Database Usage Example
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## Database Usage Example

In Database Example we created several functions that will open and close the databases that we will use for our inventory application. We now make use of those functions to load inventory data into the two databases that we use for this application.

Again, remember that you can find the complete implementation for these functions in:

    DB_INSTALL/examples_c/getting_started

where `DB_INSTALL` is the location where you placed your DB distribution.

Example 3.1 VENDOR Structure

We want to store data related to an inventory system. There are two types of information that we want to manage: inventory data and related vendor contact information. To manage this information, we could create a structure for each type of data, but to illustrate storing mixed data without a structure we refrain from creating one for the inventory data.

For the vendor data, we add the VENDOR structure to the same file as holds our STOCK_DBS structure. Note that the VENDOR structure uses fixed-length fields. This is not necessary and in fact could represent a waste of resources if the number of vendors stored in our database scales to very large numbers. However, for simplicity we use fixed-length fields anyway, especially given that our sample data contains so few vendor records.

Note that for the inventory data, we will store the data by marshaling it into a buffer, described below.

    /* File: gettingstarted_common.h */
    #include <db.h>

    ...

    typedef struct vendor {
        char name[MAXFIELD];             /* Vendor name */
        char street[MAXFIELD];           /* Street name and number */
        char city[MAXFIELD];             /* City */
        char state[3];                   /* Two-digit US state code */
        char zipcode[6];                 /* US zipcode */
        char phone_number[13];           /* Vendor phone number */
        char sales_rep[MAXFIELD];        /* Name of sales representative */
        char sales_rep_phone[MAXFIELD];  /* Sales rep's phone number */
    } VENDOR;

Example 3.2 example_database_load

Our initial sample application will load database information from several flat files. To save space, we won't show all the details of this example program. However, as always you can find the complete implementation for this program here:

    DB_INSTALL/examples_c/getting_started

where `DB_INSTALL` is the location where you placed your DB distribution.

We begin with the normal include directives and forward declarations:

    /* example_database_load.c */
    #include "gettingstarted_common.h"

    /* Forward declarations */
    int load_vendors_database(STOCK_DBS, char *);
    int pack_string(char *, char *, int);
    int load_inventory_database(STOCK_DBS, char *);

Next we begin our `main()` function with the variable declarations and command line parsing that is normal for most command line applications:

    /*
     * Loads the contents of vendors.txt and inventory.txt into
     * Berkeley DB databases.
     */
    int
    main(int argc, char *argv[])
    {
        STOCK_DBS my_stock;
        int ret, size;
        char *basename, *inventory_file, *vendor_file;

        /* Initialize the STOCK_DBS struct */
        initialize_stockdbs(&my_stock);

       /*
        * Initialize the base path. This path is used to
        * identify the location of the flat-text data
        * input files.
        */
        basename = "./";

        /*
         * Parse the command line arguments here and determine
         * the location of the flat text files containing the
         * inventory data here. This step is omitted for clarity.
         */

        /*
         * Identify the files that will hold our databases
         * This function uses information obtained from the
         * command line to identify the directory in which
         * the database files reside.
         */
        set_db_filenames(&my_stock);

        /* Find our input files */
        size = strlen(basename) + strlen(INVENTORY_FILE) + 1;
        inventory_file = malloc(size);
        snprintf(inventory_file, size, "%s%s", basename, INVENTORY_FILE);

        size = strlen(basename) + strlen(VENDORS_FILE) + 1;
        vendor_file = malloc(size);
        snprintf(vendor_file, size, "%s%s", basename, VENDORS_FILE);

        /* Open all databases */
        ret = databases_setup(&my_stock, "example_database_load", stderr);
        if (ret != 0) {
                fprintf(stderr, "Error opening databases\n");
                databases_close(&my_stock);
                return (ret);
        }

        ret = load_vendors_database(my_stock, vendor_file);
        if (!ret) {
            fprintf(stderr, "Error loading vendors database.\n");
            databases_close(&my_stock);
            return (ret);
        }
        ret = load_inventory_database(my_stock, inventory_file);
        if (!ret) {
            fprintf(stderr, "Error loading inventory database.\n");
            databases_close(&my_stock);
            return (ret);
        }

        /* close our environment and databases */
        databases_close(&my_stock);

        printf("Done loading databases.\n");
        return (0);
    }

Notice that there is not a lot to this function because we have pushed off all the database activity to other places. In particular our databases are all opened and configured in `databases_setup()` which we implemented in The databases_setup() Function.

Next we show the implementation of `load_vendors_database()`. We load this data by scanning (line by line) the contents of the `vendors.txt` into a VENDOR structure. Once we have a line scanned into the structure, we can store that structure into our vendors database.

Note that we use the vendor's name as the key here. In doing so, we assume that the vendor's name is unique in our database. If it was not, we would either have to select a different key, or architect our application such that it could cope with multiple vendor records with the same name.

    /*
     * Loads the contents of the vendors.txt file into
     * a database.
     */
    int
    load_vendors_database(STOCK_DBS my_stock, char *vendor_file)
    {
        DBT key, data;
        FILE *ifp;
        VENDOR my_vendor;
        char buf[MAXLINE];

        /* Open the vendor file for read access */
        ifp = fopen(vendor_file, "r");
        if (ifp == NULL) {
            fprintf(stderr, "Error opening file '%s'\n", vendor_file);
            return(-1);
        }

        /* Iterate over the vendor file */
        while(fgets(buf, MAXLINE, ifp) != NULL) {
            /* zero out the structure */
            memset(&my_vendor, 0, sizeof(VENDOR));
            /* Zero out the DBTs */
            memset(&key, 0, sizeof(DBT));
            memset(&data, 0, sizeof(DBT));

            /*
             * Scan the line into the structure.
             * Convenient, but not particularly safe.
             * In a real program, there would be a lot more
             * defensive code here.
             */
            sscanf(buf,
              "%20[^#]#%20[^#]#%20[^#]#%3[^#]#%6[^#]#%13[^#]#%20[^#]#%20[^\n]",
              my_vendor.name, my_vendor.street,
              my_vendor.city, my_vendor.state,
              my_vendor.zipcode, my_vendor.phone_number,
              my_vendor.sales_rep, my_vendor.sales_rep_phone);

            /*
             * Now that we have our structure we can load it
             * into the database.
             */

            /* Set up the database record's key */
            key.data = my_vendor.name;
            key.size = strlen(my_vendor.name) + 1;

            /* Set up the database record's data */
            data.data = &my_vendor;
            data.size = sizeof(my_vendor);

            /*
             * Note that given the way we built our struct, there are extra
             * bytes in it. Essentially we're using fixed-width fields with
             * the unused portion of some fields padded with zeros. This
             * is the easiest thing to do, but it does result in a bloated
             * database. Look at load_inventory_data() for an example of how
             * to avoid this.
             */

            /* Put the data into the database.
             * Omitting error handling for clarity.
             */
            my_stock.vendor_dbp->put(my_stock.vendor_dbp, 0, &key, &data, 0);
        } /* end vendors database while loop */

        /* Close the vendor.txt file */
        fclose(ifp);
        return(0);
    }

Finally, we need to write the `load_inventory_database()` function. We made this function a bit more complicated than is necessary by avoiding the use of a structure to manage the data. Instead, we manually pack all our inventory data into a single block of memory, and store that data in the database.

While this complicates our code somewhat, this approach allows us to use the smallest amount of space possible for the data that we want to store. The result is that our cache can be smaller than it might otherwise be and our database will take less space on disk than if we used a structure with fixed-length fields.

For a trivial dataset such as what we use for these examples, these resource savings are negligible. But if we were storing hundreds of millions of records, then the cost savings may become significant.

Before we actually implement our inventory loading function, it is useful to create a simple utility function that copies a character array into a buffer at a designated offset:

    /*
     * Simple little convenience function that takes a buffer, a string,
     * and an offset and copies that string into the buffer at the
     * appropriate location. Used to ensure that all our strings
     * are contained in a single contiguous chunk of memory.
     */
    int
    pack_string(char *buffer, char *string, int start_pos)
    {
        int string_size = strlen(string) + 1;

        memcpy(buffer+start_pos, string, string_size);

        return(start_pos + string_size);
    }

That done, we can now load the inventory database:

    /*
     * Loads the contents of the inventory.txt file into
     * a database.
     */
    int
    load_inventory_database(STOCK_DBS my_stock, char *inventory_file)
    {
        DBT key, data;
        char buf[MAXLINE];
        void *databuf;
        int bufLen, dataLen;
        FILE *ifp;

        /*
         * Rather than lining everything up nicely in a struct, we're being
         * deliberately a bit sloppy here. This function illustrates how to
         * store mixed data that might be obtained from various locations
         * in your application.
         */
        float price;
        int quantity;
        char category[MAXFIELD], name[MAXFIELD];
        char vendor[MAXFIELD], sku[MAXFIELD];

        /* Load the inventory database */
        ifp = fopen(inventory_file, "r");
        if (ifp == NULL) {
            fprintf(stderr, "Error opening file '%s'\n", inventory_file);
            return(-1);
        }

        /* Get our buffer. MAXDATABUF is some suitably large number */
        databuf = malloc(MAXDATABUF);

        /*
         * Read the inventory.txt file line by line, saving each line off to the
         * database as we go.
         */
        while(fgets(buf, MAXLINE, ifp) != NULL) {
            /*
             * Scan the line into the appropriate buffers and variables.
             * Convenient, but not particularly safe. In a real
             * program, there would be a lot more defensive code here.
             */
            sscanf(buf,
              "%20[^#]#%20[^#]#%f#%i#%20[^#]#%20[^\n]",
              name, sku, &price, &quantity, category, vendor);

            /*
             * Now pack it into a single contiguous memory location for
             * storage.
             */
            memset(databuf, 0, MAXDATABUF);
            bufLen = 0;
            dataLen = 0;

            /*
             * We first store the fixed-length elements. This makes our code to
             * retrieve this data from the database a little bit easier.
             */

            /* First discover how long the data element is. */
            dataLen = sizeof(float);
            /* Then copy it to our buffer */
            memcpy(databuf, &price, dataLen);
            /*
             * Then figure out how much data is actually in our buffer.
             * We repeat this pattern for all the data we want to store.
             */
            bufLen += dataLen;

            /* Rinse, lather, repeat. */
            dataLen = sizeof(int);
            memcpy(databuf + bufLen, &quantity, dataLen);
            bufLen += dataLen;

            bufLen = pack_string(databuf, name, bufLen);
            bufLen = pack_string(databuf, sku, bufLen);
            bufLen = pack_string(databuf, category, bufLen);
            bufLen = pack_string(databuf, vendor, bufLen);

            /*
             * Now actually save the contents of the buffer off
             * to our database.
             */

            /* Zero out the DBTs */
            memset(&key, 0, sizeof(DBT));
            memset(&data, 0, sizeof(DBT));

            /*
             * The key is the item's SKU. This is a unique value, so we need not
             * support duplicates for this database.
             */
            key.data = sku;
            key.size = strlen(sku) + 1;

            /* The data is the information that we packed into databuf. */
            data.data = databuf;
            data.size = bufLen;

            /* Put the data into the database */
            my_stock.vendor_dbp->put(my_stock.inventory_dbp, 0, &key, &data, 0);
        } /* end vendors database while loop */

        /* Cleanup */
        fclose(ifp);
        if (databuf != NULL)
            free(databuf);

        return(0);
    }

In the next chapter we provide an example that shows how to read the inventory and vendor databases.

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