This error occurs when MongoDB fails to create an index on a collection due to various constraints like duplicate keys, insufficient permissions, memory limits, or index key size violations. Index creation is critical for query performance but can fail on populated collections with existing data issues.
The "CannotCreateIndex" error indicates that MongoDB was unable to complete an index creation operation on a collection. This can happen for several reasons: the collection contains data that violates the index constraints (like duplicate values for a unique index), the MongoDB user lacks the necessary permissions, memory or disk space is insufficient during the index build process, or the index entry size exceeds MongoDB's limits. Index creation is a critical operation that improves query performance but requires careful consideration when working with populated collections. MongoDB uses a combination of memory (default 200MB per command) and temporary disk files to build indexes, and the process can fail if resources are exhausted or data quality issues exist. In replica sets, index builds can also hang or fail due to connectivity issues between voting nodes when using commitQuorum settings. Understanding the specific cause is essential for resolving the error effectively.
If creating a unique index, ensure no duplicate values exist in the field(s):
// Check for duplicates in the field
db.collection.aggregate([
{ $group: { _id: "$fieldName", count: { $sum: 1 } } },
{ $match: { count: { $gt: 1 } } },
{ $sort: { count: -1 } }
])If duplicates exist, clean them up before creating the unique index, or use the prepareUnique option (MongoDB 6.0+):
db.collection.createIndex(
{ fieldName: 1 },
{ unique: true, prepareUnique: true }
)Ensure your MongoDB user has the necessary privileges:
// Check current user privileges
db.runCommand({ connectionStatus: 1 })
// Grant createIndex permission if needed (as admin)
db.grantRolesToUser("yourUsername", [
{ role: "dbOwner", db: "yourDatabase" }
])The user needs either the dbOwner role or a custom role with createIndex privilege on the target database.
MongoDB has limits on index entry sizes. Verify that your indexed fields don't exceed these limits:
// Check document sizes in the field
db.collection.find().forEach(doc => {
const size = Object.bsonsize({ field: doc.fieldName });
if (size > 1024) {
print("Large field in doc: " + doc._id + ", size: " + size);
}
})For MongoDB 4.2+, the index key limit is 1024 bytes. If you have larger values, consider:
- Using a hashed index instead
- Indexing only a prefix of the field
- Redesigning your schema to use smaller identifiers
Index creation can fail due to insufficient resources. Check available resources:
# Check disk space
df -h
# Monitor MongoDB logs during index creation
tail -f /var/log/mongodb/mongod.logIncrease the memory limit for index builds if needed:
// Create index with increased memory limit (MongoDB 4.2+)
db.adminCommand({
setParameter: 1,
maxIndexBuildMemoryUsageMegabytes: 500
})
db.collection.createIndex({ fieldName: 1 })MongoDB unique indexes allow only one document with a null value. If multiple documents have null values, the index creation fails:
// Check for multiple null values
db.collection.countDocuments({ fieldName: null })
// Option 1: Remove or update documents with null values
db.collection.updateMany(
{ fieldName: null },
{ $unset: { fieldName: "" } }
)
// Option 2: Use a sparse index to exclude null values
db.collection.createIndex(
{ fieldName: 1 },
{ unique: true, sparse: true }
)
// Option 3: Use a partial filter expression
db.collection.createIndex(
{ fieldName: 1 },
{
unique: true,
partialFilterExpression: { fieldName: { $exists: true, $ne: null } }
}
)MongoDB doesn't allow creating an index with a different name if the same key pattern already exists:
// List existing indexes
db.collection.getIndexes()
// Drop the conflicting index if needed
db.collection.dropIndex("old_index_name")
// Create the new index
db.collection.createIndex(
{ fieldName: 1 },
{ name: "new_index_name" }
)If using Spring Data MongoDB, disable automatic index creation and manage indexes manually:
# application.properties
spring.data.mongodb.auto-index-creation=falseThen create indexes explicitly:
@Component
public class IndexInitializer implements ApplicationRunner {
@Autowired
private MongoTemplate mongoTemplate;
@Override
public void run(ApplicationArguments args) {
mongoTemplate.indexOps(YourEntity.class)
.ensureIndex(new Index()
.on("fieldName", Sort.Direction.ASC)
.unique()
);
}
}Replica Set Considerations: When creating indexes in a replica set, be aware of the commitQuorum setting. If set to votingMembers and some voting nodes become unreachable, the index build can hang indefinitely. Consider using a specific number instead of "votingMembers" for more predictable behavior.
Background Index Builds: In MongoDB 4.2+, index builds use an optimized build process that holds an exclusive lock only at the beginning and end of the build. However, they still impact performance on large collections and can take hours to complete.
Index Build Monitoring: Use db.currentOp() to monitor ongoing index builds:
db.currentOp({
$or: [
{ op: "command", "command.createIndexes": { $exists: true } },
{ op: "none", "msg": /^Index Build/ }
]
})Memory Limit Tuning: The default 200MB memory limit for index builds can be increased using the maxIndexBuildMemoryUsageMegabytes parameter, but be cautious not to exhaust system memory. MongoDB will spill to temporary disk files when the limit is reached.
Collation and Indexes: When creating indexes with specific collations, ensure the collation matches your query patterns. Indexes with different collations on the same fields are treated as separate indexes and require unique names.
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