Module discovery 

DNS-based dynamic upstream discovery (DW-042, feature analysis 4.4).

When an upstream configures dns_discovery, a background DiscoveryTasks resolves the hostname via DNS (A or SRV records), updates the upstream’s endpoint set in the UpstreamLb live, and re-resolves when the record TTL expires (or at refresh_interval_s, whichever comes first). Endpoint scale-up/down is reflected without a restart — new connections use the updated set; existing connections stay alive until they close.

Resolution

DnsResolver wraps a hickory_resolver::TokioAsyncResolver. A records are resolved to (IpAddr, ttl) pairs; SRV records to (IpAddr, port, ttl) triples (the SRV target is resolved to an IP by the resolver’s recursive lookup). The resolver is configured with the SYSTEM resolver’s name servers when /etc/resolv.conf is readable (so container-internal names resolve), and the public Google resolvers otherwise.

Refresh cycle

Each cycle: resolve, update the endpoint set, sleep min(refresh_interval_s, record_ttl), repeat. On DNS failure: fail_open: true keeps the last resolved set; fail_open: false clears endpoints (the upstream answers 503 until DNS recovers). The min_endpoints floor prevents shrinking below a configured minimum — if a resolution yields fewer endpoints, the previous set is kept.

Task lifecycle

DiscoveryTasks owns one [tokio::task::JoinSet] for every discovery task. The operator (dwara-bin) calls DiscoveryTasks::respawn after every snapshot swap (startup and each reload): all previous tasks are aborted and new ones spawned for the new generation’s upstreams. Dropping DiscoveryTasks (or calling DiscoveryTasks::abort_all) aborts every task — the graceful-shutdown path. This mirrors crate::dataplane::active::ActiveProbes (DW-013), the closest precedent for a long-running dataplane task.

Consul watch and Kubernetes EndpointSlice watch are DEFERRED to a future milestone — DNS is the first discovery source.

Structs

DiscoveryTasks

Owns every DNS discovery task for the running generation. Call DiscoveryTasks::respawn on startup and after every snapshot swap; dropping it (or shutdown) aborts all tasks. Mirrors crate::dataplane::active::ActiveProbes (DW-013), the closest precedent for a long-running dataplane task.

DnsCache

A TTL-cached async DNS resolver for the pooled dial path (PERF-05, #170).

DnsResolver

A TTL-aware DNS resolver wrapping hickory_resolver::TokioResolver.