LMDB database atomic get and the put in C

Question

I need to get the key from LMDB database and then depending on this key put some additional data into value of this key back and it is necessary to make this key unwritable for others during this operations.

The library I use - github.com/LMDB/lmdb/tree/mdb.master/libraries/liblmdb, and the lmdb.h file - github.com/LMDB/lmdb/blob/mdb.master/libraries/liblmdb/lmdb.h

How is it possible in LMDB?

Here is my basic code:

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

#include "lmdb.h"


#define BUF_SIZE 1024
#define CACHE_SIZE 1UL * 1024UL * 1024UL * 1024UL

#define E(expr) CHECK((rc = (expr)) == MDB_SUCCESS, #expr)
#define CHECK(test, msg) ((test) ? (void)0 : ((void)fprintf(stderr, \
    "%s:%d: %s: %s\n", __FILE__, __LINE__, msg, mdb_strerror(rc)), abort()))

/* Must create somedb folder first */
#define DBDIR "./somedb"


/* Put the key to db */
int putdb(char *, char *);

/* Put and get the key from db */
int getandputdb(char *, char *);

/* Get the key from db */
int getdb(char *);


int main(int argc, char * argv[])
{
    putdb("key", "value");
    getdb("key");

    /* Must to be atomic */
    getandputdb("KeyToGet", "ValueToBePutAfterGet");

    return 0;
}

/* Put to db */
int putdb(char *keyvalue, char *value) {

    int rc;
    MDB_env *env;
    MDB_dbi dbi;
    MDB_txn *txn;
    MDB_val key, data, rdata;
    MDB_cursor *cursor;
    char sval[32];

    //printf("put %s %s\n",keyvalue, value);

    E(mdb_env_create(&env));

    /* Set the cache size */
    E(mdb_env_set_mapsize(env, CACHE_SIZE));
    E(mdb_env_open(env, DBDIR, 0, 0664));

    /* Put some data */
    E(mdb_txn_begin(env, NULL, 0, &txn));
    E(mdb_dbi_open(txn, NULL, 0, &dbi));

    key.mv_data = keyvalue;
    key.mv_size = strlen(keyvalue);
    data.mv_data = value;
    data.mv_size = strlen(value);

    E(mdb_put(txn, dbi, &key, &data, 0));

    E(mdb_txn_commit(txn));

    mdb_dbi_close(env, dbi);
    mdb_env_close(env);

    return 0;
}

/* Get from lmdb */
int getdb(char *thekey) {

    int rc;
    MDB_env *env;
    MDB_dbi dbi;
    MDB_txn *txn;
    MDB_val key, data, rdata;
    MDB_cursor *cursor;

    //printf("get %s\n", thekey);

    E(mdb_env_create(&env));

    /* Set the cache size */
    E(mdb_env_set_mapsize(env, CACHE_SIZE));
    E(mdb_env_open(env, DBDIR, 0, 0664));

    /* Get some data */
    E(mdb_txn_begin(env, NULL, 0, &txn));
    E(mdb_dbi_open(txn, NULL, 0, &dbi));

    key.mv_data = thekey;
    key.mv_size = strlen(thekey);

    rc = mdb_get(txn, dbi, &key, &data);
    if (!rc) {
        char *valuen = (char *) malloc(data.mv_size + 1);
        memcpy(valuen, data.mv_data, data.mv_size);
        valuen[data.mv_size] = 0;

        printf("%s %s\n", (char *) key.mv_data, valuen);

        free(valuen);
    } else {
        printf("No such key\n");
    }

    //printf("%s %d, %s %d\n", (char *) key.mv_data, key.mv_size, (char *)data.mv_data, data.mv_size);

    E(mdb_txn_commit(txn));


    mdb_dbi_close(env, dbi);
    mdb_env_close(env);

    return 0;
}

/* Put and get from db */
int getandputdb(char *keyvalue, char *value) {

    int rc;
    MDB_env *env;
    MDB_dbi dbi;
    MDB_txn *txn;
    MDB_val key, data, rdata;
    MDB_cursor *cursor;
    char sval[32];

    /*
     * Atomic operation, how to do this?
     * Get the key value and then put the value with some changes back in the atomic operation.
     */



    return 0;

}

Everything is working pretty well but how to do the atomic transaction of get, then check some conditions and then put?

Answer 1

• Create a transaction with mdb_txn_begin. Make sure you do NOT flag it as MDB_RDONLY.
• Get a value with mdb_get
• Change the value as needed.
• Put the new value with mdb_put
• Finish the transaction with mdb_txn_commit

This is pretty much how you run transactions in any database system. See also sample code in the mtest*.c files.