How To Debug RocksDB Source Code

 RocksDB is one of the popular open source embedded key value database used by several other popular systems. It is a derivative of LevelDB which is developed by Google. More and more opensource as wells as commercial systems started using RocksDB due to its high performance, flexibility and tuning features.

I wanted to read the source code but the code base grown over the years. I tried to search some of the function implementations and it is not very easy to find due to lot if abstraction. RocksDB site provides very good information on the architecture, internal mechanisms, configuration options and API etc. It showed a sample program in Getting Started. I wanted to use this program to debug the RocksDB code.

First we need to build the RocksDB with debug information enabled in the library. For this you can download the source code from github.

  git clone https://github.com/facebook/rocksdb.git

In order to compile the code you need to install/download dependent libraries. The best place to get this information is in INSTALL.md. In this file there are instruction for each supported OS type. Please follow and install all the required libraries. The latest code uses gcc version of 7.0 or greater. I am using CentOS 7.8. It support GCC version of 4.8.5. I need to get the gcc source code and compile it and install it to get the GCC version 7.3.0. The GCC compilation tool a while.Once the GCC installed successfully I compiled the RocksDB code. 

 Here is my sample program based on the Getting Started guide.

 

$ cat Test.cc
#include <iostream>
#include "assert.h"
#include "rocksdb/db.h"

int main(int argc, char** argv) {
    rocksdb::DB* db;
    rocksdb::Options options;
    options.create_if_missing = true;
    rocksdb::Status s =
      rocksdb::DB::Open(options, "/tmp/testdb", &db);
    assert(s.ok());
    std::string value = "Hello World";
    std::string key = "key";
    std::string value1;

    if (s.ok()) s = db->Put(rocksdb::WriteOptions(), key, value);
    if (s.ok()) s = db->Get(rocksdb::ReadOptions(), key, &value1);
    if (s.ok()) std::cout << value1 << std::endl;;

    delete db;
    return 0;
}

To compile you need to include all the dependent libraries. 

g++ -v -I./include -I/usr/include -g -o Test Test.cc \
-std=gnu++17 -lzstd -lpthread -lsnappy -lbz2 -llz4 \
-lz -ldl ./librocksdb_debug.a

I used -v flag to see more information when compile is reports any errors, -g flag to include debug information for the test program. Once you compile the sample code successfully then you can start debugging the code using gdb.

The dynamic libraries linked to program are 

$ ldd Test
    linux-vdso.so.1 =>  (0x00007ffd52ffb000)
    libzstd.so.1 => /usr/local/lib/libzstd.so.1 (0x00007f26d5a7a000)
    libpthread.so.0 => /usr/lib64/libpthread.so.0 (0x00007f26d585e000)
    libsnappy.so.1 => /usr/lib64/libsnappy.so.1 (0x00007f26d5658000)
    libbz2.so.1 => /usr/lib64/libbz2.so.1 (0x00007f26d5448000)
    liblz4.so.1 => /usr/lib64/liblz4.so.1 (0x00007f26d5239000)
    libz.so.1 => /usr/lib64/libz.so.1 (0x00007f26d5023000)
    libdl.so.2 => /usr/lib64/libdl.so.2 (0x00007f26d4e1f000)
    libstdc++.so.6 => /usr/local/lib64/libstdc++.so.6 (0x00007f26d4a9d000)
    libm.so.6 => /usr/lib64/libm.so.6 (0x00007f26d479b000)
    libgcc_s.so.1 => /usr/local/lib64/libgcc_s.so.1 (0x00007f26d4584000)
    libc.so.6 => /usr/lib64/libc.so.6 (0x00007f26d41b6000)
    /lib64/ld-linux-x86-64.so.2 (0x00007f26d5cef000) 

Please let me know if you encounter any issue.  I spent lot of time to figure out the things to debug this simple program. Hope this helps. Enjoy debugging the code. I also created another blog article on RocksDB environment which abstracts the operating system specifics from the DB code.