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Cover image for ClickHouse ReplacingMergeTree Engine: The Airport & Flights Metaphor
Santhosh Balasa
Santhosh Balasa

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ClickHouse ReplacingMergeTree Engine: The Airport & Flights Metaphor


To comprehend the inner workings of ClickHouse’s ReplacingMergeTree engine, let’s embark on a journey through an airport’s operations. This metaphorical exploration will elucidate the similarities between the dynamic environment of an airport and ClickHouse’s functionalities.

  1. Passengers & Boarding Passes = Data Entries

    • Each data entry in ClickHouse resembles a passenger holding a boarding pass.
    • The boarding pass carries essential details, notably a flight number and version.

  2. The Lounge = Temporary Data Storage

    • Upon their arrival, passengers settle in the airport lounge, mirroring the initial storage of data in ClickHouse.
    • This space hosts first-time travelers and those who have exchanged their initial passes for updated versions.

  3. The Role of the Gate Agent = ReplacingMergeTree Engine

    • The gate agent, tasked with verifying boarding passes, ensures passengers with the most recent version for a specific flight board the plane.
    • This gate agent’s role mirrors how ClickHouse’s ReplacingMergeTree engine functions — ensuring only the latest version of data is stored, while older versions are seamlessly replaced.

  4. Flight Seating & Merging

    • The airplane features various sections. Initially, as passengers board, they might spread across several areas.
    • Periodically, the cabin crew reseats passengers to ensure optimal seat utilization, representing the merging aspect of the ReplacingMergeTree engine, where smaller data parts are integrated into larger, more organized segments.

  5. In-flight = Main Data Storage

    • Once aboard, passengers represent the consolidated and up-to-date data.
    • The meticulous efforts of the gate agent and cabin crew guarantee an organized and efficient flight experience, much like how data is managed in ClickHouse.

  6. Departure & Lounge Management = Efficient Storage Management

    • After each boarding call, passengers with outdated boarding passes (older versions of data) are asked to exit the lounge to keep it spacious and organized.
    • This process ensures that the lounge remains comfortable and efficient for incoming passengers, just as ClickHouse prioritizes the latest versions of data and efficiently manages storage space.


Navigating through an airport’s bustling operations offers insightful parallels to understanding ClickHouse’s ReplacingMergeTree engine.

The careful management of passengers — from boarding to seating — serves as a fitting metaphor for how ClickHouse seamlessly manages, replaces, and merges data for optimal performance.

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