MarkLogic Server 11.0 Product Documentation

Content Processing Framework Guide — Chapter 5

Condition Modules Must Return a Boolean

Condition modules must return a boolean value (true or false).

In one common scenario, a condition module checks either the existence or the value of an element in the document or in its properties document. If it exists, then the module returns true and the document needs processing for the current phase of the pipeline. Another scenario is that the condition performs some specialized logic based on some part of the document contents. The logic does not even need to pertain to the document, as long as the module returns true or false.

Condition Modules Should Not Update Documents

A condition module should only return a boolean value--it should not perform any other work. This is an assumption of the pipeline design. If you do perform updates in conditions, it will change the document from the state it was in when the event occurred (potentially causing an additional trigger to fire). Do not perform any document updates in a condition module; doing updates in conditions can cause non-deterministic behavior.

Action Modules Use try/catch With cpf:success and cpf:failure

The mechanism for transitioning a document from one state to the next is carried out by two functions in the cpf.xqy XQuery module: cpf:success and cpf:failure. These functions handle the logic to advance a document either to the on-success or to the on-failure state specified in the pipeline.

Action modules must call either cpf:success or cpf:failure exactly once. By using these functions in a try/catch expression in your XQuery code, it is easy to either advance the document to the success state when the code runs without exceptions, or to catch any exceptions in the XQuery code and then put the document in a failure state. This ensures that exactly one of these functions is called. In the XQuery code, you concatenate the cpf:success call following your action code, and return cpf:failure in the catch clause (which is run only if the try clause throws an exception).

The try/catch expression ensures that the state is advanced in the same transaction that performs the document processing. This way, if processing is interrupted, the state of the document always matches the actual state of the content processing.

The following sample code (from the link-rename-action.xqy module) shows how to use cpf:success and cpf:failure in a try/catch expression.

xquery version "1.0-ml";
import module namespace cpf = "http://marklogic.com/cpf"
   at "/MarkLogic/cpf/cpf.xqy";
import module namespace lnk = "http://marklogic.com/cpf/links"
   at "/MarkLogic/cpf/links.xqy";

declare variable $cpf:document-uri as xs:string external;
declare variable $cpf:transition as node() external;

try {
   lnk:propagate-rename( $cpf:document-uri )
   ,
   cpf:success( $cpf:document-uri, $cpf:transition, () )
}
catch ($e) {
   cpf:failure( $cpf:document-uri, $cpf:transition, $e, () )
}

Action Modules Operate On a Single Document

Use care if your action modules modify multiple documents. As a general rule, action modules should only modify the document being processed; they should not modify any other documents without fully understanding the implications. Creating side effects by modifying other documents within a single transaction can cause triggers to fire on updates, which can (potentially) cause multiple updates to the same document to be queued in the task server. Therefore, you should only modify other documents if you fully understand the consequences (or if you are sure there are no triggers on the other updates).

If your action modules modify multiple documents, you must design your application to handle the side effects. Each time a document that is in the scope of a domain is updated, a trigger fires, which can initiate a new set of processing. If your application must do this, make sure you carefully think through the side effects that will occur.

Action Modules Must Be a Single Transaction

An action module must execute as a single transaction; it should not update the document more than once. Avoid using xdmp:eval or xdmp:invoke to run other transactions from within an action module. The Content Processing Framework assumes that action modules will perform a single transaction.

If you do perform multiple update transactions in an action module, you should understand the implications. Transitions are initiated as transactions complete. Having multiple transactions complete in a single action module can lead to multiple transitions being initiated in parallel, leading to unpredictable results. While it is possible to do this, your application design must take this into account. For more details, see Design Patterns For Content Applications.

Using XSLT Stylesheets Instead of Action Modules

If you specify a path to an XSLT stylesheet instead of a path to an XQuery module in a pipeline, then the framework will invoke the stylesheet with the appropriate variables. For an action module, the stylesheet must return the new contents of the document. The framework takes care of invoking the stylesheet for you, and it takes care of the other mechanics (the try/catch and calling cpf:success or cpf:failure); your stylesheet should not directly perform any updates, as the framework will take care of that for you. Note that this is different from XQuery actions, which actually perform the update. If your stylesheet produces multiple result documents in an action module, the first result document is the content that updates the document under CPF control. Any subsequent result documents in the stylesheet output are saved to the database at the URIs specified in the stylesheet.