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Overview

SQLite4 works with run-time interchangeable storage engines with thefollowing properties:

• The storage engine works with key/value pairs where both the key and the value are byte arrays of arbitrary length and with no restrictions on content.

• All keys are unique.

• Keys sort in lexicographical order (as if sorted using the memcmp() library function). When one key is a prefix of another, the shorter key occurs first.

• Transaction commit and rollback is handled by the storage engine.

SQLite4 comes with two built-in storage engines. A log-structured merge-tree(LSM) storage engine is used for persistent on-disk databases and anin-memory binary tree storage engine is used for TEMP databases.Future versions of SQLite4 might add other built-in storage engines.

Applicates can add new storage engines to SQLite4 at run-time. Thepurpose of this document is to describe how that is done.

Adding A New Storage Engine

Each storage engine implements a 'factory function'. The factory functioncreates an object that defines a single connection to a single databasefile. The signature of the factory function is as follows:

SQLite4 will invoke the factory function whenever it needs to open aconnection to a database file. The first argument is therun-time environment in use by the database connection.The third argument is the name of the database file to be opened.The fourth argument is zero or more boolean flags that are hints to thefactory telling it how the database will be used. The factory shouldcreate a new sqlite4_kv_storage object describing the connection to thedatabase file and return a pointer to that object in the addressspecified by 2nd argument, then return SQLITE_OK. Or, if something goeswrong, the factory should return an appropriate error code.

We need to add some mechanism for the factory to return detailederror information back up to the caller.

To add a new storage engine to SQLite4, use the sqlite4_env_config()interface to register the factory function for the storage engine withthe run-time environment that will be using the storage engine. For example:

The example above adds the factory 'exampleStorageEngine()' to the run-time environment as the 'main' storage engine. The 'main' storageengine is used by default for persistent databases. The 'temp' storageengine is used by default for transient and 'TEMP' databases. Storageengines with other names can be registered and used by specifying thestorage engine name in the 'kv=' query parameter of the URI passed tosqlite4_open().

Storage engines stack. The built-in includes two storage engines for'main' and 'temp': the LSM and binary-tree storage engines, respectively.If you push a new 'main' storage engine, the new one will take precedenceover the built-in storage engine. Later, you can call sqlite4_env_config()with the SQLITE_ENVCONFIG_POP_KVSTORE argument to remove the added storageengine and restore the built-in LSM storage engine as the 'main' storageengine. The built-in storage engines cannot be popped from the stack.

The SQLITE_ENVCONFIG_GET_KVSTORE operator for sqlite3_env_config() isavailable for querying the current storage engines.

Storage Engine Implementation

The sqlite4_kvstore object that the factory function returns has thefollowing basis:

Useful subclasses of the sqlite4_kvstore base class will almostcertainly want to add additional fields at after the basis.The most interesting part of the sqlite4_kvstore object is surelythe virtual method table, which looks like this:

The storage engine implementation will need to provide implementations forall of the methods in the sqlite4_kv_methods object. Note that the firsttwo fields, iVersion and szSelf, are present to support future extensions.The iVersion field should always currently be 1, but might be larger forlater enhanced versions. And the szSelf field should be equal tosizeof(sqlite4_kv_methods).

Search operations involve a cursor object whose basis is the following:

As before, actual implementations will more than likely want to extendthis object by adding additional fields onto the end.

Storage Engine Methods

SQLite invokes the xBegin, xCommit, and xRollback methods changethe transaction level of the storage engine. The transaction levelis a non-negative integer that is initialized to zero. The transactionlevel must be at least 1 in order for content to be read. Thetransaction level must be at least 2 for content to be modified.

The xBegin method increases transaction level. The increase may be nomore than 1 unless the transaction level is initially 0 in which caseit can be increased immediately to 2. Increasing the transaction levelto 1 or more makes a 'snapshot' of the database file such that changesmade by other connections are not visible. An xBegin call may failwith SQLITE_BUSY if the initial transaction level is 0 or 1.

A read-only database will fail an attempt to increase xBegin above 1. Animplementation that does not support nested transactions will fail anyattempt to increase the transaction level above 2.

The xCommitPhaseOne and xCommitPhaseTwo methods implement a 2-phasecommit that lowers the transaction level to the value given in thesecond argument, making all the changes made at higher transaction levelspermanent. A rollback is still possible following phase one. Ifpossible, errors should be reported during phase one so that amultiple-database transaction can still be rolled back if thephase one fails on a different database. Implementations that do notsupport two-phase commit can implement xCommitPhaseOne as a no-op functionreturning SQLITE_OK.

The xRollback method lowers the transaction level to the value given inits argument and reverts or undoes all changes made at higher transactionlevels. An xRollback to level N causes the database to revert to the stateit was in on the most recent xBegin to level N+1.

The xRevert(N) method causes the state of the database file to go backto what it was immediately after the most recent xCommit(N). Higher-levelsubtransactions are cancelled. This call is equivalent to xRollback(N-1)followed by xBegin(N) but is atomic and might be more efficient.

The xReplace method replaces the value for an existing entry with thegiven key, or creates a new entry with the given key and value if noprior entry exists with the given key. The key and value pointers passedinto xReplace belong to the caller and will likely be destroyed when thecall to xReplace returns so the xReplace routine must make its owncopy of that information.

A cursor is at all times pointing to ether an entry in the database orto EOF. EOF means 'no entry'. Cursor operations other than xCloseCursor will fail if the transaction level is less than 1.

The xSeek method moves a cursor to an entry in the database that matchesthe supplied key as closely as possible. If the dir argument is 0, thenthe match must be exact or else the seek fails and the cursor is leftpointing to EOF. If dir is negative, then an exact match isfound if it is available, otherwise the cursor is positioned at the largestentry that is less than the search key or to EOF if the store contains noentry less than the search key. If dir is positive, then an exist matchis found if it is available, otherwise the cursor is left pointing thethe smallest entry that is larger than the search key, or to EOF if thereare no entries larger than the search key.

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The return code from xSeek might be one of the following:

SQLITE_OK The cursor is left pointing to any entry that exactly matchings the probe key.

SQLITE_INEXACT The cursor is left pointing to the nearest entry to the probe it could find, either before or after the probe, according to the dir argument.

SQLITE_NOTFOUND No suitable entry could be found. Either dir0 and there was no exact match, or dir<0 and the probe is smaller than every entry in the database, or dir>0 and the probe is larger than every entry in the database.

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xSeek might also return some error code like SQLITE_IOERR orSQLITE_NOMEM.

The xNext method will only be called following an xSeek with a positive dir,or another xNext. The xPrev method will only be called following an xSeekwith a negative dir or another xPrev. Both xNext and xPrev will returnSQLITE_OK on success and SQLITE_NOTFOUND if they run off the end of thedatabase. Both routines might also return error codes such asSQLITE_IOERR, SQLITE_CORRUPT, or SQLITE_NOMEM.

Values returned by xKey and xData must remain stable untilthe next xSeek, xNext, xPrev, xReset, xDelete, or xCloseCursor on the samecursor. This is true even if the transaction level is reduced to zero,or if the content of the entry is changed by xInsert, xDelete on a differentcursor, or xRollback. The content returned by repeated calls to xKey andxData is allowed (but is not required) to change if xInsert, xDelete, orxRollback are invoked in between the calls, but the content returned byevery call must be stable until the cursor moves, or is reset or closed.The cursor owns the values returned by xKey and xData and must takeresponsiblity for freeing memory used to hold those values when appropriate.If the SQLite core needs to keep a key or value beyond the time when itis guaranteed to be stable, it will make its own copy.

The xDelete method deletes the entry that the cursor is currentlypointing at. However, subsequent xNext or xPrev calls behave as if theentries is not actually deleted until the cursor moves. In other wordsit is acceptable to xDelete an entry out from under a cursor. SubsequentxNext or xPrev calls on that cursor will work the same as if the entryhad not been deleted. Two cursors can be pointing to the same entry andone cursor can xDelete and the other cursor is expected to continuefunctioning normally, including responding correctly to subsequentxNext and xPrev calls.

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To be continued...