LevelDB C++ API中如何获取异步写入操作完成通知？

Hey there, let's tackle your question directly:

Unfortunately, the native LevelDB C++ API does not provide a built-in callback mechanism to notify you when an asynchronous write completes. When you use the default asynchronous mode (WriteOptions::sync=false), calling DB::Write() returns immediately after the write is added to LevelDB's internal write queue. The actual persistence to disk is handled by background threads, but LevelDB doesn't expose a way to get notified when that background work finishes.

If you need this kind of completion notification, you'll have to build a wrapper on top of LevelDB's existing interfaces. Here are two practical approaches:

1. Track Writes with Marker Keys + Background Polling #

This method lets you verify when an asynchronous write has been fully persisted to disk:

• Whenever you issue an asynchronous write (via WriteBatch or single put), add a unique marker key-value pair to the batch (e.g., __write_complete_marker_<UUID> with a timestamp or task ID as the value).
• Spin up a dedicated background thread that periodically checks for the presence of these marker keys in LevelDB's persistent storage. To ensure you're reading from flushed SST files (not just the in-memory memtable), you can use ReadOptions::snapshot to capture a point-in-time view, or query LevelDB's properties like leveldb.num-immutable-mem-table to check if pending memtables have been flushed.
• Once a marker is detected, trigger your desired callback logic and remove the marker to avoid reprocessing.

2. Use a Thread Pool to Simulate Async Callbacks with Sync Writes #

If you don't want to deal with polling, this approach trades off some throughput for cleaner callback semantics:

• Maintain a dedicated thread pool in your application.
• When you need to perform an "asynchronous write with callback", submit the write task (including your WriteBatch/put operations and callback function) to the thread pool.
• In the thread pool worker, execute the write with WriteOptions::sync=true (blocking the worker thread until the write is fully persisted), then run your callback function once the write completes.
• This way, your main thread never blocks (you get the async behavior you want), and you get a reliable notification when the write is fully committed to disk.

It's worth noting that LevelDB's design prioritizes write throughput for asynchronous operations, which is why it doesn't expose native completion callbacks—most use cases don't need to track when background persistence finishes, only that the write has been accepted into LevelDB's queue.

内容的提问来源于stack exchange，提问作者slowcheetah