Fix cascading 503 on cold-cache tile loading#210
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Two-part fix for tiles graying out instead of progressively loading: 1. Client-side: Add concurrency pool (6 slots) to retryTileLayer.js. Prevents thundering herd where ~35 tiles blast the server simultaneously, overwhelming both per-IP and global outbound budgets. 2. Server-side: Two-phase per-IP outbound budget in SendTileRequestCoreAsync. Previously, IsRateLimitExceeded incremented the per-IP counter on every request, even those rejected by the global budget. This caused retries to find the counter already past the limit and fail immediately (cascading 503). Now uses WouldExceedRateLimit (peek, no increment) for fast-fail, then RecordRateLimitHit only after global budget is acquired.
Two-part fix for tiles graying out instead of progressively loading: 1. Client-side (retryTileLayer.js): Add concurrency pool (6 slots). Prevents thundering herd where ~35 tiles blast the server simultaneously, overwhelming both per-IP and global outbound budgets. 2. Server-side (TileCacheService/RateLimitHelper): Two-phase per-IP budget. Previously, IsRateLimitExceeded incremented the per-IP counter on every request, even those rejected by the global budget. This caused retries to find the counter already past the limit and fail immediately (cascading 503). Now uses WouldExceedRateLimit (peek without increment) for fast-fail, then RecordRateLimitHit only after global budget is acquired — so only actual upstream fetches count against the per-IP limit. Closes #206
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Summary
retryTileLayer.js— tiles queue and stream in progressively instead of blasting ~35 simultaneous requests that overwhelm server budgetsIsRateLimitExceededincremented on every request (including rejected ones), causing retries to snowball past the limit → cascading 503 → all tiles gray out permanentlyWouldExceedRateLimit(peek),RecordRateLimitHit(increment), andPeekCounttoRateLimitHelperfor the check-then-record patternCloses #206
Test plan
dotnet buildpasses (0 errors)dotnet testpasses (1407 tests)code-reviewersubagent (passed)