Idempotent filters in Mule ensure only unique messages are processed by checking message IDs. They can use different object stores like in-memory, file-based, or managed stores to check for duplicate message IDs. This document demonstrates configuring idempotent filters with simple-text-file-store, in-memory-store, and managed-store object stores. The examples show how idempotent filters can prevent duplicate messages from being processed in a Mule flow.

1. By Anirban Sen Chowdhary

2. Idempotent Filter is an important message filter in Mule platform which ensures
that only unique messages are received by a service by checking the unique ID of
the incoming message.
We use Idempotent Filter to avoid any duplicate message in our Mule flow

3. Idempotent Filter can be configured various ways. We can use Object store with
Idempotent Filters.
Mule provides two types of object stores:-
• In-memory store – Prior to Mule 3.5.0, in-memory store was the default. As of
Mule 3.5.0, persistent store is the default.
• Persistent store – Mule persists data when an object store is explicitly configured
to be persistent. Mule creates a default persistent store in the file system.
Here we will see various configuration with Idempotent Filter

4. Idempotent Filter with simple-text-file-store :-
We can configure it following way :-
<idempotent-message-filter idExpression="#[message:payload]" doc:name="Idempotent
Message" throwOnUnaccepted="true" onUnaccepted="ValidationFailFlow">
<simple-text-file-store directory="./idempotent" />
</idempotent-message-filter>
Here you can see that we have configured our Idempotent Filter with simple-text-
file-store

5. You will see that we will run our Mule application it will generate text file in our
Mule application where it will store all the message with ID to avoid the duplicate :-

6. Idempotent Filter with in-memory-store :-
We can configure it following way :-
<idempotent-message-filter idExpression="#[message:payload]" doc:name="Idempotent Message"
throwOnUnaccepted="true" onUnaccepted="ValidationFailFlow">
<in-memory-store name="myInMemoryObjectStore" entryTTL="120"
expirationInterval="3600" maxEntries="60000" />
</idempotent-message-filter>
Here you can see that we have configured our Idempotent Filter with in-memory-
store

7. Idempotent Filter with managed-store:-
We can configure it following way :-
<idempotent-message-filter idExpression="#[message:payload]" doc:name="Idempotent Message"
throwOnUnaccepted="true" onUnaccepted="ValidationFailFlow">
<managed-store storeName="myNonPersistentManagedObjectStore" maxEntries="-1"
entryTTL="20000" expirationInterval="5000"/>
</idempotent-message-filter>
Here you can see that we have configured our Idempotent Filter with managed-store

8. Now, we can configure with any of the options shown here, we will get following Mule
flow :-

9. Our Mule Config will be :-

10. So, we will test our application by hitting the following url
http://localhost:8081/path/test for the first time we will get :-
For the first time we will see browser display “passed” as the
message payload is new and unique (path/test is the message
payload here)

11. And then if we hit the same url http://localhost:8081/path/test again, will get the
following :-
For the second time we will see browser display “EXCEPTION” as
the message payload is now same and duplicate (path/test is the
message payload here)

12. Hope I am able to make it clear on how to implement Idempotent Filter with different
ways and you can now try by your own and experiment with this powerful filter of
Mule.
Hope you enjoyed this simpler version