A Simple Analogy

AWS S3 is like a massive, organized warehouse. You can store any file, organize it with labels (tags), control who accesses it, and retrieve it instantly from anywhere. You pay only for what you store.

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What Is AWS S3?

Amazon S3 (Simple Storage Service) is object storage for files of any size. It's durable, highly available, and scales infinitely.

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Core Concepts

| Concept | Meaning | |---------|---------| | Bucket | Top-level container (like folder) | | Object | File stored in bucket | | Key | File path/name (photos/pic1.jpg) | | ACL | Access control list | | Lifecycle | Rules for object expiration |

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.NET Usage with AWS SDK

using Amazon.S3;
using Amazon.S3.Model;

var s3Client = new AmazonS3Client();

// Upload file
var putRequest = new PutObjectRequest
{
    BucketName = "my-bucket",
    Key = "uploads/document.pdf",
    FilePath = "/path/to/document.pdf",
    ContentType = "application/pdf"
};
await s3Client.PutObjectAsync(putRequest);

// Download file
var getRequest = new GetObjectRequest
{
    BucketName = "my-bucket",
    Key = "uploads/document.pdf"
};
using var response = await s3Client.GetObjectAsync(getRequest);
await response.ResponseStream.CopyToAsync(File.Create("local.pdf"));

// Delete file
await s3Client.DeleteObjectAsync("my-bucket", "uploads/document.pdf");

// Generate presigned URL (temporary download link)
var urlRequest = new GetPreSignedUrlRequest
{
    BucketName = "my-bucket",
    Key = "uploads/document.pdf",
    Expires = DateTime.UtcNow.AddHours(1)
};
var url = s3Client.GetPreSignedURL(urlRequest);

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Practical Example

public class FileService
{
    private readonly IAmazonS3 _s3Client;
    private readonly string _bucketName = "my-app-storage";
    
    public async Task<string> UploadFileAsync(IFormFile file, string userId)
    {
        var key = $"user-uploads/{userId}/{Guid.NewGuid()}-{file.FileName}";
        
        using (var stream = file.OpenReadStream())
        {
            var putRequest = new PutObjectRequest
            {
                BucketName = _bucketName,
                Key = key,
                InputStream = stream,
                ContentType = file.ContentType
            };
            
            await _s3Client.PutObjectAsync(putRequest);
        }
        
        return key;
    }
    
    public async Task<string> GetDownloadUrlAsync(string key)
    {
        var request = new GetPreSignedUrlRequest
        {
            BucketName = _bucketName,
            Key = key,
            Expires = DateTime.UtcNow.AddHours(1)
        };
        
        return _s3Client.GetPreSignedURL(request);
    }
}

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Best Practices

1. Use versioning: Track file history
2. Enable encryption: Protect data at rest
3. Set lifecycle policies: Archive old files
4. Use presigned URLs: Temporary access without AWS credentials
5. Monitor costs: Track storage usage

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Related Concepts

• CloudFront CDN for distribution
• S3 event notifications
• S3 lifecycle policies
• Server access logging
• Cross-region replication

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Summary

AWS S3 provides scalable, durable object storage. Master bucket organization, access control, and presigned URLs to build secure file management systems.