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Product: Hibernate Shards
If you want to adopt a shard architecture, but don't want to start from scratch, you may want to consider Hibernate's sharding system.
Hibernate Shards is a framework that is designed to encapsulate and minimize this complexity by adding support for horizontal partitioning to Hibernate Core.
Hibernate Shards key features:
* Standard Hibernate programming model - Hibernate Shards allows you to continue using the Hibernate APIs you know and love: SessionFactory, Session, Criteria, Query. If you already know how to use Hibernate, you already know how to use Hibernate Shards.
* Flexible sharding strategies - Distribute data across your shards any way you want. Use one of the default strategies we provide or plug in your own application-specific logic.
* Support for virtual shards - Think your sharding strategy is never going to change? Think again. Adding new shards and redistributing your data is one of the toughest operational challenges you will face once you've deployed your shard-aware application. Hibernate Sharding supports virtual shards, a feature designed to simplify the process of resharding your data.
* Free/open source - Hibernate Shards is licensed under the LGPL (Lesser GNU Public License)
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Re: Product: Hibernate Shards
In a single-shard environment this query can be easily answered, but in a multi-shard environment it's a little bit trickier. Why? Because just getting the average from each shard isn't enough to calculate the average across all shards. In order to calculate this piece of information we need not just the average but the number of records from each shard. This is exactly what we do, and the performance hit (doing an extra count as part of each query) is probably negligible. Now, if we wanted the median we'd be in trouble (just adding the count to the query would not provide enough information to perform the calculation), but at the moment Criteria doesn't expose a median function so we'll deal with that if and when it becomes and issue.
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