The document describes Netflix's Rebalance API for SolrCloud, which provides strategies and capabilities for scaling Solr clusters while maintaining service level agreements (SLAs). The Rebalance API includes scaling strategies for operations like auto-sharding, redistributing data, and replacing nodes with zero downtime. It also includes allocation strategies that determine core placement. The goal is to allow for fine-grained SLA management when indexing and querying large datasets across multiple data centers. The Rebalance API has been open sourced and is aimed at forming the basis for auto-scaling in Solr.