Loadtest: make MySQL max open conns tunable (#44802)

[getvictor]

Replace the hardcoded FLEET_MYSQL_MAX_OPEN_CONNS=10 in the loadtest terraform (both the root ecs.tf stack and the infra/ stack) with a mysql_max_open_conns variable, applied to both the writer and read-replica pools.

Also document per-tier connection pool sizing in the AWS reference architecture: 20 for R-class instances (25k+ hosts), 10 for the smaller T-class instances (5k/10k hosts) whose low default max_connections (~90/135) leaves little headroom.

Related issue: Resolves #44802

Summary by CodeRabbit

• New Features
  • Added a configurable setting for MySQL connection limits in the load testing environment.
  • Connection settings now use a shared input value instead of fixed defaults, making it easier to tune database load behavior across container types.

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Pull request overview

Updates the loadtesting Terraform stacks to make Fleet’s MySQL connection pool size configurable instead of hardcoded, so operators can tune per-container DB connections based on Aurora capacity and loadtesting scale.

Changes:

• Adds a mysql_max_open_conns Terraform variable (default 10) to both loadtesting Terraform stacks.
• Wires mysql_max_open_conns into both writer and read-replica Fleet MySQL pool env vars (FLEET_MYSQL_MAX_OPEN_CONNS and FLEET_MYSQL_READ_REPLICA_MAX_OPEN_CONNS).
• Updates AWS reference architecture documentation (content excluded by policy).

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
infrastructure/loadtesting/terraform/variables.tf	Adds mysql_max_open_conns variable for the root loadtesting stack.
infrastructure/loadtesting/terraform/infra/variables.tf	Adds mysql_max_open_conns variable for the infra/ loadtesting stack.
infrastructure/loadtesting/terraform/infra/locals.tf	Applies the variable to both writer and read-replica pool env vars.
infrastructure/loadtesting/terraform/ecs.tf	Applies the variable to ECS task env vars for both writer and read-replica pools.
docs/Deploy/Reference-Architectures.md	Updates connection pool sizing guidance (content excluded).

Files excluded by content exclusion policy (1)

• docs/Deploy/Reference-Architectures.md

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📝 Walkthrough

Walkthrough

A new Terraform input variable mysql_max_open_conns (type number, default 10) is declared in both infrastructure/loadtesting/terraform/variables.tf and infrastructure/loadtesting/terraform/infra/variables.tf. The previously hardcoded "10" values for FLEET_MYSQL_MAX_OPEN_CONNS and FLEET_MYSQL_READ_REPLICA_MAX_OPEN_CONNS in ecs.tf and infra/locals.tf are replaced with tostring(var.mysql_max_open_conns).

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (1 warning, 1 inconclusive)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description is missing most required template sections and checklist items, so it is largely incomplete.	Fill out the required template sections, especially the checklist and testing items, or remove non-applicable lines.
Linked Issues check	❓ Inconclusive	The Terraform changes satisfy the variable update, but the docs update in docs/Deploy/Reference-Architectures.md is excluded by !**/*.md, so that objective can't be verified.	Re-run review with the markdown file included or confirm the reference-architecture update separately.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title concisely states the main change: making loadtest MySQL connection limits tunable.
Out of Scope Changes check	✅ Passed	No clearly unrelated code changes are evident; the edits all support tuning loadtest MySQL connection limits.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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• Create PR with unit tests
• Commit unit tests in branch victor/44802-mysql-conns-tunable

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Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@infrastructure/loadtesting/terraform/infra/variables.tf`:
- Around line 46-50: The `mysql_max_open_conns` variable description is using
the wrong sizing term and an outdated formula. Update the description in
`variables.tf` to reference `fleet_task_count` instead of `fleet_containers`,
and make it clear that this setting applies to both the writer and read-replica
pools so the worst-case connection math reflects the total impact. Keep the
change localized to the `variable "mysql_max_open_conns"` block and adjust the
wording to match the stack’s actual budget.

In `@infrastructure/loadtesting/terraform/variables.tf`:
- Around line 35-39: Update the mysql_max_open_conns variable description to
reflect the correct worst-case connection budget: it should mention that this
setting applies to both the writer and read-replica pools, and that a single
Aurora instance can see roughly 2 * fleet_containers * mysql_max_open_conns when
reader traffic collapses onto the writer. Keep the change limited to the
variable description in variables.tf so the wording matches the actual
connection behavior.

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Fix all unresolved CodeRabbit comments on this PR:

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📥 Commits

Reviewing files that changed from the base of the PR and between 21c3a3f and e79934d.

⛔ Files ignored due to path filters (1)

• docs/Deploy/Reference-Architectures.md is excluded by !**/*.md

📒 Files selected for processing (4)

• infrastructure/loadtesting/terraform/ecs.tf
• infrastructure/loadtesting/terraform/infra/locals.tf
• infrastructure/loadtesting/terraform/infra/variables.tf
• infrastructure/loadtesting/terraform/variables.tf

[getvictor]

@rachaelshaw Please approve the doc part of this PR