S3

AWS S3

Amazon Simple Storage Service (S3) provides scalable object storage with industry-leading durability (99.999999999%). S3 is fundamental to AWS—used for data lakes, backups, static websites, and as storage for many other AWS services.

Table of Contents

• Core Concepts
• Common Patterns
• CLI Reference
• Best Practices
• Troubleshooting
• References

Core Concepts

Buckets

Containers for objects. Bucket names are globally unique across all AWS accounts.

Objects

Files stored in S3, consisting of data, metadata, and a unique key (path). Maximum size: 5 TB.

Storage Classes

Versioning

Keeps multiple versions of an object. Essential for data protection and recovery.

Common Patterns

Create a Bucket with Best Practices

AWS CLI:

# Create bucket (us-east-1 doesn't need LocationConstraint)
aws s3api create-bucket \
  --bucket my-secure-bucket-12345 \
  --region us-west-2 \
  --create-bucket-configuration LocationConstraint=us-west-2

# Enable versioning
aws s3api put-bucket-versioning \
  --bucket my-secure-bucket-12345 \
  --versioning-configuration Status=Enabled

# Block public access
aws s3api put-public-access-block \
  --bucket my-secure-bucket-12345 \
  --public-access-block-configuration \
    BlockPublicAcls=true,IgnorePublicAcls=true,BlockPublicPolicy=true,RestrictPublicBuckets=true

# Enable encryption
aws s3api put-bucket-encryption \
  --bucket my-secure-bucket-12345 \
  --server-side-encryption-configuration '{
    "Rules": [{"ApplyServerSideEncryptionByDefault": {"SSEAlgorithm": "AES256"}}]
  }'

boto3:

import boto3

s3 = boto3.client('s3', region_name='us-west-2')

# Create bucket
s3.create_bucket(
    Bucket='my-secure-bucket-12345',
    CreateBucketConfiguration={'LocationConstraint': 'us-west-2'}
)

# Enable versioning
s3.put_bucket_versioning(
    Bucket='my-secure-bucket-12345',
    VersioningConfiguration={'Status': 'Enabled'}
)

# Block public access
s3.put_public_access_block(
    Bucket='my-secure-bucket-12345',
    PublicAccessBlockConfiguration={
        'BlockPublicAcls': True,
        'IgnorePublicAcls': True,
        'BlockPublicPolicy': True,
        'RestrictPublicBuckets': True
    }
)

Upload and Download Objects

# Upload a single file
aws s3 cp myfile.txt s3://my-bucket/path/myfile.txt

# Upload with metadata
aws s3 cp myfile.txt s3://my-bucket/path/myfile.txt \
  --metadata "environment=production,version=1.0"

# Download a file
aws s3 cp s3://my-bucket/path/myfile.txt ./myfile.txt

# Sync a directory
aws s3 sync ./local-folder s3://my-bucket/prefix/ --delete

# Copy between buckets
aws s3 cp s3://source-bucket/file.txt s3://dest-bucket/file.txt

Generate Presigned URL

import boto3
from botocore.config import Config

s3 = boto3.client('s3', config=Config(signature_version='s3v4'))

# Generate presigned URL for download (GET)
url = s3.generate_presigned_url(
    'get_object',
    Params={'Bucket': 'my-bucket', 'Key': 'path/to/file.txt'},
    ExpiresIn=3600  # URL valid for 1 hour
)

# Generate presigned URL for upload (PUT)
upload_url = s3.generate_presigned_url(
    'put_object',
    Params={
        'Bucket': 'my-bucket',
        'Key': 'uploads/newfile.txt',
        'ContentType': 'text/plain'
    },
    ExpiresIn=3600
)

Configure Lifecycle Policy

cat > lifecycle.json << 'EOF'
{
  "Rules": [
    {
      "ID": "MoveToGlacierAfter90Days",
      "Status": "Enabled",
      "Filter": {"Prefix": "logs/"},
      "Transitions": [
        {"Days": 90, "StorageClass": "GLACIER"}
      ],
      "Expiration": {"Days": 365}
    },
    {
      "ID": "DeleteOldVersions",
      "Status": "Enabled",
      "Filter": {},
      "NoncurrentVersionExpiration": {"NoncurrentDays": 30}
    }
  ]
}
EOF

aws s3api put-bucket-lifecycle-configuration \
  --bucket my-bucket \
  --lifecycle-configuration file://lifecycle.json

Event Notifications to Lambda

aws s3api put-bucket-notification-configuration \
  --bucket my-bucket \
  --notification-configuration '{
    "LambdaFunctionConfigurations": [
      {
        "LambdaFunctionArn": "arn:aws:lambda:us-east-1:123456789012:function:ProcessS3Upload",
        "Events": ["s3:ObjectCreated:*"],
        "Filter": {
          "Key": {
            "FilterRules": [
              {"Name": "prefix", "Value": "uploads/"},
              {"Name": "suffix", "Value": ".jpg"}
            ]
          }
        }
      }
    ]
  }'

CLI Reference

High-Level Commands (aws s3)

Low-Level Commands (aws s3api)

Useful Flags

• --recursive: Process all objects in prefix
• --exclude/--include: Filter objects
• --dryrun: Preview changes
• --storage-class: Set storage class
• --acl: Set access control (prefer policies instead)

Best Practices

Security

• Block public access at account and bucket level
• Enable versioning for data protection
• Use bucket policies over ACLs
• Enable encryption (SSE-S3 or SSE-KMS)
• Enable access logging for audit
• Use VPC endpoints for private access
• Enable MFA Delete for critical buckets

Performance

• Use Transfer Acceleration for distant uploads
• Use multipart upload for files > 100 MB
• Randomize key prefixes for high-throughput (less relevant with 2024 improvements)
• Use byte-range fetches for large file downloads

Cost Optimization

• Use lifecycle policies to transition to cheaper storage
• Enable Intelligent-Tiering for unpredictable access
• Delete incomplete multipart uploads:

  {
    "Rules": [{
      "ID": "AbortIncompleteMultipartUpload",
      "Status": "Enabled",
      "Filter": {},
      "AbortIncompleteMultipartUpload": {"DaysAfterInitiation": 7}
    }]
  }
  

• Use S3 Storage Lens to analyze storage patterns

Troubleshooting

Access Denied Errors

Causes:

1. Bucket policy denies access
2. IAM policy missing permissions
3. Public access block preventing access
4. Object owned by different account
5. VPC endpoint policy blocking

Debug steps:

# Check your identity
aws sts get-caller-identity

# Check bucket policy
aws s3api get-bucket-policy --bucket my-bucket

# Check public access block
aws s3api get-public-access-block --bucket my-bucket

# Check object ownership
aws s3api get-object-attributes \
  --bucket my-bucket \
  --key myfile.txt \
  --object-attributes ObjectOwner

CORS Errors

Symptom: Browser blocks cross-origin request

Fix:

aws s3api put-bucket-cors --bucket my-bucket --cors-configuration '{
  "CORSRules": [{
    "AllowedOrigins": ["https://myapp.com"],
    "AllowedMethods": ["GET", "PUT", "POST"],
    "AllowedHeaders": ["*"],
    "ExposeHeaders": ["ETag"],
    "MaxAgeSeconds": 3600
  }]
}'

Slow Uploads

Solutions:

• Use multipart upload for large files
• Enable Transfer Acceleration
• Use aws s3 cp with --expected-size for large files
• Check network throughput to the region

403 on Presigned URL

Causes:

• URL expired
• Signer lacks permissions
• Bucket policy blocks access
• Region mismatch (v4 signatures are region-specific)

Fix: Ensure signer has permissions and use correct region.

References

• S3 User Guide
• S3 API Reference
• S3 CLI Reference
• boto3 S3

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