Design of AlsoSQL: Drizzle JSON HTTP Server

This particular project was proposed by Henrik at Drizzle Day 2011. A week later, Stewart published version 0.1 of AlsoSQL. Later I and Henrik worked on version 0.2 and he talked about it at Drizzle Day 2012 .

AlsoSQL:Drizzle JSON HTTP Server, is capable of SQL-over-HTTP with key-value operation in pure json. Henrik explained about its working and functionality in this particular post. 

Here,I am going to talk about design of AlsoSQL and the various problems I faced.

Up-to version 0.2 , the API wasn't properly object oriented, so we planned to do re-factoring first.

After going through the code-base of AlsoSQL, I realized the need of design pattern. So we looked through various of them and chose Fascade design pattern for future development.

Here is the rough design of AlsoSQL (I called it rough because it might change in future).

Description:

HttpHandler is used to handle the http request ,parsing and validating json from that request and sending back response.

SQLGenerator simply generates the sql string corresponding to request type using input json.

SQLExecutor executes the sql and returns resultset.

SQLToJsonGenerator generates the output json corresponding to request type.

DBAccess is an interface to access generators and executor.

The main reason to design it in such a way is that it can be used over storage engine in future directly.

Problems Faced:

<config.h> , I always forgot to include this header file and it floods error on my terminal.

Another one , I need to get LAST_INSERT_ID() and I want to get it in a single query with REPLACE query. Still working on it.

Debug Drizzle Code with GDB

From last few days , I have been working on a bug in JSON Server of Drizzle. And the bug is of Segmentation fault , which is not easy to recognize by just reading the code-base. So , the best way to get rid of this problem is  to debug the code. But How ?
I used Visual Studio once during my internships to debug a project. So ,I tried to find out some IDEs that can be use with drizzle . But I figured it out as we can debug a C++ code-base easily and efficiently with GDB .

Here are few steps for debugging:

First of all build your server and enable debugging:

mohit@mohit-PC:~$ cd repos/drizzle/drizzle-json_server/

mohit@mohit-PC:~/repos/drizzle/drizzle-json_server$ ./config/autorun.sh && ./configure --with-debug && make install -j2

Default installation path is /usr/local/ but you can change it with --prefix = /installation_path/
Start server and debug the code:

mohit@mohit-PC:~/repos/drizzle/drizzle-json_server$ gdb /usr/local/sbin/drizzled >

Now set arguments needed to start server with specific plugin (In my case , plugin is json_server)

(gdb) set args --plugin-add=json_server

Set breakpoints,you can do that with this command:
 b <filename>:<line-number> or break <filename>:<line-number>

(gdb) break json_server.cc:1093

Since the json_server plugin is not loaded yet , hence it prompts with :

No source file named json_server.cc.
Make breakpoint pending on future shared library load? (y or [n])

Go with "y"
Run your server now :

(gdb) r

You will get something like this:

Starting program: /usr/local/sbin/drizzled --plugin-add=json_server
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/i386-linux-gnu/libthread_db.so.1".

Now you can use various commands of GDB to debug your code.
List of these commands are mentioned here.

Problem Faced :

Once I started drizzle server with plugin. I got a message:

Using host libthread_db library "/lib/i386-linux-gnu/libthread_db.so.1".
/usr/local/sbin/drizzled: relocation error: /usr/local/sbin/drizzled: symbol _ZN8drizzled7message6AccessC1Ev, version DRIZZLE7 not defined in file libdrizzledmessage.so.0 with link time reference
[Inferior 1 (process 23532) exited with code 0177]

I was unable to get a single line of this problem, Thank to Henrik for solution.

Solution:

mohit@mohit-PC:~$ sudo rm /etc/ld.so.cache
mohit@mohit-PC:~$ ldconfig

Toru and Padraig previously posted on this topic which may also be helpful.
Also You can find documentation on this at Drizzle wiki.

Calculating Value During Compile Time In C++

"If you think it's simple, then you have misunderstood the problem" - Bjarne Stroustrup

Recently one of my colleague asked me a question:

Print the value of (1 + 100 * 10 - 5 / 2 + 30) expression during compile time. And he allowed me to use any programming language.

For a while , I was not able to get the answer of this question. But after spending few minutes , I found out it's a good conceptual question.
The answer of this problem is simple, if you know about Templates in C++ a bit.

What are Templates?
Templates are a feature of the C++ programming language that allow functions and classes to operate with generic types. This allows a function or class to work on many different data types without being rewritten for each one.


The solution is based on Template Meta-Programming.
Template meta-programming is a technique in which templates are used by a compiler to generate temporary source code, which is merged by the compiler with the rest of the source code and then compiled. More on wiki.

#include <iostream>

using namespace std;
template<unsigned int n>
struct Expression {
   static const float result = n;
};

template<unsigned int exp>
struct _{ operator char() { return exp;} };

int main() {
        char(_<Expression<1 + 100 * 10 - 5 / 2 + 30>::result>());
        return 0;
}

If you want to explore more , try to print Factorial<n> ;)

"Undefined Reference" While Using Templates In C++

Did you get "collect2: ld returned 1 exit status" ??

If yes , then there is some linker's problem. Lets solve it.

I designed a template for one of my assignment , But I was unaware of "Declaring & Defining" fundamentals of templates in C++.

Here is some code :

//Add.h

#include "Basic.h"
using namespace std;
template <class T>
class Add{

        int x;
        int y;
        
public:
      Add(int x,int y);
      Dump();
};

Add.cpp

#include "Add.h"

template <class T>
Add<T>::Add(int x,int y):x(1),y(2){};


template <class T>
Add<T>::Dump(){
 cout<<x<<"  "<<y;
}

Output:
collect2: ld returned 1 exit status

Reason:
Templates are just a recipe , they are not code. In order to compile this code, it must be generated first, which needs to know templates definition & definition parameters which are passed to it. And compiler does not know about separate compilation (compiler doesn't know about anything about code outside of a file).

Solution:
Put the definitions in the header files itself :)

//Add.h

#include "Basic.h"
using namespace std;
template <class T>
class Add{

        int x;
        int y;
        
public:
      Add(int x,int y);
      Dump();
};

template <class T>
Add<T>::Add(int x,int y):x(1),y(2){};

template <class T>
Add<T>::Dump(){
 cout<<x<<"  "<<y;
}

"Bored Of Segmentation Fault"

Generally , the way to use "Pointers" defines , the way to tackle "Segmentation Fault".

Are you bored of segmentation fault, lets try to make it interesting. Without a fault (segmentation),we can't
learn it. :)
So,

Here is small conversation between me and my friend:


Me: (After spending almost a half day, over a silly mistake. I went to him) : Dude, I am bored of this segmentation fault ?  Help me to get rid of it.

Friend (After a cute smile) : Do you know what it is ??

Me: Yes , its a silly fault which can't be debug.

Friend: No man , You can debug it use GDB debbuger. And its just a  "attempt to access a memory that a processing unit cannot physically address".

Me(reminds me OS course): Ohoo.. Segmentation (approach to memory management and protection in the operating system). Got It. ....... But why it occurs.. even I didn't plan it to occur :)

Friend: Hmm .. Nice question .. reasons for it -
            - A buffer overflow.
            - use of unintialized pointers.
            - derefrencing Null pointers.
            - attempting a memory ehich is stranger for program.
            - exceeding the allowable stack size.

Me: Ohoo .. I think I figure out why it occurs in my code.

Here is that code(sample one):

Code #1:

                 
                     void func(string word,const int n)
                     {
                         int key[n];
                         for(int j=0;j<n*n;j++)
                         key[n]=word[j];
                     }

                     int main()
                      {
                         string word;
                         int len;
                         cin>>word;
                         len=word.length();
                         const int n=sqrt(len);
                        func(word,n);
                     }

I found segmentation fault in above code, but with small change I figure out a small lesson.

Code #2:

                    void func(string word,int n)
                     {
                         const int n = sqrt(len);
                         int key[n];
                         for(int j=0;j<n*n;j++)
                         key[n]=word[j];
                     }

Is this correct solution??-  No.
But it get you out of segmentation fault for a while.
Now question is how ??

As we use 'n' in parameter in Code #1, means it get place on stack. While in second case , on heap. 

Code # 3 (Correct solution):
               

                  void func(string word,const int n)
                     {
                         int key[n*n];
                         for(int j=0;j<n*n;j++)
                         key[n]=word[j];
                     }