S3 Fundamentals

☁️ What is Amazon S3?

Amazon S3 is an object storage service that provides industry-leading scalability, availability, durability, and security.

✅ Store virtually unlimited objects like files, logs, images, backups, etc., in a bucket, which is like a container or folder.

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📦 Core Concepts

Concept	Description
Bucket	A globally unique container for objects (like a folder)
Object	Any file/data stored in a bucket (e.g., .jpg, .pdf, .zip)
Key	The unique identifier for an object within a bucket
Region	Buckets are created in a specific AWS region
Storage Classes	Optimize cost/performance: Standard, IA, Glacier, etc.

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🛡️ 1. Bucket Policy

A Bucket Policy is a JSON-based resource policy that controls access to a specific S3 bucket.

🔹 Example: Allow public read access

{
  "Version": "2012-10-17",
  "Statement": [{
    "Sid": "PublicReadGetObject",
    "Effect": "Allow",
    "Principal": "*",
    "Action": "s3:GetObject",
    "Resource": "arn:aws:s3:::mybucket/*"
  }]
}

📛 Be careful: Public access must also be enabled (see section 3).

🔹 Use Cases:

• Grant access to IAM users/roles

• Allow CloudFront access

• Allow cross-account access

• Define conditions using IPs, user agents, etc.

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🪪 2. ACL (Access Control List)

ACLs are legacy access controls that define read/write permissions per grantee (e.g., specific AWS account, everyone).

🔹 ACL Types:

• Bucket ACL: Controls access to the bucket

• Object ACL: Controls access to specific files

🔹 Example: Make object publicly readable

aws s3api put-object-acl --bucket mybucket --key myphoto.jpg --acl public-read

⚠️ ACLs are not recommended for fine-grained control. Use Bucket Policies or IAM.

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🔓 3. Public Access Settings

Even if a bucket policy or ACL grants public access, S3 Public Access Block Settings can override and block it.

🔹 Block Public Access Options:

• Block public ACLs

• Block public bucket policies

• Block new public ACLs

• Ignore public ACLs

🔒 These settings are enabled by default for new buckets (to prevent accidental exposure).

🔹 Disable (CLI Example):

aws s3api put-public-access-block \
  --bucket mybucket \
  --public-access-block-configuration \
  BlockPublicAcls=false,IgnorePublicAcls=false,BlockPublicPolicy=false,RestrictPublicBuckets=false

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🌐 4. Static Website Hosting

You can host a static website (HTML/CSS/JS) using Amazon S3.

🔹 Steps:

1. Enable static hosting:

   • Set index document (e.g., index.html)

   • (Optional) Set error document (e.g., error.html)

2. Make your files publicly readable (via policy or ACL)

3. Access via S3 website endpoint:

   http://<bucket-name>.s3-website-<region>.amazonaws.com
   

🔹 Example Static Hosting Policy:

{
  "Statement": [{
    "Effect": "Allow",
    "Principal": "*",
    "Action": "s3:GetObject",
    "Resource": "arn:aws:s3:::mywebsitebucket/*"
  }]
}

✅ Use CloudFront for HTTPS + domain support.

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🕓 5. Versioning

Versioning keeps multiple versions of an object in the same bucket.

🔹 Enable Versioning:

aws s3api put-bucket-versioning \
  --bucket mybucket \
  --versioning-configuration Status=Enabled

🔹 Benefits:

• Restore accidentally deleted or overwritten files

• Protect against ransomware/data corruption

🔹 Notes:

• Deleted objects still remain (you get a delete marker)

• Can be combined with Lifecycle Policies for cost savings

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🔸 1. Maximum Size of an Object in Amazon S3

Operation	Limit
Single PUT upload	5 GB maximum
Maximum object size	5 TB (via multipart upload)

✅ If your file is larger than 5 GB, you must use multipart upload.

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🔸 2. What is Multipart Upload?

Multipart upload lets you upload a large object in parts (chunks), in parallel, and resume if a part fails.

🧠 Why Use Multipart Upload?

• Upload files larger than 5 GB (up to 5 TB)

• Faster uploads (upload parts in parallel)

• Resume uploads after failure (only re-upload failed parts)

• Pause and resume support

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🔸 3. How Multipart Upload Works

1. Initiate upload – Get an uploadId

2. Upload parts – Each part (5 MB – 5 GB) is uploaded separately

3. Complete upload – S3 assembles all parts into a single object

💡 Parts are numbered (1–10,000). So max parts = 10,000.

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🔸 4. Multipart Upload Size Rule

Object Size	Recommended Upload Method
≤ 5 GB	Single PUT upload
> 5 GB & ≤ 5 TB	Use Multipart Upload
> 5 TB	❌ Not supported in S3

Each part must be at least 5 MB, except the last part.

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🔧 Example: AWS CLI Multipart Upload

aws s3 cp largefile.zip s3://mybucket/ --storage-class STANDARD

CLI will automatically use multipart upload for files > 8 MB.

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🔚 Summary

Feature	Value
Max object size	5 TB
PUT upload limit	5 GB
Multipart required	For > 5 GB objects
Part size	≥ 5 MB (last part can be less)
Max parts	10,000

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