1. 1. Scalability: Galera Cluster is a synchronous multi-master cluster that allows for active-active database setups, providing scalability by distributing load across multiple nodes. QuestDB, on the other hand, is a high-performance time-series database that focuses on query performance and scalability by utilizing optimized storage and indexing techniques specifically designed for time-series data.
2. 2. Replication: Galera Cluster uses synchronous replication, meaning that every write operation must be synchronously replicated to all nodes in the cluster before it is considered committed. In contrast, QuestDB utilizes asynchronous replication, where write operations are acknowledged once they are locally committed and then asynchronously replicated to other nodes.
3. 3. Data Model: Galera Cluster supports distributed cluster-wide transactions and provides full ACID compliance. It follows a standard relational data model with support for features like foreign keys, transactions, and complex joins. QuestDB, on the other hand, focuses specifically on time-series data and utilizes a high-performance column-oriented storage format optimized for efficient querying of time-series data.
4. 4. Query Performance: QuestDB is designed to offer exceptional query performance for time-series data, providing specialized functions and optimized storage formats like compressed columns. It also supports distributed query execution across multiple nodes. Galera Cluster focuses more on delivering synchronous replication and ensuring data integrity across the cluster, with query performance being a secondary consideration.
5. 5. Clustering Mechanism: Galera Cluster utilizes the Galera Replication Plugin, which is integrated directly into the MariaDB or MySQL servers. It operates on a multi-master model where each node is capable of accepting write operations. QuestDB, on the other hand, does not utilize Galera replication and instead relies on its own clustering mechanism for reading and writing data across multiple nodes.
6. 6. Community and Ecosystem: Galera Cluster benefits from being built on top of popular relational database systems like MariaDB and MySQL, which have large and active communities, extensive documentation, and support from various tools and frameworks. QuestDB is a newer and more focused database solution dedicated to time-series data, with a growing community and ecosystem that is specifically tailored towards time-series use cases.

In summary, Galera Cluster provides synchronous multi-master replication for distributed databases, while QuestDB focuses on high-performance time-series data storage and querying with its own clustering mechanism. Galera Cluster offers full ACID compliance and a standard relational data model, while QuestDB specializes in optimized storage and query performance for time-series data.