cache: a byte budget of 256 bytes per entry bounds the cached answers, evicting by clock, and a full or failed copy is counted instead of swallowed

2026-10-22 08:54:59 +02:00
parent d951d03b7d
commit e4e405f8e9
11 changed files with 344 additions and 96 deletions
@@ -75,11 +75,11 @@ Within one source, as compiled entries, an exact `x` in `.list` together with `x
 ### 6. DNS cache bounded in bytes

 - `DnsCache` keeps its entry count and adds a budget on live response bytes, tracked separately from `memoryBytes` (which also counts slots and index). `put` evicts by CLOCK until the copy fits. A response larger than the whole budget is not cached and is counted. A replacement keeps the old entry if allocating the new copy fails. A failed copy allocation is counted; today `handler.zig:918` discards it with `catch {}` and `DnsCache.Stats` has no field for it. The answer already obtained is still sent. The budget, the live bytes and both counters appear in cache diagnostics.
- New config key `cache.max_bytes`, through the config field checklist. Default from the Pi's measured live response bytes.
+- The budget is not a config key (owner, 2026-10-21: an operator has no basis for choosing it). It follows `cache.size`: 256 bytes per entry (about 2.4 MiB at the default 10000), about 2.5 times the Pi's measured average of 100 bytes per cached response, and never less than one 65535-byte message, so a small cache still caches any answer a large one would. The live bytes and the budget are reported as `nxdns_cache_response_bytes` and `nxdns_cache_max_bytes`.

 ## Propagation

- Config: `cache.max_bytes`; the changed default of `logging.query_log_buffer_max`. Config field checklist for the new key: model key + round-trip drift guards, validation + reference, settings API view, openapi.yaml, admin types + settings control, contract samples regenerated, docs config reference, CHANGELOG.
+- Config: the changed default of `logging.query_log_buffer_max`. No new key (item 6).
 - Diagnostics: the item-0 counters, `ConcurrencyUnavailable`, oversized cache responses, cache live bytes and budget.
 - Docs: the user docs make no memory claim. The measured RSS, refresh peak and thread count at the Pi's blocklist size go into the measurement record below and the release's CHANGELOG entry.