chore(zero-cache): export a view-syncer drain-time estimate (#45)

Author: tmm

packages/zero-cache/src/services/view-syncer/drain-coordinator.ts

@@ -5,7 +5,7 @@ type DrainListener = () => void;
 
 // The target (additional) utilization to impose on the server
 // that receives the drained connections.
-const TARGET_UTILIZATION = 0.6;
+export const TARGET_UTILIZATION = 0.6;
 
 /**
  * There are two types of drains:

packages/zero-cache/src/services/view-syncer/view-syncer.ts

@@ -182,6 +182,11 @@ export interface ViewSyncer {
 
   readonly queryCount: number;
   readonly rowCount: number;
+  /**
+   * The time its queries took to hydrate, which its client group's
+   * reconnect costs the view-syncer it lands on.
+   */
+  readonly hydrationTimeMs: number;
   readonly createdAtMs: number;
   readonly servedVersion: string | null;
   readonly servingLagEligible: boolean;
@@ -1136,6 +1141,10 @@ export class ViewSyncerService implements ViewSyncer, ActivityBasedService {
     return this.#pipelines.initialized() ? this.#pipelines.queries().size : 0;
   }
 
+  get hydrationTimeMs(): number {
+    return this.#totalHydrationTimeMs();
+  }
+
   pipelineHashes(): readonly PipelineHashInfo[] {
     if (!this.#pipelines.initialized()) {
       return [];

packages/zero-cache/src/workers/syncer.ts

@@ -34,7 +34,10 @@ import type {
   SingletonService,
 } from '../services/service.ts';
 import type {ConnectionContextManager} from '../services/view-syncer/connection-context-manager.ts';
-import {DrainCoordinator} from '../services/view-syncer/drain-coordinator.ts';
+import {
+  DrainCoordinator,
+  TARGET_UTILIZATION,
+} from '../services/view-syncer/drain-coordinator.ts';
 import type {ViewSyncer} from '../services/view-syncer/view-syncer.ts';
 import type {ClientGroupStatusMessage, Worker} from '../types/processes.ts';
 import type {Subscription} from '../types/subscription.ts';
@@ -520,6 +523,21 @@ export class Syncer implements SingletonService {
       result.observe(total);
     });
 
+    getOrCreateGauge('sync', 'drain-time', {
+      description:
+        'How long a graceful shutdown would take to release every client ' +
+        'group, pacing their disconnects by their hydration times as drain() ' +
+        'does. A pod drains its workers in parallel, so its drain takes the ' +
+        'longest of theirs.',
+      unit: 's',
+    }).addCallback(result => {
+      let hydrationTimeMs = 0;
+      for (const vs of this.#viewSyncers.getServices()) {
+        hydrationTimeMs += vs.hydrationTimeMs;
+      }
+      result.observe(hydrationTimeMs / TARGET_UTILIZATION / 1000);
+    });
+
     getOrCreateGauge(
       'sync',
       'rows',