Connecting an agent and its strategy

[Claudio Jolowicz]

I am trying to find a solution to the following design problem
(code at the bottom):

We are implementing a trader agent that can trade with other traders
on an electronical trading platform. To make the trader more
extensible, we have defined a strategy interface and implemented this
interface for different trading strategies. The problem relates to how
to connect the trader and the strategy.

The problem is tricky because the strategy has to call back the
trader, for instance to get information about orders (i.e. prices) on
the trading platform, and to initiate own orders. Technically, the
strategy constructor takes an argument of type StrategyCallback. The
latter is an interface which is implemented by the trader class.
The quick and dirty approach would be the following:

The user (1) instantiates the trader and then (2) uses the reference
to the trader to instantiate the strategy. After that (3), the user
calls a method of the trader that initializes the trader's strategy,
passing it the reference to the strategy.

The problem is that the initialization takes place outside of, and
after the constructor. Also, the user has to worry about internals of
the trader.
A bit more elegant is the following approach:

The strategy cannot be instantiated before the trader. It would be
nice to let the trader instantiate it. But the trader doesn't know the
exact type of the strategy. So let's make the trader a template. Like
this, the initialization can take place in the trader's constructor
and is hidden from the user.

We have implemented this approach, and it works. However, it doesn't
really solve the design problem. Every instantiation of the trader
template has a different type. The trader class has to be recompiled
whenever we write a new strategy for it. For the same reason, it is
awkward to manage a collection of traders with different strategies.
Is there another way?
struct Strategy;
struct StrategyCallback;

/// trader class aggregating a strategy
class Trader
: public StrategyCallback
{
public:
//construction
Trader(trading::Strategy*);
~Trader();
private:
Trader(const& Trader);
Trader& operator=(const Trader&);

//private data
Strategy* m_pStrategy;
};

/// interface representing a trader's strategy.
struct Strategy
{
virtual ~Strategy() { /*cleanup*/ }

/// reevaluate situation.
virtual void evaluate() = 0;

/// the strategy has reached the goal.
virtual bool succeeded() = 0;

/// the strategy has failed.
virtual bool failed() = 0;

/// perform the strategy.
virtual void trade() = 0;
};

/// callback interface for strategy.
struct StrategyCallback
{
/// access orders.
virtual OrderIseq getOrders() = 0;

/// create an order.
virtual trading::Order* createOrder(
double dPrice,
double dVolume) = 0;
};

--
Claudio Jolowicz

Department of Computing
180 Queen's Gate
South Kensington campus
Imperial College
LONDON SW7 2AZ

31 Humbolt Road
Fulham
LONDON W6 8QH

mobile: [phone]
mail to: cj***@doc.ic.ac.uk
webpage: www.doc.ic.ac.uk/~cj603

[tom_usenet]

On Thu, 8 Apr 2004 07:15:32 +0100, Claudio Jolowicz
<cj***@doc.ic.ac.uk> wrote:

We are implementing a trader agent that can trade with other traders
on an electronical trading platform. To make the trader more
extensible, we have defined a strategy interface and implemented this
interface for different trading strategies. The problem relates to how
to connect the trader and the strategy.

The problem is tricky because the strategy has to call back the
trader, for instance to get information about orders (i.e. prices) on
the trading platform, and to initiate own orders. Technically, the
strategy constructor takes an argument of type StrategyCallback. The
latter is an interface which is implemented by the trader class. [snip]A bit more elegant is the following approach:

The strategy cannot be instantiated before the trader. It would be
nice to let the trader instantiate it. But the trader doesn't know the
exact type of the strategy. So let's make the trader a template. Like
this, the initialization can take place in the trader's constructor
and is hidden from the user.

We have implemented this approach, and it works. However, it doesn't
really solve the design problem. Every instantiation of the trader
template has a different type. The trader class has to be recompiled
whenever we write a new strategy for it. For the same reason, it is
awkward to manage a collection of traders with different strategies.

Is there another way?
You could change the Strategy interface to take a StrategyCallback in
the relevent method calls (such as evaluate and trade). That way there
is no direct composition relationship between a Strategy and the
"callback", which might better decouple them (and remove the circular
reference).
struct Strategy;
struct StrategyCallback;
It might be better to call that OrderHandler, or similar, to better
express its purpose.

/// trader class aggregating a strategy
class Trader
: public StrategyCallback
{
public:
//construction
Trader(trading::Strategy*);
~Trader();
private:
Trader(const& Trader);
Trader& operator=(const Trader&);

//private data
Strategy* m_pStrategy;

Obviously a smart pointer would be appropriate here (and in the
constructor).

Another solution would be to use a StrategyFactory. e.g.

class StrategyFactory
{
public:
virtual ~StrategyFactory(){};

typedef shared_ptr<Strategy> StrategyPtr;

virtual StrategyPtr createStrategy(OrderHandler& handler) = 0;
};

template <class Strat>
class ConcreteStrategyFactory: public StrategyFactory
{
public:
virtual StrategyPtr createStrategy(OrderHandler& handler)
{
return StrategyPtr(new Strat(handler));
}
};

and pass that into the Trader constructor (note that the handler must
outlive the Strategy - avoid circular smart pointer references by
using a weak_ptr or a reference to the handler as above).

Tom
--
C++ FAQ: http://www.parashift.com/c++-faq-lite/
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html

[tom_usenet]

On Thu, 8 Apr 2004 07:15:32 +0100, Claudio Jolowicz
<cj***@doc.ic.ac.uk> wrote:

We are implementing a trader agent that can trade with other traders
on an electronical trading platform. To make the trader more
extensible, we have defined a strategy interface and implemented this
interface for different trading strategies. The problem relates to how
to connect the trader and the strategy.

The problem is tricky because the strategy has to call back the
trader, for instance to get information about orders (i.e. prices) on
the trading platform, and to initiate own orders. Technically, the
strategy constructor takes an argument of type StrategyCallback. The
latter is an interface which is implemented by the trader class. [snip]A bit more elegant is the following approach:

The strategy cannot be instantiated before the trader. It would be
nice to let the trader instantiate it. But the trader doesn't know the
exact type of the strategy. So let's make the trader a template. Like
this, the initialization can take place in the trader's constructor
and is hidden from the user.

We have implemented this approach, and it works. However, it doesn't
really solve the design problem. Every instantiation of the trader
template has a different type. The trader class has to be recompiled
whenever we write a new strategy for it. For the same reason, it is
awkward to manage a collection of traders with different strategies.

Is there another way?
You could change the Strategy interface to take a StrategyCallback in
the relevent method calls (such as evaluate and trade). That way there
is no direct composition relationship between a Strategy and the
"callback", which might better decouple them (and remove the circular
reference).
struct Strategy;
struct StrategyCallback;
It might be better to call that OrderHandler, or similar, to better
express its purpose.

/// trader class aggregating a strategy
class Trader
: public StrategyCallback
{
public:
//construction
Trader(trading::Strategy*);
~Trader();
private:
Trader(const& Trader);
Trader& operator=(const Trader&);

//private data
Strategy* m_pStrategy;

Obviously a smart pointer would be appropriate here (and in the
constructor).

Another solution would be to use a StrategyFactory. e.g.

class StrategyFactory
{
public:
virtual ~StrategyFactory(){};

typedef shared_ptr<Strategy> StrategyPtr;

virtual StrategyPtr createStrategy(OrderHandler& handler) = 0;
};

template <class Strat>
class ConcreteStrategyFactory: public StrategyFactory
{
public:
virtual StrategyPtr createStrategy(OrderHandler& handler)
{
return StrategyPtr(new Strat(handler));
}
};

and pass that into the Trader constructor (note that the handler must
outlive the Strategy - avoid circular smart pointer references by
using a weak_ptr or a reference to the handler as above).

Tom
--
C++ FAQ: http://www.parashift.com/c++-faq-lite/
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html

[Karl Heinz Buchegger]

Claudio Jolowicz wrote:

The quick and dirty approach would be the following:

The user (1) instantiates the trader and then (2) uses the reference
to the trader to instantiate the strategy. After that (3), the user
calls a method of the trader that initializes the trader's strategy,
passing it the reference to the strategy.

The problem is that the initialization takes place outside of, and
after the constructor. Also, the user has to worry about internals of
the trader.

Hmm.

class StrategyCallback
{
public:
virtual OrderIseq getOrders() = 0;
virtual trading::Order* createOrder( double dPrice, double dVolume) = 0;
};

class Strategy
{
public:
Strategy();

void UseWith( StrategyCallback& TheTrader );
};

class Trader : public StrategyCallback
{
public:
Trader( Strategy& UseStrategy ) : m_Strategy( UseStrategy )
{
m_Strategy.UseWith( *this );
};

protected:
Strategy m_Strategy;
};

int main()
{
Strategy MyStrat;
Trader MyTrader( MyStrat );
}

Seems acceptable to me, without having the user of the classes have to know
to much details about the Trader class. A Trader cannot be instatiated without
a strategy, thus it cannot be forgotten to give one to it. The passed strategy
is copied at the moment it is given to the trader thus you can throw away the
MyStrat in main without affecting the MyTrader (may or may not be what you want).

--
Karl Heinz Buchegger
kb******@gascad.at

[Karl Heinz Buchegger]

Claudio Jolowicz wrote:

The quick and dirty approach would be the following:

The user (1) instantiates the trader and then (2) uses the reference
to the trader to instantiate the strategy. After that (3), the user
calls a method of the trader that initializes the trader's strategy,
passing it the reference to the strategy.

The problem is that the initialization takes place outside of, and
after the constructor. Also, the user has to worry about internals of
the trader.

Hmm.

class StrategyCallback
{
public:
virtual OrderIseq getOrders() = 0;
virtual trading::Order* createOrder( double dPrice, double dVolume) = 0;
};

class Strategy
{
public:
Strategy();

void UseWith( StrategyCallback& TheTrader );
};

class Trader : public StrategyCallback
{
public:
Trader( Strategy& UseStrategy ) : m_Strategy( UseStrategy )
{
m_Strategy.UseWith( *this );
};

protected:
Strategy m_Strategy;
};

int main()
{
Strategy MyStrat;
Trader MyTrader( MyStrat );
}

Seems acceptable to me, without having the user of the classes have to know
to much details about the Trader class. A Trader cannot be instatiated without
a strategy, thus it cannot be forgotten to give one to it. The passed strategy
is copied at the moment it is given to the trader thus you can throw away the
MyStrat in main without affecting the MyTrader (may or may not be what you want).

--
Karl Heinz Buchegger
kb******@gascad.at