# Best practices for bulk put items in DynamoDB
The following are the best practices for using bulk put items.
## Split your input across multiple similarly sized S3 objects
DynamoDB reads and processes S3 objects in parallel, so a set of similarly sized objects completes faster than one large object or a mix of very large and very small objects. Each operation supports up to 50,000 S3 objects, and each object can be up to 5 GB. If your dataset has more than 50,000 objects, consolidate them into fewer, larger objects.
## Stay within the size limit for your performance tier
Each performance tier has its own limit on the total input size of one operation. To load a dataset larger than the limit, split it into multiple operations, or use the Accelerated performance tier if your table supports it. For the limits, see [Quotas for bulk put items](ServiceQuotas.md#limits-bulk-put-items).
## Distribute writes evenly across the key space
Bulk put items performs best when your data has high-cardinality partition keys that are spread evenly across the key space. If an S3 object holds items sorted by partition key, consecutive writes target the same partition, which creates a rolling hot partition and slows the operation. Randomize the order of sorted data before you start the operation. For more information, see [Distributing write activity efficiently during data upload in DynamoDB](bp-partition-key-data-upload.md).
## Validate your input before you start
Run a validation pass on a sample of your files before you submit a large operation, because a single record that DynamoDB cannot parse stops it reading the rest of that object, and can end the whole operation `FAILED`. For the conditions that stop an object, see [Validation errors that stop an object](BulkPutItems.Format.md#BulkPutItems.Format.ValidationErrors).
## Use condition expressions to protect existing data
If you must not overwrite existing or newer data, add an insert-only condition or a version guard to every record, including keys that your application writes while the operation runs. For the syntax and examples, see [Per-item condition expressions](BulkPutItems.Format.md#BulkPutItems.Format.Conditions) and [Concurrent writes to the same keys](BulkPutItems.Considerations.md#BulkPutItems.Considerations.ConcurrentWrites).
## Do not modify S3 objects during an active operation
Keep your source S3 objects unchanged while an operation is in progress. Modifying an input object can cause the operation to fail with failure code `ObjectModifiedInS3DuringImport`. Wait for the operation to finish before you change the source files, and restart it with a stable version of the objects if you see this error.
## Use a client token when you retry a start request
Pass a [ClientToken](https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_StartBulkPutItems.html#DDB-StartBulkPutItems-request-ClientToken) with [StartBulkPutItems](https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_StartBulkPutItems.html) whenever you might retry the request, so that a retry returns the existing operation instead of starting a second one. For how the token works, see [Starting a bulk put items operation](BulkPutItems.Requesting.md#BulkPutItems.Requesting.Start).
## Protect downstream stream consumers
On a table with DynamoDB Streams or a Kinesis data stream enabled, every bulk write produces a stream record at the operation write rate. Consumers sized for normal traffic can fall behind. DynamoDB Streams retains records for 24 hours. Set [MaxWriteRequestUnits](https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_StartBulkPutItems.html#DDB-StartBulkPutItems-request-MaxWriteRequestUnits) to cap the bulk put items write rate at a level your consumers can sustain.
Do not set `MaxWriteRequestUnits` lower than you need. A low cap slows the operation, so a large operation might not finish within the maximum processing time and ends `FAILED` with failure code `Timeout`, leaving the remaining items unwritten. Choose the highest rate your consumers can sustain. For more information, see [Status values](BulkPutItems.Requesting.md#BulkPutItems.Requesting.Describe.Status).