diff --git a/java/arcs/core/host/ParticleContext.kt b/java/arcs/core/host/ParticleContext.kt
index 8b0c474f71b..3ae7f57bfb3 100644
--- a/java/arcs/core/host/ParticleContext.kt
+++ b/java/arcs/core/host/ParticleContext.kt
@@ -184,7 +184,7 @@ class ParticleContext(
         // Trigger the StorageProxy sync request for each readable handle. Once
         // the StorageEvent.READY notifications have all, been received, we can
         // call particle.onReady (handled by notify below).
-        awaitingReady.forEach { it.maybeInitiateSync() }
+        awaitingReady.toSet().forEach { it.maybeInitiateSync() }
       }
 
       log.debug { "runParticleAsync finished" }
diff --git a/java/arcs/core/host/Utils.kt b/java/arcs/core/host/Utils.kt
index ee500c3279c..3cff626affc 100644
--- a/java/arcs/core/host/Utils.kt
+++ b/java/arcs/core/host/Utils.kt
@@ -21,6 +21,8 @@ import arcs.core.entity.Reference
 import arcs.core.host.api.HandleHolder
 import arcs.core.host.api.Particle
 import arcs.core.storage.StorageKey
+import arcs.core.storage.StorageKeyProtocol
+import arcs.core.storage.referencemode.ReferenceModeStorageKey
 import arcs.core.type.Tag
 import arcs.flags.BuildFlags
 import kotlin.reflect.KClass
@@ -151,11 +153,18 @@ object NoOpArcHostParticle : Particle {
 
 /**
  * Examines all [Plan.HandleConnection]s in a given [Plan.Partition] and returns true if and only if
- * every connection with a matching key is both a [Tag.CollectionType] and uses a [HandleMode]
- * that cannot read.
+ * every connection with a matching key is both a [Tag.CollectionType], uses a [HandleMode]
+ * that cannot read, and is a database backed (as opposed to volatile or ramdisk) key.
  */
 fun isWriteOnlyStorageKey(partition: Plan.Partition, key: StorageKey): Boolean =
   BuildFlags.WRITE_ONLY_STORAGE_STACK &&
     partition.particles.flatMap { it.handles.values }.filter { it.storageKey == key }.all {
-      !it.mode.canRead && it.type.tag == Tag.Collection
+      !it.mode.canRead && it.type.tag == Tag.Collection && isDatabaseKey(it.storageKey)
     }
+
+/** True iff and key is backed by a database, not a ramdisk or volatile. */
+fun isDatabaseKey(key: StorageKey): Boolean = when (key.protocol) {
+  StorageKeyProtocol.ReferenceMode -> isDatabaseKey((key as ReferenceModeStorageKey).backingKey)
+  StorageKeyProtocol.Database -> true
+  else -> false
+}
diff --git a/java/arcs/core/storage/WriteOnlyStorageProxyImpl.kt b/java/arcs/core/storage/WriteOnlyStorageProxyImpl.kt
index 5f042f0607a..88de09a22ac 100644
--- a/java/arcs/core/storage/WriteOnlyStorageProxyImpl.kt
+++ b/java/arcs/core/storage/WriteOnlyStorageProxyImpl.kt
@@ -45,12 +45,29 @@ class WriteOnlyStorageProxyImpl<Data : CrdtData, Op : CrdtOperation, T> private
   // WriteOnly proxies do not perform sync requests.
   override fun prepareForSync() = Unit
 
-  override fun maybeInitiateSync() = Unit
+  override fun maybeInitiateSync() {
+    notifyCallback?.let { callback ->
+      scheduler.schedule(
+        HandleCallbackTask(callbackId!!, "notify(READY)") {
+          callback(StorageEvent.READY)
+        }
+      )
+    }
+    callbackId = null
+    notifyCallback = null
+  }
+
+  private var callbackId: CallbackIdentifier? = null
+  private var notifyCallback: ((StorageEvent) -> Unit)? = null
 
   // WriteOnly proxies are immediately ready.
   override fun registerForStorageEvents(id: CallbackIdentifier, notify: (StorageEvent) -> Unit) {
     checkNotClosed()
-    notify(StorageEvent.READY)
+    require(callbackId == null && notifyCallback == null) {
+      "You can only registerForStorageEvents once."
+    }
+    callbackId = id
+    notifyCallback = notify
   }
 
   // WriteOnly proxies can't have errors as there are no events.
diff --git a/javatests/arcs/core/host/UtilsTest.kt b/javatests/arcs/core/host/UtilsTest.kt
index acf3734cbc3..dc5dbd481b8 100644
--- a/javatests/arcs/core/host/UtilsTest.kt
+++ b/javatests/arcs/core/host/UtilsTest.kt
@@ -28,6 +28,7 @@ import arcs.core.entity.ForeignReferenceCheckerImpl
 import arcs.core.host.api.HandleHolder
 import arcs.core.storage.api.DriverAndKeyConfigurator
 import arcs.core.storage.driver.RamDisk
+import arcs.core.storage.keys.DatabaseStorageKey
 import arcs.core.storage.keys.RamDiskStorageKey
 import arcs.core.storage.referencemode.ReferenceModeStorageKey
 import arcs.core.storage.testutil.testStorageEndpointManager
@@ -171,9 +172,13 @@ class UtilsTest(private val params: Params) {
     }
   }
 
-  private fun generateHandle(key: String, type: Type) =
+  private fun generateHandle(key: String, type: Type, db: Boolean = true) =
     Plan.Handle(
-      RamDiskStorageKey(key),
+      if (!db) RamDiskStorageKey(key)
+      else ReferenceModeStorageKey(
+        backingKey = DatabaseStorageKey.Persistent("backing$key", "1234a", dbName = "test"),
+        storageKey = DatabaseStorageKey.Persistent("entity$key", "1234a", dbName = "test"),
+      ),
       type,
       emptyList()
     )
@@ -213,6 +218,22 @@ class UtilsTest(private val params: Params) {
     assertThat(isWriteOnlyStorageKey(partition, handle.storageKey)).isTrue()
   }
 
+  @Test
+  fun isWriteOnlyStorageKey_withOneParticleAndAllWriteOnlyRamDiskConnections_isFalse() {
+    BuildFlags.WRITE_ONLY_STORAGE_STACK = true
+
+    val handle = generateHandle("foo", collectionType, db = false)
+    val connection = generateConnection(handle, collectionType, HandleMode.Write)
+    val particle = generateParticle("foo", "bar" to connection)
+
+    val partition = Plan.Partition(
+      "fooId",
+      "fooHost",
+      listOf(particle)
+    )
+    assertThat(isWriteOnlyStorageKey(partition, handle.storageKey)).isFalse()
+  }
+
   @Test
   fun isWriteOnlyStorageKey_withOneParticleAndOneSingletonConnection_isFalse() {
     BuildFlags.WRITE_ONLY_STORAGE_STACK = true
diff --git a/javatests/arcs/core/storage/WriteOnlyStorageProxyImplTest.kt b/javatests/arcs/core/storage/WriteOnlyStorageProxyImplTest.kt
index 2b1d583802e..66ced3d23a8 100644
--- a/javatests/arcs/core/storage/WriteOnlyStorageProxyImplTest.kt
+++ b/javatests/arcs/core/storage/WriteOnlyStorageProxyImplTest.kt
@@ -161,10 +161,12 @@ class WriteOnlyStorageProxyImplTest {
   }
 
   @Test
-  fun storageEventReadyImmediatelyCalled() = runTest {
+  fun storageEventReadyCalledAfterMaybeInitiateSync() = runTest {
     val proxy = mockProxy()
     val callback: () -> Unit = mock()
     proxy.registerForStorageEvents(callbackId) { callback() }
+    proxy.prepareForSync()
+    proxy.maybeInitiateSync()
     verify(callback).invoke()
   }
 
diff --git a/third_party/java/arcs/flags/flags.bzl b/third_party/java/arcs/flags/flags.bzl
index b7f820582ed..00d647aad88 100644
--- a/third_party/java/arcs/flags/flags.bzl
+++ b/third_party/java/arcs/flags/flags.bzl
@@ -85,7 +85,7 @@ ARCS_BUILD_FLAGS = [
         name = "write_only_storage_stack",
         desc = "Optimized write-only storage stack.",
         bug_id = "b/181723292",
-        status = "NOT_READY",
+        status = "LAUNCHED",
         stopwords = [
             "write.?only.?storage.?stack",
             "DatabaseOp",