Deduplicate Stage


Deduplicate is an Akka Streams GraphStage to drop identical (consecutive or non-consecutive) elements in a stream.


Add the following dependency to your build.sbt or scala build file:

"org.squbs" %% "squbs-ext" % squbsVersion


The usage is very similar to standard Akka Stream stages:

val result = Source("a" :: "b" :: "b" :: "c" :: "a" :: "a" :: "a" :: "c" :: Nil).

// Output: ("a" :: "b" :: "c" :: Nil)

Deduplicate keeps a registry of already seen elements. To prevent the registry growing unboundedly, it allows to specify the number of duplicates for each message. Once duplicateCount is reached that element is removed from the registry. In the following example, duplicateCount is specified as 2, so, "a" will not be dropped when seen the third time:

val result = Source("a" :: "b" :: "b" :: "c" :: "a" :: "a" :: "a" :: "c" :: Nil).

// Output: ("a" :: "b" :: "c" :: "a" :: Nil)

Please note, duplicateCount prevents registry from ever growing when the number of duplicates are known. However, there is still the potential of memory leaks. For instance, if duplicateCount is set to 2, an element will be kept in the registry until the duplicate is seen; however, there might be scenarios where duplicate never shows up, e.g., a filter or drop is used. So, be aware of consequences of Duplicate in your use case.

You can also provide a different registry implementation to Deduplicate that cleans itself periodically. But, you should do this only if you are certain that a duplicate would not be seen after a given time frame; otherwise, the duplication logic might be corrupted.

Configuring registry key and registry implementation

Deduplicate uses the element itself as the key to the registry by default. However, it also accepts a function to map to a key from the element. For instance, if the elements in the stream are tuples of type (Int, String) and you would like to identify duplicates only based on the first field of the tuple, you can pass a function as follows:

val deduplicate = Deduplicate((element: (Int, String)) => element._1, 2)

Deduplicate also allows the registry to replaced with a different implementation of type java.util.Map[Key, MutableLong].

val deduplicate = Deduplicate(2, new util.TreeMap[String, MutableLong]())