When Kappa first appeared as an architecture style (introduced by Jay Kreps) I was really fond of this new approach. I carried out several projects that went with Kafka as the main “thing” and not having the trade-offs as Lambda. But the more complex projects got, the more I figured out that it isn’t the answer to everything and that we ended up with Lambda again … somehow.

First of all, what is the benefit of Kappa and the trade-off with Lambda? It all started with Jay Kreps in his blog post when he questioned the Lambda Architecture. Basically, with different layers in the Lambda Architecture (Speed Layer, Batch Layer and Presentation Layer) you need to use different tools and programming languages. This leads to code complexity and the risk that you end up having inconsistent versions of your processing capabilities. A change to the logic on the one layer requires changes on the other layer as well. Complexity is basically one thing we want to remove from our architecture at all times, so we should also do it with Data Processing.

The Kappa Architecture came with the promise to put everything into one system: Apache Kafka. The speed that data can be processed with it is tremendous and also the simplicity is great. You only need to change code once and not twice or three times as compared to Lambda. This leads to cheaper labour costs as well, as less people are necessary to maintain and produce code. Also, all our data is available at our fingertips, without major delays as with batch processing. This brings great benefits to business units as they don’t need to wait forever for processing.

However, my initial statement was about something else – that I mistrust Kappa Architecture. I implemented this architecture style at several IoT projects, where we had to deal with sensor data. There was no question if Kappa is the right thing – as we were in a rather isolated Use-Case. But as soon as you have to look at a Big Data architecture for a large enterprise (and not only into isolated use-cases) you end up with one major issue around Kappa: Cost.

In use-cases where data don’t need to be available within minutes, Kappa seems to be an overkill. Especially in the cloud, Lambda brings major cost benefits with Object Storages in combination with automated processing capabilities such as Azure Databricks. In enterprise environments, cost does matter and an architecture should also be cost efficient. This also holds true when it comes to the half-live of data which I was recently writing about. Basically, data that looses its value fast should be stored on cheap storage systems at the very beginning already.

An easy way to compare Kappa to Lambda is the comparison per Terabyte stored or processed. Basically, we will use a scenario to store 32 TB. With a Kappa Architecture running 24/7, this would mean that we have an estimated 16.000$ per month to spend (no discounts, no reserved instances – pay as you go pricing; E64 CPUs with 64 cores per node, 432 GB Ram and E80 SSDs attached with 32TB per disk). If we would use Lambda and only process once per day, this would mean that we need 32TB on a Blob Store – that costs 680$ per month. Now we would take the cluster above for processing with Spark and use it 1 hour per day: 544$. Summing up, this would equal to 1.224$ per month – a cost ratio of 1:13.

However, this is a very easy calculation and it can still be optimised on both sides. In the broader enterprise context, Kappa is only a specialisation of Lambda but won’t exist all alone at all time. Kappa vs. Lambda can only be selected by the use-case, and this is what I recommend you to do.

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