I see that Appian uses theses two technologies, but I can't find in the documentation the use of Zookeeper and Kafka.
What are they used for ?
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kafka: A distributed messaging and persistence layer, used as transaction log for the Appian Engines.
The Appian engines synchronously store write transactions in Kafka as they are applied. For troubleshooting purposes it is sometimes useful to be able to look at the contents of the transaction log for an engine to see how many transactions were applied in a given time period, what types of transactions are flowing through the system, or what parameters were used in a particularly slow transaction. Zookeeper: A distributed consensus framework, used to coordinate leadership decisions for the Appian Engines