Added: incubator/nifi/site/trunk/docs/nifi-docs/html/developer-guide.html URL: http://svn.apache.org/viewvc/incubator/nifi/site/trunk/docs/nifi-docs/html/developer-guide.html?rev=1682431&view=auto ============================================================================== --- incubator/nifi/site/trunk/docs/nifi-docs/html/developer-guide.html (added) +++ incubator/nifi/site/trunk/docs/nifi-docs/html/developer-guide.html Fri May 29 11:51:20 2015 @@ -0,0 +1,3031 @@ +<!-- + Licensed to the Apache Software Foundation (ASF) under one or more + contributor license agreements. See the NOTICE file distributed with + this work for additional information regarding copyright ownership. + The ASF licenses this file to You under the Apache License, Version 2.0 + (the "License"); you may not use this file except in compliance with + the License. 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+.hide-for-print{display:none!important} +.show-for-print{display:inherit!important}} +</style> +<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.2.0/css/font-awesome.min.css";> +</head> +<body class="article"> +<div id="header"> +<h1>NiFi Developer’s Guide</h1> +<div class="details"> +<span id="author" class="author">Apache NiFi Team</span><br> +<span id="email" class="email"><a href="mailto:d...@nifi.incubator.apache.org";>d...@nifi.incubator.apache.org</a></span><br> +</div> +<div id="toc" class="toc"> +<div id="toctitle">Table of Contents</div> +<ul class="sectlevel1"> +<li><a href="developer-guide.html#introduction">Introduction</a></li> +<li><a href="developer-guide.html#components">NiFi Components</a></li> +<li><a href="developer-guide.html#processor_api">Processor API</a> +<ul class="sectlevel2"> +<li><a href="developer-guide.html#supporting_api">Supporting API</a></li> +<li><a href="developer-guide.html#AbstractProcessor">AbstractProcessor API</a></li> +<li><a href="developer-guide.html#component-lifecycle">Component Lifecycle</a></li> +<li><a href="developer-guide.html#reporting-processor-activity">Reporting Processor Activity</a></li> +</ul> +</li> +<li><a href="developer-guide.html#documenting-a-component">Documenting a Component</a> +<ul class="sectlevel2"> +<li><a href="developer-guide.html#documenting-properties">Documenting Properties</a></li> +<li><a href="developer-guide.html#documenting-relationships">Documenting Relationships</a></li> +<li><a href="developer-guide.html#documenting-capability-and-keywords">Documenting Capability and Keywords</a></li> +<li><a href="developer-guide.html#documenting-flowfile-attribute-interaction">Documenting FlowFile Attribute Interaction</a></li> +<li><a href="developer-guide.html#documenting-related-components">Documenting Related Components</a></li> +<li><a href="developer-guide.html#advanced-documentation">Advanced Documentation</a></li> +</ul> +</li> +<li><a href="developer-guide.html#common-processor-patterns">Common Processor Patterns</a> +<ul class="sectlevel2"> +<li><a href="developer-guide.html#ingress">Data Ingress</a></li> +<li><a href="developer-guide.html#data-egress">Data Egress</a></li> +<li><a href="developer-guide.html#route-based-on-content-one-to-one">Route Based on Content (One-to-One)</a></li> +<li><a href="developer-guide.html#route-based-on-content-one-to-many">Route Based on Content (One-to-Many)</a></li> +<li><a href="developer-guide.html#route-streams-based-on-content-one-to-many">Route Streams Based on Content (One-to-Many)</a></li> +<li><a href="developer-guide.html#route-based-on-attributes">Route Based on Attributes</a></li> +<li><a href="developer-guide.html#split-content-one-to-many">Split Content (One-to-Many)</a></li> +<li><a href="developer-guide.html#update-attributes-based-on-content">Update Attributes Based on Content</a></li> +<li><a href="developer-guide.html#enrich-modify-content">Enrich/Modify Content</a></li> +</ul> +</li> +<li><a href="developer-guide.html#error-handling">Error Handling</a> +<ul class="sectlevel2"> +<li><a href="developer-guide.html#exceptions-within-the-processor">Exceptions within the Processor</a></li> +<li><a href="developer-guide.html#exceptions-within-a-callback-ioexception-runtimeexception">Exceptions within a callback: IOException, RuntimeException</a></li> +<li><a href="developer-guide.html#penalization-vs-yielding">Penalization vs. Yielding</a></li> +<li><a href="developer-guide.html#session-rollback">Session Rollback</a></li> +</ul> +</li> +<li><a href="developer-guide.html#general-design-considerations">General Design Considerations</a> +<ul class="sectlevel2"> +<li><a href="developer-guide.html#consider-the-user">Consider the User</a></li> +<li><a href="developer-guide.html#cohesion-and-reusability">Cohesion and Reusability</a></li> +<li><a href="developer-guide.html#naming-convensions">Naming Conventions</a></li> +<li><a href="developer-guide.html#processor-behavior-annotations">Processor Behavior Annotations</a></li> +<li><a href="developer-guide.html#data-buffering">Data Buffering</a></li> +</ul> +</li> +<li><a href="developer-guide.html#controller-services">Controller Services</a> +<ul class="sectlevel2"> +<li><a href="developer-guide.html#developing-controller-service">Developing a ControllerService</a></li> +<li><a href="developer-guide.html#interacting-with-controller-service">Interacting with a ControllerService</a></li> +</ul> +</li> +<li><a href="developer-guide.html#reporting-tasks">Reporting Tasks</a> +<ul class="sectlevel2"> +<li><a href="developer-guide.html#developing-a-reporting-task">Developing a Reporting Task</a></li> +</ul> +</li> +<li><a href="developer-guide.html#testing">Testing</a> +<ul class="sectlevel2"> +<li><a href="developer-guide.html#instantiate-testrunner">Instantiate TestRunner</a></li> +<li><a href="developer-guide.html#add-controllerservices">Add ControllerServices</a></li> +<li><a href="developer-guide.html#set-property-values">Set Property Values</a></li> +<li><a href="developer-guide.html#enqueue-flowfiles">Enqueue FlowFiles</a></li> +<li><a href="developer-guide.html#run-the-processor">Run the Processor</a></li> +<li><a href="developer-guide.html#validate-output">Validate Output</a></li> +<li><a href="developer-guide.html#mocking-external-resources">Mocking External Resources</a></li> +<li><a href="developer-guide.html#additional-testing-capabilities">Additional Testing Capabilities</a></li> +</ul> +</li> +<li><a href="developer-guide.html#nars">NiFi Archives (NARs)</a></li> +<li><a href="developer-guide.html#how-to-contribute-to-apache-nifi">How to contribute to Apache NiFi</a> +<ul class="sectlevel2"> +<li><a href="developer-guide.html#technologies">Technologies</a></li> +<li><a href="developer-guide.html#where-to-start">Where to Start?</a></li> +<li><a href="developer-guide.html#supplying-a-contribution">Supplying a contribution</a></li> +<li><a href="developer-guide.html#contact-us">Contact Us</a></li> +</ul> +</li> +</ul> +</div> +</div> +<div id="content"> +<div class="sect1"> +<h2 id="introduction"><a class="anchor" href="developer-guide.html#introduction"></a>Introduction</h2> +<div class="sectionbody"> +<div class="paragraph"> +<p>The intent of this Developer Guide is to provide the reader with the information needed to understand how Apache NiFi (incubating) +extensions are developed and help to explain the thought process behind developing the components. It provides an introduction to +and explanation of the API that is used to develop extensions. It does not, however, go into great detail about each +of the methods in the API, as this guide is intended to supplement the JavaDocs of the API rather than replace them. +This guide also assumes that the reader is familiar with Java 7 and Apache Maven.</p> +</div> +<div class="paragraph"> +<p>This guide is written by developers for developers. It is expected that before reading this +guide, you have a basic understanding of NiFi and the concepts of dataflow. If not, please see the <a href="overview.html">NiFi Overview</a> +and the <a href="user-guide.html">NiFi User Guide</a> to familiarize yourself with the concepts of NiFi.</p> +</div> +</div> +</div> +<div class="sect1"> +<h2 id="components"><a class="anchor" href="developer-guide.html#components"></a>NiFi Components</h2> +<div class="sectionbody"> +<div class="paragraph"> +<p>NiFi provides several extension points to provide developers the +ability to add functionality to the application to meet their needs. The following list provides a +high-level description of the most common extension points:</p> +</div> +<div class="ulist"> +<ul> +<li> +<p>Processor</p> +<div class="ulist"> +<ul> +<li> +<p>The Processor interface is the mechanism through which NiFi exposes access to +<a href="developer-guide.html#flowfile">FlowFile</a>s, their attributes, and their content. The Processor is the basic building +block used to comprise a NiFi dataflow. This interface is used to accomplish +all of the following tasks:</p> +<div class="ulist"> +<ul> +<li> +<p>Create FlowFiles</p> +</li> +<li> +<p>Read FlowFile content</p> +</li> +<li> +<p>Write FlowFile content</p> +</li> +<li> +<p>Read FlowFile attributes</p> +</li> +<li> +<p>Update FlowFile attributes</p> +</li> +<li> +<p>Ingest data</p> +</li> +<li> +<p>Egress data</p> +</li> +<li> +<p>Route data</p> +</li> +<li> +<p>Extract data</p> +</li> +<li> +<p>Modify data</p> +</li> +</ul> +</div> +</li> +</ul> +</div> +</li> +<li> +<p>ReportingTask</p> +<div class="ulist"> +<ul> +<li> +<p>The ReportingTask interface is a mechanism that NiFi exposes to allow metrics, +monitoring information, and internal NiFi state to be published to external +endpoints, such as log files, e-mail, and remote web services.</p> +</li> +</ul> +</div> +</li> +<li> +<p>ControllerService</p> +<div class="ulist"> +<ul> +<li> +<p>A ControllerService provides shared state and functionality across Processors, other ControllerServices, +and ReportingTasks within a single JVM. An example use case may include loading a very +large dataset into memory. By performing this work in a ControllerService, the data +can be loaded once and be exposed to all Processors via this service, rather than requiring +many different Processors to load the dataset themselves.</p> +</li> +</ul> +</div> +</li> +<li> +<p>FlowFilePrioritizer</p> +<div class="ulist"> +<ul> +<li> +<p>The FlowFilePrioritizer interface provides a mechanism by which <a href="developer-guide.html#flowfile">FlowFile</a>s +in a queue can be prioritized, or sorted, so that the FlowFiles can be processed in an order +that is most effective for a particular use case.</p> +</li> +</ul> +</div> +</li> +<li> +<p>AuthorityProvider</p> +<div class="ulist"> +<ul> +<li> +<p>An AuthorityProvide is responsible for determining which privileges and roles, if any, +a given user should be granted.</p> +</li> +</ul> +</div> +</li> +</ul> +</div> +</div> +</div> +<div class="sect1"> +<h2 id="processor_api"><a class="anchor" href="developer-guide.html#processor_api"></a>Processor API</h2> +<div class="sectionbody"> +<div class="paragraph"> +<p>The Processor is the most widely used Component available in NiFi. +Processors are the only Component +to which access is given to create, remove, modify, or inspect +FlowFiles (data and attributes).</p> +</div> +<div class="paragraph"> +<p>All Processors are loaded and instantiated using Java’s ServiceLoader +mechanism. This means that all +Processors must adhere to the following rules:</p> +</div> +<div class="ulist"> +<ul> +<li> +<p>The Processor must have a default constructor.</p> +</li> +<li> +<p>The Processor’s JAR file must contain an entry in the META-INF/services directory named +<code>org.apache.nifi.processor.Processor</code>. This is a text file where each line contains the +fully-qualified class name of a Processor.</p> +</li> +</ul> +</div> +<div class="paragraph"> +<p>While <code>Processor</code> is an interface that can be implemented directly, it +will be extremely rare to do so, as +the <code>org.apache.nifi.processor.AbstractProcessor</code> is the base class +for almost all Processor implementations. The <code>AbstractProcessor</code> class provides a significant +amount of functionality, which makes the task of developing a Processor much easier and more convenient. +For the scope of this document, we will focus primarily on the <code>AbstractProcessor</code> class when dealing +with the Processor API.</p> +</div> +<div class="paragraph"> +<div class="title">Concurrency Note</div> +<p>NiFi is a highly concurrent framework. This means that all extensions +must be thread-safe. If unfamiliar with writing concurrent software in Java, it is highly +recommended that you familiarize yourself with the principles of Java concurrency.</p> +</div> +<div class="sect2"> +<h3 id="supporting_api"><a class="anchor" href="developer-guide.html#supporting_api"></a>Supporting API</h3> +<div class="paragraph"> +<p>In order to understand the Processor API, we must first understand - +at least at a high level - several supporting classes and interfaces, which are discussed below.</p> +</div> +<div class="sect3"> +<h4 id="flowfile"><a class="anchor" href="developer-guide.html#flowfile"></a>FlowFile</h4> +<div class="paragraph"> +<p>A FlowFile is a logical notion that correlates a piece of data with a +set of Attributes about that data. +Such attributes include a FlowFile’s unique identifier, as well as its +name, size, and any number of other +flow-specific values. While the contents and attributes of a FlowFile +can change, the FlowFile object is +immutable. Modifications to a FlowFile are made possible by the ProcessSession.</p> +</div> +</div> +<div class="sect3"> +<h4 id="process_session"><a class="anchor" href="developer-guide.html#process_session"></a>ProcessSession</h4> +<div class="paragraph"> +<p>The ProcessSession, often referred to as simply a "session," provides +a mechanism by which FlowFiles can be created, destroyed, examined, cloned, and transferred to other +Processors. Additionally, a ProcessSession provides mechanism for creating modified versions of +FlowFiles, by adding or removing attributes, or by modifying the FlowFile’s content. The ProcessSession +also exposes a mechanism for emitting provenance events that provide for the ability to track the +lineage and history of a FlowFile. After operations are performed on one or more FlowFiles, a +ProcessSession can be either committed or rolled back.</p> +</div> +</div> +<div class="sect3"> +<h4 id="process_context"><a class="anchor" href="developer-guide.html#process_context"></a>ProcessContext</h4> +<div class="paragraph"> +<p>The ProcessContext provides a bridge between a Processor and the framework. It provides information +about how the Processor is currently configured and allows the Processor to perform +Framework-specific tasks, such as yielding its resources so that the framework will schedule other +Processors to run without consuming resources unnecessarily.</p> +</div> +</div> +<div class="sect3"> +<h4 id="property_descriptor"><a class="anchor" href="developer-guide.html#property_descriptor"></a>PropertyDescriptor</h4> +<div class="paragraph"> +<p>PropertyDescriptor defines a property that is to be used by a +Processor, ReportingTask, or ControllerService. +The definition of a property includes its name, a description of the +property, an optional default value, +validation logic, and an indicator as to whether or not the property +is required in order for the Processor +to be valid. PropertyDescriptors are created by instantiating an +instance of the <code>PropertyDescriptor.Builder</code> +class, calling the appropriate methods to fill in the details about +the property, and finally calling +the <code>build</code> method.</p> +</div> +</div> +<div class="sect3"> +<h4 id="validator"><a class="anchor" href="developer-guide.html#validator"></a>Validator</h4> +<div class="paragraph"> +<p>A PropertyDescriptor may specify one or more Validators that can be +used to ensure that the user-entered value +for a property is valid. If a Validator indicates that a property +value is invalid, the Component will not be +able to be run or used until the property becomes valid.</p> +</div> +</div> +<div class="sect3"> +<h4 id="validation_context"><a class="anchor" href="developer-guide.html#validation_context"></a>ValidationContext</h4> +<div class="paragraph"> +<p>When validating property values, a ValidationContext can be used to +obtain ControllerServices, +create PropertyValue objects, and compile and evaluate property values +using the Expression Language.</p> +</div> +</div> +<div class="sect3"> +<h4 id="property_value"><a class="anchor" href="developer-guide.html#property_value"></a>PropertyValue</h4> +<div class="paragraph"> +<p>All property values returned to a Processor are returned in the form +of a PropertyValue object. This +object has convenience methods for converting the value from a String +to other forms, such as numbers +and time periods, as well as providing an API for evaluating the +Expression Language.</p> +</div> +</div> +<div class="sect3"> +<h4 id="relationship"><a class="anchor" href="developer-guide.html#relationship"></a>Relationship</h4> +<div class="paragraph"> +<p>Relationships define the routes to which a FlowFile may be transfered +from a Processor. Relationships +are created by instantiating an instance of the <code>Relationship.Builder</code> +class, calling the appropriate methods +to fill in the details of the Relationship, and finally calling the +<code>build</code> method.</p> +</div> +</div> +<div class="sect3"> +<h4 id="processor_initialization_context"><a class="anchor" href="developer-guide.html#processor_initialization_context"></a>ProcessorInitializationContext</h4> +<div class="paragraph"> +<p>After a Processor is created, its <code>initialize</code> method will be called +with an <code>InitializationContext</code> object. +This object exposes configuration to the Processor that will not +change throughout the life of the Processor, +such as the unique identifier of the Processor.</p> +</div> +</div> +<div class="sect3"> +<h4 id="ProcessorLog"><a class="anchor" href="developer-guide.html#ProcessorLog"></a>ProcessorLog</h4> +<div class="paragraph"> +<p>Processors are encouraged to perform their logging via the +<code>ProcessorLog</code> interface, rather than obtaining +a direct instance of a third-party logger. This is because logging via +the ProcessorLog allows the framework +to render log messages that exceed s a configurable severity level to +the User Interface, allowing those who +monitor the dataflow to be notified when important events occur. +Additionally, it provides a consistent logging +format for all Processors by logging stack traces when in DEBUG mode +and providing the Processor’s unique +identifier in log messages.</p> +</div> +</div> +</div> +<div class="sect2"> +<h3 id="AbstractProcessor"><a class="anchor" href="developer-guide.html#AbstractProcessor"></a>AbstractProcessor API</h3> +<div class="paragraph"> +<p>Since the vast majority of Processors will be created by extending the +AbstractProcessor, it is the +abstract class that we will examine in this section. The +AbstractProcessor provides several methods that +will be of interest to Processor developers.</p> +</div> +<div class="sect3"> +<h4 id="processor-initialization"><a class="anchor" href="developer-guide.html#processor-initialization"></a>Processor Initialization</h4> +<div class="paragraph"> +<p>When a Processor is created, before any other methods are invoked, the +<code>init</code> method of the +AbstractProcessor will be invoked. The method takes a single argument, +which is of type +<code>ProcessorInitializationContext</code>. The context object supplies the +Processor with a ProcessorLog, +the Processor’s unique identifier, and a ControllerServiceLookup that +can be used to interact with the +configured ControllerServices. Each of these objects is stored by the +AbstractProcessor and may be obtained by +subclasses via the <code>getLogger</code>, <code>getIdentifier</code>, and +<code>getControllerServiceLookup</code> methods, respectively.</p> +</div> +</div> +<div class="sect3"> +<h4 id="exposing-processor-s-relationships"><a class="anchor" href="developer-guide.html#exposing-processor-s-relationships"></a>Exposing Processor’s Relationships</h4> +<div class="paragraph"> +<p>In order for a Processor to transfer a FlowFile to a new destination +for follow-on processing, the +Processor must first be able to expose to the Framework all of the +Relationships that it currently supports. +This allows users of the application to connect Processors to one +another by creating +Connections between Processors and assigning the appropriate +Relationships to those Connections.</p> +</div> +<div class="paragraph"> +<p>A Processor exposes the valid set of Relationships by overriding the +<code>getRelationships</code> method. +This method takes no arguments and returns a <code>Set</code> of <code>Relationship</code> +objects. For most Processors, this Set +will be static, but other Processors will generate the Set +dynamically, based on user configuration. +For those Processors for which the Set is static, it is advisable to +create an immutable Set in the Processor’s +constructor or init method and return that value, rather than +dynamically generating the Set. This +pattern lends itself to cleaner code and better performance.</p> +</div> +</div> +<div class="sect3"> +<h4 id="exposing-processor-properties"><a class="anchor" href="developer-guide.html#exposing-processor-properties"></a>Exposing Processor Properties</h4> +<div class="paragraph"> +<p>Most Processors will require some amount of user configuration before +they are able to be used. The properties +that a Processor supports are exposed to the Framework via the +<code>getSupportedPropertyDescriptors</code> method. +This method takes no arguments and returns a <code>List</code> of +<code>PropertyDescriptor</code> objects. The order of the objects in the +List is important in that it dictates the order in which the +properties will be rendered in the User Interface.</p> +</div> +<div class="paragraph"> +<p>A <code>PropertyDescriptor</code> object is constructed by creating a new +instance of the <code>PropertyDescriptor.Builder</code> object, +calling the appropriate methods on the builder, and finally calling +the <code>build</code> method.</p> +</div> +<div class="paragraph"> +<p>While this method covers most of the use cases, it is sometimes +desirable to allow users to configure +additional properties whose name are not known. This can be achieved +by overriding the +<code>getSupportedDynamicPropertyDescriptor</code> method. This method takes a +<code>String</code> as its only argument, which +indicates the name of the property. The method returns a +<code>PropertyDescriptor</code> object that can be used to validate +both the name of the property, as well as the value. Any +PropertyDescriptor that is returned from this method +should be built setting the value of <code>isDynamic</code> to true in the +<code>PropertyDescriptor.Builder</code> class. The default +behavior of AbstractProcessor is to not allow any dynamically created +properties.</p> +</div> +</div> +<div class="sect3"> +<h4 id="validating-processor-properties"><a class="anchor" href="developer-guide.html#validating-processor-properties"></a>Validating Processor Properties</h4> +<div class="paragraph"> +<p>A Processor is not able to be started if its configuration is not +valid. Validation of a Processor property can +be achieved by setting a Validator on a PropertyDescriptor or by +restricting the allowable values for a +property via the PropertyDescriptor.Builder’s <code>allowableValues</code> method +or <code>identifiesControllerService</code> method.</p> +</div> +<div class="paragraph"> +<p>There are times, though, when validating a Processor’s properties +individually is not sufficient. For this purpose, +the AbstractProcessor exposes a <code>customValidate</code> method. The method +takes a single argument of type <code>ValidationContext</code>. +The return value of this method is a <code>Collection</code> of +<code>ValidationResult</code> objects that describe any problems that were +found during validation. Only those ValidationResult objects whose +<code>isValid</code> method returns <code>false</code> should be returned. +This method will be invoked only if all properties are valid according +to their associated Validators and Allowable Values. +I.e., this method will be called only if all properties are valid +in-and-of themselves, and this method allows for +validation of a Processor’s configuration as a whole.</p> +</div> +</div> +<div class="sect3"> +<h4 id="responding-to-changes-in-configuration"><a class="anchor" href="developer-guide.html#responding-to-changes-in-configuration"></a>Responding to Changes in Configuration</h4> +<div class="paragraph"> +<p>It is sometimes desirable to have a Processor eagerly react when its +properties are changed. The <code>onPropertyModified</code> +method allows a Processor to do just that. When a user changes the +property values for a Processor, the +<code>onPropertyModified</code> method will be called for each modified property. +The method takes three arguments: the PropertyDescriptor that +indicates which property was modified, +the old value, and the new value. If the property had no previous +value, the second argument will be <code>null</code>. If the property +was removed, the third argument will be <code>null</code>. It is important to +note that this method will be called regardless of whether +or not the values are valid. This method will be called only when a +value is actually modified, rather than being +called when a user updates a Processor without changing its value. At +the point that this method is invoked, it is guaranteed +that the thread invoking this method is the only thread currently +executing code in the Processor, unless the Processor itself +creates its own threads.</p> +</div> +</div> +<div class="sect3"> +<h4 id="performing-the-work"><a class="anchor" href="developer-guide.html#performing-the-work"></a>Performing the Work</h4> +<div class="paragraph"> +<p>When a Processor has work to do, it is scheduled to do so by having +its <code>onTrigger</code> method called by the framework. +The method takes two arguments: a <code>ProcessContext</code> and a +<code>ProcessSession</code>. The first step in the <code>onTrigger</code> method +is often to obtain a FlowFile on which the work is to be performed by +calling one of the <code>get</code> methods on the ProcessSession. +For Processors that ingest data into NiFi from external sources, this +step is skipped. The Processor is then free to examine +FlowFile attributes; add, remove, or modify attributes; read or modify +FlowFile content; and transfer FlowFiles to the appropriate +Relationships.</p> +</div> +</div> +<div class="sect3"> +<h4 id="when-processors-are-triggered"><a class="anchor" href="developer-guide.html#when-processors-are-triggered"></a>When Processors are Triggered</h4> +<div class="paragraph"> +<p>A Processor’s <code>onTrigger</code> method will be called only when it is +scheduled to run and when work exists for the Processor. +Work is said to exist for a Processor if any of the following conditions is met:</p> +</div> +<div class="ulist"> +<ul> +<li> +<p>A Connection whose destination is the Processor has at least one +FlowFile in its queue</p> +</li> +<li> +<p>The Processors has no incoming Connections</p> +</li> +<li> +<p>The Processor is annotated with the @TriggerWhenEmpty annotation</p> +</li> +</ul> +</div> +<div class="paragraph"> +<p>Several factors exist that will contribute to when a Processor’s +<code>onTrigger</code> method is invoked. First, the Processor will not +be triggered unless a user has configured the Processor to run. If a +Processor is scheduled to run, the Framework periodically +(the period is configured by users in the User Interface) checks if +there is work for the Processor to do, as described above. +If so, the Framework will check downstream destinations of the +Processor. If any of the Processor’s outbound Connections is full, +by default, the Processor will not be scheduled to run.</p> +</div> +<div class="paragraph"> +<p>However, the <code>@TriggerWhenAnyDestinationAvailable</code> annotation may be +added to the Processor’s class. In this case, the requirement +is changed so that only one downstream destination must be "available" +(a destination is considered "available" if the Connection’s +queue is not full), rather than requiring that all downstream +destinations be available.</p> +</div> +<div class="paragraph"> +<p>Also related to Processor scheduling is the <code>@TriggerSerially</code> +annotation. Processors that use this Annotation will never have more +than one thread running the <code>onTrigger</code> method simultaneously. It is +crucial to note, though, that the thread executing the code +may change from invocation to invocation. Therefore, care must still +be taken to ensure that the Processor is thread-safe!</p> +</div> +</div> +</div> +<div class="sect2"> +<h3 id="component-lifecycle"><a class="anchor" href="developer-guide.html#component-lifecycle"></a>Component Lifecycle</h3> +<div class="paragraph"> +<p>The NiFi API provides lifecycle support through use of Java +Annotations. The <code>org.apache.nifi.annotations.lifecycle</code> package +contains +several annotations for lifecycle management. The following +Annotations may be applied to Java methods in a NiFi component to +indicate to +the framework when the methods should be called. For the discussion of +Component Lifecycle, we will define a NiFi component as a +Processor, ControllerServices, or ReportingTask.</p> +</div> +<div class="sect3"> +<h4 id="onadded"><a class="anchor" href="developer-guide.html#onadded"></a>@OnAdded</h4> +<div class="paragraph"> +<p>The <code>@OnAdded</code> annotation causes a method to be invoked as soon as a +component is created. The +component’s <code>initialize</code> method (or <code>init</code> method, if subclasses +<code>AbstractProcessor</code>) will be invoked after the component is +constructed, +followed by methods that are annotated with <code>@OnAdded</code>. If any method +annotated with <code>@OnAdded</code> throws an Exception, an error will +be returned to the user, and that component will not be added to the +flow. Furthermore, other methods with this +Annotation will not be invoked. This method will be called only once +for the lifetime of a component. +Methods with this Annotation must take zero arguments.</p> +</div> +</div> +<div class="sect3"> +<h4 id="onremoved"><a class="anchor" href="developer-guide.html#onremoved"></a>@OnRemoved</h4> +<div class="paragraph"> +<p>The <code>@OnRemoved</code> annotation causes a method to be invoked before a +component is removed from the flow. +This allows resources to be cleaned up before removing a component. +Methods with this annotation must take zero arguments. +If a method with this annotation throws an Exception, the component +will still be removed.</p> +</div> +</div> +<div class="sect3"> +<h4 id="onscheduled"><a class="anchor" href="developer-guide.html#onscheduled"></a>@OnScheduled</h4> +<div class="paragraph"> +<p>This annotation indicates that a method should be called every time +the component is scheduled to run. Because ControllerServices +are not scheduled, using this annotation on a ControllerService does +not make sense and will not be honored. It should be +used only for Processors and Reporting Tasks. If any method with this +annotation throws an Exception, other methods with this +annotation will not be invoked, and a notification will be presented +to the user. In this case, methods annotated with +<code>@OnUnscheduled</code> are then triggered, followed by methods with the +<code>@OnStopped</code> annotation (during this state, if any of these +methods throws an Exception, those Exceptions are ignored). The +component will then yield its execution for some period of time, +referred to as the "Administrative Yield Duration," which is a value +that is configured in the <code>nifi.properties</code> file. Finally, the +process will start again, until all of the methods annotated with +<code>@OnScheduled</code> have returned without throwing any Exception. +Methods with this annotation may take zero arguments or may take a +single argument. If the single argument variation is used, +the argument must be of type <code>ProcessContext</code> if the component is a +Processor or <code>ConfigurationContext</code> if the component +is a ReportingTask.</p> +</div> +</div> +<div class="sect3"> +<h4 id="onunscheduled"><a class="anchor" href="developer-guide.html#onunscheduled"></a>@OnUnscheduled</h4> +<div class="paragraph"> +<p>Methods with this annotation will be called whenever a Processor or +ReportingTask is no longer scheduled to run. At that time, many threads +may still be active in the Processor’s <code>onTrigger</code> method. If such a method +throws an Exception, a log message will be generated, and the +Exception will be otherwise +ignored and other methods with this annotation will still be invoked. +Methods with this annotation may take zero arguments or may take a +single argument. +If the single argument variation is used, the argument must be of type +<code>ProcessContext</code> if the component is a Processor or +<code>ConfigurationContext</code> if the +component is a ReportingTask.</p> +</div> +</div> +<div class="sect3"> +<h4 id="onstopped"><a class="anchor" href="developer-guide.html#onstopped"></a>@OnStopped</h4> +<div class="paragraph"> +<p>Methods with this annotation will be called when a Processor or +ReportingTask is no longer scheduled to run +and all threads have returned from the <code>onTrigger</code> method. If such a +method throws an Exception, +a lot message will be generated, and the Exception will otherwise be +ignored; other methods with +this annotation will still be invoked. Methods with this annotation +must take zero arguments.</p> +</div> +</div> +<div class="sect3"> +<h4 id="onshutdown"><a class="anchor" href="developer-guide.html#onshutdown"></a>@OnShutdown</h4> +<div class="paragraph"> +<p>Any method that is annotated with the <code>@OnShutdown</code> annotation will be +called when NiFi is successfully +shut down. If such a method throws an Exception, a log message will be +generated, and the +Exception will be otherwise ignored and other methods with this +annotation will still be invoked. +Methods with this annotation must take zero arguments. Note: while +NiFi will attempt to invoke methods +with this annotation on all components that use it, this is not always +possible. For example, the process +may be killed unexpectedly, in which case it does not have a chance to +invoke these methods. Therefore, +while methods using this annotation can be used to clean up resources, +for instance, they should not be +relied upon for critical business logic.</p> +</div> +</div> +</div> +<div class="sect2"> +<h3 id="reporting-processor-activity"><a class="anchor" href="developer-guide.html#reporting-processor-activity"></a>Reporting Processor Activity</h3> +<div class="paragraph"> +<p>Processors are responsible for reporting their activity so that users +are able to understand what happens +to their data. Processors should log events via the ProcessorLog, +which is accessible via the InitializationContext +or by calling the <code>getLogger</code> method of <code>AbstractProcessor</code>.</p> +</div> +<div class="paragraph"> +<p>Additionally, Processors should use the <code>ProvenanceReporter</code> +interface, obtained via the ProcessSession’s +<code>getProvenanceReporter</code> method. The ProvenanceReoprter should be used +to indicate any time that content is +received from an external source or sent to an external location. The +ProvenanceReporter also has methods for +reporting when a FlowFile is cloned, forked, or modified, and when +multiple FlowFiles are merged into a single FlowFile +as well as associating a FlowFile with some other identifier. However, +these functions are less critical to report, as +the framework is able to detect these things and emit appropriate +events on the Processor’s behalf. Yet, it is a best practice +for the Processor developer to emit these events, as it becomes +explicit in the code that these events are being emitted, and +the developer is able to provide additional details to the events, +such as the amount of time that the action took or +pertinent information about the action that was taken. If the +Processor emits an event, the framework will not emit a duplicate +event. Instead, it always assumes that the Processor developer knows +what is happening in the context of the Processor +better than the framework does. The framework may, however, emit a +different event. For example, if a Processor modifies both the +content of a FlowFile and its attributes and then emits only an +ATTRIBUTES_MODIFIED event, the framework will emit a CONTENT_MODIFIED +event. The framework will not emit an ATTRIBUTES_MODIFIED event if any +other event is emitted for that FlowFile (either by the +Processor or the framework). This is due to the fact that all +Provenance Events know about the attributes of the FlowFile before the +event occurred as well as those attributes that occurred as a result +of the processing of that FlowFile, and as a result the +ATTRIBUTES_MODIFIED is generally considered redundant and would result +in a rendering of the FlowFile lineage being very verbose. +It is, however, acceptable for a Processor to emit this event along +with others, if the event is considered pertinent from the +perspective of the Processor.</p> +</div> +</div> +</div> +</div> +<div class="sect1"> +<h2 id="documenting-a-component"><a class="anchor" href="developer-guide.html#documenting-a-component"></a>Documenting a Component</h2> +<div class="sectionbody"> +<div class="paragraph"> +<p>NiFi attempts to make the user experience as simple and convenient as +possible by providing significant amount of documentation +to the user from within the NiFi application itself via the User +Interface. In order for this to happen, of course, Processor +developers must provide that documentation to the framework. NiFi +exposes a few different mechanisms for supplying documentation to +the framework.</p> +</div> +<div class="sect2"> +<h3 id="documenting-properties"><a class="anchor" href="developer-guide.html#documenting-properties"></a>Documenting Properties</h3> +<div class="paragraph"> +<p>Individual properties can be documented by calling the <code>description</code> +method of a PropertyDescriptor’s builder as such:</p> +</div> +<div class="listingblock"> +<div class="content"> +<pre class="highlight"><code class="language-java" data-lang="java">public static final PropertyDescriptor MY_PROPERTY = new PropertyDescriptor.Builder() + .name("My Property") + .description("Description of the Property") + ... + .build();</code></pre> +</div> +</div> +<div class="paragraph"> +<p>If the property is to provide a set of allowable values, those values +are presented to the user in a drop-down field in the UI. +Each of those values can also be given a description:</p> +</div> +<div class="listingblock"> +<div class="content"> +<pre class="highlight"><code class="language-java" data-lang="java">public static final AllowableValue EXTENSIVE = new AllowableValue("Extensive", "Extensive", + "Everything will be logged - use with caution!"); +public static final AllowableValue VERBOSE = new AllowableValue("Verbose", "Verbose", + "Quite a bit of logging will occur"); +public static final AllowableValue REGULAR = new AllowableValue("Regular", "Regular", + "Typical logging will occur"); + +public static final PropertyDescriptor LOG_LEVEL = new PropertyDescriptor.Builder() + .name("Amount to Log") + .description("How much the Processor should log") + .allowableValues(REGULAR, VERBOSE, EXTENSIVE) + .defaultValue(REGULAR.getValue()) + ... + .build();</code></pre> +</div> +</div> +</div> +<div class="sect2"> +<h3 id="documenting-relationships"><a class="anchor" href="developer-guide.html#documenting-relationships"></a>Documenting Relationships</h3> +<div class="paragraph"> +<p>Processor Relationships are documented in much the same way that +properties are - by calling the <code>description</code> method of a +Relationship’s builder:</p> +</div> +<div class="listingblock"> +<div class="content"> +<pre class="highlight"><code class="language-java" data-lang="java">public static final Relationship MY_RELATIONSHIP = new Relationship.Builder() + .name("My Relationship") + .description("This relationship is used only if the Processor fails to process the data.") + .build();</code></pre> +</div> +</div> +</div> +<div class="sect2"> +<h3 id="documenting-capability-and-keywords"><a class="anchor" href="developer-guide.html#documenting-capability-and-keywords"></a>Documenting Capability and Keywords</h3> +<div class="paragraph"> +<p>The <code>org.apache.nifi.annotations.documentation</code> package provides Java +annotations that can be used to document components. The +CapabilityDescription +annotation can be added to a Processor, Reporting Task, or Controller +Service and is intended to provide a brief description of the +functionality +provided by the component. The Tags annotation has a <code>value</code> variable +that is defined to be an Array of Strings. As such, it is used +by providing multiple values as a comma-separated list of <code>String</code>s +with curly braces. These values are then incorporated into the UI by +allowing +users to filter the components based on a tag (i.e., a keyword). +Additionally, the UI provides a tag cloud that allows users to select +the tags that +they want to filter by. The tags that are largest in the cloud are +those tags that exist the most on the components in that instance of +NiFi. An +example of using these annotations is provided below:</p> +</div> +<div class="listingblock"> +<div class="content"> +<pre class="highlight"><code class="language-java" data-lang="java">@Tags({"example", "documentation", "developer guide", "processor", "tags"}) +@CapabilityDescription("Example Processor that provides no real functionality but is provided" + + " for an example in the Developer Guide") +public static final ExampleProcessor extends Processor { + ... +}</code></pre> +</div> +</div> +</div> +<div class="sect2"> +<h3 id="documenting-flowfile-attribute-interaction"><a class="anchor" href="developer-guide.html#documenting-flowfile-attribute-interaction"></a>Documenting FlowFile Attribute Interaction</h3> +<div class="paragraph"> +<p>Many times a processor will expect certain FlowFile attributes be set on in-bound FlowFiles in order +for the processor to function properly. In other cases a processor may update or +create FlowFile attributes on the out-bound FlowFile. Processor developers may document both of these +behaviors using the <code>ReadsAttribute</code> and <code>WritesAttribute</code> documentation annotations. These attributes are used to generate documentation +that gives users a better understanding of how a processor will interact with the flow.</p> +</div> +<div class="paragraph"> +<p>Note: Because Java 7 does not support +repeated annotations on a type, you may need to use <code>ReadsAttributes</code> and <code>WritesAttributes</code> to indicate +that a processor reads or writes multiple FlowFile attributes. This annotation can only be applied to Processors. An example is listed below:</p> +</div> +<div class="listingblock"> +<div class="content"> +<pre class="highlight"><code class="language-java" data-lang="java">@WritesAttributes({ @WritesAttribute(attribute = "invokehttp.status.code", description = "The status code that is returned"), + @WritesAttribute(attribute = "invokehttp.status.message", description = "The status message that is returned"), + @WritesAttribute(attribute = "invokehttp.response.body", description = "The response body"), + @WritesAttribute(attribute = "invokehttp.request.url", description = "The request URL"), + @WritesAttribute(attribute = "invokehttp.tx.id", description = "The transaction ID that is returned after reading the response"), + @WritesAttribute(attribute = "invokehttp.remote.dn", description = "The DN of the remote server") }) +public final class InvokeHTTP extends AbstractProcessor {</code></pre> +</div> +</div> +</div> +<div class="sect2"> +<h3 id="documenting-related-components"><a class="anchor" href="developer-guide.html#documenting-related-components"></a>Documenting Related Components</h3> +<div class="paragraph"> +<p>Often Processors and ControllerServices are related to one another. Sometimes its a put/get relation as in <code>PutFile</code> and <code>GetFile</code>. +Sometimes a Processor uses a ControllerService like <code>InvokeHTTP</code> and <code>StandardSSLContextService</code>. Sometimes one ControllerService uses another +like <code>DistributedMapCacheClientService</code> and <code>DistributedMapCacheServer</code>. Developers of these extension points may relate these +different components using the <code>SeeAlso</code> tag. This annotation links these components in the documentation. +<code>SeeAlso</code> can be applied to Processors, ControllerServices and ReportingTasks. An example of how to do this is listed below:</p> +</div> +<div class="listingblock"> +<div class="content"> +<pre class="highlight"><code class="language-java" data-lang="java">@SeeAlso(GetFile.class) +public class PutFile extends AbstractProcessor {</code></pre> +</div> +</div> +</div> +<div class="sect2"> +<h3 id="advanced-documentation"><a class="anchor" href="developer-guide.html#advanced-documentation"></a>Advanced Documentation</h3> +<div class="paragraph"> +<p>When the documentation methods above are not sufficient, NiFi provides +the ability to expose more advanced documentation to the user via the +"Usage" documentation. When a user right-clicks on a Processor, NiFi +provides a "Usage" menu item in the context menu. Additionally, the +UI exposes a "Help" link in the top-right corner, from which the same +Usage information can be found.</p> +</div> +<div class="paragraph"> +<p>The advanced documentation of a Processor is provided as an HTML file named <code>additionalDetails.html</code>. +This file should exist within a directory whose name is the +fully-qualified +name of the Processor, and this directory’s parent should be named +<code>docs</code> and exist in the root of the Processor’s jar. +This file will be linked from a generated HTML file that will contain +all the Capability, Keyword, PropertyDescription and Relationship information, +so it will not be necessary to duplicate that. This is a place +to provide a rich explanation of what this Processor is doing, what kind of +data it expects and produces, and what FlowFile attributes it expects and produces. +Because this documentation is in an HTML format, you may include images and tables +to best describe this component. The same methods can be used to provide advanced +documentation for Processors, ControllerServices and ReportingTasks.</p> +</div> +</div> +</div> +</div> +<div class="sect1"> +<h2 id="common-processor-patterns"><a class="anchor" href="developer-guide.html#common-processor-patterns"></a>Common Processor Patterns</h2> +<div class="sectionbody"> +<div class="paragraph"> +<p>While there are many different Processors available to NiFi users, the +vast majority of them fall into +one of several common design patterns. Below, we discuss these +patterns, when the patterns are appropriate, +reasons we follow these patterns, and things to watch out for when +applying such patterns. Note that the patterns +and recommendations discussed below are general guidelines and not +hardened rules.</p> +</div> +<div class="sect2"> +<h3 id="ingress"><a class="anchor" href="developer-guide.html#ingress"></a>Data Ingress</h3> +<div class="paragraph"> +<p>A Processor that ingests data into NiFi has a single Relationship +names <code>success</code>. This Processor generates +new FlowFiles via the ProcessSession <code>create</code> method and does not pull +FlowFiles from incoming Connections. +The Processor name starts with "Get" or "Listen," depending on whether +it polls an external source or exposes +some interface to which external sources can connect. The name ends +with the protocol used for communications. +Processors that follow this pattern include <code>GetFile</code>, <code>GetSFTP</code>, +<code>ListenHTTP</code>, and <code>GetHTTP</code>.</p> +</div> +<div class="paragraph"> +<p>This Processor may create or initialize a Connection Pool in a method +that uses the <code>@OnScheduled</code> annotation. +However, because communications problems may prevent connections from +being established or cause connections +to be terminated, connections themselves are not created at this +point. Rather, the connections are +created or leased from the pool in the <code>onTrigger</code> method.</p> +</div> +<div class="paragraph"> +<p>The <code>onTrigger</code> method of this Processor begins by leasing a +connection from the Connection Pool, if possible, +or otherwise creates a connection to the external service. When no +data is available from the +external source, the <code>yield</code> method of the ProcessContext is called by +the Processor and the method returns so +that this Processor avoids continually running and depleting resources +without benefit. Otherwise, this +Processor then creates a FlowFile via the ProcessSession’s <code>create</code> +method and assigns an appropriate +filename and path to the FlowFile (by adding the <code>filename</code> and <code>path</code> +attributes), as well as any other +attributes that may be appropriate. An OutputStream to the FlowFile’s content is +obtained via the ProcessSession’s <code>write</code> method, passing a new +OutputStreamCallback (which is usually +an anonymous inner class). From within this callback, the Processor is +able to write to the FlowFile and streams +the content from the external resource to the FlowFile’s OutputStream. +If the desire is to write the entire contents +of an InputStream to the FlowFile, the <code>importFrom</code> method of +ProcessSession may be more convenient to use than the +<code>write</code> method.</p> +</div> +<div class="paragraph"> +<p>When this Processor expects to receive many small files, it may be +advisable to create several FlowFiles from a +single session before committing the session. Typically, this allows +the Framework to treat the content of the +newly created FlowFiles much more efficiently.</p> +</div> +<div class="paragraph"> +<p>This Processor generates a Provenance event indicating that it has +received data and specifies from +where the data came. This Processor should log the creation of the +FlowFile so that the FlowFile’s +origin can be determined by analyzing logs, if necessary.</p> +</div> +<div class="paragraph"> +<p>This Processor acknowledges receipt of the data and/or removes the +data from the external source in order +to prevent receipt of duplicate files. <strong>This is done only after the +ProcessSession by which the FlowFile was +created has been committed!</strong> Failure to adhere to this principle may +result in data loss, as restarting NiFi +before the session has been committed will result in the temporary +file being deleted. Note, however, that it +is possible using this approach to receive duplicate data because the +application could be restarted after +committing the session and before acknowledging or removing the data +from the external source. In general, though, +potential data duplication is preferred over potential data loss. The +connection is finally returned or added to +the Connection Pool, depending on whether the connection was leased +from the Connection Pool to begin with or +was created in the <code>onTrigger</code> method.</p> +</div> +<div class="paragraph"> +<p>If there is a communications problem, the connection is typically +terminated and not returned (or added) to +the Connection Pool. Connections to remote systems are torn down and +the Connection Pool shutdown in a method +annotated with the <code>@OnStopped</code> annotation so that resources can be reclaimed.</p> +</div> +</div> +<div class="sect2"> +<h3 id="data-egress"><a class="anchor" href="developer-guide.html#data-egress"></a>Data Egress</h3> +<div class="paragraph"> +<p>A Processor that publishes data to an external source has two +Relationships: <code>success</code> and <code>failure</code>. The +Processor name starts with "Put" followed by the protocol that is used +for data transmission. Processors +that follow this pattern include <code>PutEmail</code>, <code>PutSFTP</code>, and +<code>PostHTTP</code> (note that the name does not +begin with "Put" because this would lead to confusion, since PUT and +POST have special meanings when dealing with +HTTP).</p> +</div> +<div class="paragraph"> +<p>This Processor may create or initialize a Connection Pool in a method +that uses the <code>@OnScheduled</code> annotation. +However, because communications problems may prevent connections from +being established or cause connections +to be terminated, connections themselves are not created at this +point. Rather, the connections are +created or leased from the pool in the <code>onTrigger</code> method.</p> +</div> +<div class="paragraph"> +<p>The <code>onTrigger</code> method first obtains a FlowFile from the +ProcessSession via the <code>get</code> method. If no FlowFile is +available, the method returns without obtaining a connection to the +remote resource.</p> +</div> +<div class="paragraph"> +<p>If at least one FlowFile is available, the Processor obtains a +connection from the Connection Pool, if possible, +or otherwise creates a new connection. If the Processor is neither +able to lease a connection from the Connection Pool +nor create a new connection, the FlowFile is routed to <code>failure</code>, the +event is logged, and the method returns.</p> +</div> +<div class="paragraph"> +<p>If a connection was obtained, the Processor obtains an InputStream to +the FlowFile’s content by invoking the +<code>read</code> method on the ProcessSession and passing an InputStreamCallback +(which is often an anonymous inner class) +and from within that callback transmits the contents of the FlowFile +to the destination. The event is logged +along with the amount of time taken to transfer the file and the data +rate at which the file was transferred. +A SEND event is reported to the ProvenanceReporter by obtaining the +reporter from the ProcessSession via the +<code>getProvenanceReporter</code> method and calling the <code>send</code> method on the +reporter. The connection is returned or added +to the Connection Pool, depending on whether the connection was leased +from the pool or newly created by the +<code>onTrigger</code> method.</p> +</div> +<div class="paragraph"> +<p>If there is a communications problem, the connection is typically +terminated and not returned (or added) to +the Connection Pool. If there is an issue sending the data to the +remote resource, the desired approach for handling the +error depends on a few considerations. If the issue is related to a +network condition, the FlowFile is generally +routed to <code>failure</code>. The FlowFile is not penalized because there is +not necessary a problem with the data. Unlike the +case of the <a href="developer-guide.html#ingress">Data Ingress</a> Processor, we typically do not call <code>yield</code> on +the ProcessContext. This is because in the case of +ingest, the FlowFile does not exist until the Processor is able to +perform its function. However, in the case of a Put Processor, +the DataFlow Manager may choose to route <code>failure</code> to a different +Processor. This can allow for a "backup" system to be +used in the case of problems with one system or can be used for load +distribution across many systems.</p> +</div> +<div class="paragraph"> +<p>If a problem occurs that is data-related, one of two approaches should +be taken. First, if the problem is likely to +sort itself out, the FlowFile is penalized and then routed to +<code>failure</code>. This is the case, for instance, with PutFTP, +when a FlowFile cannot be transferred because of a file naming +conflict. The presumption is that the file will eventually +be removed from the directory so that the new file can be transferred. +As a result, we penalize the FlowFile and route to +<code>failure</code> so that we can try again later. In the other case, if there +is an actual problem with the data (such as the data does +not conform to some required specification), a different approach may +be taken. In this case, it may be advantageous +to break apart the <code>failure</code> relationship into a <code>failure</code> and a +<code>communications failure</code> relationship. This allows the +DataFlow Manager to determine how to handle each of these cases +individually. It is important in these situations to document
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