CSC 213 – Large Scale Programming

What is “the BTree?”  Common multi-way tree implementation  Every BTree has an order (“BTree of order m ”) ‏  m / 2 to m children per internal node  Root node has m or fewer elements  Many variants exist to improve some failing  Each variant is specialized for some niche use  Minor differences only between each variant  This lecture will stick with vanilla BTrees

BTree Order  Order selected to minimize paging  Elements & references to kids in full node fills page  Nodes have at least m / 2 elements, even at their smallest  In memory guarantees each page is at least 50% full  How many pages touched during operation?

Removal from BTree  Swap element with successor in a leaf node  Similar to (2,4) node removal  If removal node left with under m / 2 elements  See if can move element from sibling to parent & steal element from parent  Else, merge with sibling & steal element from parent  But this might propagate underflow to parent node!  Remind anyone else of another structure?

Where to Find BTrees  Often used to implement databases  Contain lots of data -- more than machine’s RAM  Perform lots of data accesses, insertions  Need simple, efficient organization  Databases must store data permanently  Losing information may cause significant problems  RAM contents lost when powered off  But storing files on hard drive is s — l — o —w

Database Implementation  Maintain BTree in memory…  … but maintain copies of records on disk  Nodes have unique ID & location in file  Immediately write changes to disk  Always keep file as up-to-date copy  Just re-read file in case of program crash  Ignore virtual memory & instead use file  Records stored in random order within file  Execution may change element order

Better Ways To Access Data  BTrees cannot read & write file sequentially  Must jump around in file instead  Need way of specify each record within file  Java’s solution: RandomAccessFile

RandomAccessFile  Can create new files or use existing one raf = new RandomAccessFile(“f.txt”,“rw”);  Creates (or rewrites) the file named f.txt  When problem arises, throws IOException  Allows reading & writing to the file from within program  File can be used and modified using raf

Reading RandomAccessFile  Read RandomAccessFile instance using:  boolean readBoolean(), int readInt(), double readDouble()…  Reads and returns the appropriate value  int read(byte[] b)‏  Reads up to b.length bytes & stores back in b  Returns number of bytes read

Writing RandomAccessFile  Write RandomAccessFile using:  void writeInt(int i), void writeDouble(double d)…  Writes value at next location in the file  When at the end, will extend the file  Overwrites file, erasing data that had been there  void write(byte[] b)‏  Write contents of b to the file  As it is needed, will overwrite/extend file

Typical File I/O  Ordinarily we read and write files sequentially RandomAccessFile raf = new …; char c = ‘’; while (c != ‘s’) { c = raf.readChar(); } This is an example file we access raf :

 Ordinarily we read and write files sequentially RandomAccessFile raf = new …; char c = ‘’; while (c != ‘s’) { c = raf.readChar(); raf.writeChar(c); } Typical File I/O This is an example file we access

Typical File I/O  Ordinarily we read and write files sequentially RandomAccessFile raf = new …; char c = ‘’; while (c != ‘s’) { c = raf.readChar(); raf.writeChar(c); } TTis is an example file we access

Typical File I/O  Ordinarily we read and write files sequentially RandomAccessFile raf = new …; char c = ‘’; while (c != ‘s’) { c = raf.readChar(); raf.writeChar(c); } TTii is an example file we access

Typical File I/O  Ordinarily we read and write files sequentially RandomAccessFile raf = new …; char c = ‘’; while (c != ‘s’) { c = raf.readChar(); raf.writeChar(c); } TTii s an example file we access

Typical File I/O  Ordinarily we read and write files sequentially RandomAccessFile raf = new …; char c = ‘’; while (c != ‘s’) { c = raf.readChar(); raf.writeChar(c); } TTii ssan example file we access

Skipping Around The File  Read & write anywhere in RandomAccessFile  void seek(long pos) moves to position in file  Positions specified as bytes from beginning of file

RandomAccessFile I/O  Ordinarily we read and write files sequentially RandomAccessFile raf = new …; char c; raf.seek(raf.length()-1); c = raf.readChar(); raf.seek(0); raf.writeChar(c); This is an example file we access

RandomAccessFile I/O  Ordinarily we read and write files sequentially RandomAccessFile raf = new …; char c; raf.seek(raf.length()-1); c = raf.readChar(); raf.seek(0); raf.writeChar(c); shis is an example file we access

How Does This Work?  Use positions to simplify everything  Element contains position of record within file  Simplify building nodes from start of program  Record new nodes at end of file  Stores node table of contents at file start  Node records position of each of its children

For Next Lecture  Start week #14 assignment (due on Tuesday) ‏  Contains 3 problems to reinforce lecture topics  Provides practice for labs & final  Often helps build up to project  Programming project #4 now available  Read sections , of book  Will complete semester by looking at graphs  Graphs are very important data structure