Prevents single-tenant overload from exhausting shared DB connection …

[vasilchev]

…pool. Enforces fair-share admission control with contention-aware bursting.

What Changed

New components:

• TenantThrottle — fair-share gate (hawkbit-core)
• ThrottlingDataSourceDecorator — JDBC interception layer (hawkbit-autoconfigure)
• ConnectionThrottleGate — permit lifecycle tied to connection close
• REST exception mapper → HTTP 429 with Retry-After

Modified:

• DataSource wrapped via BeanPostProcessor when enabled
• System work (no tenant context) exempt
• Nested transactions (REQUIRES_NEW) bypass reentrancy

Configuration Modes

1. No Throttle (current default)

hawkbit.throttle.enabled=false
DataSource untouched. Zero behavior change.

2. Hard Cap (strict limit, fast-reject)

hawkbit.throttle.enabled=true
hawkbit.throttle.limit=5           # each tenant max 5 concurrent connections
hawkbit.throttle.timeout=0         # reject immediately when over limit
hawkbit.throttle.threshold=-1      # no threshold (always enforce limit)
Behavior: Tenant holding 5 connections → 6th request gets HTTP 429 instantly. Other tenants unaffected. Safe on platform threads.

3. Contention-Aware Burst (threshold + backpressure)

hawkbit.throttle.enabled=true
hawkbit.throttle.limit=10          # ceiling per tenant
hawkbit.throttle.timeout=20s       # wait up to 20s before 429
hawkbit.throttle.threshold=8       # fairness kicks in at 8/10 pool utilization
spring.threads.virtual.enabled=true  # REQUIRED for timeout>0