is for errors that happen outside the bounds of a subsystem.
int HFPage::available_space(void)
This routine should return the amount of space available for a new record that is left on the page. For instance, if all slots are full and there are 100 bytes of free space on the page, this method should return (100 - sizeof(slot_t)) bytes. This accounts for the fact that sizeof(slot_t) bytes must be reserved for a new slot and cannot be used by a new record.
bool HFPage::empty(void)
Returns true if the page has no records in it, and false otherwise.

Handing in Your Code

Use hsp to hand in the entire directory containing your code. Do not submit the individual files, but rather submit the directory as a whole. For example, if I am sitting in the directory above BufMgr, I would execute the following command:

hsp musicant CS395 HFPage

Good luck!