zlay#

an AT Protocol relay in zig. subscribes to every PDS on the network, verifies commit signatures, and serves the merged event stream to downstream consumers via com.atproto.sync.subscribeRepos.

live instance: zlay.waow.tech — metrics

design#

• direct PDS crawl — the bootstrap relay (bsky.network) is called once at startup for the host list via listHosts, then all data flows directly from each PDS.

• optimistic signature validation — on signing key cache miss, the frame passes through immediately and the DID is queued for background resolution. the first commit from an unknown account is unvalidated; all subsequent commits are verified against the cached key. >99.9% cache hit rate after warmup.

• inline collection index — indexes (DID, collection) pairs directly in the event processing pipeline using RocksDB with two column families: rbc for collection-to-DID lookups and cbr for DID-to-collection cleanup. serves listReposByCollection from the relay process — no sidecar. the index design draws on fig's work on lightrail, which uses adjacent keys from CAR slices to enumerate collections.

• one OS thread per PDS — predictable memory, no garbage collector. ~2,750 threads is fine; most are blocked on websocket reads. thread stacks are set to 2 MB (zig's default is 16 MB).

endpoints#

dependencies#

build#

requires zig 0.15 and a C/C++ toolchain (for RocksDB).

zig build                          # build
zig build test                     # run tests
zig build -Doptimize=ReleaseSafe   # release build

configuration#

see docs/deployment.md for production deployment and docs/backfill.md for collection index backfill.

numbers#