LevelDB supports both single key-value writes and batch writes. These two types of operations are essentially handled the same way - they’re both encapsulated in a WriteBatch object, which helps improve write operation efficiency.

In LevelDB, WriteBatch is implemented using a simple data structure that contains a series of write operations. These operations are serialized (converted to byte streams) and stored in an internal string. Each operation includes an operation type (such as insert or delete), key, and value (for insert operations).

When a WriteBatch is committed to the database, its contents are parsed and applied to both the WAL log and memtable. Regardless of how many operations a WriteBatch contains, they are processed and logged as a single unit.

If you’ve worked on any non-trivial C++ project, you’ve likely encountered process coredumps. A coredump is a mechanism where the operating system records the current memory state of a program when it encounters a severe error during execution.

There are many reasons why a C++ process might coredump, including:

1. Illegal Memory Access: This includes dereferencing null pointers, accessing freed memory, array bounds violations, etc.
2. Stack Overflow: Caused by infinite recursion or large arrays allocated on the stack
3. Segmentation Fault: Attempting to write to read-only memory or accessing unmapped memory regions
4. Uncaught Exceptions: Program termination due to unhandled exceptions

When encountering a coredump, we typically need to examine the core file for problem analysis and debugging. Analyzing core files can be challenging as it requires a deep understanding of C++’s memory model, exception handling mechanisms, and system calls.

Rather than focusing on core file analysis methods, this article will present several real-world cases to help developers proactively avoid these errors in their code.

LevelDB has some interesting macros that I rarely use in my daily coding. These macros are defined in thread_annotations.h and can be used to detect potential thread safety issues at compile time using Clang’s thread safety analysis tool.