Modeling a simple system in Mason...

In this post I'm describing how to implement a simple agent model in Mason multiagent simulation environment. See introductory post for additional details about this endeavour.

Installing Mason

Mason installation is easy. Just download the newest archive and unpack it somewhere on the disk. That's all that has to be done. In the following text I'm referring to this unpacked installation, and anything done is done within that directory. It doesn't have to, but it is easier for a start.

Running simulation

The next thing is how to run Mason simulation. But it turns out to be easy. As an example I'll show you how to run Tutorial2 example. This example simulates Conway's game of life and has a GUI that can be used to control the simulation. So, go to the directory where you unpacked archive that you've downloaded in the previous step and then enter sim/app/tutorial1and2 subdirectory. Java file is already precompiled, but nevertheless, we'll compile it again because it is easy and instructive. To compile Tutorial2 issue the following command:

CLASSPATH=../../../jar/mason.17.jar javac Tutorial2.java

Note that Mason framework is in mason.17.jar and that you have to specify it to Java compiler using CLASSPATH variable. The previous command shouldn't give you any messages. To run compiled example, issue the following command:

CLASSPATH=../../../jar/mason.17.jar:. java sim.app.tutorial1and2.Tutorial2

All in all, compiling and running models built using Mason framework is relatively straightforward.

Evolving the target system

The idea I'll pursue in this section is to gradually build a simulation system. The simulation system will be represented by one class that will instantiate and control all the other classes. Those other classes I'll call agents. There will be an agent that represents a job, one for server(s) and one for a queue that will hold jobs until the server is free to take them.

The simplest possible simulation

We'll start with the simplest possible simulation in Mason, and that is the following one:

package hr.fer.zemris.queue;

import sim.engine.*;

public class QueueSystem extends SimState
{
    public QueueSystem(long seed)
    {
        super(seed);
    }

    public static void main(String[] args)
    {
        doLoop(QueueSystem.class, args);
        System.exit(0);
    }
}

To compile it you have to place it into hr/fer/zemris/queue directory (corresponds to package statement at the beginning of the source). I'll assume that this directory is in the mason's toplevel directory. The name of the Java file has to be QueueSystem.java. In order to compile it, issue the following command:

CLASSPATH=jar/mason.17.jar javac hr/fer/zemris/queue/QueueSystem.java

and run it in the following way:

$ CLASSPATH=jar/mason.17.jar:. java hr/fer/zemris/queue/QueueSystem
MASON Version 17.  For further options, try adding ' -help' at end.
Job: 0 Seed: -1713501367
Starting hr.fer.zemris.queue.QueueSystem
Exhausted

Don't forget that dot at the end of the CLASSPATH variable's value, or else, you'll get an error about being unable to find a class.

This simulation is a very simple one and, as expected, it doesn't do anything useful. All it does is call doLoop method of SimState class which will instantiate QueueSystem object. In our case, we didn't specify anything for the simulation, so nothing happens.

In the following text this simulation will be extended so that it create and coordinate other agents.

First agent

Ok, let's create an agent. Our initial agent will, again, be very simple. It will only print it was instantiated, but nothing else. So, here it is:

package hr.fer.zemris.queue;

import sim.engine.*;

public class Server implements Steppable
{
    public Server()
    {
        System.out.println("Instantiated one Server");
    }

    public void step(final SimState state)
    {
        System.out.println("step() method called");
    }
}

Note that we have to define step() method, because it is required by Steppable interface. But, for the moment, it doesn't do anything.

Ok, to compile this agent, use the usual command:

CLASSPATH=jar/mason.17.jar javac hr/fer/zemris/queue/Server.java

Again, I assumed that you are positioned into mason's root directory, the agent is placed within hr/fer/zemris/queue directory and it is called Server.java.

Note that you can not directly run agents, at least not in this form (i.e. without main method). So, we'll instantiate and schedule execution of our agent in the main class that represents the whole simulation. The change is simple, in the class QueueSystem.java add the following method:

public void start()
{
    super.start();

    Server server = new Server();
    schedule.scheduleOnce(server);
}

Now, recompile QueueSystem.java class, and run it:

$ CLASSPATH=jar/mason.17.jar:. java hr/fer/zemris/queue/QueueSystem
MASON Version 17. For further options, try adding ' -help' at end.
Job: 0 Seed: -1710667392
Starting hr.fer.zemris.queue.QueueSystem
Instantiated one Server
step() method called
Exhausted

Note the lines in bold. First line is printed when constructor of our simple agent was called. The second one is outputted when agent's step() method was called. Note that step method was called only once, and that is because we used method scheduleOnce() that schedules a single occurrence of an event. Try to change scheduleOnce() into scheduleRepeating() and see what will change.

There is also a question of when this event was called. We used a simple version of schedule methods that schedule execution 1 time unit in the future, i.e. in getTime() + 1.0. Well, at least documentation says so! Try to check it by youself. Hint: to get current time in agent's step() method use state.schedule.getTime() method.

Creating jobs

Jobs are a bit different. They are not created at the start of the simulation, but instead are created dynamically according to Poisson distribution. So, what I'm going to do is to create class named JobFactory that will create Job. Each job will be represented using the following class:

package hr.fer.zemris.queue;

import sim.engine.*;

public class Job
{
    public double createTime;
    public double processingTime;
    public double finishTime;
}

Note that job isn't agent! It doesn't have step() method neither it's subclassed from some Mason's class. What I decided is that Job class will only have fields to keep statistical data and that's it.

To create jobs, I written JobFactory agent. Here is the agent:

package hr.fer.zemris.queue;

import sim.engine.*;
import sim.util.distribution.*;
import ec.util.MersenneTwisterFast;

public class JobFactory implements Steppable
{
    private Poisson poisson;
    private Exponential exponential;
    private QueueSystem queueSystem;

    public JobFactory(double lambda, double mu, QueueSystem qs)
    {
        MersenneTwisterFast randomGenerator = new MersenneTwisterFast();
        poisson = new Poisson(lambda, randomGenerator);
        exponential = new Exponential(mu, randomGenerator);
        queueSystem = qs;
    }

    public void step(final SimState state)
    {
        double currentTime = state.schedule.getTime();
        double nextEventTime = currentTime + poisson.nextDouble();

        Job job = new Job();
        job.createTime = currentTime;
        job.processingTime = exponential.nextDouble();
        queueSystem.pushNewJob(job);

        state.schedule.scheduleOnce(nextEventTime, this);
    }
}

So, how this JobFactory agent works? First, we have a constructor. Constructor instantiates two classes, Poisson and Exponential, that will be used to generate random numbers from respective distributions. The first two parameters of the constructor define distributions' mean values. The third parameter is used for sending newly created jobs into a system queue.

Note that, apart from generating new jobs according to the Poisson distribution, we also have to specify for how long will a single job be processed within the server. I think that a natural place to determine this is when the job is created since it is the characteristic of the job itself.

I thought about sending Job objects directly to the server agent. But the problem with that approach is that server has to schedule itself in case there are no other jobs waiting, i.e. the job immediately enters server. Namely, server has to wake up when some job is finished and remove it from the system.

But, in order to be able to do scheduling I had to have access to SimState object, which is accessible only from step() method. Now, I could save this object internally, but it would be a hack. Namely, I would have to somehow provoke step() to be executed immediately at the beginning. Oh, yeah, I could send SimState object via constructor. But in the end, I gave up from pursuing this approach as I haven't been able to find someone else already doing this (nor in the examples directory, nor on the Internet).

The second part of the JobFactory class, and the its workhorse, is the method step(). What this method does is create a new Job class initializes its processing time (job.processingTime) and adds it to the queue of jobs waiting for the server (via call to the method queueSystem.pushNewJob). Finally, this method draws new random number for the Poisson distribution which defines when a new job will be created. It schedules itself at that point in time.

Ok, our simulation class, QueueSystem, has to have a method for accepting new jobs. This method has name pushNewJob, and the code is the following:

public void pushNewJob(Job job)
{
    jobQueue.add(job);

    if (jobQueue.size() == 1)
        schedule.scheduleOnce(schedule.getTime() + job.processingTime, server);
}

jobQueue is a linked list, i.e. FIFO queue, that is used to hold jobs while being processed in Server and waiting for the Server. The job that is in front of the queue is the job that is currently processed by Server. Maybe I should have written code a bit differently, i.e. so that Server holds the job it processes in some internal attribute, but I did it this way and I didn't bother to rewrite it.

Apart from adding new job to a queue there is one additional thing I had to do. In case there is no job in queue, that means the server is idle, and it is not scheduled for the execution! So, the if statement checks this condition, and if the server is idle it schedules its execution when jobs is finished! Otherwise, server will execute at some point and it will take next job and schedule itself. We'll come to that part a bit later.

One more thing hasn't been specified with respect to QueueSystem, namely jobQueue and activation of JobFactory. Server isn't activated until there is a job, and that is handled by pushNewJob method.

So, in order to take care of that case, here is the new start() method of QueueSystem simulation/class:

public void start()
{
    double alpha = 3;
    double beta = 5;

    super.start();

    jobQueue = new LinkedList<job>();

    server = new Server(jobQueue);

    jobFactory = new JobFactory(alpha, beta, this);
    schedule.scheduleOnce(jobFactory.getFirstInvocationTimeStamp(), jobFactory);
}

So, what's going on in this method. There are alpha and beta parameters for M/M/1 queue. Next, I'm initializing FIFO queue, jobQueue. It's defined as follows as a QueueSystem's class atribute:

Queue<job> jobQueue;

Then, server agent is instantiated. Note that I'm sending queue to server. That is necessary since server has to take jobs from a queue. I'm also instantiating JobFactory agent. Finally, I'm scheduling initial run of JobFactory.

There is a small probelm. Namely, I have to schedule first invocation according to Poisson distribution. It is not correct to invoke it immediately, at least not in the form I wrote it. And, this class, QueueSystem, doesn't have access to poison distribution in order to get first random number. It would be also error to create another Poisson distribution. So, I added a method to JobFactory class/agent that will return me first random number. It is the following method:

public double getFirstInvocationTimeStamp()
{
    return exponential.nextDouble();
}

and you should place it in JobFactory agent/class.

Ok, the final piece of puzzle, Server agent. First, constructor is now a bit different, namely, it has to take queue reference:

public Server(Queue jq)
{
    jobQueue = (LinkedList)jq;
}

step() method is also a bit more involved:

public void step(final SimState state)
{
    Job job = jobQueue.remove();
    job.finishTime = state.schedule.getTime();

    jobs++;
    systemTimeAvg = systemTimeAvg + (job.finishTime - job.createTime - systemTimeAvg) / jobs;
    jobNumberAvg = jobNumberAvg + (jobQueue.size() - jobNumberAvg) / jobs;
    currentStep++;
    if (skipSteps == currentStep) {
        System.out.println(systemTimeAvg + " " + jobNumberAvg);
        currentStep = 0;
    }

    if (jobQueue.size() > 0) {
        job = jobQueue.peek();
        state.schedule.scheduleOnce(state.schedule.getTime() + job.processingTime, this);
    }
}

What does this method do? First, it pops a job from the front of the queue, the job that was processed within the server. Then, it updates and prints some statistics. Finally, it checks if there is another job in the queue, and if it is, it schedules invocation of itself when that particular job has to finish.

Basically, that's it.

GnuCash - evidencija plaćanja debitnom karticom

Evidencija plaćanja debitnom karticom gotovo je identična evidenciji plaćanja u gotovini, jedino što je tijekom upisa odakle se povlače novci potrebno staviti tekući račun - budući da vam debitna kartica skida s tekućeg računa.

U tom smislu, pretpostavimo da ste platili debitnom karticom gablec u restoranu. Kako bi to upisali, prvo je potrebno odrediti u koju stavku će to biti zavedeno. Možete si otvoriti zaseban račun za to, ili kao u mom slučaju, možete voditi unutar stavke Hrana. Dakle, kada upišete u tu stavku račun, to će izgledati otprilike kao na idućoj slici:

Dakle, jedina razlika je što se kao izvor sredstava navodi tekući račun.

Lecture about agile software development

On November 22nd and 25th, 2013, I gave a lecture about agile software development to a group of employees of Croatian Telecom. This lecture was part of their internal education in which they were educated about different IT and business technologies. The goal of the lecture was not to teach them specifics of agile software development but to give them some kind of an overview of agile development, to contrast it to a more traditional methods, more specifically to waterfall development model, and to compare them mutually. With that in mind, I ended up with a presentation that has the following parts:

1. Introduction and motivation
2. Why is software development hard?
3. Traditional methodologies
4. Principles of Agile software development.
5. Some specific agile software development methodologies.
6. Problems encountered when introducing agile methods.
7. Experiences gained during introduction of agile methods from some companies.
8. History of agile methods.
9. Conclusion.

The presentation is in Croatian, and available here. In case there is demand, I'll translate it to English. I plan to enhance and improve the presentation (there is lot of place to do so). If I upload newer version I'll expand this post. But also, filename will be changed (it includes date of a release). Accompanying this presentations are references I deemed the most interesting for those employes. They can be downloaded as a single ZIP file. Note that those references are in English!

I'm using some pictures from the Internet in the presentation and I hope that I didn't broke any licence or copyright agreements. The same holds for papers in ZIP file which are given for purely educational purposes. In case I didn't do right something, please notify me.

GnuCash - evidencija plaćanja gotovinom

Pretpostavimo kako ste si ujutro u pekari kupili sendvič za doručak. Dobili ste račun i odlučili ste to sada upisati u GnuCash. Račun je iznosio 14kn.

Kako bi upisali tu stavku u GnuCash, pokrenite program i potom odaberite gdje ćete voditi te troškove. Ja sam odlučio voditi zasebno troškove o hrani i za to sam si otvorio odgovarajuću stavku:

Svjestan sam kako navedena stavka nije baš najpreciznija, ali kao što rekoh, u hodu ću rješavati sve probleme. Kako bi se upisao račun potrebno je dva puta kliknuti na stavku, u ovom slučaju Hrana, te se potom otvara kartica gdje je potrebno upisati što je kupljeno. Inače, moguće je voditi evidenciju o svakoj kupljenoj stavci, međutim, ja sam se odlučio voditi evidenciju samo o računima pri čemu razmišljam da svaki račun slikam fotoparatom (nemam skener!) i pohranjujem slike na računalo. :) Vidjet ćemo kako će to ići. U međuvremenu, ovako izgleda upisan račun:

Primjetite da sam upisao i broj računa. To radim kako bi mogao povezati svaku stavku sa svakim računom, da kasnije ne moram pretraživati po iznosu. Još bolje bi bilo kada bi mogao negdje upisati JIR ili ZKI, ali nećemo pretjerivati. ;)

Primjetite također da je u kolonu Transfer upisano odakle sam uzimao novce za plaćanje. Plaćao sam gotovinom, iz novčanika, te je zato odabrana opcija Assets: Novčanik - Kune.

Stanje nakon te transakcije je sada sljedeće:

Kao što se vidi, u novčaniku sad moram imati 186Kn. Ako ih nema tamo, onda imam problem. :)

To je to što se tiče upisivanja računa. U osnovi, svaki drugi račun koji je plaćen gotovinom iz novčanika upisuje se na isti način.

Vođenje osobnog knjigovodstva upotrebom GnuCash alata - Uvod i postavljanje okoline

Već neko vrijeme razmišljam kako bi bilo dobro voditi evidenciju gdje trošim novce. Svako malo mi se desi da se uhvatim u razmišljanju gdje sam uspio potrošiti plaću i naravno da se ne mogu sjetiti. Drugo pitanje koje me muči s vremena na vrijeme je: Koliko novaca mi odlazi na režije mjesečno, godišnje? U tom smislu, prošao sam nekoliko faza. Prva faza je bila redovito skidanje izvoda iz banke te sam u LibreCalcu računao jednostavni zbroj potrošnje svaki mjesec koji sam potom uspoređivao s plaćom. To je bilo OK, ali nedovoljno precizno. Recimo, nisam znao koliko sam mjesečno trošio na režije unatrag zadnje godine dana (ili više). Zbog toga sam prešao na iduću fazu. Druga faza je bila vođenje evidencije u LibreCalcu gdje sam svaki mjesec upisivao koliko sam potrošio na što. To je bilo bolje, međutim, još uvijek nisam bio zadovoljan jer sam mislio kako su mi mogućnosti upravljanja i analize podataka relativno ograničene. Pri tome nisu ograničene zbog mogućnosti same aplikacije (LibreCalc) već zbog mog vođenja evidencije. Konačno, to me je potaknulo da krenem u treću fazu, korištenje programa GnuCash. U biti, imao sam jedan izlet u taj program puno ranije, međutim, nisam bio uporan i s obzirom kako je ipak nužno upoznati se s nekim temeljnim pojmovima iz knjigovodstva, brzo sam odustao. No, ovaj puta sam ozbiljnije odlučio pokušati koristiti tu aplikaciju.

Tome je pripomogao i relativno dobar uvod u Gnucash program sam pronašao na YouTubeu. Naime, na temelju tog videa sam se odlučio na pristup od jednostavnijeg prema složenijem, tj. da započnem koristiti program na najjednostanviji mogući način, baš kao što se u videu i pokazuje. S vremenom bi potom proširivao količinu podataka kako se pokazuje potreba za njima. Dodatno, odlučio sam iskoristiti mogućnosti prilagodbe različitih podataka kako bi što bolje reflektirali specifičnosti Hrvatske, odnosno, onoga što meni treba.

Inače, razlog zašto sam se odlučio za GnuCash je činjenica kako je besplatan, radi na svim bitnim platformama (Windows, Linux, Mac OS X) i često se spominje kao vrlo kvalitetan program. Osim osobnog knjigovodstva, moguće je voditi i poslovanje u njemu, a ima i Python API pa se može proširivati.

Uglavnom, nakon instalacije GnuCasha i njegovog prvog pokretanja otvara se dijalog New Account Hierarchy Setup.

Nakon odabira tipke Forward, nudi se odabir podrazumijevane valute. Tu nema puno nedoumica, u Hrvatskoj je podrazumijevana valuta kuna, tj. HRK. Konačno, dolazi odabir načina vođenja stavki. Tu sam odabrao opciju 'A Simple Checkbook'.

Konačno, još sam dva puta odabrao Forward i potom Apply. Zatim je trebalo odabrati mjesto na disku gdje će biti pohranjena datoteka. Pripazite kako GnuCash automatski čuva stare verzije datoteke, a istovremeno vodi evidenciju o promjenama (logs). U tom smislu, kreirajte zaseban direktorij u kojemu će biti pohranjeni ti podaci i u njega spremite datoteku.

Nakon toga, pobrisao sam sve što mi se nije sviđalo, te sam preveo i dodao određene stvari. Konkretno, izmjene koje sam napravio su sljedeće:

• Checking Account sam preveo kao Tekući račun i premjestio ga direktno pod Assets stavku. Tekući račun je za fizičke osobe (pojedince) u HR temeljna stvar.
• Current Assets stavku sam izbrisao.
• Nadalje, pod Assets sam još dodao stavke Žiro račun i Novčanik - Kune. Ako imate gotovinu u devizama dodajte još stavku Novčanik - EUR (ili koja vam je već valuta u pitanju). Ovdje bi, osim toga, dodali i razne fondove u kojima držite novce, ako ih imate. U osnovi, dodajete sve gdje držite novce. U krajnjem slučaju, ako u nekoj čarapi držite novce, onda dodajete to tu.
• Riječ Expenses sam preveo u Troškovi i potom sam tamo dodao stavke Auto, Internet, Mobilni telefon, Hrana, Režije. Pod Auto dodatno Gorivo, Servis, Registracija i Ostalo, te pod režije Struja, Voda, i slično.
• Konačno, riječ Income sam pretvorio u Prihodi, a onda sam i tu dodao podstavku Plaća. Ako tijekom vremena budem imao nekakav drugi prihod, onda ću ga dodatni naknadno. Slično vrijedi i za troškove.

Promjene su se svodile na uređivanje postojeće stavke, odnosno, dodavanje nove. Dodavanje nove stavke obavlja se tako da kliknete desnom tipkom na stavku unutar koje želite dodati novu stavku i potom odaberete opciju New Account... Promjena postojeće stavke se obavlja slično, kliknete desnom tipkom na stavku koju želite izmjeniti i potom odaberete opciju Edit Account...

Nakon svih tih mojih uređivanja, ono što sam ja imao bilo je sljedeće:

Mogao sam tu još uređivati pojedine elemente, ali za sada sam stao na ovom stanju. S vremenom ću to poboljšavati i mijenjati.

Bitna stvar je da s GnuCashom počinjete voditi evidenciju o stanju svojih računa i troškovima počevši od jednog specifičnog trenutka. Taj trenutak može biti proizvoljan, ali stvar je u tome da negdje morate upisati koliko čega imate u tom početnom trenutku. Ja sam odabrao kako ću početi voditi knjigovodstvo od dana kada sam započeo upotrebljavati GnuCash. Za definiranje početnog stanja služi stavka Equity. Unutar te stavke ima stavka Opening Balances. Tamo sam dodao stavke koje su opisivale koliko imam na tekućem računu, deviznom, u novčaniku i slično. S obzirom da sam imao nešto deviza, a Opening Balances je u kunama, dodao sam i podstavku Devize. Tamo sam potom upisao eure. Možda ima nekakav način da se u jednu podstavku upisuju različite valute, ali trenutno to ne znam napraviti. Osim toga, GnuCash je prilično fleksibilan i moguće je naknadno napraviti reorganizaciju. Prema tome, nema potrebe težiti savršenstvu odmah od prve već je moguće s vremenom poboljšavati proces.

Primjera radi, pretpostavit ću da sam na tekućem računu započeo s 3000kn, na žiro računau 50kn, te na deviznom računu 100EUR. Dodatno, u novčaniku sam imao 200kn i 10EUR. Prvo sam, dakle, dodao sve kune koje imam. Dodavanje se obavlja tako da se dva puta klikne na opciju Opening balances te se potom upisuju odgovarajuće stavke. Nakon upisa, stanje je bilo sljedeće:

Iduće što je trebalo upisati su euri. S obzirom da ne znam način kako se može na stavke koje su definirane u kunama (Opening Balances je u kunama) upisivati druga valuta, konkretno euri, zbog toga sam otvorio novu kategoriju pod Equity naziva Početno stanje - EUR. Istovremeno s tim, stavku Opening Balances sam promijenio u Početno stanje - Kune.

U tu stavku sam potom upisao početne vrijednosti u Eurima:

Eto, to je što se tiče postavljanja inicijalnog stanja. Sada slijedi skupljanje računa i vođenje evidencije o onome što potrošite i što zaradite. O tome ću u drugim postovima:

1. Evidencija pristigle plaće.
2. Evidencija troškova na MasterCard kartici.
3. Plaćanje gotovinom u trgovini.
4. Plaćanje debitnom karticom u trgovini.
5. Podmirenje troškova kreditne kartice.

Za kraj, par napomena:

1. Volio bi kada bi me neki knjigovođa ispravio u vezi pojmova koje sam upotrebljavao, a također, ako koji savjet oko boljeg korištenja ovog alata bi jako dobro došao. S vremenom će se sigurno iskristalizirati nekakve prakse, no, bolje je preskočiti lutanje ako je moguće. :)
2. Ako ima koji stručnjak za GnuCash alat, također bi volio da malo prokomentira moj post, a i kasnije postove!
3. Kada započnete voditi evidenciju troškova u GnuCash-u, ili bilo kojem sličnom alatu, to trebate raditi detaljno jer u suprotnom neće biti poklapanja između stanja vaših računa (i novčanika) i onoga što piše u programu te je cijeli postupak besmislen.