简单数据库实现——Part5 - 持久存储到磁盘
目前的数据库在终止程序重新启动后会丢失所有记录，所以我们会通过将整个数据库保存到一个文件来保存记录。
我们已经通过将行序列化到页面大小的内存快中，为了增加持久性，我们可以简单的将内存块写入文件并在下次程序启动时读入内存。
为了简化这一过程，我们将进行一个称为寻呼机(pager)的抽象。我们向寻呼机询问页码x，寻呼机返回给我们一块内存。它会首先查看他的缓存(cache)，如果缓存中不存在，那么它会从磁盘(disk)中将数据复制到内存中。
寻呼机(pager)访问页面缓存和文件，表对象通过寻呼机请求页面：
+typedef struct {
+ int file_descriptor;
+ uint32_t file_length;
+ void* pages[TABLE_MAX_PAGES];
+} Pager;
+typedef struct {
- void* pages[TABLE_MAX_PAGES];
+ Pager* pager;uint32_t num_rows;} Table;
将 new_table() 重命名为 db_open() ，因为它现在的作用是连接数据库：
- 打开数据库文件
- 初始化寻呼机(pager)数据结构
- 初始化表(table)数据结构
-Table* new_table() {
+Table* db_open(const char* filename) {
+ Pager* pager = pager_open(filename);
+ uint32_t num_rows = pager->file_length / ROW_SIZE;
+Table* table = malloc(sizeof(Table));
- table->num_rows = 0;
+ table->pager = pager;
+ table->num_rows = num_rows;return table;}
db_open() 依次调用 paper_open()，这将打开数据文件并跟踪其大小。它还将页面缓存初始化为 NULL 。
函数的使用：
open()函数(APUE 3.3和4.5节)：
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int open(const char *pathname, int oflag,.../*, mode_t mode */);
/* 返回：若成功为文件描述符若出错为-1 */
pathname是要打开或创建的文件的名字。oflag参数可用来说明此函数的多个选择项。
O_RDWR读、写打开O_CREAT若此文件不存在则创建它。使用此选择项时，需同时说明第三个参数mode，用其说明该新文件的存取许可权位。S_IWUSR用户-写S_IRUSR用户-读
lseek()函数(APUE 3.6节)：
#include <sys/types.h>
#include <unistd.h>
off_t lseek(int filedes, off_t offset, int whence);
/*返回：若成功为新的文件位移若出错为-1 */
对参数offset的解释与参数whence的值有关。
- 若
whence是SEEK_END，则将该文件的位移量设置为文件长度加offset，offset可为正或负。
若lseek成功执行，则返回新的文件位移量。
+Pager* pager_open(const char* filename) {
+ int fd = open(filename,
+ O_RDWR | // Read/Write mode
+ O_CREAT, // Create file if it does not exist
+ S_IWUSR | // User write permission
+ S_IRUSR // User read permission
+ );
+
+ if (fd == -1) {
+ printf("Unable to open file\n");
+ exit(EXIT_FAILURE);
+ }
+
+ off_t file_length = lseek(fd, 0, SEEK_END);
+
+ Pager* pager = malloc(sizeof(Pager));
+ pager->file_descriptor = fd;
+ pager->file_length = file_length;
+
+ for (uint32_t i = 0; i < TABLE_MAX_PAGES; i++) {
+ pager->pages[i] = NULL;
+ }
+
+ return pager;
+}
我们现在将获取页面移动到它自己的方法 get_page() 中。
void* row_slot(Table* table, uint32_t row_num) {
uint32_t page_num = row_num / ROWS_PER_PAGE;
- void* page = table->pages[page_num];
- if (page == NULL) {
- // Allocate memory only when we try to access page
- page = table->pages[page_num] = malloc(PAGE_SIZE);
- }
+ void* page = get_page(table->pager, page_num);uint32_t row_offset = row_num % ROWS_PER_PAGE;uint32_t byte_offset = row_offset * ROW_SIZE;return page + byte_offset;}
get_page()方法可以处理缓存丢失。我们假设页面一个接一个保存在数据库文件中：第0页位于偏移量0，第1页位于偏移量4096，第2页位于偏移量8192，等等。如果请求的页面位于文件的边界之外，我们知道它应该是空的，所以我们只需分配一些内存并返回它。当我们稍后将缓存更新到磁盘时，该页面将被添加到文件中。
+void* get_page(Pager* pager, uint32_t page_num) {
+ if (page_num > TABLE_MAX_PAGES) {
+ printf("Tried to fetch page number out of bounds. %d > %d\n", page_num,
+ TABLE_MAX_PAGES);
+ exit(EXIT_FAILURE);
+ }
+
+ if (pager->pages[page_num] == NULL) {
+ // Cache miss. Allocate memory and load from file.
+ void* page = malloc(PAGE_SIZE);
+ uint32_t num_pages = pager->file_length / PAGE_SIZE;
+
+ // We might save a partial page at the end of the file
+ if (pager->file_length % PAGE_SIZE) {
+ num_pages += 1;
+ }
+
+ if (page_num <= num_pages) {
+ lseek(pager->file_descriptor, page_num * PAGE_SIZE, SEEK_SET);
+ ssize_t bytes_read = read(pager->file_descriptor, page, PAGE_SIZE);
+ if (bytes_read == -1) {
+ printf("Error reading file: %d\n", errno);
+ exit(EXIT_FAILURE);
+ }
+ }
+
+ pager->pages[page_num] = page;
+ }
+
+ return pager->pages[page_num];
+}
现在，我们等待将缓存更新到磁盘，直到用户关闭数据库连接。当用户退出时，我们将调用一个名为db_close()的新方法。这个方法将会：
- 将缓存更新到磁盘
- 关闭数据库文件
- 释放寻呼机和表数据结构的内存
+void db_close(Table* table) {
+ Pager* pager = table->pager;
+ uint32_t num_full_pages = table->num_rows / ROWS_PER_PAGE;
+
+ for (uint32_t i = 0; i < num_full_pages; i++) {
+ if (pager->pages[i] == NULL) {
+ continue;
+ }
+ pager_flush(pager, i, PAGE_SIZE);
+ free(pager->pages[i]);
+ pager->pages[i] = NULL;
+ }
+
+ // There may be a partial page to write to the end of the file
+ // This should not be needed after we switch to a B-tree
+ uint32_t num_additional_rows = table->num_rows % ROWS_PER_PAGE;
+ if (num_additional_rows > 0) {
+ uint32_t page_num = num_full_pages;
+ if (pager->pages[page_num] != NULL) {
+ pager_flush(pager, page_num, num_additional_rows * ROW_SIZE);
+ free(pager->pages[page_num]);
+ pager->pages[page_num] = NULL;
+ }
+ }
+
+ int result = close(pager->file_descriptor);
+ if (result == -1) {
+ printf("Error closing db file.\n");
+ exit(EXIT_FAILURE);
+ }
+ for (uint32_t i = 0; i < TABLE_MAX_PAGES; i++) {
+ void* page = pager->pages[i];
+ if (page) {
+ free(page);
+ pager->pages[i] = NULL;
+ }
+ }
+ free(pager);
+ free(table);
+}
+
-MetaCommandResult do_meta_command(InputBuffer* input_buffer) {
+MetaCommandResult do_meta_command(InputBuffer* input_buffer, Table* table) {
if (strcmp(input_buffer->buffer, ".exit") == 0) {
+ db_close(table);exit(EXIT_SUCCESS);} else {
return META_COMMAND_UNRECOGNIZED_COMMAND;
在我们当前的设计中，文件的长度编码了数据库中的行数，因此我们要在文件的末尾写入部分页面。这就是为什么pager_flush()同时接受页码和大小。它不是最好的设计，它会在我们开始实现B树时去除。
+void pager_flush(Pager* pager, uint32_t page_num, uint32_t size) {
+ if (pager->pages[page_num] == NULL) {
+ printf("Tried to flush null page\n");
+ exit(EXIT_FAILURE);
+ }
+
+ off_t offset = lseek(pager->file_descriptor, page_num * PAGE_SIZE, SEEK_SET);
+
+ if (offset == -1) {
+ printf("Error seeking: %d\n", errno);
+ exit(EXIT_FAILURE);
+ }
+
+ ssize_t bytes_written =
+ write(pager->file_descriptor, pager->pages[page_num], size);
+
+ if (bytes_written == -1) {
+ printf("Error writing: %d\n", errno);
+ exit(EXIT_FAILURE);
+ }
+}
最后，我们需要接受文件名作为命令行参数。不要忘记添加额外的参数来do_meta_command。
int main(int argc, char* argv[]) {
- Table* table = new_table();
+ if (argc < 2) {
+ printf("Must supply a database filename.\n");
+ exit(EXIT_FAILURE);
+ }
+
+ char* filename = argv[1];
+ Table* table = db_open(filename);
+InputBuffer* input_buffer = new_input_buffer();while (true) {
print_prompt();read_input(input_buffer);if (input_buffer->buffer[0] == '.') {
- switch (do_meta_command(input_buffer)) {
+ switch (do_meta_command(input_buffer, table)) {
通过这些更改，我们可以关闭然后重新打开数据库，并且我们的记录仍然在那里。
~ ./db mydb.db
db > insert 1 cstack foo@bar.com
Executed.
db > insert 2 voltorb volty@example.com
Executed.
db > .exit
~
~ ./db mydb.db
db > select
(1, cstack, foo@bar.com)
(2, voltorb, volty@example.com)
Executed.
db > .exit
~
实践结果：
原文中还研究了数据库文件中数据的存储方式，可参见原文，这里暂时不翻译了。
结论
现在如果在没有键入.exit的情况下终止了程序，更改将会丢失。另外我们要将所有的页面写回磁盘，其中也包含了读取后没有改变的页面。这是以后要解决的问题。
接下来我们要介绍游标(cursors)，它使实现B树更简单。
修改的部分
+#include <errno.h>
+#include <fcntl.h>#include <stdbool.h>#include <stdio.h>#include <stdlib.h>#include <string.h>#include <stdint.h>
+#include <unistd.h>struct InputBuffer_t {
char* buffer;
@@ -62,9 +65,16 @@ const uint32_t PAGE_SIZE = 4096;const uint32_t ROWS_PER_PAGE = PAGE_SIZE / ROW_SIZE;const uint32_t TABLE_MAX_ROWS = ROWS_PER_PAGE * TABLE_MAX_PAGES;+typedef struct {
+ int file_descriptor;
+ uint32_t file_length;
+ void* pages[TABLE_MAX_PAGES];
+} Pager;
+typedef struct {
uint32_t num_rows;
- void* pages[TABLE_MAX_PAGES];
+ Pager* pager;} Table;@@ -84,32 +94,81 @@ void deserialize_row(void *source, Row* destination) {
memcpy(&(destination->email), source + EMAIL_OFFSET, EMAIL_SIZE);}+void* get_page(Pager* pager, uint32_t page_num) {
+ if (page_num > TABLE_MAX_PAGES) {
+ printf("Tried to fetch page number out of bounds. %d > %d\n", page_num,
+ TABLE_MAX_PAGES);
+ exit(EXIT_FAILURE);
+ }
+
+ if (pager->pages[page_num] == NULL) {
+ // Cache miss. Allocate memory and load from file.
+ void* page = malloc(PAGE_SIZE);
+ uint32_t num_pages = pager->file_length / PAGE_SIZE;
+
+ // We might save a partial page at the end of the file
+ if (pager->file_length % PAGE_SIZE) {
+ num_pages += 1;
+ }
+
+ if (page_num <= num_pages) {
+ lseek(pager->file_descriptor, page_num * PAGE_SIZE, SEEK_SET);
+ ssize_t bytes_read = read(pager->file_descriptor, page, PAGE_SIZE);
+ if (bytes_read == -1) {
+ printf("Error reading file: %d\n", errno);
+ exit(EXIT_FAILURE);
+ }
+ }
+
+ pager->pages[page_num] = page;
+ }
+
+ return pager->pages[page_num];
+}
+void* row_slot(Table* table, uint32_t row_num) {
uint32_t page_num = row_num / ROWS_PER_PAGE;
- void *page = table->pages[page_num];
- if (page == NULL) {
- // Allocate memory only when we try to access page
- page = table->pages[page_num] = malloc(PAGE_SIZE);
- }
+ void *page = get_page(table->pager, page_num);uint32_t row_offset = row_num % ROWS_PER_PAGE;uint32_t byte_offset = row_offset * ROW_SIZE;return page + byte_offset;}-Table* new_table() {
- Table* table = malloc(sizeof(Table));
- table->num_rows = 0;
+Pager* pager_open(const char* filename) {
+ int fd = open(filename,
+ O_RDWR | // Read/Write mode
+ O_CREAT, // Create file if it does not exist
+ S_IWUSR | // User write permission
+ S_IRUSR // User read permission
+ );
+
+ if (fd == -1) {
+ printf("Unable to open file\n");
+ exit(EXIT_FAILURE);
+ }
+
+ off_t file_length = lseek(fd, 0, SEEK_END);
+
+ Pager* pager = malloc(sizeof(Pager));
+ pager->file_descriptor = fd;
+ pager->file_length = file_length;
+for (uint32_t i = 0; i < TABLE_MAX_PAGES; i++) {
- table->pages[i] = NULL;
+ pager->pages[i] = NULL;}
- return table;
+
+ return pager;}-void free_table(Table* table) {
- for (int i = 0; table->pages[i]; i++) {
- free(table->pages[i]);
- }
- free(table);
+Table* db_open(const char* filename) {
+ Pager* pager = pager_open(filename);
+ uint32_t num_rows = pager->file_length / ROW_SIZE;
+
+ Table* table = malloc(sizeof(Table));
+ table->pager = pager;
+ table->num_rows = num_rows;
+
+ return table;}InputBuffer* new_input_buffer() {
@@ -142,10 +201,76 @@ void close_input_buffer(InputBuffer* input_buffer) {
free(input_buffer);}+void pager_flush(Pager* pager, uint32_t page_num, uint32_t size) {
+ if (pager->pages[page_num] == NULL) {
+ printf("Tried to flush null page\n");
+ exit(EXIT_FAILURE);
+ }
+
+ off_t offset = lseek(pager->file_descriptor, page_num * PAGE_SIZE,
+ SEEK_SET);
+
+ if (offset == -1) {
+ printf("Error seeking: %d\n", errno);
+ exit(EXIT_FAILURE);
+ }
+
+ ssize_t bytes_written = write(
+ pager->file_descriptor, pager->pages[page_num], size
+ );
+
+ if (bytes_written == -1) {
+ printf("Error writing: %d\n", errno);
+ exit(EXIT_FAILURE);
+ }
+}
+
+void db_close(Table* table) {
+ Pager* pager = table->pager;
+ uint32_t num_full_pages = table->num_rows / ROWS_PER_PAGE;
+
+ for (uint32_t i = 0; i < num_full_pages; i++) {
+ if (pager->pages[i] == NULL) {
+ continue;
+ }
+ pager_flush(pager, i, PAGE_SIZE);
+ free(pager->pages[i]);
+ pager->pages[i] = NULL;
+ }
+
+ // There may be a partial page to write to the end of the file
+ // This should not be needed after we switch to a B-tree
+ uint32_t num_additional_rows = table->num_rows % ROWS_PER_PAGE;
+ if (num_additional_rows > 0) {
+ uint32_t page_num = num_full_pages;
+ if (pager->pages[page_num] != NULL) {
+ pager_flush(pager, page_num, num_additional_rows * ROW_SIZE);
+ free(pager->pages[page_num]);
+ pager->pages[page_num] = NULL;
+ }
+ }
+
+ int result = close(pager->file_descriptor);
+ if (result == -1) {
+ printf("Error closing db file.\n");
+ exit(EXIT_FAILURE);
+ }
+ for (uint32_t i = 0; i < TABLE_MAX_PAGES; i++) {
+ void* page = pager->pages[i];
+ if (page) {
+ free(page);
+ pager->pages[i] = NULL;
+ }
+ }
+
+ free(pager);
+ free(table);
+}
+MetaCommandResult do_meta_command(InputBuffer* input_buffer, Table *table) {
if (strcmp(input_buffer->buffer, ".exit") == 0) {
close_input_buffer(input_buffer);
- free_table(table);
+ db_close(table);exit(EXIT_SUCCESS);} else {
return META_COMMAND_UNRECOGNIZED_COMMAND;
@@ -182,6 +308,7 @@ PrepareResult prepare_insert(InputBuffer* input_buffer, Statement* statement) {
return PREPARE_SUCCESS;}
+PrepareResult prepare_statement(InputBuffer* input_buffer,Statement* statement) {
if (strncmp(input_buffer->buffer, "insert", 6) == 0) {
@@ -227,7 +354,14 @@ ExecuteResult execute_statement(Statement* statement, Table *table) {
}int main(int argc, char* argv[]) {
- Table* table = new_table();
+ if (argc < 2) {
+ printf("Must supply a database filename.\n");
+ exit(EXIT_FAILURE);
+ }
+
+ char* filename = argv[1];
+ Table* table = db_open(filename);
+InputBuffer* input_buffer = new_input_buffer();while (true) {
print_prompt();