SQL Relay Database Connection Daemons

Most database API's work like this:

The SQL Relay connection daemon class works this way too. Writing a connection daemon mainly consists of wrapping database API methods in corresponding connection class methods. More specifically, to write a new connection daemon, you create classes which inherit from the "connection" and "cursor" classes and implement their pure virtual methods.

Below is an example. On the left is pseudocode and on the right, the code for the MySQL connection.

Header File

// Copyright (c) 2000-2001  David Muse
// See the file COPYING for more information

#ifndef MYCONNECTION_H
#define MYCONNECTION_H

#define NUM_CONNECT_STRING_VARS however many connect string vars you have

#include <connection.h>

#include < whatever header files you need >

#include < your header file >

class myconnection;

class mycursor : public cursor {
        friend class myconnection;
        private:
                        mycursor(connection *conn);
                        ~mycursor();

		... override the cursor class's pure virtual methods ...

                int     executeQuery(char *query, long length);
                char    *getErrorMessage(int *liveconnection);
                void    returnRowCounts();
                void    returnColumnCount();
                void    returnColumnInfo();
                int     noRowsToReturn();
                int     skipRow();
                int     fetchRow();
                void    returnRow();
                void    cleanUpData();

		... you'll need a result set descriptor
		    possibly some other descriptors ...

		... you'll probably need variables for
		    the number of rows, columns,
		    affected rows and an error message ...

		... and a pointer to the connection class ...
                myconnection *myconn;
};

class myconnection : public connection {
        friend class mycursor;
        public:
                        myconnection();
                        ~myconnection();
        private:

		... override the connection class's pure virtual methods ...

                int             getNumberOfConnectStringVars();
                void            handleConnectString();
                int             logIn();
                cursor          *initCursor();
                void            deleteCursor(cursor *curs);
                void            logOut();
                int             isTransactional();
                int             ping();
                char            *identify();
                unsigned short  *autoCommitOn();
                unsigned short  *autoCommitOff();
                void            commit();
                void            rollback();


		... you'll need a connection descriptor here ...

		... you'll also need variables for the 
		    connect string arguments ...
};

#endif
// Copyright (c) 2000-2001  David Muse
// See the file COPYING for more information

#ifndef MYSQLCONNECTION_H
#define MYSQLCONNECTION_H

#define NUM_CONNECT_STRING_VARS 6

#include <sqlrconnection.h>

#include <mysql.h>

class mysqlconnection;

class mysqlcursor : public sqlrcursor {
        friend class mysqlconnection;
        private:
                        mysqlcursor(sqlrconnection *conn);
                int     executeQuery(const char *query, long length,
                                        unsigned short execute);
                char    *getErrorMessage(int *liveconnection);
                void    returnRowCounts();
                void    returnColumnCount();
                void    returnColumnInfo();
                int     noRowsToReturn();
                int     skipRow();
                int     fetchRow();
                void    returnRow();
                void    cleanUpData();

                MYSQL_RES       *mysqlresult;
                MYSQL_FIELD     *mysqlfield;
                MYSQL_ROW       mysqlrow;
                int             ncols;
                int             nrows;
                int             affectedrows;
                int             queryresult;

                mysqlconnection *mysqlconn;
};

class mysqlconnection : public sqlrconnection {
        friend class mysqlcursor;
        public:
                        mysqlconnection();
        private:
                int     getNumberOfConnectStringVars();
                void    handleConnectString();
                int     logIn();
                sqlrcursor      *initCursor();
                void    deleteCursor(sqlrcursor *curs);
                void    logOut();
                int     isTransactional();
#if MYSQL_VERSION_ID>=32200
                int     ping();
#endif
                char    *identify();
                unsigned short  autoCommitOn();
                unsigned short  autoCommitOff();
                int     commit();
                int     rollback();

                MYSQL   mysql;
                int     connected;

                char    *db;
                char    *host;
                char    *port;
                char    *socket;
};

#endif


.C File

// Copyright (c) 1999-2001  Your Name Here
// See the file COPYING for more information

#include < your header file >

	... you need to include this file too ...
#include <datatypes.h>

	... you may need to include some other headers ...
// Copyright (c) 1999-2001  David Muse
// See the file COPYING for more information

#include <mysqlconnection.h>
#if MYSQL_VERSION_ID>=32200
        #include <errmsg.h>
#endif

#include <datatypes.h>

#include <config.h>

#include <stdlib.h>
#include <string.h>


myconnection::myconnection() {
        connected=0;

	... initialize whatever you need to here ...
}


mysqlconnection::mysqlconnection() {
        connected=0;
}


myconnection::~myconnection() {

	... deallocate whatever you initialized ...
}


	... the mysqlconnection has no destructor ...


int     myconnection::getNumberOfConnectStringVars() {
        return NUM_CONNECT_STRING_VARS;
}


int     mysqlconnection::getNumberOfConnectStringVars() {
        return NUM_CONNECT_STRING_VARS;
}


void    myconnection::handleConnectString() {

	... set your connect string variables 
            using the connectStringValue()
            method ...

}


void    mysqlconnection::handleConnectString() {
        setUser(connectStringValue("user"));
        setPassword(connectStringValue("password"));
        db=connectStringValue("db");
        host=connectStringValue("host");
        port=connectStringValue("port");
        socket=connectStringValue("socket");
}


int     mysqlconnection::logIn() {

        ... set some default values for any
            missing connect string variables ...

        ... connect to the database ...

        ... return 1 for success, 0 for failure ...

}


int     mysqlconnection::logIn() {


        // handle host
        char    *hostval;
        if (host && host[0]) {
                hostval=host;
        } else {
                hostval="";
        }

        // handle port
        int     portval;
        if (port && port[0]) {
                portval=atoi(port);
        } else {
                portval=0;
        }

        // handle socket
        char    *socketval;
        if (socket && socket[0]) {
                socketval=socket;
        } else {
                socketval=NULL;
        }

        // handle db
        char    *dbval;
        if (db && db[0]) {
                dbval=db;
        } else {
                dbval="";
        }

        // initialize database connection structure
#if MYSQL_VERSION_ID>=32200
        if (!mysql_init(&mysql)) {
                return 0;
        }
#endif

        // log in
        char    *user=getUser();
        char    *password=getPassword();
#ifdef MYSQL_VERSION_ID
        #if MYSQL_VERSION_ID>=32200
                if (!mysql_real_connect(&mysql,hostval,user,password,dbval,
                                                portval,socketval,0)) {
        #else
                if (!mysql_real_connect(&mysql,hostval,user,password,
                                                portval,socketval,0)) {
        #endif
#else
        if (!mysql_connect(&mysql,hostval,user,password)) {
#endif
                logOut();
                return 0;
        } else {
#if MYSQL_VERSION_ID<32200
                if (!mysql_select_db(&mysql,dbval)) {
                        logOut();
                        return 0;
                }
#endif
                connected=1;
                return 1;
        }
}


cursor  *myconnection::initCursor() {
        return (cursor *)new mycursor((connection *)this);
}


sqlrcursor      *mysqlconnection::initCursor() {
        return (sqlrcursor *)new mysqlcursor((sqlrconnection *)this);
}


void    myconnection::deleteCursor(cursor *curs) {
        delete (mycursor *)curs;
}


void    mysqlconnection::deleteCursor(sqlrcursor *curs) {
        delete (mysqlcursor *)curs;
}


void    myconnection::logOut() {

	... disconnect from the database ...
}


void    mysqlconnection::logOut() {
        connected=0;
        mysql_close(&mysql);
}


int     myconnection::ping() {

	... test to see if the database connection
            is still good, return 1 if it is, and
            0 if it's not ...
}


#if MYSQL_VERSION_ID>=32200
int     mysqlconnection::ping() {
        if (!mysql_ping(&mysql)) {
                return 1;
        }
        return 0;
}
#endif


char    *myconnection::identify() {

	... return the name of your database ...
}


char    *mysqlconnection::identify() {
        return "mysql";
}


int     myconnection::isTransactional() {

	... you only need this method if your
            database is not transactional ...

        ... return a 0 ...
}


int     mysqlconnection::isTransactional() {
        return 0;
}


unsigned short    myconnection::autoCommmitOn() {

	... set autocommit on ...

	... return 1 for success and 0 for failure ...
}


unsigned short  mysqlconnection::autoCommitOn() {
        // do nothing
        return 1;
}


unsigned short    myconnection::autoCommmitOff() {

	... set autocommit off ...

	... return 1 for success and 0 for failure ...
}


unsigned short  mysqlconnection::autoCommitOff() {
        // do nothing
        return 1;
}


void     myconnection::commit() {

	... if your database has a special commit 
	    function, call it here ...

	... if your database is non-transactional,
	    make this function do nothing ...

        ... if your database is transactional but
            doesn't have a special commit function,
            don't even implement this method at all,
	    let the base class provide it ...
}


int     mysqlconnection::commit() {
        // do nothing
        return 1;
}


void     myconnection::rollback() {

	... if your database has a special rollback 
	    function, call it here ...

	... if your database is non-transactional,
	    make this function do nothing ...

        ... if your database is transactional but
            doesn't have a special rollback function,
            don't even implement this method at all,
	    let the base class provide it ...
}


int     mysqlconnection::rollback() {
        // do nothing
        return 1;
}


mycursor::mycursor(connection *conn) : cursor(conn) {

        mysqlconn=(mysqlconnection *)conn;
        errmesg=NULL;

	... initialize whatever you need to here ...
}


mysqlcursor::mysqlcursor(sqlrconnection *conn) : sqlrcursor(conn) {
        mysqlconn=(mysqlconnection *)conn;
        mysqlresult=NULL;
}


mycursor::~mycursor() {

	... deallocate whatever you initialized ...
}


	... the mysqlcursor has no destructor ...


int     mycursor::executeQuery(char *query, long length) {

	... check for a query that creates a temporary
	    table if your database supports them ...

	... set row and column counts to 0 ...

        Note: Some databases have a 3 step query
              execution process involving bind variables.
              See the information at the end of this
              document for how to handle those
              databases.

	... if your database doesn't support real
	    binds, then create a strstream buffer,
	    call fakeInputBinds() and pass the
	    result into your execute query command
	    (unless the result is NULL which means
	    there were no variables to bind in which
	    case you must execute the original query) ...

        ... get the result set ...

        ... get information about the result set 
            such as the row and column counts
            and the number of affected rows 
            (affected rows are the number of rows
            affected by an insert, update or delete) ...

        ... return 1 for success, 0 for failure ...
}


int     mysqlcursor::executeQuery(const char *query, long length,
                                                unsigned short execute) {

        checkForTempTable(query,length);

        // initialize counts
        ncols=0;
        nrows=0;

        // initialize result set
        mysqlresult=NULL;

        // fake binds
        stringbuffer    *newquery=fakeInputBinds(query);

        // execute the query
        if (newquery) {
                if (queryresult=mysql_real_query(&mysqlconn->mysql,
                                        newquery->getString(),
                                        strlen(newquery->getString()))) {
                        delete newquery;
                        return 0;
                }
                delete newquery;
        } else {
                if (queryresult=mysql_real_query(&mysqlconn->mysql,
                                                        query,length)) {
                        return 0;
                }
        }

        // get the affected row count
        affectedrows=mysql_affected_rows(&mysqlconn->mysql);

        // store the result set
        if ((mysqlresult=mysql_store_result(&mysqlconn->mysql))==
                                                (MYSQL_RES *)NULL) {

                // if there was an error then return failure, otherwise
                // the query must have been some DML or DDL
                char    *err=mysql_error(&mysqlconn->mysql);
                if (err && err[0]) {
                        return 0;
                } else {
                        return 1;
                }
        }

        // get the column count
        ncols=mysql_num_fields(mysqlresult);

        // get the row count
        nrows=mysql_num_rows(mysqlresult);

        return 1;
}


char    *mycursor::getErrorMessage(int *liveconnection) {

	... call the database API to get an error message ...

        ... if that error is a down database, set the
            "liveconnection" parameter to 1 ...

        ... return the error message string ...
}


char    *mysqlcursor::getErrorMessage(int *liveconnection) {

        *liveconnection=1;
        char    *err=mysql_error(&mysqlconn->mysql);
#if MYSQL_VERSION_ID>=32200
        if (queryresult==CR_SERVER_GONE_ERROR || queryresult==CR_SERVER_LOST) {
                *liveconnection=0;
        }
#else
        if (strstr(err,"mysql server has gone away")) {
                *liveconnection=0;
        }
#endif
        return err;
}


void    mycursor::returnColumnCount() {

	... use the sendColumnCount() method to 
	    return the number of columns in the result set...
}


void    mysqlcursor::returnColumnCount() {
        conn->sendColumnCount(ncols);
}


void    mycursor::returnRowCounts() {

	... use the sendRowCounts() method to return the 
	    number of rows and/or affected rows in the result set...

	... if your database api doesn't supply one or both
	    of these values, return -1 for whichever it doesn't
	    supply ...
}


void    mysqlcursor::returnRowCounts() {

        // send row counts
        conn->sendRowCounts((long)nrows,(long)affectedrows);
}


void    mycursor::returnColumnInfo() {

	... It is possible to send back column types as strings or
	    id's.  Using id's is faster and preferred, however
	    if your database requires users to define their own
	    column types and has no built-in types, you can return
	    strings instead. ...

        ... if the query isn't a select, return immediately ...

        ... Position yourself at the first column.
            This is important because returnColumnInfo()
            may be called more than 1 time per result
            set (if the user suspends the result set
            for example) and you don't want to be
            off the end of the column list from the
            previous call.  Some database API's provide
            methods for accessing the columns by index
            or require you to store the definitions
            in your own array.  For those databases, 
            this positioning is not necessary. ...

        ... Run through the columns of the result set, 
            for each, use the sendColumnDefinition()
	    (or sendColumnDefinitionString()) method
	    to return the name, type and size of each.
	    When using sendColumnDefinition(), the
	    column type should be one of the types in
	    the datatypes.h file, so you'll have to
	    establish a mapping between one of them and
	    the database API's data types.  If your
	    database has a type that's not in that file,
	    add it and submit a patch! ...
}


void    mysqlcursor::returnColumnInfo() {

        conn->sendColumnTypeFormat(COLUMN_TYPE_IDS);

        // for DML or DDL queries, return no column info
        if (!mysqlresult) {
                return;
        }

        // some useful variables
        int     type;
        int     length;

        // position ourselves at the first field
        mysql_field_seek(mysqlresult,0);

        // for each column...
        for (int i=0; i<ncols; i++) {

                // fetch the field
                mysqlfield=mysql_fetch_field(mysqlresult);

                // append column type to the header
                if (mysqlfield->type==FIELD_TYPE_STRING) {
                        type=STRING_DATATYPE;
                        length=(int)mysqlfield->length;
                } else if (mysqlfield->type==FIELD_TYPE_VAR_STRING) {
                        type=CHAR_DATATYPE;
                        length=(int)mysqlfield->length+1;
                } else if (mysqlfield->type==FIELD_TYPE_DECIMAL) {
                        type=DECIMAL_DATATYPE;
                        if (mysqlfield->decimals>0) {
                                length=(int)mysqlfield->length+2;
                        } else if (mysqlfield->decimals==0) {
                                length=(int)mysqlfield->length+1;
                        }
                        if (mysqlfield->length<mysqlfield->decimals) {
                                length=(int)mysqlfield->decimals+2;
                        }
                } else if (mysqlfield->type==FIELD_TYPE_TINY) {
                        type=TINYINT_DATATYPE;
                        length=1;
                } else if (mysqlfield->type==FIELD_TYPE_SHORT) {
                        type=SMALLINT_DATATYPE;
                        length=2;
                } else if (mysqlfield->type==FIELD_TYPE_LONG) {
                        type=INT_DATATYPE;
                        length=4;
                } else if (mysqlfield->type==FIELD_TYPE_FLOAT) {
                        type=FLOAT_DATATYPE;
                        if (mysqlfield->length<=24) {
                                length=4;
                        } else {
                                length=8;
                        }
                } else if (mysqlfield->type==FIELD_TYPE_DOUBLE) {
                        type=REAL_DATATYPE;
                        length=8;
                } else if (mysqlfield->type==FIELD_TYPE_LONGLONG) {
                        type=BIGINT_DATATYPE;
                        length=8;
                } else if (mysqlfield->type==FIELD_TYPE_INT24) {
                        type=MEDIUMINT_DATATYPE;
                        length=3;
                } else if (mysqlfield->type==FIELD_TYPE_TIMESTAMP) {
                        type=TIMESTAMP_DATATYPE;
                        length=4;
                } else if (mysqlfield->type==FIELD_TYPE_DATE) {
                        type=DATE_DATATYPE;
                        length=3;
                } else if (mysqlfield->type==FIELD_TYPE_TIME) {
                        type=TIME_DATATYPE;
                        length=3;
                } else if (mysqlfield->type==FIELD_TYPE_DATETIME) {
                        type=DATETIME_DATATYPE;
                        length=8;
#if MYSQL_VERSION_ID>=32200
                } else if (mysqlfield->type==FIELD_TYPE_YEAR) {
                        type=YEAR_DATATYPE;
                        length=1;
                } else if (mysqlfield->type==FIELD_TYPE_NEWDATE) {
                        type=NEWDATE_DATATYPE;
                        length=1;
#endif
                } else if (mysqlfield->type==FIELD_TYPE_NULL) {
                        type=NULL_DATATYPE;
#ifdef MYSQL_VERSION_ID
                } else if (mysqlfield->type==FIELD_TYPE_ENUM) {
                        type=ENUM_DATATYPE;
                        // 1 or 2 bytes delepending on the # of enum values
                        // (65535 max)
                        length=2;
                } else if (mysqlfield->type==FIELD_TYPE_SET) {
                        type=SET_DATATYPE;
                        // 1,2,3,4 or 8 bytes delepending on the # of members
                        // (64 max)
                        length=8;
#endif
                // For some reason, tinyblobs, mediumblobs and longblobs
                // all show up as FIELD_TYPE_BLOB despite field types being
                // defined for those types.  tinyblobs have a length
                // of 255 though, so that can be used for something.  medium
                // and long blobs both have the same length though.  Go
                // figure.  Also, the word TEXT and BLOB appear to be
                // interchangable.  We'll use BLOB because it appears to be
                // more standard than TEXT.  I wonder if this will be changed
                // in a future incarnation of mysql.  I also wonder what
                // happens on a 64 bit machine.
                } else if (mysqlfield->type==FIELD_TYPE_TINY_BLOB) {
                        type=TINY_BLOB_DATATYPE;
                        length=(int)mysqlfield->length+2;
                } else if (mysqlfield->type==FIELD_TYPE_MEDIUM_BLOB) {
                        type=MEDIUM_BLOB_DATATYPE;
                        length=(int)mysqlfield->length+3;
                } else if (mysqlfield->type==FIELD_TYPE_LONG_BLOB) {
                        type=LONG_BLOB_DATATYPE;
                        length=(int)mysqlfield->length+4;
                } else if (mysqlfield->type==FIELD_TYPE_BLOB) {
                        if ((int)mysqlfield->length==255) {
                                type=TINY_BLOB_DATATYPE;
                                length=(int)mysqlfield->length+2;
                        } else {
                                type=BLOB_DATATYPE;
                                length=(int)mysqlfield->length+3;
                        }
                } else {
                        type=UNKNOWN_DATATYPE;
                        length=(int)mysqlfield->length;
                }

                // send column definition
                // for mysql, length is actually precision
                conn->sendColumnDefinition(mysqlfield->name,
                                        strlen(mysqlfield->name),
                                        type,length,
                                        mysqlfield->length,
                                        mysqlfield->decimals,
                                        !(IS_NOT_NULL(mysqlfield->flags)),
                                        IS_PRI_KEY(mysqlfield->flags),0);
        }
}


int     mycursor::noRowsToReturn() {

	... test to see if you're at the end of the
            result set, if so, return 1, otherwise
            return 0 ...

        ... if there is no good way to know this
	    for your database, just return 0 ...
}


int     mysqlcursor::noRowsToReturn() {

        // for DML or DDL queries, return no data
        if (!mysqlresult) {
                return 1;
        }
        return 0;
}


int     mycursor::skipRow() {

	... If your API can skip rows in the result set without
	    actually fetching them, then do that here.  If not
	    just call fetchRow() ...
}


int     mysqlcursor::skipRow() {
        return fetchRow();
}


int     mycursor::fetchRow() {

	... fetch a row, return a 1 for success and 0 for failure ...
}


int     mysqlcursor::fetchRow() {

        return ((mysqlrow=mysql_fetch_row(mysqlresult))!=NULL);
}


void    mycursor::returnRow() {

	... run through each column of the result 
            set and return it using the sendField()
            method ...
}


void    mysqlcursor::returnRow() {

        for (int col=0; col<ncols; col++) {

                if (mysqlrow[col]) {
                        conn->sendField(mysqlrow[col],strlen(mysqlrow[col]));
                } else {
                        conn->sendNullField();
                }
        }
}


void    mycursor::cleanUpData() {

	... if you allocated any memory between
            getting the query and returning the
            result set, deallocate it here ...

	... Note, this method shouldn't arbitrarily
	    deallocate memory.  It is possible that
	    it might be called before the first
	    query has been run, after a query failed
	    to run, etc. ...
}


void    mysqlcursor::cleanUpData() {
        if (mysqlresult!=(MYSQL_RES *)NULL) {
                mysql_free_result(mysqlresult);
                mysqlresult=NULL;
        }
}

If your database has a lightweight facility for switching from one user to another without just logging out and logging back in as a different user, you should consider overriding the changeUser() method.

You may elect to override some more of the cursor class's virtual methods as well.

When a session is ended (either on purpose or accident) the connection daemon sends either a commit or rollback (depending on how it's configured) to the database if any DML or DDL queries were executed. The following methods are used to implement this functionality.

The queryIsNotSelect() method returns 0 if the query is a SELECT and 1 if it is not. Similarly, the queryIsCommitOrRollback() method returns 1 if the query is either a COMMIT or ROLLBACK and 0 if it is not. The connection class implements these methods by parsing the text of the query. Many database API's have their own means of identifying the query type. If yours does, then you should use it to override these methods.

Some databases have a 3 part process for executing queries or procedural code. In Part 1, the database figures out what steps it will take to run the query or code. In Part 2, local variables or values are "bound" to variables in those steps. In Part 3, the steps are executed. If your database has a process like this, you should override these methods.

The prepareQuery() method should execute Part 1 functions. The inputBindValue() and outputBindValue() methods should handle functions for Part 2. And the executeQuery() method should handle Part 3.

If your database uses a character other than a colon as a prefix for it's named bind variables, then you should override the bindVariablePrefix() method to return that character. Similarly, if your database uses a value other than 0 in it's bind functions to indicate that a value is NULL and a value other than -1 to indicate that a value is non-NULL then you should override the nullBindValue() and nonNullBindValue() methods to return the appropriate values. You should also override the bindValueIsNull() method to return 0 if the value passed into it corresponds to your non-NULL indicator and 1 if the value passed into it corresponds to your NULL indicator.

If your database API has a function for determining whether the database is up or down (a ping method) then you should implement the ping() method. The example MySQL connection does this. However, if your database has no such method, the connection class can run a simple query and check for a "database is down" error. You can specify a query to run by implementing the pingQuery() method instead of the ping() method.