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Amazon DocumentDB is finally included in the discount! Significant cost savings are possible with Database Savings Plans.

This is Kobayashi (@berlinbytes) from the Service Reliability Group (SRG) of the Media Division.
#SRGThe Service Reliability Group primarily provides comprehensive support for the infrastructure surrounding our media services, focusing on improving existing services, launching new ones, and contributing to open-source software (OSS).
 
This article is,CyberAgent Group SRE Advent Calendar 2025This is the article for day 13.
 
With the announcement of "Database Savings Plans for AWS Databases" at the recent AWS re:Invent 2025, Amazon DocumentDB is finally included in Savings Plans! For those who felt that DocumentDB had weaker discount options compared to Aurora and RDS, this is a very welcome update. This was also long-awaited and exciting news for us.
 
 
This article summarizes the overview of Database Savings Plans, provides an example of discounts using Amazon DocumentDB pricing from the official documentation, and offers a simple cost estimate for a production configuration in the Tokyo region.
Please note that all figures in this article are estimates based on USD. For actual unit prices and exchange rates, please be sure to check the latest information on the official AWS pricing page.
 

What are Database Savings Plans?

First, let's explain the main topic of this discussion: Database Savings Plans. In short, Database Savings Plans are "Savings Plans specifically for database services." Similar to Compute Savings Plans, they work by committing to a fixed amount of usage (in dollars/hour) for one year, which then provides a discount on on-demand pricing. The main features are as follows:
  • The contract period is one year (currently only one-year contracts are offered).
  • The amount you committed to will be automatically discounted from the on-demand fee.
  • The services covered are diverse, including Aurora, RDS, DynamoDB, ElastiCache, Amazon DocumentDB, Neptune, Keyspaces, Timestream, and DMS.
  • Applicable across instance families, sizes, regions, and database engines.
  • Database RIs and DynamoDB Reserved Capacity cannot be stacked for the same usage.
According to the official explanation, the approximate (upper limit) of the discount rate is as follows:
  • Serverless solutions: Up to 35% off
  • Provisioned instances (Aurora, RDS, DocumentDB, etc.): Up to 20% off
The key point here is that "Amazon DocumentDB Instances" and "Amazon DocumentDB Serverless" are included in this package. This makes it possible to optimize costs while maintaining flexibility.
 

Amazon DocumentDB pricing model

Next, let's summarize the pricing structure for DocumentDB. The pricing for Amazon DocumentDB (with MongoDB compatibility) is mainly composed of the following four elements:
  1. On-demand instances
      • Cluster compute instances (instance class × number of instances)
  1. Database I/O
      • Read and write I/O to storage volumes (in units of millions of I/O)
  1. Database Storage
      • Cluster storage volume capacity (GB/month)
  1. Backup storage
      • Backup usage exceeding data limit (GB/month)
Furthermore, you can choose from the following two storage configurations:
  • Standard (pay-as-you-go I/O configuration)
    • You are charged for four things: instances, storage, I/O, and backups.
    • This option is suitable if I/O is likely to account for less than 25% of the total cluster cost.
  • I/O-Optimized (Configuration including I/O)
    • Billing is based on three components: instance, storage, and backup (I/O is free).
    • This option is suitable if I/O is likely to account for more than 25% of the total cluster cost.
The Database Savings Plans, the theme of this discussion, apply discounts to the pricing of "On-Demand Instances" (or Serverless DCUs). Please note that storage and I/O charges are not included in the discount.

Estimated pricing based on the official documentation's example (Standard configuration)

First, let's look at the cost savings potential of the Standard configuration using a setup similar to the example on the official AWS DocumentDB pricing page.
 
 

US East (N. Virginia) / db.r8g.large ×2

The official documentation's Example 1 (Standard configuration) uses r5 instances, but we will perform a calculation using r8g instances to qualify for Savings Plans.
  • Region: US East (N. Virginia)
  • Instance class: db.r8g.large
  • Number of instances: 2 (1 Primary + 1 Replica)
  • Storage: 50 GB
  • Backup storage: 50 GB
  • I/O: 200M (200 million) I/Os / month
The monthly cost for this configuration is as follows:
  • Instance cost: 0.304 USD × 730 hours × 2 instances = 443.84 USD
  • Storage cost: 50 GB × 0.10 USD = 5.00 USD
  • I/O cost: 200M × 0.20 USD = 40.00 USD
Total: 443.84 + 5.00 + 40.00 = 488.84 USD/month

Image after applying Database Savings Plans

We will apply Database Savings Plans to this configuration and calculate the cost with the instance portion discounted.
  • Instance cost (after discount): 0.243 USD × 730 hours × 2 instances = 354.78 USD
  • Storage cost: 5.00 USD (unchanged)
  • I/O cost: 40.00 USD (unchanged)
Total: 354.78 + 5.00 + 40.00 = 399.78 USD/month
The amount and percentage of reduction in this case are as follows:
  • Savings: 488.84 USD - 399.78 USD = 89.06 USD / month
  • Reduction rate: about 18%
The reduction amounted to approximately $89.06 USD, representing a reduction of about 18% across the entire cluster. This demonstrates that the instance discount significantly contributed to the overall cost reduction.

Estimated cost for a Tokyo region configuration with 3 x db.r8g.8xlarge machines (I/O-Optimized configuration).

Next, we will perform calculations based on a more practical production workload. Here, we will use instances equipped with the latest Graviton4 processor and select an I/O-Optimized configuration with no I/O charges.

ap-northeast-1（Tokyo）/ db.r8g.8xlarge ×3

  • Region: ap-northeast-1 (Tokyo)
  • Instance class: db.r8g.8xlarge
  • Number of instances: 3 (1 Primary + 2 Replica)
  • Storage: 1 TB
  • Backup storage: 1 TB
The monthly cost for this configuration is as follows:
  • Instance cost: 5.863 USD × 730 hours × 3 instances = 12,839.97 USD
  • Storage cost: 1,000 GB × 0.30 USD = 300.00 USD
  • Backup cost: 1,000 GB × 0.02 USD = 20.00 USD
Total: 12,839.97 + 300 + 20 = 13,159.97 USD/month

Monthly fee when Database Savings Plans are applied

Next, we calculate the cost after applying Database Savings Plans. The discount applies only to the instance portion.
  • Instance cost (after discount): 4.690 USD × 730 hours × 3 instances = 10,271.1 USD
  • Storage cost: 300.00 USD (unchanged)
  • Backup cost: 20.00 USD (unchanged)
Total: 10,271.10 + 300 + 20 = 10,591.10 USD/month
The amount and percentage of reduction in this case are as follows:
  • Savings: 13,159.97 USD - 10,591.10 USD = 2,568.87 USD / month
  • Reduction rate: approx. 19.5%
In an I/O-Optimized configuration, I/O charges are eliminated, but the instance cost is set higher. Therefore, the instance cost accounts for a very large proportion of the total cluster cost. As a result, the 20% discount on the instance portion is almost directly reflected in the overall cluster cost, resulting in a high reduction of approximately 19.5%.
Thus, I/O-Optimized configurations and Database Savings Plans are a very compatible combination.

Steps to consider when implementing Database Savings Plans

When considering purchasing Database Savings Plans, including DocumentDB, we recommend following these steps:
  1. Understanding On-Demand Costs For existing clusters, aggregate your DocumentDB on-demand usage from Cost Explorer or your invoices. For new deployments, use the AWS Pricing Calculator to create an estimate.
  1. Determining the Scope of Application Consider whether to include other database services such as Aurora, RDS, ElastiCache, and DynamoDB, in addition to DocumentDB, under the Savings Plans. Database Savings Plans can be applied across engines and regions, providing flexibility to accommodate potential future database migrations or configuration changes.
  1. Check the recommended values. Check the "Savings Plans Recommendations" in the AWS Billing and Cost Management console. Based on your past usage, you will be presented with a suggested optimal commit amount ($/hour).
  1. How to use Reserved Instances (RIs) and Savings Plans: For RDS instances that you plan to continue using without changing the configuration for the next three years, apply RIs, which offer a higher discount rate. For DocumentDB or Aurora instances that may require configuration changes or migration in the future, apply Savings Plans, which offer greater flexibility. It is effective to choose the appropriate plan based on the characteristics of your workload.

summary

With the announcement of Database Savings Plans at re:Invent 2025, Amazon DocumentDB can finally benefit from Savings Plans.
In DocumentDB's pricing structure, Savings Plans discount the instance portion (or Serverless DCU). For example, a Standard configuration can result in an overall reduction of approximately 18%, while an I/O-Optimized configuration, which has a higher proportion of instance costs, can expect a reduction of nearly 20% overall.
In particular, the flexibility to apply across regions, instance families, and database engines is a significant advantage in rapidly changing system environments. We encourage you to consider implementing this solution after reviewing your current costs and anticipated future configuration changes.
 
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