The answer is a connection pool that is not releasing idle connections. This is the most likely cause of RDS connection exhaustion because connection pooling, while efficient, can accumulate and hold open connections indefinitely if the pool lacks proper timeout or validation settings, eventually saturating the database’s max_connections limit even when the application is not actively using them. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding of how application-layer configurations directly impact database availability, often appearing as a trap where candidates blame instance size or network ACLs instead of the pool’s lifecycle management. A common memory tip is to remember that connection exhaustion is rarely about the default max_connections value—db.r5.large defaults to 1000, which is sufficient—but about connections that are “leaked” or never returned. Think of it as a sink with the drain closed: the pool keeps filling, but nothing flows out.
DBS-C01 Workload-Specific Database Design Practice Question
This DBS-C01 practice question tests your understanding of workload-specific database design. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Exhibit
Refer to the exhibit.
Error log from application connecting to Amazon RDS for MySQL:
"ERROR 1040 (08004): Too many connections"
Current RDS configuration: db.r5.large, max_connections = 1000 (default).
An application is receiving the error shown in the exhibit. The application uses connection pooling. The RDS instance is a db.r5.large with max_connections set to 1000. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Refer to the exhibit.
Error log from application connecting to Amazon RDS for MySQL:
"ERROR 1040 (08004): Too many connections"
Current RDS configuration: db.r5.large, max_connections = 1000 (default).
A
The security group is blocking incoming connections.
Why wrong: Blocked connections would result in timeout, not 'Too many connections'.
B
The max_connections parameter is set too low for the instance size.
Why wrong: 1000 is the default for db.r5.large.
C
The connection pool in the application is not releasing idle connections.
Leaked connections accumulate, exhausting the max_connections limit.
D
The RDS instance is in a different VPC than the application.
Why wrong: Cross-VPC connectivity issues would prevent connection establishment.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The connection pool in the application is not releasing idle connections.
Option C is correct because connection pooling can accumulate connections if not properly configured, leading to exhaustion. Option A is wrong because 1000 is the default for db.r5.large. Option B is wrong because network ACLs affect connectivity, not the number of connections once established. Option D is wrong because the error indicates connections are being accepted but exhausted.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The security group is blocking incoming connections.
Why it's wrong here
Blocked connections would result in timeout, not 'Too many connections'.
✗
The max_connections parameter is set too low for the instance size.
Why it's wrong here
1000 is the default for db.r5.large.
✓
The connection pool in the application is not releasing idle connections.
Why this is correct
Leaked connections accumulate, exhausting the max_connections limit.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
The RDS instance is in a different VPC than the application.
Why it's wrong here
Cross-VPC connectivity issues would prevent connection establishment.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
How this comes up in practice
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Got this wrong? Here's your next step.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related DBS-C01 ACL questions on filtering logic and placement.
Workload-Specific Database Design — This question tests Workload-Specific Database Design — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: The connection pool in the application is not releasing idle connections. — Option C is correct because connection pooling can accumulate connections if not properly configured, leading to exhaustion. Option A is wrong because 1000 is the default for db.r5.large. Option B is wrong because network ACLs affect connectivity, not the number of connections once established. Option D is wrong because the error indicates connections are being accepted but exhausted.
What should I do if I get this DBS-C01 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related DBS-C01 ACL questions on filtering logic and placement.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Standard ACLs match source addresses.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company's application is logging the error shown in the exhibit. The application is deployed on Amazon EC2 and connects to an Amazon RDS for MySQL Multi-AZ DB instance. Which configuration change is most likely to resolve this issue?
easy
A.Add an additional standby instance in a third Availability Zone.
B.Increase the connection pool timeout in the application configuration.
C.Create a read replica and direct write traffic to it.
✓ D.Increase the DB instance class to handle more concurrent connections.
Why D: The error log indicates that the application is hitting the maximum number of connections allowed by the RDS DB instance. Increasing the DB instance class (Option D) provides more memory and CPU resources, which allows the instance to support a higher `max_connections` value (calculated as `DBInstanceClassMemory / 12582880` for MySQL). This directly resolves the connection limit issue without changing the application's connection pool behavior or architecture.
Last reviewed: Jun 20, 2026
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