在C程序中实现类似Redis的SCAN机制的LevelDB大规模key分批扫描-CFANZ编程社区

要在C程序中实现类似Redis的SCAN机制的LevelDB大规模key分批扫描，您可以通过以下方式进行实现：

批量迭代：维护一个迭代器，从上次扫描位置继续扫描，直到指定批量大小为止。
标记位置：在每次迭代完成后保存当前迭代器的位置，以便下次迭代从这个位置继续。

下面是一个示例程序，展示如何实现这种机制：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <leveldb/c.h>

#define BATCH_SIZE 10  // 每次扫描的键值对数量

void scan_batch(leveldb_t* db, const char* start_key, size_t start_key_len) {
    leveldb_readoptions_t* read_options = leveldb_readoptions_create();
    leveldb_iterator_t* it = leveldb_create_iterator(db, read_options);
    
    // 定位到起始键
    if (start_key != NULL) {
        leveldb_iter_seek(it, start_key, start_key_len);
    } else {
        leveldb_iter_seek_to_first(it);
    }

    // 扫描批量键值对
    int count = 0;
    for (; leveldb_iter_valid(it) && count < BATCH_SIZE; leveldb_iter_next(it), ++count) {
        size_t key_len, value_len;
        const char* key = leveldb_iter_key(it, &key_len);
        const char* value = leveldb_iter_value(it, &value_len);

        printf("Key: %.*s, Value: %.*s\n", (int)key_len, key, (int)value_len, value);
    }

    // 检查是否有更多键值对
    if (leveldb_iter_valid(it)) {
        size_t key_len;
        const char* next_start_key = leveldb_iter_key(it, &key_len);
        printf("Next batch start key: %.*s\n", (int)key_len, next_start_key);
    } else {
        printf("End of database reached.\n");
    }

    // 清理
    leveldb_iter_destroy(it);
    leveldb_readoptions_destroy(read_options);
}

int main() {
    // 创建并打开一个新的LevelDB数据库
    leveldb_options_t* options = leveldb_options_create();
    leveldb_options_set_create_if_missing(options, 1);

    char* err = NULL;
    leveldb_t* db = leveldb_open(options, "testdb", &err);
    if (err != NULL) {
        fprintf(stderr, "Open fail.\n");
        return 1;
    }

    // 写一些键值对到数据库中
    leveldb_writeoptions_t* write_options = leveldb_writeoptions_create();
    for (int i = 0; i < 50; ++i) {
        char key[20], value[20];
        snprintf(key, sizeof(key), "key%d", i);
        snprintf(value, sizeof(value), "value%d", i);
        leveldb_put(db, write_options, key, strlen(key), value, strlen(value), &err);
        if (err != NULL) {
            fprintf(stderr, "Write fail.\n");
            return 1;
        }
    }

    // 分批次扫描数据库
    const char* start_key = NULL;
    size_t start_key_len = 0;
    for (int batch_num = 0; batch_num < 5; ++batch_num) {
        printf("Batch %d:\n", batch_num + 1);
        scan_batch(db, start_key, start_key_len);

        // 获取下一个批次的起始键
        leveldb_readoptions_t* read_options = leveldb_readoptions_create();
        leveldb_iterator_t* it = leveldb_create_iterator(db, read_options);
        if (start_key != NULL) {
            leveldb_iter_seek(it, start_key, start_key_len);
        } else {
            leveldb_iter_seek_to_first(it);
        }

        for (int i = 0; i < BATCH_SIZE && leveldb_iter_valid(it); ++i) {
            leveldb_iter_next(it);
        }

        if (leveldb_iter_valid(it)) {
            start_key = leveldb_iter_key(it, &start_key_len);
        } else {
            start_key = NULL;
        }

        leveldb_iter_destroy(it);
        leveldb_readoptions_destroy(read_options);

        if (start_key == NULL) {
            break;
        }
    }

    // 清理
    leveldb_writeoptions_destroy(write_options);
    leveldb_close(db);
    leveldb_options_destroy(options);

    return 0;
}