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As far as I understand your point, in your architecture (EC2 and OpenSearch Serverless in the same Availability Zone), the cost structure is broken down into three main categories. Here is the breakdown:
1. Data Transfer (Networking)
- Intra-AZ Transfer: Data transfer between an EC2 instance and a VPC Interface Endpoint located in the same Availability Zone is $0.00 per GB.
- Pre-requisite: You must use a VPC Interface Endpoint (AWS PrivateLink). If you send data to the public OpenSearch Serverless endpoint, you will incur standard Data Transfer Out charges ($0.09/GB).
2. VPC Endpoint Charges (The "Hidden" Cost)
- Even if the network transfer is free, AWS Interface Endpoints have their own pricing:
- Hourly Charge: Approximately $0.01 per hour for the endpoint in your AZ.
- Data Processing: You are charged $0.01 per GB for data processed by the interface endpoint. This is often what users mistake for "data transfer" costs.
3. OpenSearch Serverless Capacity (OCU)
- Remember that OpenSearch Serverless has a minimum capacity (typically 2 OCUs: 1 for indexing, 1 for search).
- Each OCU costs roughly $0.24 per hour. This fixed cost (approx. $350/month) is usually significantly higher than the data transfer costs for small to medium workloads.
Minimum Monthly Cost:
2 x OCUs x $0.24/hour x 24hours x 30 days = about 345.60 USD per month
This fixed cost is usually significantly higher than the data transfer costs for small to medium workloads.
| Cost Component | Charge (Same AZ | Charge (Cross-AZ) |
|---|---|---|
| Data Transfer (EC2 Out) | $0.00 / GB | $0.01 / GB |
| VPC Endpoint Processing | $0.01 / GB | $0.01 / GB |
| VPC Endpoint Hourly | ~$0.01 / Hr | ~$0.02 / Hr |
To keep costs at the absolute minimum, ensure your VPC Endpoint has an ENI (Elastic Network Interface) in the exact same subnet/AZ where your EC2 instance resides. This avoids the $0.01/GB Inter-AZ fee.
PS: I always use the AWS Cost Calculator to determine the exact price. You do not even need an AWS account, as it is accessible without logging in. -> https://calculator.aws/
See also:
Yes, and how would the random internet know how much data you are sending?
Data transfer fees is a fixed rate but it's always a variable since you cannot control how much data you are transferring. For most regions, transferring data to outside VPC or intra-AZ could be about $100 TB - it can vary slightly by regions. It's best if you monitor your cost for a few days or month and take measurements on how much is acceptable for your case.
that is a fair point - only the owner knows the exact data throughput. however, the goal of my breakdown was to clarify the unit pricing, which is often the missing piece of the puzzle.
In this specific architecture, the volume actually matters less than you might think for two reasons:
Fixed vs. Variable Cost: The OpenSearch Serverless OCUs (Indexing/Search) create a high fixed floor of roughly $345.60/month (2 OCUs at $0.24/hr).
Minimal Processing Fee: Since the VPC Endpoint processing fee is only $0.01 per GB, even a significant transfer of 100 GB would only add $1.00 to the bill. While I don't know the exact data volume, we can conclude that for most small to medium workloads, the fixed OCU costs will vastly outweigh the variable data transfer costs. To get your exact number, you can check the 'BytesThroughput' metric for your VPC Endpoint in Amazon CloudWatch and plug it into the AWS Calculator -> https://calculator.aws/ .