Cognos TM1 Rand() function - it is NOT quite random after all !

Let me start the post with a Dilbert cartoon on Random number generator :)

Ok, so we have Rand() function in Cognos TM1 to generate random numbers,

1. Which can be used in Rules as well as in Turbo Integrator (TI)
2. Which is uniformly distributed between 0 and 1
3. That generates a decimal with 9 digit precision (theoretically up to a billion combination of numbers)
4. Whose seed is generated at the start of TM1 server

While I was analyzing the random number distribution, I noticed certain odd behavior.  Upon narrowing down the possible culprit, what I found out is that, IBM Cognos TM1 Rand () function can generate up to 65,536 unique numbers.  Thereafter it starts repeating the numbers.  To test this behavior, enter below piece of code in the prolog of a TI process and execute it.

i = 1; iMax = 65536; WHILE ( i <= iMax *2 );
    IF (i <= iMax);
        vFile = 'sk1.txt';
    ELSE;
        vFile = 'sk2.txt';
    ENDIF;
    vFile = GetProcessErrorFileDirectory | vFile ;
    AsciiOutput (vFile, NumberToString (Rand()) );
    i = i + 1; END;

It will generate 2 files where in

• sk1.txt captures 1st to 65,536 Rand() calls
• sk2.txt captures 65,537 onwards all the way to 131,072 Rand() calls

A screen grab showing the first several lines of these 2 files is below:

Once a TM1 server is ready then you can get up to 65,536 unique random numbers.  After that it is no longer random, it is predictable (with 100% accuracy). Besides, this number is on the server basis (not user basis)
Now let's modify the code slightly to prove the point.

vFile = 'sk3.txt';
vFile = GetProcessErrorFileDirectory | vFile ;
AsciiOutput (vFile, 'Code run by: ' | TM1User () );
i = 1;
iMax = 65536;
WHILE ( i <= iMax);
AsciiOutput (vFile, NumberToString (Rand()) );
i = i + 1;
END;

Run this process.  It will now create sk3.txt.  Log out from architect and log back in with a different user.  Modify the file name to sk4.txt, save and run the process

vFile = 'sk4.txt';

Let's compare sk3.txt with sk4.txt

 The random numbers generated between the 2 users are exactly the same i.e., the unique times IBM Cognos TM1 can generate random numbers is 65,536 and it is system wide (irrespective of users).  If the TM1 service associated with the server is restarted, then the slate is wiped clean.