leveldb
[Synchronous Writes]
By default, each write
to leveldb is asynchronous: it returns after
pushing the write from the process into the operating system.
默认所有到leveldb的写操作都是异步, 在把数据从进程交给操作系统后该写操作就返回.
The transfer from operating system memory to the underlying persistent storage happens asynchronously.
从操作系统到下层的持久存储是一个异步的过程.
The sync flag can be
turned on for a particular write to make the write operation not return until
the data being written has been pushed all the way to persistent
storage.
sync标致可打开, 以使得写操作到直至把数据写入到持久存储才返回.
The downside of asynchronous writes is that a crash of the machine may cause the last few updates to be lost.
异步写操作不利的一面是, 机器的崩溃会导致最后几个操作丢失.
Note that a crash of just the writing process will
not cause any loss since even when sync is
false, an update is pushed from the process memory into the operating system
before it is considered done.
当sync为false时,写进程的崩溃不会导致异步写操作丢失, 国为数据已经被写到了操作系统.
Asynchronous writes can often be used safely. For example, when loading a large amount of data into the database you can handle lost updates by restarting the bulk load after a crash. A hybrid scheme is also possible where every Nth write is synchronous, and in the event of a crash, the bulk load is restarted just after the last synchronous write finished by the previous run.
[Concurrency]
A
database may only be opened by one process at a time.
The leveldb implementation acquires a lock
from the operating system to prevent misuse. Within a single process, the
same leveldb::DB object may be safely shared by
multiple concurrent threads. I.e., different threads may write
into or fetch iterators or call Get on the
same database without any external synchronization (the
leveldb implementation will automatically do the required
synchronization). However other objects (like Iterator and
WriteBatch) may require external synchronization. If two threads share such an
object, they must protect access to it using their own locking
protocol.
[Iteration]
The following example demonstrates how to print all key,value pairs in a database.
The following variation
shows how to process just the keys in the
range [start,limit):
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