The increased bucket limits fundamentally change the landscape for S3 multi-tenant data partitioning. The old recommendations were heavily influenced by the bucket limits. Let's break down the implications and discuss when each strategy might be suitable now:

Prefix-per-Tenant:

Pros:

• Simpler Management (Initially): One bucket means fewer IAM policies and less overhead at first. You manage permissions within the prefix structure.
• Cost Optimization (Potentially): Data lifecycle management is easier to configure at the bucket level. You can transition all tenants' data to cheaper storage classes (e.g., Glacier) simultaneously. Aggregate usage across tenants can help qualify for bulk pricing or discounts.
• Reduced Operational Overhead (Potentially): Easier to monitor and maintain a single bucket than thousands.
• Strong eventual consistency: Accesses and updates to a single prefix are strongly consistent, although this is the case for all accesses with S3.

Cons:

• IAM Complexity at Scale: Managing granular permissions per tenant becomes extremely complex using IAM policies and prefix conditions. Maintaining and auditing these policies can be a nightmare. As complexity increases, you end up building your own permissioning system on top of S3.
• Potential Performance Bottlenecks: While S3 scales incredibly well, you can hit performance limitations within a single bucket if a particular tenant generates a disproportionate amount of traffic to their prefix. S3's key-naming guidelines can help mitigate this, but it's a factor. The more diverse the traffic, the less likely you are to face such problems
• Difficult Usage Tracking per Tenant: While S3 Storage Lens can provide insights, getting accurate, real-time cost and usage attribution per tenant is more challenging. You often need to rely on custom logging and analytics.
• Data Isolation Concerns: While IAM policies should enforce isolation, any misconfiguration can lead to one tenant accessing another's data. This risk is lower with bucket-per-tenant.
• Reduced Granularity of Security and Compliance: Bucket-level compliance features (like object locking) or security features (like S3 Inventory for audit) apply to all tenants in the bucket. This can make it harder to tailor security and compliance policies to individual tenants.

Bucket-per-Tenant:

Pros:

• Strongest Data Isolation: Each tenant has their own isolated environment. Misconfiguration of one bucket is less likely to impact other tenants.
• Simplified IAM Management: IAM policies are simpler because you are managing permissions at the bucket level, which aligns naturally with tenant boundaries. Each bucket has its own set of IAM permissions.
• Granular Control: You can apply different lifecycle policies, security settings, and compliance controls to each tenant's bucket as needed. This allows for customized experiences.
• Improved Usage Tracking and Cost Attribution: Native S3 features and AWS Cost Explorer can easily provide accurate cost and usage data per bucket (and therefore, per tenant).
• Easier Scaling and Management with Automation: Infrastructure-as-Code (IaC) tools like CloudFormation, Terraform, or the AWS CDK make it easy to automate the creation and management of thousands of buckets.
• Simplified Data Replication: You can replicate buckets to different regions or accounts on a per-tenant basis.

Cons:

• Increased Initial Complexity: Setting up thousands of buckets can seem daunting initially, but IaC tools make this manageable.
• Increased Operational Overhead (Potentially, but addressed by automation): Monitoring and managing thousands of buckets can be more complex than a single bucket, but automation and centralized logging/monitoring solutions can mitigate this.
• Potential for Resource Exhaustion: You need to carefully manage your AWS resource limits (IAM roles, policies, etc.) to avoid hitting account limits.
• Potential for Inconsistent Application of Organization Policies. You need to take extra measures to verify that the application of policies like KMS key usage is consistent.

So, When to Use Which?

The increased bucket limits make bucket-per-tenant the preferred approach in many, if not most, situations that involve a high degree of isolation. However, there are still cases where prefix-per-tenant may be appropriate:

• Very Small Number of Tenants (e.g., < 10): If you have a very small number of tenants and predictable usage patterns, the simplicity of a single bucket might outweigh the cons.
• Very Low Security/Compliance Requirements: If data isolation is not a primary concern, and you prioritize simplicity over granular control.
• Aggregated Billing/Pricing Requirements: If you only offer an "all in one" service, and offer no granularity around tenants, aggregating the cost savings into a single bucket might be ideal
• Extreme Cost Sensitivity and Consistent Tenant Requirements: If all tenants require the exact same lifecycle policies, data retention, security controls, and you have extreme budget limitations, a single bucket with prefix-per-tenant might be a cost-effective solution (but be aware of the trade-offs).