Sunday, February 23, 2020

Debatching(Splitting) XML Message in Logic Apps - ForEach and SplitOn


When working on an Enterprise integration Project, it is very likely to have a scenario where the message would be received in the form of Batches containing multiple messages within a single message. So if processing has to be done on individual messages, how it is done?

The design pattern used here is Splitter, where a Splitter/Debatcher splits the composite/Batched message into individual messages and further processes each message separately.

Splitter Pattern

Now depending on the message size, whether aggregation of the Processing results of each individual message is required, whether fast processing is to be done etc ,above pattern can be adjusted with little variation in approach.

Splitter Pattern with self processing
For example , say you want to stop processing of batch messages if any of message is faulty or you want to do some actions only after all messages are processed, then above approach can be used. Here single instance of process/workflow/splitter will split the batch message into individual messages and process them one by one. 

So it has control as a whole, as it has state of individual messages with it. Thus if any message processing fails you can stop further message processing and take corrective measures.

Above pattern in BizTalk it is done by using Orchestration - Debatching XML Batch Message in Orchestration

And in Logic app it is implemented by using For each control
Splitter Patern with multiple instance
But if that is not the case, i.e. if it is Fire and Forget scenario (no need to maintain state) then each splitted messages can be processed by a dedicated instance of process, it surely makes processing faster.

Above pattern in BizTalk it is implemented at Receive Port using Pipeline Debatching XML Batch Message at Receive Port

And in Logic app it is implemented by using SplitOn property on trigger.

In both the above approaches Xpath plays an important role, using which we tell which repeating node is to be looked for and then splitting/debatching is to be done.

XPath is a query language for selecting nodes from an XML document, basically it describes a way to locate and process items in XML documents by using an addressing syntax based on a path through the document's logical structure or hierarchy.

Note: In both the approaches message debatching happens, but in second approach the process/workflow also gets splitted (it scales out based on splitted messages by spinning new instances of itself). However in first approach single instance does all the work.

Steps to implement Splitter/Debatching Pattern in Logic App


Say we want to debatch/split PurchaseOrders message having multiple Purchase Order in it

xml Purchase Order

Splitting using For each control

Create Logic app Instance

Create DebatchXMLMessageLogic App using for each

Add http trigger, followed by ForEach and Response Action
foreach la

It is in For each control where we specify the xpath of the repeating node (which is to be splitted)
foreach with xpath

xpath(xml(triggerBody()),'//*[local-name()="PurchaseOrder" and namespace-uri()

Using xpath above - we are asking to traverse through the triggerBody and look out of PurchaseOrder node and if found add entry in  For each array.(All occurrence of PurchaseOrder nodes are added).

Now we add next steps within For each by using Add an action, here I have added Send an email step. So Send an email will be executed based on items in For each array, if there are 3 items then 3 times email will be sent out.

Read Error related to this -- The template language function 'xpath' expects its first parameter to be an XML object

Note : The default behavior of For each loop is to run in parallel and each iteration in it runs in different
thread, however it can be made to run in sequential form by setting its Degree of Parallelism to 1.

Another example using For each debatching -- Inserting Multiple Records In On Prem SQL Using Logic App

Splitting using SplitOn property on trigger

Create Logic app Instance

In Logic app Designer add HTTP trigger followed by send email action (just keeping it simple :) )

split on la

We need to set splitOn property on trigger, but no where it is visible -- Not able to understand why Microsoft has not provided.

No problem, we can still do it by going to Logic app code view, and under triggers, add splitOn property with value as xpath expression (same xpath is used as in For each above)

Spliton property in logic app code view

Save it, and now you should be able to see in Designer view as well in trigger Settings

split on property in trigger settings

Setting this property forces Logic App engine to create equivalent number of instances as that of number of repeating nodes, so if there are three repeating nodes in message posted to this logic app, then 3 different instance of this logic app will be instantiated.

Note: You can't use SplitOn with a synchronous response pattern , read Error related to this
-- The workflow with 'Response' action type should not have triggers with 'splitOn' property


Using Postman, sent a request having three Purchase node in it to both the logic app

And three emails were sent by each logic app, however in first logic app single instance
did it whereas in second logic app 3 separate instances did it. It can be checked in Logic
App run history

Logic App run history with Foreach control

for each logic app history

Logic App run history with SplitOn property on trigger

split on la history

    1 comment:

    1. I am getting error, triggerBody() method is returning null.


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