diff --git a/src/app/handleCommand.ts b/src/app/handleCommand.ts
index d76f44c..ce88d06 100644
--- a/src/app/handleCommand.ts
+++ b/src/app/handleCommand.ts
@@ -9,7 +9,7 @@ import { Future } from '@/lib/Future';
 import { Result, Failure } from '@/lib/Result';
 import { Repositories, Projections, allProjections } from '@/app/projections';
 import { Services } from '@/app/integrations';
-import { WithProjectionStore } from '@/app/projectionStore';
+import { WithProjectionStore, Mode } from '@/app/projectionStore';
 import { WithEventStore } from '@/app/eventStore';
 
 type CommandHandler<Command> = (v: {
@@ -39,7 +39,7 @@ function handleCommand<Command>(
 ): express.Handler {
   return router.route((req) =>
     decodeCommand(decoder, req).chain((command) =>
-      withProjectionStore(onStoreError, (projectionStore) =>
+      withProjectionStore(onStoreError, Mode.ReadOnly, (projectionStore) =>
         withEventStore(onStoreError, (store) =>
           handler({
             command,
diff --git a/src/app/handleProjection.ts b/src/app/handleProjection.ts
index ef7884c..4ac001c 100644
--- a/src/app/handleProjection.ts
+++ b/src/app/handleProjection.ts
@@ -19,6 +19,7 @@ import { Projections, Repositories, allProjections } from '@/app/projections';
 import {
   MongoProjectionStore,
   WithProjectionStore,
+  Mode,
 } from '@/app/projectionStore';
 
 type ProjectionHandler<E> = (v: {
@@ -44,7 +45,7 @@ function handleProjection<E extends Event<any>>(
   return router.route((req) =>
     decodeEvent(decoder, req)
       .chain(({ event, info }) =>
-        withProjectionStore(onProjectionStoreError, (store) =>
+        withProjectionStore(onProjectionStoreError, Mode.ReadWrite, (store) =>
           handler({
             event,
             info,
diff --git a/src/app/handleQuery.ts b/src/app/handleQuery.ts
index 670392c..3e145cb 100644
--- a/src/app/handleQuery.ts
+++ b/src/app/handleQuery.ts
@@ -7,7 +7,7 @@ import * as router from '@/lib/router';
 import { Future } from '@/lib/Future';
 import { Result, Failure } from '@/lib/Result';
 import { Projections, Repositories, allProjections } from '@/app/projections';
-import { WithProjectionStore } from '@/app/projectionStore';
+import { WithProjectionStore, Mode } from '@/app/projectionStore';
 import { internalServerError } from '@/app/responses';
 
 type QueryHandler<Query> = (v: {
@@ -29,7 +29,7 @@ function handleQuery<Query>(
 ): express.Handler {
   return router.route((req) =>
     decodeQuery(decoder, req).chain((query) =>
-      withProjectionStore(onProjectionStoreError, (store) =>
+      withProjectionStore(onProjectionStoreError, Mode.ReadOnly, (store) =>
         handler({
           query,
           projections: allProjections(repositories, store),
diff --git a/src/app/handleReaction.ts b/src/app/handleReaction.ts
index d96049c..9aac529 100644
--- a/src/app/handleReaction.ts
+++ b/src/app/handleReaction.ts
@@ -15,9 +15,9 @@ import { AmbarResponse, ErrorMustRetry } from '@/lib/ambar';
 import { Future } from '@/lib/Future';
 import { Maybe } from '@/lib/Maybe';
 import { decodeEvent } from '@/app/handleProjection';
-
-type Projections = {};
-type Services = {};
+import { Services } from '@/app/integrations';
+import { Repositories, Projections, allProjections } from '@/app/projections';
+import { WithProjectionStore, Mode } from '@/app/projectionStore';
 
 type ReactionController<E extends Event<any>> = {
   decoder: Decoder<Maybe<E>>;
@@ -32,7 +32,7 @@ type ReactionHandler<E> = (v: {
   store: EventStore;
 }) => Future<AmbarResponse, void>;
 
-const onEventStoreError = (err: Error) => new Ambar.ErrorMustRetry(err.message);
+const onStoreError = (err: Error) => new Ambar.ErrorMustRetry(err.message);
 
 type WithStoreGeneric = <E, T>(
   onError: (e: Error) => E,
@@ -47,31 +47,34 @@ const wrapWithEventStore = (
   withEventStore: WithStoreGeneric,
 ): WithStoreConcrete =>
   function (f) {
-    return withEventStore(onEventStoreError, (store) => f(store));
+    return withEventStore(onStoreError, (store) => f(store));
   };
 
 function handleReaction<E extends Event<any>>(
   withEventStore: WithStoreConcrete,
-  projections: Projections,
+  withProjectionStore: WithProjectionStore,
   services: Services,
+  repositories: Repositories,
   { decoder, handler }: ReactionController<E>,
 ): express.Handler {
   return router.route((req) =>
     decodeEvent(decoder, req)
       .chain(({ event, info }) =>
-        withEventStore((store) =>
-          handler({
-            event,
-            info,
-            projections,
-            services,
-            store,
-          }).chainRej((r) =>
-            r instanceof Ambar.Success
-              ? Future.resolve(undefined)
-              : r instanceof Ambar.ErrorMustRetry
-                ? Future.reject(r)
-                : (r satisfies never),
+        withProjectionStore(onStoreError, Mode.ReadOnly, (projectionStore) =>
+          withEventStore((store) =>
+            handler({
+              event,
+              info,
+              projections: allProjections(repositories, projectionStore),
+              services,
+              store,
+            }).chainRej((r) =>
+              r instanceof Ambar.Success
+                ? Future.resolve(undefined)
+                : r instanceof Ambar.ErrorMustRetry
+                  ? Future.reject(r)
+                  : (r satisfies never),
+            ),
           ),
         ),
       )
diff --git a/src/app/integrations.ts b/src/app/integrations.ts
index bf802c2..3ec8eb3 100644
--- a/src/app/integrations.ts
+++ b/src/app/integrations.ts
@@ -6,6 +6,7 @@ import { Mongo } from '@/lib/mongo';
 import {
   MongoProjectionStore,
   WithProjectionStore,
+  Mode,
 } from '@/app/projectionStore';
 import { EmailService } from '@/app/services/email';
 import { FileStorageService } from '@/app/services/file-storage';
@@ -77,11 +78,11 @@ async function configureDependencies(): Promise<Dependencies> {
       f(new PostgresEventStore(t, schemas, table)),
     );
 
-  const withProjectionStore: WithProjectionStore = (onError, f) =>
-    mongo.withTransaction(onError, (t) => f(new MongoProjectionStore(t)));
+  const withProjectionStore: WithProjectionStore = (onError, mode, f) =>
+    mongo.withTransaction(onError, (t) => f(new MongoProjectionStore(t, mode)));
 
   const repositories = await mongo.withTransactionP(async (t) =>
-    initializeRepositories(new MongoProjectionStore(t)),
+    initializeRepositories(new MongoProjectionStore(t, Mode.ReadOnly)),
   );
 
   const services = initializeServices();
diff --git a/src/app/projectionStore.ts b/src/app/projectionStore.ts
index bb53ad7..d5cb36b 100644
--- a/src/app/projectionStore.ts
+++ b/src/app/projectionStore.ts
@@ -5,6 +5,7 @@ export {
   type JsonDoc,
   type RepositoryArgs,
   type WithProjectionStore,
+  Mode,
 };
 
 import { Collection } from 'mongodb';
@@ -53,11 +54,20 @@ const schemaIdAndValue = <T>(schema: Schema<T>): Schema<IdAndDoc<T>> => {
 
 type WithProjectionStore = <E, T>(
   onError: (e: Error) => E,
+  mode: Mode,
   f: (s: MongoProjectionStore) => Future<E, T>,
 ) => Future<E, T>;
 
+enum Mode {
+  ReadOnly = 'ReadOnly',
+  ReadWrite = 'ReadWrite',
+}
+
 class MongoProjectionStore {
-  constructor(private transaction: MongoTransaction) {}
+  constructor(
+    private transaction: MongoTransaction,
+    private readonly readOnly: Mode,
+  ) {}
 
   // Initialize a repository, creating the collection and indexes if needed.
   async createRepository<T>(args: RepositoryArgs<T>): Promise<Repository<T>> {
@@ -122,6 +132,9 @@ class MongoProjectionStore {
     document: T,
     options?: InsertOneOptions,
   ): Promise<void> {
+    if (this.readOnly == Mode.ReadOnly) {
+      throw new Error('Trying to write to read-only projection store');
+    }
     const schema = schemaIdAndValue(repository.values.schema);
     const _id = repository.values.toId(document);
     await this.transaction.insertOne(
@@ -137,6 +150,9 @@ class MongoProjectionStore {
     document: T,
     options: InsertOneOptions = {},
   ): Promise<void> {
+    if (this.readOnly == Mode.ReadOnly) {
+      throw new Error('Trying to write to read-only projection store');
+    }
     const schema = schemaIdAndValue(repository.values.schema);
     const _id = repository.values.toId(document);
     await this.transaction.replaceOne(
diff --git a/src/index.ts b/src/index.ts
index a8362fa..8c0a86f 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -46,6 +46,7 @@ async function main() {
       endpoint,
       handleReaction(
         wrapWithEventStore(withEventStore),
+        withProjectionStore,
         services,
         repositories,
         controller,