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Correlating BPEL processes

Correlations are used in runtime environments where there are multiple instances of the same process running. The sets allow two partners to initialize a BPEL process transaction, temporarily suspend activity, and then recognize each other again when that transaction resumes.


Correlation sets

Correlation sets are initialized with values from process inbound or outbound messages.

Correlation sets are used in situations where many similar conversations are occurring simultaneously, and might proceed something like this:

  1. A partner call is made, and an instance of the process is initialized.
  2. Correlation tokens are gathered for the partner.
  3. Communication is terminated for some reason. Usually because one of the participants needs to gather some more information independently.
  4. Communication is reestablished, the tokens gathered earlier are used to confirm that the same partner is interacting with the same instance of the process.

  5. The process continues running.

To illustrate how a correlation set might be used in a real world setting, imagine that you call a travel agent to ask about a trip. In this, the initial conversation, you tell the agent what you're interested in, and she gives you a quote. You tell her that you want to think about it, so she gathers some specific contact information about you (name and phone number), and you end the conversation. A week later, you have made up your mind, and you call the travel agent again and give her your contact information. With this, she is able to identify you, and pulls up your quote on her computer. You conclude your business.

In this example, the correlation set is the contact information that the agent stores so that she'll be able to recognize you in future correspondence. She does not stop everything else that she's doing while you decide what you want to do, instead she conducts similar business with other customers, and picks up your request where it left off when you reinitiate contact.

A process must have a correlation set if it has more than one receive or pick activity.


Correlation properties

Correlation properties contain a name and a data type and define the correlation sets with which they are associated. In addition, you can map a message parameter to a correlation property using a property alias.


Property aliases

In the previous example, mention was made of tokens. Correlation tokens refer to specific message parts in your process. Sometimes you may find that you want to correlate a property to a message part from another interface. They may have what is essentially the same kind of content, but use different names, so you will need to tell the system that they are analogous. You would do this with an alias.

For example, you would use an alias to tell the system that whenever it encounters a message part called lastName, it should assume that it means the same as the other message part called familyName.

Aliases are especially useful in abstract BPEL processes that can't always fully expose their data. These kinds of processes need to make all of the business logic apparent to each of its partners, but often can't afford to reveal sensitive business data. In this context, aliases allow for the fact that two message parts may have the same value, but are called something by one partner, and something entirely different by another. In other words, aliases recognize that the partners may be using the same data in the process but are labelling it differently.

For a real world example, you use aliases most every time that you use a snack-based vending machine. When you look through the window of the vending machine to choose the snack that you want to buy, you don't identify the snack by its proper name, you identify it with a code, usually a combination of a letter and a number.

For example, you may want to buy an oatmeal cookie, but you don't enter the word "cookie" on the keypad, instead you enter a code that is unique to that item (for example F6). The vending machine then knows exactly which snack to dispense.


Example

To see some examples of how to use correlation in a BPEL process that you can build and run yourself, go to http://publib.boulder.ibm.com/bpcsamp/index.html, click Samples for v7.0Advanced BPEL features, and choose from either of the following samples:

BPEL process editor

Building BPEL processes


Related reference:
Correlation tab: BPEL process editor
Defaults tab: BPEL process editor
Description tab: business state machine editor