Amazon S3: Basic Concepts

Amazon S3 is an reliable, scalable, online object storage that stores files.

• Bucket: A bucket is a container in Amazon S3 where the files are uploaded. Files are stored in buckets. We need at least one bucket to store the files.
• Bucket name has to be unique because it is shared by all users.
• Buckets can't have nested buckets but can have nested directories.
• Maximum of 100 buckets can be created in a single account.
• There is no size limit on buckets.
• We can't rename a bucket once created.
• Buckets can be accessed via HTTP URLs as follows.
• http://<BUCKET_NAME>.s3.amazonaws.com/<OBJECT_NAME>
• http://s3.amazonaws.com/<BUCKET_NAME>/<OBJECT_NAME>
• Buckets can be managed via
• REST-Style HTTP Interface
• SOAP Interface
• The access logging feature if enabled, keeps track of bucket requests such as request type, resources accessed, date and time when requested.
• Object: An object is a file on Amazon S3. Each object is assigned a unique identifier. Every object is stored in a bucket. Objects consist of data and metadata.
• Objects can be managed via
• REST-style HTTP Interface
• SOAP Interface
• Objects can be downloaded via
• HTTP GET Interface
• BitTorrent protocol
• Every object is assigned a key as an identifier and is unique. 
• Objects can be added to a folder in either of two ways
• Add Files option - Individual files can be uploaded using this option.
• Enable enhanced uploader - This option is used when we need to upload whole folders.
• There are two options under Set Details section on files.
• Use Reduced Redundancy Storage - Non critical data can be set to use reduced redundancy storage. Using this will store the file at lower levels of redundancy compared to standard storage class.
• Use Server Side Encryption
• This is for security. Data is encrypted while storing. When object is accessed, Amazon S3 decrypts the data.
• Use server side encryption has two options.
• Use the Amazon S3 service master key
• Use an AWS Key Management Service master key
• Key: A key is a unique identifier for an object within the bucket. Combination of bucket, key and version ID uniquely identifies each object. 
• Region: We might want to choose the geographical region where Amazon S3 will store buckets.
• Folder: The folders in Amazon S3 are S3 files that are used to put Amazon S3 objects together under one group. This is analogous to Directory.
• Versioning: Versioning helps us to retrieve old objects. We can retrieve deleted and updated objects. When an object is deleted, Amazon S3 inserts a delete marker rather than deleting it permanently.
• Versioning is enabled at bucket level.
• Versioning can be enabled in any of the following states.
• Unversioned - the default
• Versioning enabled
• Versioning suspended.
• By default, when versioning is enabled Amazon S3 stores all versions of an object.
• To control the limit of versions, enable "Lifecycle rules" for the object. These rules will delete the old files.
• Data consistency Model: S3 provides eventual consistency for read-after-write. 
• If we make a GET request to an object after an update request, we might get old data if update is not complete, else we will get latest data.
• S3 would return old data or updated data but will never return partial data.
• Amazon S3 provides high consistency by replication data across multiple servers. 
• If a PUT request is successful, data is safely stored across multiple servers.
• If changes have to be made to a file, the change has to be replicated across all locations and this will take time. Any GET request during this time period might return old data until change is fully propagated.