feat(mongoose): force secondaryPreferred if no secondaries in cluster

- Trigger early load of mongoose before any other import and implicit
  load of mongoose model
- Add a global schema plugin that inspects schema for read preference
  - if `secondary`, check the cluster using mongoose connection
  - if cluster without secondaries, force read pref to be
    `secondaryPreferred`

replacements/src/app/loaders/mongoose.ts

@@ -1,9 +1,18 @@
 import { MongoMemoryServer } from 'mongodb-memory-server-core'
-import mongoose, { Connection } from 'mongoose'
+import mongoose, { Connection, type Schema } from 'mongoose'
 
 import config from '../config/config'
 import { createLoggerWithLabel } from '../config/logger'
 
+const readPrefSecondarySchemas = [] as Schema[]
+
+// Invoke this at module load time
+mongoose.plugin((schema) => {
+  if (schema.get('read') === 'secondary') {
+    readPrefSecondarySchemas.push(schema)
+  }
+})
+
 const logger = createLoggerWithLabel(module)
 
 export default async (): Promise<Connection> => {
@@ -90,27 +99,44 @@ export default async (): Promise<Connection> => {
     })
   })
 
-  // Seed db with initial agency if we have none
+  mongoose.set('debug', (collectionName, method, query, doc) => {
+    logger.info({
+      message: `${collectionName}.${method}`,
+      meta: {
+        action: 'mongoose',
+        query,
+        doc,
+      },
+    })
+  })
+
+  // Inspect cluster topology from client
+  const client = mongoose.connection.getClient()
+  const {
+    description: { servers, type },
+    // eslint-disable-next-line @typescript-eslint/ban-ts-comment
+    // @ts-ignore
+  } = client.topology
+
   if (
-    process.env.INIT_AGENCY_DOMAIN &&
-    process.env.INIT_AGENCY_SHORTNAME &&
-    process.env.INIT_AGENCY_FULLNAME
+    type === 'ReplicaSetWithPrimary' &&
+    !servers
+      .values()
+      .some(({ type }: { type: string }) => type === 'RSSecondary')
   ) {
-    const Agency = mongoose.model('Agency')
-    const agencyCount = await Agency.count()
-    if (agencyCount === 0) {
-      await mongoose.connection.collection(Agency.collection.name).updateOne(
-        { shortName: process.env.INIT_AGENCY_SHORTNAME },
-        {
-          $setOnInsert: {
-            shortName: process.env.INIT_AGENCY_SHORTNAME,
-            fullName: process.env.INIT_AGENCY_FULLNAME,
-            emailDomain: [process.env.INIT_AGENCY_DOMAIN],
-            logo: '/logo192.png',
-          },
-        },
-        { upsert: true },
-      )
+    // There are no secondary nodes in ReplicaSetWithPrimary cluster.
+    // Queries with `secondary` read preferences will fail, so rewrite
+    // those to be `secondaryPreferred`.
+    logger.warn({
+      message:
+        'ReplicaSetWithPrimary cluster has no secondary nodes. ' +
+        'Forcing secondary read preference to secondaryPreferred.',
+      meta: {
+        action: 'schema',
+      },
+    })
+    for (const schema of readPrefSecondarySchemas) {
+      schema.set('read', 'secondaryPreferred')
     }
   }
 

replacements/src/app/server.ts

@@ -0,0 +1,27 @@
+import './loaders/datadog-tracer'
+import './loaders/mongoose'
+
+import config from './config/config'
+import { createLoggerWithLabel } from './config/logger'
+import loadApp from './loaders'
+
+const logger = createLoggerWithLabel(module)
+
+const initServer = async () => {
+  const app = await loadApp()
+
+  // Configure aws-sdk based on environment
+  // sdk is later used to upload images to S3
+  await config.configureAws()
+
+  app.listen(config.port)
+
+  logger.info({
+    message: `[${config.nodeEnv}] Connected to port ${config.port}`,
+    meta: {
+      action: 'initServer',
+    },
+  })
+}
+
+void initServer()