Getting Started with Apache NiFi

For Windows users, navigate to the folder where NiFi was installed. Within this folder is a subfolder named bin. Navigate to this subfolder and double-click the run-nifi.bat file.

This will launch NiFi and leave it running in the foreground. To shut down NiFi, select the window that was launched and hold the Ctrl key while pressing C.

For Linux and OS X users, use a Terminal window to navigate to the directory where NiFi was installed. To run NiFi in the foreground, run bin/nifi.sh run. This will leave the application running until the user presses Ctrl-C. At that time, it will initiate shutdown of the application.

To run NiFi in the background, instead run bin/nifi.sh start. This will initiate the application to begin running. To check the status and see if NiFi is currently running, execute the command bin/nifi.sh status. NiFi can be shutdown by executing the command bin/nifi.sh stop.

Currently, installing NiFi as a service is supported only for Linux and Mac OS X users. To install the application as a service, navigate to the installation directory in a Terminal window and execute the command bin/nifi.sh install to install the service with the default name nifi. To specify a custom name for the service, execute the command with an optional second argument that is the name of the service. For example, to install NiFi as a service with the name dataflow, use the command bin/nifi.sh install dataflow.

Once installed, the service can be started and stopped using the appropriate commands, such as sudo service nifi start and sudo service nifi stop. Additionally, the running status can be checked via sudo service nifi status.

We can now begin creating our dataflow by adding a Processor to our canvas. To do this, drag the Processor icon () from the top-left of the screen into the middle of the canvas (the graph paper-like background) and drop it there. This will give us a dialog that allows us to choose which Processor we want to add:

We have quite a few options to choose from. For the sake of becoming oriented with the system, let’s say that we just want to bring in files from our local disk into NiFi. When a developer creates a Processor, the developer can assign "tags" to that Processor. These can be thought of as keywords. We can filter by these tags or by Processor name by typing into the Filter box in the top-right corner of the dialog. Type in the keywords that you would think of when wanting to ingest files from a local disk. Typing in keyword "file", for instance, will provide us a few different Processors that deal with files. Filtering by the term "local" will narrow down the list pretty quickly, as well. If we select a Processor from the list, we will see a brief description of the Processor near the bottom of the dialog. This should tell us exactly what the Processor does. The description of the GetFile Processor tells us that it pulls data from our local disk into NiFi and then removes the local file. We can then double-click the Processor type or select it and choose the Add button. The Processor will be added to the canvas in the location that it was dropped.

Now that we have added the GetFile Processor, we can configure it by right-clicking on the Processor and choosing the Configure menu item. The provided dialog allows us to configure many different options that can be read about in the User Guide, but for the sake of this guide, we will focus on the Properties tab. Once the Properties tab has been selected, we are given a list of several different properties that we can configure for the Processor. The properties that are available depend on the type of Processor and are generally different for each type. Properties that are in bold are required properties. The Processor cannot be started until all required properties have been configured. The most important property to configure for GetFile is the directory from which to pick up files. If we set the directory name to ./data-in, this will cause the Processor to start picking up any data in the data-in subdirectory of the NiFi Home directory. We can choose to configure several different Properties for this Processor. If unsure what a particular Property does, we can hover over the Help icon ( ) next to the Property Name with the mouse in order to read a description of the property. Additionally, the tooltip that is displayed when hovering over the Help icon will provide the default value for that property, if one exists, information about whether or not the property supports the Expression Language (see the Expression Language / Using Attributes in Property Values section below), and previously configured values for that property.

In order for this property to be valid, create a directory named data-in in the NiFi home directory and then click the Ok button to close the dialog.

Each Processor has a set of defined "Relationships" that it is able to send data to. When a Processor finishes handling a FlowFile, it transfers it to one of these Relationships. This allows a user to configure how to handle FlowFiles based on the result of Processing. For example, many Processors define two Relationships: success and failure. Users are then able to configure data to be routed through the flow one way if the Processor is able to successfully process the data and route the data through the flow in a completely different manner if the Processor cannot process the data for some reason. Or, depending on the use case, it may simply route both relationships to the same route through the flow.

Now that we have added and configured our GetFile processor and applied the configuration, we can see in the top-left corner of the Processor an Alert icon ( ) signaling that the Processor is not in a valid state. Hovering over this icon, we can see that the success relationship has not been defined. This simply means that we have not told NiFi what to do with the data that the Processor transfers to the success Relationship.

In order to address this, let’s add another Processor that we can connect the GetFile Processor to, by following the same steps above. This time, however, we will simply log the attributes that exist for the FlowFile. To do this, we will add a LogAttributes Processor.

We can now send the output of the GetFile Processor to the LogAttribute Processor. Hover over the GetFile Processor with the mouse and a Connection Icon ( ) will appear over the middle of the Processor. We can drag this icon from the GetFile Processor to the LogAttribute Processor. This gives us a dialog to choose which Relationships we want to include for this connection. Because GetFile has only a single Relationship, success, it is automatically selected for us.

Clicking on the Settings tab provides a handful of options for configuring how this Connection should behave:

We can give the Connection a name, if we like. Otherwise, the Connection name will be based on the selected Relationships. We can also set an expiration for the data. By default, it is set to "0 sec" which indicates that the data should not expire. However, we can change the value so that when data in this Connection reaches a certain age, it will automatically be deleted (and a corresponding EXPIRE Provenance event will be created).

The backpressure thresholds allow us to specify how full the queue is allowed to become before the source Processor is no longer scheduled to run. This allows us to handle cases where one Processor is capable of producing data faster than the next Processor is capable of consuming that data. If the backpressure is configured for each Connection along the way, the Processor that is bringing data into the system will eventually experience the backpressure and stop bringing in new data so that our system has the ability to recover.

Finally, we have the Prioritizers on the right-hand side. This allows us to control how the data in this queue is ordered. We can drag Prioritizers from the "Available prioritizers" list to the "Selected prioritizers" list in order to activate the prioritizer. If multiple prioritizers are activated, they will be evaluated such that the Prioritizer listed first will be evaluated first and if two FlowFiles are determined to be equal according to that Prioritizer, the second Prioritizer will be used.

For the sake of this discussion, we can simply click Add to add the Connection to our graph. We should now see that the Alert icon has changed to a Stopped icon ( ). The LogAttribute Processor, however, is now invalid because its success Relationship has not been connected to anything. Let’s address this by signaling that data that is routed to success by LogAttribute should be "Auto Terminated," meaning that NiFi should consider the FlowFile’s processing complete and "drop" the data. To do this, we configure the LogAttribute Processor. On the Settings tab, in the right-hand side we can check the box next to the success Relationship to Auto Terminate the data. Clicking OK will close the dialog and show that both Processors are now stopped.

At this point, we have two Processors on our graph, but nothing is happening. In order to start the Processors, we can click on each one individually and then right-click and choose the Start menu item. Alternatively, we can select the first Processor, and then hold the Shift key while selecting the other Processor in order to select both. Then, we can right-click and choose the Start menu item. As an alternative to using the context menu, we can select the Processors and then click the Start icon in the Operate palette.

Once started, the icon in the top-left corner of the Processors will change from a stopped icon to a running icon. We can then stop the Processors by using the Stop icon in the Operate palette or the Stop menu item.

Once a Processor has started, we are not able to configure it anymore. Instead, when we right-click on the Processor, we are given the option to view its current configuration. In order to configure a Processor, we must first stop the Processor and wait for any tasks that may be executing to finish. The number of tasks currently executing is shown near the top-right corner of the Processor, but nothing is shown there if there are currently no tasks.

With each Processor having the ability to expose multiple different Properties and Relationships, it can be challenging to remember how all of the different pieces work for each Processor. To address this, you are able to right-click on a Processor and choose the Usage menu item. This will provide you with the Processor’s usage information, such as a description of the Processor, the different Relationships that are available, when the different Relationships are used, Properties that are exposed by the Processor and their documentation, as well as which FlowFile Attributes (if any) are expected on incoming FlowFiles and which Attributes (if any) are added to outgoing FlowFiles.

The toolbar that provides users the ability to drag and drop Processors onto the graph includes several other components that can be used to build a dataflow. These components include Input and Output Ports, Funnels, Process Groups, and Remote Process Groups. Due to the intended scope of this document, we will not discuss these elements here, but information is readily available in the Building a Dataflow section of the User Guide.

Each FlowFile has a minimum set of Attributes:

Note that the uuid, entryDate, lineageStartDate, and fileSize attributes are system-generated and cannot be changed.

NiFi provides several different Processors out of the box for extracting Attributes from FlowFiles. A list of commonly used Processors for this purpose can be found above in the Attribute Extraction section. This is a very common use case for building custom Processors, as well. Many Processors are written to understand a specific data format and extract pertinent information from a FlowFile’s content, creating Attributes to hold that information, so that decisions can then be made about how to route or process the data.

In addition to having Processors that are able to extract particular pieces of information from FlowFile content into Attributes, it is also common for users to want to add their own user-defined Attributes to each FlowFile at a particular place in the flow. The UpdateAttribute Processor is designed specifically for this purpose. Users are able to add a new property to the Processor in the Configure dialog by clicking the "+" button in the top-right corner of the Properties tab. The user is then prompted to enter the name of the property and then a value. For each FlowFile that is processed by this UpdateAttribute Processor, an Attribute will be added for each user-defined property. The name of the Attribute will be the same as the name of the property that was added. The value of the Attribute will be the same as the value of the property.

The value of the property may contain the Expression Language, as well. This allows Attributes to be modified or added based on other Attributes. For example, if we want to prepend the hostname that is processing a file as well as the date to a filename, we could do this by adding a property with the name filename and the value ${hostname()}-${now():format('yyyy-dd-MM')}-${filename}. While this may seem confusing at first, the section below on Expression Language / Using Attributes in Property Values will help to clear up what is going on here.

In addition to always adding a defined set of Attributes, the UpdateAttribute Processor has an Advanced UI that allows the user to configure a set of rules for which Attributes should be added when. To access this capability, in the Configure dialog’s Properties tab, click the Advanced button at the bottom of the dialog. This will provide a UI that is tailored specifically to this Processor, rather than the simple Properties table that is provided for all Processors. Within this UI, the user is able to configure a rules engine, essentially, specifying rules that must match in order to have the configured Attributes added to the FlowFile.

One of the most powerful features of NiFi is the ability to route FlowFiles based on their Attributes. The primary mechanism for doing this is the RouteOnAttribute Processor. This Processor, like UpdateAttribute, is configured by adding user-defined properties. Any number of properties can be added by clicking the "+" button in the top-right corner of the Properties tab in the Processor’s Configure dialog.

Each FlowFile’s Attributes will be compared against the configured properties to determine whether or not the FlowFile meets the specified criteria. The value of each property is expected to be an Expression Language expression and return a boolean value. For more on the Expression Language, see the Expression Language / Using Attributes in Property Values section below.

After evaluating the Expression Language expressions provided against the FlowFile’s Attributes, the Processor determines how to route the FlowFile based on the Routing Strategy selected. The most common strategy is the "Route to Property name" strategy. With this strategy selected, the Processor will expose a Relationship for each property configured. If the FlowFile’s Attributes satisfy the given expression, a copy of the FlowFile will be routed to the corresponding Relationship. For example, if we had a new property with the name "begins-with-r" and the value "${filename:startsWith(\'r')}" then any FlowFile whose filename starts with the letter 'r' will be routed to that Relationship. All other FlowFiles will be routed to 'unmatched'.

As we extract Attributes from FlowFiles' contents and add user-defined Attributes, they don’t do us much good as an operator unless we have some mechanism by which we can use them. The NiFi Expression Language allows us to access and manipulate FlowFile Attribute values as we configure our flows. Not all Processor properties allow the Expression Language to be used, but many do. In order to determine whether or not a property supports the Expression Language, a user can hover over the Help icon ( ) in the Properties tab of the Processor Configure dialog. This will provide a tooltip that shows a description of the property, the default value, if any, and whether or not the property supports the Expression Language.

For properties that do support the Expression Language, it is used by adding an expression within the opening ${ tag and the closing } tag. An expression can be as simple as an attribute name. For example, to reference the uuid Attribute, we can simply use the value ${uuid}. If the Attribute name begins with any character other than a letter, or if it contains a character other than a number, a letter, a period (.), or an underscore (_), the Attribute name will need to be quoted. For example, ${My Attribute Name} will be invalid, but ${'My Attribute Name'} will refer to the Attribute My Attribute Name.

In addition to referencing Attribute values, we can perform a number of functions and comparisons on those Attributes. For example, if we want to check if the filename attribute contains the letter 'r' without paying attention to case (upper case or lower case), we can do this by using the expression ${filename:toLower():contains('r')}. Note here that the functions are separated by colons. We can chain together any number of functions to build up more complex expressions. It is also important to understand here that even though we are calling filename:toLower(), this does not alter the value of the filename Attribute in anyway but rather just gives us a new value to work with.

We can also embed one expression within another. For example, if we wanted to compare the value of the attr1 Attribute to the value of the attr2 Attribute, we can do this with the following expression: ${attr1:equals( ${attr2} )}.

The Expression Language contains many different functions that can be used in order to perform the tasks needed for routing and manipulating Attributes. Functions exist for parsing and manipulating strings, comparing string and numeric values, manipulating and replacing values, and comparing values. A full explanation of the different functions available is out of the scope of this document, but the Expression Language Guide provides far greater detail for each of the functions.

In addition, this Expression Language guide is built in to the application so that users are able to easily see which functions are available and see their documentation while typing. When setting the value of a property that supports the Expression Language, if the cursor is within the Expression Language start and end tags, pressing Ctrl + Space on the keyword will provide a pop-up of all of the available functions and will provide auto-complete functionality. Clicking on or using the keyboard to navigate to one of the functions listed in the pop-up will cause a tooltip to show, which explains what the function does, the arguments that it expects, and the return type of the function.

Near the top of the NiFi screen under the Component toolbar is a bar that is referred to as the Status Bar. It contains a few important statistics about the current health of NiFi. The number of Active Threads can indicate how hard NiFi is currently working, and the Queued stat indicates how many FlowFiles are currently queued across the entire flow, as well as the total size of those FlowFiles.

If the NiFi instance is in a cluster, we will also see an indicator here telling us how many nodes are in the cluster and how many are currently connected. In this case, the number of active threads and the queue size are indicative of all the sum of all nodes that are currently connected.

Each Processor, Process Group, and Remote Process Group on the canvas provides several statistics about how much data has been processed by the component. These statistics provide information about how much data has been processed in the past five minutes. This is a rolling window and allows us to see things like the number of FlowFiles that have been consumed by a Processor, as well as the number of FlowFiles that have been emitted by the Processor.

The connections between Processors also expose the number of items that are currently queued.

It may also be valuable to see historical values for these metrics and, if clustered, how the different nodes compare to one another. In order to see this information, we can right-click on a component and choose the Stats menu item. This will show us a graph that spans the time since NiFi was started, or up to 24 hours, whichever is less. The amount of time that is shown here can be extended or reduced by changing the configuration in the properties file.

In the top-right corner of this dialog is a drop-down that allows the user to select which metric they are viewing. The graph on the bottom allows the user to select a smaller portion of the graph to zoom in.

In addition to the statistics provided by each component, a user will want to know if any problems occur. While we could monitor the logs for anything interesting, it is much more convenient to have notifications pop up on the screen. If a Processor logs anything as a WARNING or ERROR, we will see a "Bulletin Indicator" show up in the top-right-hand corner of the Processor. This indicator looks like a sticky note and will be shown for five minutes after the event occurs. Hovering over the bulletin provides information about what happened so that the user does not have to sift through log messages to find it. If in a cluster, the bulletin will also indicate which node in the cluster emitted the bulletin. We can also change the log level at which bulletins will occur in the Settings tab of the Configure dialog for a Processor.

If the framework emits a bulletin, we will also see a bulletin indicator highlighted at the top-right of the screen. In the Global Menu is a Bulletin Board option. Clicking this option will take us to the bulletin board where we can see all bulletins that occur across the NiFi instance and can filter based on the component, the message, etc.

Once we have performed our search, our table will be populated only with the events that match the search criteria. From here, we can choose the Info icon ( ) on the left-hand side of the table to view the details of that event:

From here, we can see exactly when that event occurred, which FlowFile the event affected, which component (Processor, etc.) performed the event, how long the event took, and the overall time that the data had been in NiFi when the event occurred (total latency).

The next tab provides a listing of all Attributes that existed on the FlowFile at the time that the event occurred:

From here, we can see all the Attributes that existed on the FlowFile when the event occurred, as well as the previous values for those Attributes. This allows us to know which Attributes changed as a result of this event and how they changed. Additionally, in the right-hand corner is a checkbox that allows the user to see only those Attributes that changed. This may not be particularly useful if the FlowFile has only a handful of Attributes, but can be very helpful when a FlowFile has hundreds of Attributes.

This is very important because it allows the user to understand the exact context in which the FlowFile was processed. It is helpful to understand 'why' the FlowFile was processed the way that it was, especially when the Processor was configured using the Expression Language.

Finally, we have the Content tab:

This tab provides us information about where in the Content Repository the FlowFile’s content was stored. If the event modified the content of the FlowFile, we will see the 'before' (input) and 'after' (output) content claims. We are then given the option to Download the content or to View the content within NiFi itself, if the data format is one that NiFi understands how to render.

Additionally, in the Replay section of the tab, there is a 'Replay' button that allows the user to re-insert the FlowFile into the flow and re-process it from exactly the point at which the event happened. This provides a very powerful mechanism, as we are able to modify our flow in real time, re-process a FlowFile, and then view the results. If they are not as expected, we can modify the flow again, and re-process the FlowFile again. We are able to perform this iterative development of the flow until it is processing the data exactly as intended.

In addition to viewing the details of a Provenance event, we can also view the lineage of the FlowFile involved by clicking on the Lineage Icon ( ) from the table view.

This provides us with a graphical representation of exactly what happened to that piece of data as it traversed the system:

From here, we can right-click on any of the events represented and click the View Details menu item to see the Event Details. This graphical representation shows us exactly which events occurred to the data. There are a few "special" event types to be aware of. If we see a JOIN, FORK, or CLONE event, we can right-click and choose to Find Parents or Expand. This allows us to see the lineage of parent FlowFiles and children FlowFiles that were created as well.

The slider in the bottom-left corner allows us to see the time at which these events occurred. By sliding it left and right, we can see which events introduced latency into the system so that we have a very good understanding of where in our system we may need to provide more resources, such as the number of Concurrent Tasks for a Processor. Or it may reveal, for example, that most of the latency was introduced by a JOIN event, in which we were waiting for more FlowFiles to join together. In either case, the ability to easily see where this is occurring is a very powerful feature that will help users to understand how the enterprise is operating.