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## Run this Tcl script to generate the sqlite.html file.#set rcsid {$Id: arch.tcl,v 1.11 2004/03/14 11:57:58 drh Exp $}puts {<html><head>  <title>Architecture of SQLite</title></head><body bgcolor=white><h1 align=center>The Architecture Of SQLite</h1>}puts "<p align=center>(This page was last modified on [lrange $rcsid 3 4] UTC)</p>"puts {<h2>Introduction</h2><table align="right" border="1" cellpadding="15" cellspacing="1"><tr><th>Block Diagram Of SQLite</th></tr><tr><td><img src="arch2.gif"></td></tr></table><p>This document describes the architecture of the SQLite library.The information here is useful to those who want to understand ormodify the inner workings of SQLite.</p><p>A block diagram showing the main components of SQLiteand how they interrelate is shown at the right.  The text thatfollows will provide a quick overview of each of these components.</p><h2>History</h2><p>There are two main C interfaces to the SQLite library:<b>sqlite_exec()</b> and <b>sqlite_compile()</b>.  Prior toversion 2.8.0 (2003-Feb-16) only sqlite_exec() was supported.For version 2.8.0, the sqlite_exec and sqlite_compile methodsexisted as peers.  Beginning with version 2.8.13, the sqlite_compilemethod is the primary interface, and sqlite_exec is implementedusing sqlite_compile.  Externally, this change is an enhancementthat maintains backwards compatibility.  But internally,the plumbing is very different.  The diagram at the right showsthe structure of SQLite for version 2.8.13 and following.</p><h2>Interface</h2><p>Much of the public interface to the SQLite library is implemented byfunctions found in the <b>main.c</b> source file though some routines arescattered about in other files where they can have access to data structures with file scope.  The<b>sqlite_get_table()</b> routine is implemented in <b>table.c</b>.<b>sqlite_step()</b> is found in <b>vdbe.c</b>.  <b>sqlite_mprintf()</b> is found in <b>printf.c</b>.The Tcl interface is implemented by <b>tclsqlite.c</b>.  Moreinformation on the C interface to SQLite is<a href="c_interface.html">available separately</a>.<p><p>To avoid name collisions with other software, all externalsymbols in the SQLite library begin with the prefix <b>sqlite</b>.Those symbols that are intended for external use (in other words,those symbols which form the API for SQLite) beginwith <b>sqlite_</b>.</p><h2>SQL Command Process</h2><p><h2>Tokenizer</h2><p>When a string containing SQL statements is to be executed, theinterface passes that string to the tokenizer.  The job of the tokenizeris to break the original string up into tokens and pass those tokensone by one to the parser.  The tokenizer is hand-coded in C.All of the code for the tokenizeris contained in the <b>tokenize.c</b> source file.</p><p>Note that in this design, the tokenizer calls the parser.  Peoplewho are familiar with YACC and BISON may be used to doing things theother way around -- having the parser call the tokenizer.  The authorof SQLite has done it both ways and finds things generally work out nicer forthe tokenizer to call the parser.  YACC has it backwards.</p><h2>Parser</h2><p>The parser is the piece that assigns meaning to tokens based ontheir context.  The parser for SQLite is generated using the<a href="http://www.hwaci.com/sw/lemon/">Lemon</a> LALR(1) parsergenerator.  Lemon does the same job as YACC/BISON, but it usesa different input syntax which is less error-prone.Lemon also generates a parser which is reentrant and thread-safe.And lemon defines the concept of a non-terminal destructor sothat it does not leak memory when syntax errors are encountered.The source file that drives Lemon is found in <b>parse.y</b>.</p><p>Becauselemon is a program not normally found on development machines, thecomplete source code to lemon (just one C file) is included in theSQLite distribution in the "tool" subdirectory.  Documentation onlemon is found in the "doc" subdirectory of the distribution.</p><h2>Code Generator</h2><p>After the parser assembles tokens into complete SQL statements,it calls the code generator to produce virtual machine code thatwill do the work that the SQL statements request.  There are manyfiles in the code generator:  <b>build.c</b>, <b>copy.c</b>,<b>delete.c</b>,<b>expr.c</b>, <b>insert.c</b>, <b>pragma.c</b>,<b>select.c</b>, <b>trigger.c</b>, <b>update.c</b>, <b>vacuum.c</b>and <b>where.c</b>.In these files is where most of the serious magic happens.<b>expr.c</b> handles code generation for expressions.<b>where.c</b> handles code generation for WHERE clauses onSELECT, UPDATE and DELETE statements.  The files <b>copy.c</b>,<b>delete.c</b>, <b>insert.c</b>, <b>select.c</b>, <b>trigger.c</b><b>update.c</b>, and <b>vacuum.c</b> handle the code generationfor SQL statements with the same names.  (Each of these files calls routinesin <b>expr.c</b> and <b>where.c</b> as necessary.)  All otherSQL statements are coded out of <b>build.c</b>.</p><h2>Virtual Machine</h2><p>The program generated by the code generator is executed bythe virtual machine.  Additional information about the virtualmachine is <a href="opcode.html">available separately</a>.To summarize, the virtual machine implements an abstract computingengine specifically designed to manipulate database files.  Themachine has a stack which is used for intermediate storage.Each instruction contains an opcode andup to three additional operands.</p><p>The virtual machine itself is entirely contained in a singlesource file <b>vdbe.c</b>.  The virtual machine also hasits own header files: <b>vdbe.h</b> that defines an interfacebetween the virtual machine and the rest of the SQLite library and<b>vdbeInt.h</b> which defines structure private the virtual machine.The <b>vdbeaux.c</b> file contains utilities used by the virtualmachine and interface modules used by the rest of the library toconstruct VM programs.</p><h2>Backend</h2><p>The backend is an abstraction layer that presents a uniform interfaceto the virtual machine for either the B-Tree drivers for disk-baseddatabases or the Red/Black Tree driver for in-memory databases.The <b>btree.h</b> source file contains the details.</p><h2>Red/Black Tree</h2><p>In-memory databases are stored in a red/black tree implementationcontain in the <b>btree_rb.c</b> source file.</p><h2>B-Tree</h2><p>An SQLite database is maintained on disk using a B-tree implementationfound in the <b>btree.c</b> source file.  A separate B-tree is used foreach table and index in the database.  All B-trees are stored in thesame disk file.  Each page of a B-tree is 1024 bytes in size.  The keyand data for an entry are stored together in an area called "payload".Up to 236 bytes of payload can be stored on the same page as the B-treeentry.  Any additional payload is stored in a chain of overflow pages.</p><p>The interface to the B-tree subsystem is defined by the header file<b>btree.h</b>.</p><h2>Page Cache</h2><p>The B-tree module requests information from the disk in 1024 bytechunks.  The page cache is reponsible for reading, writing, andcaching these chunks.The page cache also provides the rollback and atomic commit abstractionand takes care of reader/writer locking of the database file.  TheB-tree driver requests particular pages from the page cache and notifiesthe page cache when it wants to modify pages or commit or rollbackchanges and the page cache handles all the messy details of making surethe requests are handled quickly, safely, and efficiently.</p><p>The code to implement the page cache is contained in the single Csource file <b>pager.c</b>.  The interface to the page cache subsystemis defined by the header file <b>pager.h</b>.</p><h2>OS Interface</h2><p>In order to provide portability between POSIX and Win32 operating systems,SQLite uses an abstraction layer to interface with the operating system.The <b>os.c</b> file contains about 20 routines used for opening andclosing files, deleting files, creating and deleting locks on files,flushing the disk cache, and so forth.  Each of these functions containstwo implementations separated by #ifdefs: one for POSIX and the otherfor Win32.  The interface to the OS abstraction layer is defined bythe <b>os.h</b> header file.</p>}puts {<br clear="both" /><p><hr /></p><p><a href="index.html"><img src="/goback.jpg" border=0 />Back to the SQLite Home Page</a></p></body></html>}
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