The MergeContent Processor provides the ability to combine many FlowFiles into a single FlowFile. There are many reasons that a dataflow designer may want to do this. For example, it may be helpful to create batches of data before sending to a downstream system, because the downstream system is better optimized for large files than for many tiny files. NiFi itself can also benefit from this, as NiFi operates best on "micro-batches," where each FlowFile is several kilobytes to several megabytes in size.

The Processor creates several 'bins' to put the FlowFiles in. The maximum number of bins to use is set to 5 by default, but this can be changed by updating the value of the <Maximum number of Bins> property. The number of bins is bound in order to avoid running out of Java heap space. Note: while the contents of a FlowFile are stored in the Content Repository and not in the Java heap space, the Processor must hold the FlowFile objects themselves in memory. As a result, these FlowFiles with their attributes can potentially take up a great deal of heap space and cause OutOfMemoryError's to be thrown. In order to avoid this, if you expect to merge many small FlowFiles together, it is advisable to instead use a MergeContent that merges no more than say 1,000 FlowFiles into a bundle and then use a second MergeContent to merges these small bundles into larger bundles. For example, to merge 1,000,000 FlowFiles together, use MergeContent that uses a <Maximum Number of Entries> of 1,000 and route the "merged" Relationship to a second MergeContent that also sets the <Maximum Number of Entries> to 1,000. The second MergeContent will then merge 1,000 bundles of 1,000, which in effect produces bundles of 1,000,000.

How FlowFiles are Binned

How the Processor determines which bin to place a FlowFile in depends on a few different configuration options. Firstly, the Merge Strategy is considered. The Merge Strategy can be set to one of two options: "Bin Packing Algorithm," or "Defragment". When the goal is to simply combine smaller FlowFiles into one larger FlowFile, the Bin Packing Algorithm should be used. This algorithm picks a bin based on whether or not the FlowFile can fit in the bin according to its size and the <Maximum Bin Size> property and whether or not the FlowFile is 'like' the other FlowFiles in the bin. What it means for two FlowFiles to be 'like FlowFiles' is discussed at the end of this section.

The "Defragment" Merge Strategy can be used when FlowFiles need to be explicitly assigned to the same bin. For example, if data is split apart using the UnpackContent Processor, each unpacked FlowFile can be processed independently and later merged back together using this Processor with the Merge Strategy set to Defragment. In order for FlowFiles to be added to the same bin when using this configuration, the FlowFiles must have the same value for the "fragment.identifier" attribute. Each FlowFile with the same identifier must also have a unique value for the "fragment.index" attribute so that the FlowFiles can be ordered correctly. For a given "fragment.identifier", at least one FlowFile must have the "fragment.count" attribute (which indicates how many FlowFiles belong in the bin). Other FlowFiles with the same identifier must have the same value for the "fragment.count" attribute, or they can omit this attribute. NOTE: while there are valid use cases for breaking apart FlowFiles and later re-merging them, it is an anti-pattern to take a larger FlowFile, break it into a million tiny FlowFiles, and then re-merge them. Doing so can result in using huge amounts of Java heap and can result in Out Of Memory Errors. Additionally, it adds large amounts of load to the NiFi framework. This can result in increased CPU and disk utilization and often times can be an order of magnitude lower throughput and an order of magnitude higher latency. As an alternative, whenever possible, dataflows should be built to make use of Record-oriented processors, such as QueryRecord, PartitionRecord, UpdateRecord, LookupRecord, PublishKafkaRecord_2_6, etc.

In order to be added to the same bin, two FlowFiles must be 'like FlowFiles.' In order for two FlowFiles to be like FlowFiles, they must have the same schema, and if the <Correlation Attribute Name> property is set, they must have the same value for the specified attribute. For example, if the <Correlation Attribute Name> is set to "filename", then two FlowFiles must have the same value for the "filename" attribute in order to be binned together. If more than one attribute is needed in order to correlate two FlowFiles, it is recommended to use an UpdateAttribute processor before the MergeContent processor and combine the attributes. For example, if the goal is to bin together two FlowFiles only if they have the same value for the "abc" attribute and the "xyz" attribute, then we could accomplish this by using UpdateAttribute and adding a property with name "correlation.attribute" and a value of "abc=${abc},xyz=${xyz}" and then setting MergeContent's <Correlation Attribute Name> property to "correlation.attribute".

When a Bin is Merged

Above, we discussed how a bin is chosen for a given FlowFile. Once a bin has been created and FlowFiles added to it, we must have some way to determine when a bin is "full" so that we can bin those FlowFiles together into a "merged" FlowFile. There are a few criteria that are used to make a determination as to whether or not a bin should be merged.

If the <Merge Strategy> property is set to "Bin Packing Algorithm", then the following rules will be evaluated.

MergeContent exposes several different thresholds that can be used to create bins that are of an ideal size. For example, the user can specify the minimum number of FlowFiles that must be packaged together before merging will be performed. The minimum number of bytes can also be configured. Additionally, a maximum number of FlowFiles and bytes may be specified.

There are also two other conditions that will result in the contents of a Bin being merged together. The Max Bin Age property specifies the maximum amount of time that FlowFiles can be binned together before the bin is merged. This property should almost always be set, as it provides a means to set a timeout on a bin, so that even if data stops flowing to the Processor for a while (due to a problem with an upstream system, a source processor being stopped, etc.) the FlowFiles won't remain stuck in the MergeContent processor indefinitely. Additionally, the processor exposes a property for the maximum number of Bins that should be used. For some use cases, this won't matter much. However, if the Correlation Attribute property is set, this can be important. When an incoming FlowFile is to be placed in a Bin, the processor must find an appropriate Bin to place the FlowFile into, or else create a new one. If a Bin must be created, and the number of Bins that exist is greater than or equal to the value of the <Maximum Number of Bins> property, then the oldest Bin will be merged together to make room for the new one.

If the <Merge Strategy> property is set to "Defragment", then a bin is full only when the number of FlowFiles in the bin is equal to the number specified by the "fragment.count" attribute of one of the FlowFiles in the bin. All FlowFiles that have this attribute must have the same value for this attribute, or else they will be routed to the "failure" relationship. It is not necessary that all FlowFiles have this value, but at least one FlowFile in the bin must have this value or the bin will never be complete. If all of the necessary FlowFiles are not binned together by the point at which the bin times amount (as specified by the <Max Bin Age> property), then the FlowFiles will all be routed to the 'failure' relationship instead of being merged together.

Whenever the contents of a Bin are merged, an attribute with the name "merge.reason" will be added to the merged FlowFile. The below table provides a listing of all possible values for this attribute with an explanation of each.

Attribute Value Explanation
MAX_BYTES_THRESHOLD_REACHED The bin has reached the maximum number of bytes, as configured by the <Max Group Size> property. When this threshold is reached, the contents of the Bin will be merged together, even if the Minimum Number of Entries has not yet been reached.
MAX_ENTRIES_THRESHOLD_REACHED The bin has reached the maximum number of FlowFiles, as configured by the <Maximum Number of Entries> property. When this threshold is reached, the contents of the Bin will be merged together, even if the minimum number of bytes (Min Group Size) has not yet been reached.
MIN_THRESHOLDS_REACHED The bin has reached both the minimum number of bytes, as configured by the <Min Group Size> property, AND the minimum number of FlowFiles, as configured by the <Minimum Number of Entries> property. The bin has not reached the maximum number of bytes (Max Group Size) OR the maximum number of FlowFiles (Maximum Number of Entries).
TIMEOUT The Bin has reached the maximum age, as configured by the <Max Bin Age> property. If this threshold is reached, the contents of the Bin will be merged together, even if the Bin has not yet reached either of the minimum thresholds. Note that the age here is determined by when the Bin was created, NOT the age of the FlowFiles that reside within those Bins. As a result, if the Processor is stopped until it has 1 million FlowFiles queued, each one being 10 days old, but the Max Bin Age is set to "1 day," the Max Bin Age will not be met for at least one full day, even though the FlowFiles themselves are much older than this threshold. If the Processor is stopped and restarted, all Bins are destroyed and recreated, so the timer is reset.
BIN_MANAGER_FULL If an incoming FlowFile does not fit into any of the existing Bins (either due to the Maximum thresholds set, or due to the Correlation Attribute being used, etc.), then a new Bin must be created for the incoming FlowFiles. If the number of active Bins is already equal to the <Maximum number of Bins> property, the oldest Bin will be merged in order to make room for the new Bin. In that case, the Bin Manager is said to be full, and this value will be used.

Note that the attribute value is minimally named, while the textual description is far more verbose. This is done for a few reasons. Firstly, storing a large value for the attribute can be more costly, utilizing more heap space and requiring more resources to process. Secondly, it's more succinct, which makes it easier to talk about. Most importantly, though, it means that a processor such as RouteOnAttribute can be used, if necessary, to route based on the value of the attribute. In this way, the explanation can be further expanded or updated, without changing the value of the attribute and without disturbing existing flows.