Explanation:Amazon Glacier supports various vault operations.
A vault inventory refers to the list of archives in a vault. For each archive in the list, the inventoryprovides archive information such as archive ID, creation date, and size. Amazon Glacier updates thevault inventory approximately once a day, starting on the day the first archive is uploaded to thevault. A vault inventory must exist for you to be able to download it.
Downloading a vault inventory is an asynchronous operation. You must first initiate a job todownload the inventory. After receiving the job request, Amazon Glacier prepares your inventory fordownload. After the job completes, you can download the inventory data.
Given the asynchronous nature of the job, you can use Amazon Simple Notification Service (AmazonSNS) notifications to notify you when the job completes. You can specify an Amazon SNS topic foreach individual job request or configure your vault to send a notification when specific vault eventsoccur. Amazon Glacier prepares an inventory for each vault periodically, every 24 hours. If there havebeen no archive additions or deletions to the vault since the last inventory, the inventory date is notupdated. When you initiate a job for a vault inventory, Amazon Glacier returns the last inventory itgenerated, which is a point-in-time snapshot and not real-time data. You might not find it useful toretrieve vault inventory for each archive upload. However, suppose you maintain a database on theclient-side associating metadata about the archives you upload to Amazon Glacier. Then, you mightfind the vault inventory useful to reconcile information in your database with the actual vaultinventory.
Reference:
http://docs.aws.amazon.com/amazonglacier/latest/dev/working-with-vaults.html