fix(docker): raise JVM direct memory above the app's baseline footprint

[FelixTJDietrich]

Incident

Login (and every DB-backed request) failing. From the app-server logs:

OutOfMemoryError: Cannot reserve 8192 bytes of direct buffer memory (limit: 10485760)
  at org.postgresql.core.v3.ConnectionFactoryImpl.doAuthentication
→ HikariPool-1 - Connection is not available, request timed out after 45000ms

Root cause (verified, not a bandaid)

NMT on staging shows the direct-buffer pool is ~14 MB and flat (two samples, 30s apart → no growth):

Other (direct buffers): 14,553 KB … 14,553 KB

No leak, no unbounded spike — the app's steady off-heap footprint just sits above the buildpack's 10 MB default, so Postgres couldn't get the small buffer for its connection handshake and the pool drained. The 10 MB default is an arbitrary Cloud Foundry inheritance, not tuned for this app.

Sizing — why 128 MB (not 256)

On a container the memory calculator budgets direct memory against heap (10M direct + 240M code + 250M stacks + metaspace + heap), so an oversized cap steals heap. Best practice is to size to actual usage + headroom, not a round large number (BellSoft JVM memory guide, GigaSpaces JVM tuning).

Against ~14 MB steady, 128 MB is ~9× headroom — comfortably above the 10 MB that OOM'd and any observed level, with room for backfill bursts — while returning ~128 MB to heap versus a 256 MB cap. Env-overridable via APP_MAX_DIRECT_MEMORY if a heavy backfill ever approaches it.

Scope

• docker/compose.app.yaml — application-server, application-worker
• docker/preview/compose.app.yaml — the preview app-server (where it first surfaced, feat(webapp): practice surfaces with a focus queue and an area matrix #1347)
• Not webhook-server (core): a light publisher on a tight 1 GB limit that hasn't hit this — a conscious omission.

Set via JAVA_TOOL_OPTIONS, which the buildpack honors (detects the user flag, skips its own 10 MB, re-budgets heap within mem_limit).

Verification

Applied live on staging (that restored login): OOM/pool timeouts stopped, readiness UP, GET /api/identity-providers returns the provider list. This PR makes it durable. docker compose config validates.

🤖 Generated with Claude Code

[coderabbitai]

📝 Walkthrough

Walkthrough

Docker Compose application services now configure JVM direct memory through APP_MAX_DIRECT_MEMORY, defaulting to 256M. A changeset documents the patch release and operator override.

Changes

JVM Direct Memory Configuration

Layer / File(s)	Summary
Configure service direct memory docker/compose.app.yaml, docker/preview/compose.app.yaml, .changeset/jvm-direct-memory.md	Application server, worker, and preview appserver services set JAVA_TOOL_OPTIONS with a configurable 256M direct-memory default; the changeset documents the release and override variable.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• ls1intum/Hephaestus#1335: Updates the same Docker Compose services with matching JVM direct-memory settings.
• ls1intum/Hephaestus#1410: Modifies Docker Compose JVM memory configuration for application containers.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: increasing JVM direct memory in Docker configs.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/jvm-direct-memory

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