MongoDB 3.1.9 发布，此版本是个开发版本，不建议在生产环境使用。

新特性：

- Improved high-availability: config servers as replica sets and faster elections
- WiredTiger as the default storage engine
- Document validation rules
- Partial indexes
- Encryption at rest
- SpiderMonkey as the Javascript engine
- Multilingual case-sensitive full-text search and improved Unicode text search support
- Aggregation and query optimizer improvements, including a sampling operator and outer left joins
- Network stack improvements
- Tools improvements

bug 修复：

- SERVER-533: Aggregation stage to randomly sample documents
- SERVER-1448: Host sharding config data on a replica set
- SERVER-10225: Replica set failover speed improvement
- SERVER-15176: New commands for find and getMore
- SERVER-17861: Change the default storage engine to WiredTiger
- SERVER-17991: Encrypted Storage Engine Enterprise - Single system key
- SERVER-18624: listCollections command should not be O(n^2) on MMAPv1
- SERVER-20159: Out of memory on index build during initial sync even with low cacheSize parameter
- SERVER-20339: Make NetworkInterfaceASIO the default factory
- TOOLS-916: Mongo export throws reflect.Value.Type errors