relay: default BUZZ_WRITE_BATCH_MAX to 0 (batching opt-in)

Mari's repaired-T1a-base gate at bdbf2f0c1 (release binary, alternated
0/16 restarts, matched accepted counts): batching cuts DB commits/msg
~25% but regresses p50 43-58% and p99 52-74% at 500-1000 QPS. The single
global batch lane serializes all channels behind one transaction at a
time, which outweighs commit amortization on the repaired base. Ship the
machinery opt-in; flip the default only if the lane-serialization cost
is fixed and remeasured.

Co-authored-by: Dawn (sprout agent) <c6237ef84fa537c78dcee78efd2d4e59f728859c7f194da42ac51ededfa0be05@sprout-oss.stage.blox.sqprod.co>
Co-authored-by: tlongwell-block <109685178+tlongwell-block@users.noreply.github.com>
Signed-off-by: tlongwell-block <109685178+tlongwell-block@users.noreply.github.com>

crates/buzz-db/src/batch.rs

@@ -55,7 +55,9 @@ use crate::error::{DbError, Result};
 use crate::event::ThreadMetadataParams;
 use crate::Db;
 
-/// Default maximum events coalesced into one transaction.
+/// Suggested maximum events coalesced into one transaction when batching is
+/// enabled. Not the relay default — `BUZZ_WRITE_BATCH_MAX` defaults to `0`
+/// (batching off); see `Config::from_env` in `buzz-relay`.
 pub const DEFAULT_MAX_BATCH: usize = 16;
 
 /// Queue depth for pending insert requests (matches the relay's default

crates/buzz-relay/src/config.rs

@@ -68,7 +68,10 @@ pub struct Config {
     pub max_concurrent_handlers: usize,
     /// Maximum plain event inserts coalesced into one group-commit
     /// transaction (`BUZZ_WRITE_BATCH_MAX`). `0` disables batching and every
-    /// insert commits individually (the pre-batching behavior).
+    /// insert commits individually (the pre-batching behavior). Defaults to
+    /// `0`: benchmarks on the repaired-T1a base showed the single batch lane
+    /// cuts DB commits/msg ~25% but regresses p50/p99 latency at 500-1000 QPS,
+    /// so batching is opt-in until that trade-off is resolved.
     pub write_batch_max: usize,
     /// Per-connection outbound message buffer size (number of messages).
     pub send_buffer_size: usize,
@@ -433,10 +436,13 @@ impl Config {
             .and_then(|v| v.parse().ok())
             .unwrap_or(1024);
 
+        // Default off (`0`): group-commit batching measurably regressed
+        // p50/p99 latency at 500-1000 QPS on the repaired-T1a base despite
+        // saving ~25% of DB commits. Opt in explicitly per deployment.
         let write_batch_max = std::env::var("BUZZ_WRITE_BATCH_MAX")
             .ok()
             .and_then(|v| v.parse().ok())
-            .unwrap_or(buzz_db::batch::DEFAULT_MAX_BATCH);
+            .unwrap_or(0);
 
         let send_buffer_size = std::env::var("BUZZ_SEND_BUFFER")
             .ok()