Design of the Payment Service Block

Document #: 221ZR053
Authors: J. L. Abad Peiro, N. Asokan, M. Steiner, M. Waidner (ZRL)
Editor: N. Asokan (ZRL)
Reviewer: CRM, DIC, KPN
Status: Draft Version. 2, SEMPER internal
Date: January 23, 1997

Introduction

The purpose of the payment service block is to provide a unified way to access various different payment systems are available to a user. The design described here is based on the concepts described in the SEMPER activity paper 212ZR055: Designing a Generic Payment Service.

This supersedes the old SEMPER activity paper, 212ZR054: Payment Manager Overview.

An active instance of the payment service block will contain the following objects:

one PaymentManager object (in reality, the PaymentManager is a static class.)
zero or more Purse objects
zero or more PaymentTransaction objects (each corresponding to an ongoing purse transaction)
one PaymentTransactionRecord for each PaymentTransaction object. (Note: this is created each time a PaymentTransaction is created. But the PaymentTransaction object is transient (i.e. discarded at the end of a session or at the completion of the transaction) while the PaymentTransactionRecord is stored for longer periods.

Figure 1: A Payment Service Block in Action
(Courtesy Günter Karjoth)

Class and Interface Hierarchies

The following class and interface hierarchies in the architecture are significant ("Abc" is used as the name of an example payment system):

The functionality expected from payment systems is captured in the PurseServices interface hierarchy. The root PurseServices interface contains methods corresponding to services expected from all payment systems. Additional services that are specific to various payment models is captured in sub-interfaces that extend the root interface.

Interface hierarchy: PurseServices

 - PurseServices -+--- AccountBasedPurseServices
                  |
                  |
                  |
                  +--- CashLikePurseServices
                  |
                  |

These interfaces describe the generic purse services required by the various payment models. These will be implemented by: the PaymentTransaction subclass (see below) corresponding to each payment system of the appropriate model.

Purse services that are optional are defined separately. An example is the MicropaymentServices interface. Each category of optional services will be in the form of a parallel interface hierarchy similar to the PurseServices hierarchy. It is not mandatory for the subclasses of the Transaction class to implement the optional part of the purse services interface.

Class hierarchy: Purse

 - Purse-+--- AccountBasedPurse -+---AbcPurse
         |                       |
         |                       +---....
         |
         +--- CashLikePurse  -------+---....
         |                          |
         |                          +---....

A purse is a repository of information related an installed payment system. As with the PurseServices hierarchy, the Purse class hierarchy has a root class and model-specific subclasses of it. The root class defines a set of services. Some are in the form of abstract method definitions which need to be implemented by the Purse subclasses. For the rest, default implementations are provided; the subclasses may override them if needed.

Class hierarchy: PaymentTransaction

 - PaymentTransaction -+--- AbcTransaction
              |
              |
              |
              +--- ....
              |

Objects of this class hierarchy are the primary objects in the payment service block for providing payment services. A PaymentTransaction object can be thought of as an abstraction of a "transaction." The root class simply provides the functionality of a generic "transaction" (e.g.: methods to abort and to status check). Each payment system is required to have a subclass of the PaymentTransaction class that implement the PurseServices interfaces for its payment model.

Note that we could have abstract subclasses AccountBasedTransaction and CashLikeTransaction and then make AbcTransaction a subclass of one of these classes. We have not done this at this time because there doesn't seem to be a need for it (if defined, such classes would be empty at least as of now). If it becomes necessary, they can always be defined.

Currently, the model-specific functionality provided by the transaction objects is expressed by the corresponding sub-interface in the PurseServices interface hierarchy. Functionality expected from a generic transaction is expressed by the PaymentTransaction class. The adapter of a payment system will subclass the PaymentTransaction class to inherit specifications of the generic transaction functionality and implement the model-specific PurseServices sub interface to inherit specifications of the model-specific functionality. Hence there is no need for the aforementioned intermediate subclasses of PaymentTransaction.

Finally, note that subclass designers should ensure that only methods corresponding to a single transaction are executed on an PaymentTransaction object. (e.g. if someone creates an PaymentTransaction object, invokes pay() on it and then attempts to make another invocation of a purse service on the same object, the second attempt must be disallowed; however multiple invocations of doMicroPayment() or capture() on the same PaymentTransaction object is legal).

Class (hierarchy): PaymentTransactionRecord

Each PaymentTransaction object keeps any data with long-term relevance in a corresponding PaymentTransactionRecord object. Recall that the latter object is the one that is archived after the transaction is complete. Typically, the designer of the adapter for a payment system will subclass the PaymentTransactionRecord class in order to keep information specific to that payment system. The startTransaction() method in the Purse subclass should do the following:

Get a new transaction record handle (by calling the getNewTXRHandle() method in the Purse class,
Create a new transaction record object of the correct type with the newly created handle,
Perform any necessary initialisations of the transaction record object using the various setter methods,
Create a new transaction object of the correct type with the newly created transaction record,
Return the reference to this object to the caller

Class (hierarchy): ServiceType

This class hierarchy is used to represent the services defined in the PurseServices interface hierarchy. For example, the PaymentTransactionRecord object has an attribute of this class to denote the service that was provided during the corresponding transaction. Each model may have an optional subclass of the root ServiceType class. Individual adapters are free to extend the hierarchy further. The various methods in the transaction class of an adapter should use these service type identifiers to keep the transaction record up-to-date.

Class (hierarchy): PaymentTransactionState

 - PaymentTransactionState-+--- AccountBasedTransactionState-+--...
         |                                                   |
         |                                                   +---....
         |
         +--------------------- CashLikeTransactionState ----+---....
         |                                                   |
         |                                                   +---....

This class hierarchy is used to represent the states of a transaction. Again, the PaymentTransactionRecord object has an attribute of this class to denote the state of the corresponding transaction. The root class is extended for each model and adapters are free to extend it further. Each class in this hierarchy provides the following:

a public constructor with no arguments
a set of pre-defined objects representing standard state values (e.g. TS_INITIATED [in PaymentTransactionState] and TS_AB_CAPTURED [in AccountBasedTransactionState])
a setter method to set the state based on one of the pre-defined state objects (e.g. setState[in PaymentTransactionState] setAccountBasedState [in AccountBasedTransactionState] etc.)
an overriding definition of the equals() method;

The various methods in the transaction class of an adapter should use these status values to keep the transaction record up-to-date. It is left to the adapter writer to decide how to mix and match the different state values at various levels of the above hierarchy.

Class: PaymentManager

The payment manager object:

acts as a "controller" of the payment service block (i.e. keeps track of which purses and transactions are active, resumes unfinished transactions when SEMPER recovers from a crash, etc.),
provides services that involve multiple purses (e.g. exchangeCurrency), and
provides services for purse/payment-method selection (including services for negotiating parameters with the peer).

The PaymentManager class is an implementation of the above.

Incorporating a Payment System into SEMPER

To incorporate a payment system into the payment service block, the following are necessary:

a (direct) subclass of the PaymentTransaction class which implements the purse services defined in the interface corresponding to the appropriate payment model,
e.g. a AbcTransaction class that implements:
- the AccountBasedPurseServices interface, and
- optionally any of the non-mandatory purse services interfaces (e.g. the MicropaymentServices interface)
When an object of this subclass is created, it should create a corresponding object of the PaymentTransactionRecord class. If necessary, a subclass of the PaymentTransactionRecord class may be defined and used to suit the needs of specific payment systems. The PaymentTransactionRecord uses objects of other classes to record information about the transaction. Examples are:
- ServiceType
- PaymentTransactionState
Adapters can extend these classes to add more information specific to their payment system.
a subclass (or "subsubclass") of the Purse class, implementing all abstract methods in the Purse class.
e.g. a AbcPurse class that extends:
- the AccountBasedPurse class.

Together, these can be considered to constitute the "adapter" for that payment system. In addition, the following are required:

any special business applications: there will be a standard special business application for the payment service block. It would provide generic services to enable user-interaction with the payment service block. For example, it will provide a default way to create a new purse (i.e. by invoking CreatePurse method of the PaymentManager) or to "install" a new payment module into the payment service block. The class PurseManagement provides a set of methods to facilitate the creation and configuration of purses using interaction via Tinguin. Each adapter will have to provide certain standard methods that are used by the PaymentManager and PurseManagement classes. Currently, these are all expected from the subclass of the Purse class. They are:

A constructor method,

A method init() to initialise the purse during startup, and

A method setup() to perform configuration of a purse via user interaction. Default implementations are available in the root Purse class. Also, if a payment system provides more functionality than the what is assumed by the SEMPER payment service block model, special business applications will be necessary to configure, and use such functionality.

If a payment system does not provide a particular service required by the corresponding interface, the adapter still has to implement a cover method that returns a "not implemented" error/exception.

The Lifecycle of Purses

A payment system adapter contains a sub-class of the Purse class. Whenever a new adapter is installed, the name of the the Purse subclass must be registered with the PaymentManager using the registerPurseClassName service. During startup, the PaymentManager will poll each known purse class to see if it is enabled: i.e. whether a purse object of this class can be instantiated. Each purse provides a isEnabled method using which the PaymentManager can check if a Purse class is enabled. The official way to create and delete purses is to use the API provided by the PaymentManager in the form of the createPurse and deletePurse services.

Each purse also provides an initialisation service. During startup, the PaymentManager will attempt to initialise each purse. The purses that are successfully initialised are called active. Inactive purses cannot be used in any value transfer transaction. A purse can be disabled using the disable (internal to the payment block) method. Before a purse is deleted, the PaymentManager will attempt to disable it first.

Each Purse class also provides a setup service to configure purses. This service must take care of all the necessary configuration before a purse can be used in value transfer transactions. Purses that can not configured properly should fail their initialisation.

Finally, the PurseReferences class provides a standard and permanent way to refer to purses (in the absence of persistent storage, java object references are transient).

Purse Transactions

For each transaction, there will be a corresponding PaymentTransaction object. The caller would do something like:


	abcTr1 = (AbcTransaction) abcPurse1.startTransaction(<parameters>);
	abcTr1.pay(<parameters>);
	abcTr1.endTransaction();

Here are some examples of usage:

The caller wants to perform a transaction consisting of a group of related operations. For example,


	abcTr1 = (AbcTransaction) abcPurse1.startTransaction(<parameters>);
	abcTr1.startMicropayment (<parameters>);
	abcTr1.doMicropayment (<parameters>);
	[...]
	abcTr1.doMicropayment (<parameters>);
	abcTr1.stopMicropayment (<parameters>);
        abcTr1.endTransaction();

The caller may want to know the status of the transaction or abort it. E.g., by doing:
```
	state = abcTr1.getState();
or
	abcTr1.abort();
```

Protocols

Initially at least, only a simple negotiation protocol will be implemented. Messages in the negotiation protocol will be in the form of a list of lists. Each secondary list is used to negotiate the value of a single parameter (e.g. "payment service name"). The head of this list will be an object of the MessageListHead class. It contains the following attributes:

name of the parameter (e.g. "payment system name"),
name of the class describing possible values of this parameter (e.g. java.lang.String),
a list of objects of the aforementioned class, and
flags to guide the negotiation (e.g. "select from this list", "make a proposal of possible values", "reply to your query" etc.)

The PaymentManager provides various methods such as choosePS and requestPSChoice to negotiate the selection of payment systems. Further, there are methods for local selection as well (for example, based on preferences, or interaction with the user). Different purse selection policies can be implemented by combining these methods in different ways. A pair of methods selectPayingPurse and selectReceivingPurse implementing a particular policy are also provided in the PaymentManager class. Currently, we plan for synchronous negotiations between two payment managers. Later, we may extend this to asynchronous negotiations, and negotiations between two PaymentTransaction objects.

Interaction with the User

[to be done]

Notes

Access control: An object will check with the AccessControlStructure to see if an operation can be performed. The latter will check with the AccessControl Manager. A string (in the "TINGUIN language") will be passed as a parameter during these checks. The A/C Manager can ultimately display this string to the user when asking to unlock a role. The payment service block should assure that this string will have a standard appearence.

Configuration/Initialisation: Special business applications will be required for performing configuration and initialisation. There will be a standard payment service block business application to enable the user to interact directly with the payment service block. The building blocks for such a service are provided in the PurseManagement class. In addition, each payment system will come with special business applications to create a purse and perform additional system-specific tasks.
Note that if trusted I/O is required by an object in the payment service block during the execution of such a special business application, it may have to deal directly with TINGUIN (and not via the access control manager).

Interactions with Other Blocks

Configuration and Preferences Blocks: See the special section below on this topic.
Archive Block: Archive block has been integrated with the payment block at the top level. Individual adapters are responsible for integrating the archive block if they choose to use its services.
Access-control Block: [to be done] - not high priority.

Issues

In this section, we list proposed or forthcoming changes to the design and issues that need to be discussed:

Using the Payment Service Block

Transfer Manager in the SEMPER transfer layer is likely to be the most common user of services from the Payment Service Block. However, as mentioned before special Business Applications that manipulate the configuration of the payment service block will interact directly with objects in the payment service block. Also, in principle, there is nothing to prevent normal Business Applications or the Commerce Layer to interact directly with these objects as well.

In this section, we give brief examples of usage:

Simple Payment

Scenario: A user "Alice" wants to make a payment of 100 PST to a shop, "Bob Inc.". The shop is at bob.com. The user and the shop have already started a commerce session. They agree to use a common external reference string (we assume that this is available in the variable ext_ref) to identify this session. They have also not selected any specific payment mechanisms but would like to leave it to their respective payment service blocks to negotiate it.

Here is what the user does:

// seller's address
// The port number and path have no relevance -- so we just use placeholders
ComPointAddress seller_addr = new ComPointAddress ("tcp", "bob.com", 0, "");
PaymentEntity seller = new PaymentEntity ("Bob Inc.", seller_addr);

// We don't want to use any specific alias (just the name we have on
// the selected purse)
String my_alias = null;

// We have no preferences about the means of payment
Vector preferred_payment_systems = null;
Vector preferred_purses = null;

// We have no security options
Vector options = null;

// External reference (already agreed upon between payer and payee)
String ext_ref = "order-# 1233";

// Now select a purse -- this involves negotiation with peer
PurseReference chosen_purse_ref = PaymentManager.selectPayingPurse
	(seller, my_alias, amount, preferred_payment_systems,
	 preferred_purses, options, ext_ref);
Purse chosen_purse = chosen_purse_ref.getPurse();

// carry out the payment
PaymentTransaction tr = chosen_purse.startTransaction();
tr.pay(seller, amount, options, ext_ref);
tr.endTransaction();

Simultaneously, the shop does the following:

// our address (PaymentManager.init() had already registered this address
// with the communication manager
// The port number and path have no relevance -- so we just use placeholders
ComPointAddress seller_addr = new ComPointAddress ("tcp", "bob.com", 0, "");
PaymentEntity seller = new PaymentEntity ("Bob Inc.", seller_addr);

// We don't particularly care for the buyer's address since they will open
// a channel to us with the agreed upon "ext_ref"
PaymentEntity buyer = null;
// The rest, as with the buyer side
Vector preferred_payment_systems = null;
Vector preferred_purses = null;
Vector options = null;
String ext_ref = "order-# 1233";

// Now select a purse -- this involves negotiation with peer
PurseReference chosen_purse_ref = PaymentManager.selectReceivingPurse
	(buyer, seller, amount, preferred_payment_systems,
	 preferred_purses, options, ext_ref);
Purse chosen_purse = chosen_purse_ref.getPurse();

// receive the payment
PaymentTransaction tr = chosen_purse.startTransaction();
tr.receivePayment(buyer, amount, options, ext_ref);
tr.endTransaction();

The above assumes that

The necessary purses have been already created. This must be done by using a special Payment Block Business Application called purse
The initialisation methods of the PaymentManager and each purse has already been executed. This is typically done by Library.init()

Since these are not yet available, any test program should do the necessary initialisation by itself. The test programs Seller.java and Buyer.java in the Test directory of the source are examples of such test programs.

Configuration and Preferences

Preferences: The init() method of the PaymentManager> will attempt to load the its preference group from the preference manager; if none exists, one will be created with initial defaults. Each purse should also do the same. Currently, the Payment Manager understands the following preferences:

Purse Selection: The user can specify how to break ties if more than one purse or payment system turns out to be suitable to use in a transaction. Currently, two ways are supported: the payment block can prompt the user to make a choice or it can simply pick any one of them. For non-interactive installations (e.g. a merchant site), the latter choice is preferable.
Default: Prompt the user to break ties.

Configuration: The following configuration variables are understood by the payment block:

semper.configuration.language - this variable is used to specify the local language for user interaction. The value of the variable can be a two letter code for the language. Currently the following are supported: "de" (Deutsch) and "en" (English). If you would like to volunteer to do a bit of translation (10 mins.) to help us support additional languages, please let us know.Beware that the local language support is quite limited.
Default: English.
semper.payment.noListeners Unofficially, this configuration variable is also understood. If this is set, there will be no listner process to handle incoming negotiation requests. This is provided solely for the purpose of being able to run two SEMPER processes on the same machine, until a proper solution is found. This may change any time -- since the support is unofficial!
Default: False.

Token-based Definitions of Interfaces to Protocols

In defining the interfaces, we have assumed that the payment block is responsible for running its protocols: the payment manager carries out the negotiation protocol and adapters of payment systems take care of their respective payment protocols. An alternate approach, in the style of GSS-API, is to define the interface in terms of tokens. The payment block is still responsible for constructing and interpreting protocol messages; but the actual communication between peers is left to the caller.

To this end, we define a token-based PurseServices hierarchy rooted at the TPurseServices interface. The token-based interface has a start<service> method corresponding to every <service> method in the PurseServices interface hierarchy. The sub-classes of the PaymentTransaction will implement the token-based interface as well.

The SEMPER model was assumed to be synchronous in that both the payer and payee is expected to have already started business sessions and therefore, for example, invoke pay() and receivePayment() synchronously. With the token-based interface, we can also support an asynchronous model.

Forthcoming Changes

We will write a token-based definition of the negotiation services and move the actual negotiation protocol inside the implementation of these services. The existing, synchronous, versions of the negotiation services can then be re-written in terms of the token-based methods. We will need to define a Negotiation class to do this.
Make the token-based PurseServices interface the default and the old synchronous interface to be optional (this can be done perhaps after the first trials). Adapter designers are encouraged to always implement the token-based interfaces first and then implement the synchronous interface using the token-base methods.
It may be useful to have a PaymentTransactionRecord class hierarchy similar to the Purse class hierarchy (e.g. with model-specific intermediate classes)

Miscellany

The payment service block will be in package semper.payment. The documentation includes an index and a tree depicting the class structure.

Individual payment modules should be in packages like semper.payment.ecash. The semper.payment package is unaware of the various individual modules. Each payment module may provide an "installer/configurator" application which can use the services that have been already provided,such as createPurse and those in the PurseManagement class as well as any additional services speicific to that payment module.