Determining Whether a Read Was Served by a Leader or a Follower

				
					#!/usr/bin/env bash
set -euo pipefail

usage() {
  echo "Usage: $0 -t <table> -k <pk_value> -H <ysql_host> [-p <ysql_port>] [-d <db>] -n <tserver_web_list> [--delta]"
  echo
  echo "  -t  Table name (required)                         e.g. my_test"
  echo "  -k  Primary key value (required)                  e.g. 1"
  echo "  -H  YSQL host for ysqlsh (required)               e.g. 127.0.0.1"
  echo "  -p  YSQL port (default: 5433)                     e.g. 5433"
  echo "  -d  Database name (default: yugabyte)             e.g. yugabyte"
  echo "  -n  Comma-separated tserver web endpoints         e.g. 127.0.0.1:9000,127.0.0.2:9000,127.0.0.3:9000"
  echo "  --delta  Show delta since last run (state in /tmp/yb_cp_probe.<table>.<pk>.state)"
  exit 1
}

TABLE=""
PK=""
YSQL_HOST=""
YSQL_PORT="5433"
DB="yugabyte"
TSERVERS=""
SHOW_DELTA="false"

while [[ $# -gt 0 ]]; do
  case "$1" in
    -t) TABLE="$2"; shift 2;;
    -k) PK="$2"; shift 2;;
    -H) YSQL_HOST="$2"; shift 2;;
    -p) YSQL_PORT="$2"; shift 2;;
    -d) DB="$2"; shift 2;;
    -n) TSERVERS="$2"; shift 2;;
    --delta) SHOW_DELTA="true"; shift 1;;
    *) usage;;
  esac
done

[[ -z "$TABLE" || -z "$PK" || -z "$YSQL_HOST" || -z "$TSERVERS" ]] && usage

IFS=',' read -r -a TS <<< "$TSERVERS"

ysql() { ysqlsh -h "$YSQL_HOST" -p "$YSQL_PORT" -d "$DB" -Atc "$1"; }

metric_table_reads() {
  local ep="$1"
  curl -s "http://${ep}/prometheus-metrics" \
    | awk -v tbl="$TABLE" '$1 ~ /^consistent_prefix_read_requests/ && $0 ~ ("table_name=\"" tbl "\"") {sum += $2} END {printf "%.0f\n", sum+0}'
}

# Determine leader for the given key (assumes HASH partitioning and numeric PK)
LEADER="$(ysql "SELECT leader FROM yb_tablet_metadata t WHERE t.db_name=current_database() AND t.relname='${TABLE}' AND yb_hash_code(${PK}) >= t.start_hash_code AND yb_hash_code(${PK}) < t.end_hash_code LIMIT 1;")"
TABLET_ID="$(ysql "SELECT tablet_id FROM yb_tablet_metadata t WHERE t.db_name=current_database() AND t.relname='${TABLE}' AND yb_hash_code(${PK}) >= t.start_hash_code AND yb_hash_code(${PK}) < t.end_hash_code LIMIT 1;")"
HASH_CODE="$(ysql "SELECT yb_hash_code(${PK});")"

if [[ -z "$LEADER" || -z "$TABLET_ID" ]]; then
  echo "ERROR: Could not determine leader/tablet for TABLE=${TABLE} PK=${PK}. Check table name, DB, and that PK is numeric."
  exit 2
fi

LEADER_HOST="${LEADER%%:*}"

STATE_FILE="/tmp/yb_cp_probe.${TABLE}.${PK}.state"
declare -A PREV
if [[ "$SHOW_DELTA" == "true" && -f "$STATE_FILE" ]]; then
  while IFS=$'\t' read -r ep val; do
    PREV["$ep"]="$val"
  done < "$STATE_FILE"
fi

echo "================================================================================"
echo "NOTE (the missing piece): consistent_prefix_read_requests increments ONLY for"
echo "CONSISTENT PREFIX reads (the follower-read / bounded-staleness path)."
echo
echo "A normal strongly-consistent YSQL SELECT (default behavior) will NOT change this"
echo "counter, even if the read is served by the tablet LEADER."
echo
echo "Workflow:"
echo "  1) Run this script (baseline)"
echo "  2) Run a follower read (READ ONLY + yb_read_from_followers=true)"
echo "  3) Run this script again to see which node(s) served the consistent-prefix read"
echo "================================================================================"
echo

echo "== Key placement =="
echo "DB=${DB} TABLE=${TABLE} PK=${PK} hash_code=${HASH_CODE} tablet_id=${TABLET_ID} leader=${LEADER}"
echo

printf "%-18s %-10s %-12s" "tserver_web" "role" "cp_reads"
if [[ "$SHOW_DELTA" == "true" ]]; then printf " %-12s" "delta"; fi
printf "\n"
echo "---------------------------------------------------------------"

: > "$STATE_FILE.tmp"
for ep in "${TS[@]}"; do
  host="${ep%%:*}"
  role="FOLLOWER"
  [[ "$host" == "$LEADER_HOST" ]] && role="LEADER"

  cur="$(metric_table_reads "$ep")"
  printf "%-18s %-10s %-12s" "$ep" "$role" "$cur"

  if [[ "$SHOW_DELTA" == "true" ]]; then
    prev="${PREV[$ep]:-0}"
    delta=$(( cur - prev ))
    printf " %-12s" "+${delta}"
  fi
  printf "\n"

  printf "%s\t%s\n" "$ep" "$cur" >> "$STATE_FILE.tmp"
done

mv "$STATE_FILE.tmp" "$STATE_FILE" >/dev/null 2>&1 || true

echo
echo "Tip: after you run follower-read SELECTs, re-run with --delta to see increments."