MongoDB Documentation, Release 3.0.4 See also: Additional Information Changes Affecting Compatibility Index Changes Enforce Index Key Length Limit Chapter 12. Release Notes 856 • Secondaries will continue to replicate documents with an indexed field whose corresponding index entry exceeds the limit on initial sync but will print warnings in the logs. • If left unfixed, the chunk will repeatedly fail migration, effectively ceasing chunk balancing for that col- lection. Or, if chunk splits occur in response to the migration failures, this response would lead to unnec- essarily large number of chunks and an overly large config databases. • Migrations will fail for a chunk that has a document with an indexed field whose index entry exceeds the limit. • If an existing document contains an indexed field whose index entry exceeds the limit, updates on other fields that result in the relocation of a document on disk will error. • db.collection.update() and db.collection.save() operations on an indexed field will error if the updated value causes the index entry to exceed the limit. • mongorestore and mongoimport will fail to insert if the new document has an indexed field whose corresponding index entry exceeds the limit. • db.collection.insert() and other operations that perform inserts (e.g. db.collection.save() and db.collection.update() with upsert that result in in- serts) will fail to insert if the new document has an indexed field whose corresponding index entry exceeds the limit. Previous versions of MongoDB would insert but not index such documents. • Because db.collection.reIndex() , compact , and repairDatabase drop all the indexes from a collection and then recreate them sequentially, the error from the index key limit prevents these oper- ations from rebuilding any remaining indexes for the collection and, in the case of the repairDatabase command, from continuing with the remainder of the process. • db.collection.ensureIndex() , db.collection.reIndex() , compact , and repairDatabase will error and not create the index. Previous versions of MongoDB would create the index but not index such documents. SNMP Changes (page 861) Compatibility Changes in MongoDB 2.6 The following 2.6 changes can affect the compatibility with older ver- sions of MongoDB. See Release Notes for MongoDB 2.6 (page 827) for the full list of the 2.6 changes. Description MongoDB 2.6 implements a stronger enforcement of the limit on index key . Creating indexes will error if an index key in an existing document exceeds the limit: Inserts will error: Updates will error: Chunk Migration will fail: Secondary members of replica sets will warn: