Add search duration metric

Author: lalinsky

src/IndexReader.zig

@@ -39,12 +39,6 @@ pub fn search(self: *Self, hashes: []const u32, allocator: std.mem.Allocator, de
 
     results.sort();
 
-    if (results.count() == 0) {
-        metrics.searchMiss();
-    } else {
-        metrics.searchHit();
-    }
-
     return results;
 }
 

src/metrics.zig

@@ -2,12 +2,19 @@ const std = @import("std");
 const m = @import("metrics");
 
 var metrics = m.initializeNoop(Metrics);
+var arena: ?std.heap.ArenaAllocator = null;
 
 const WithIndex = struct { index: []const u8 };
 
+const SearchDuration = m.Histogram(
+    f64,
+    &.{ 0.005, 0.01, 0.05, 0.1, 0.5, 1, 5, 10 },
+);
+
 const Metrics = struct {
     search_hits: m.Counter(u64),
     search_misses: m.Counter(u64),
+    search_duration: SearchDuration,
     searches: m.Counter(u64),
     updates: m.Counter(u64),
     checkpoints: m.Counter(u64),
@@ -28,6 +35,10 @@ pub fn searchMiss() void {
     metrics.search_misses.incr();
 }
 
+pub fn searchDuration(duration_ms: i64) void {
+    metrics.search_duration.observe(@as(f64, @floatFromInt(duration_ms)) / 1000.0);
+}
+
 pub fn update(count: usize) void {
     metrics.updates.incrBy(@intCast(count));
 }
@@ -49,20 +60,24 @@ pub fn docs(index_name: []const u8, value: u32) void {
 }
 
 pub fn initializeMetrics(allocator: std.mem.Allocator, comptime opts: m.RegistryOpts) !void {
+    arena = std.heap.ArenaAllocator.init(allocator);
+    const alloc = arena.?.allocator();
+
     metrics = .{
         .search_hits = m.Counter(u64).init("search_hits_total", .{}, opts),
         .search_misses = m.Counter(u64).init("search_misses_total", .{}, opts),
+        .search_duration = SearchDuration.init("search_duration_seconds", .{}, opts),
         .searches = m.Counter(u64).init("searches_total", .{}, opts),
         .updates = m.Counter(u64).init("updates_total", .{}, opts),
         .checkpoints = m.Counter(u64).init("checkpoints_total", .{}, opts),
         .memory_segment_merges = m.Counter(u64).init("memory_segment_merges_total", .{}, opts),
         .file_segment_merges = m.Counter(u64).init("file_segment_merges_total", .{}, opts),
-        .docs = try m.GaugeVec(u32, WithIndex).init(allocator, "docs", .{}, opts),
+        .docs = try m.GaugeVec(u32, WithIndex).init(alloc, "docs", .{}, opts),
     };
 }
 
 pub fn deinitMetrics() void {
-    metrics.docs.deinit();
+    arena.?.deinit();
 }
 
 pub fn writeMetrics(writer: anytype) !void {

src/server.zig

@@ -288,6 +288,9 @@ fn getRequestBody(comptime T: type, req: *httpz.Request, res: *httpz.Response) !
 }
 
 fn handleSearch(ctx: *Context, req: *httpz.Request, res: *httpz.Response) !void {
+    const start_time = std.time.milliTimestamp();
+    defer metrics.searchDuration(std.time.milliTimestamp() - start_time);
+
     const body = try getRequestBody(SearchRequestJSON, req, res) orelse return;
 
     const index = try getIndex(ctx, req, res, true) orelse return;
@@ -306,6 +309,12 @@ fn handleSearch(ctx: *Context, req: *httpz.Request, res: *httpz.Response) !void
 
     const results = try index.search(body.query, req.arena, deadline);
 
+    if (results.count() == 0) {
+        metrics.searchMiss();
+    } else {
+        metrics.searchHit();
+    }
+
     var results_json = SearchResultsJSON{
         .results = try req.arena.alloc(SearchResultJSON, results.count()),
     };