Difference between DynamoDb PutItem vs UpdateItem?

The main difference between the two is, PutItem will Replace an entire item while UpdateItem will Update it.

Eg.

I have an item like:

userId = 1
Name= ABC
Gender= Male

If I use PutItem item with

UserId = 1
Country = India

This will replace Name and Gender and now new Item is UserId and Country. While if you want to update an item from Name = ABC to Name = 123 you have to use UpdateItem.

You can use PutItem item to update it but you need to send all the parameters instead of just the Parameter you want to update because it Replaces the item with the new attribute(Internally it Deletes the item and Adds a new item)

Hope this makes sense.

PutItem overwrites the whole item (all attributes) with the new version being passed while UpdateItem will only Update the passed attributes

Performance: PutItem may affect on the performance if you overwrite the entire item so often as it involves more operations than UpdateItem FindItem, DeleteOldVersion, and AddNewVersion

From cost prospective, it is different as well:

AWS calculates the cost based on used read/write capacity units that are completely tied to the size of the item being overwritten/updated.

In case of PutItem, the size will be the larger of the new and old versions of the item. For example, replacing a 2 KB item with a 1 KB, It will consume 2 WCUs however subsequent requests will only use 1 WCU. so if you're overwriting so often and the size of the item changes heavily that will calculate always the larger version of the item and affects on the cost.

In case of modifying items using UpdateItem, the size includes all of the item’s pre-existing attributes, not the larger version like PutItem :) but also not just the ones being added or updated :(

Slight correction in the above answer: Even the update item API takes the larger of the two item sizes(pre-update and post-update) into consideration. See documentation here.