chore: Add read/write mode support for persistent store.

Author: kinyoklion

pkgs/sdk/server/src/Internal/DataSystem/FDv2DataSystem.cs

@@ -59,7 +59,8 @@ public static FDv2DataSystem Create(Logger logger, Configuration configuration,
             var persistentStore =
                 dataSystemConfiguration.PersistentStore?.Build(clientContext.WithDataStoreUpdates(dataStoreUpdates));
 
-            var writeThroughStore = new WriteThroughStore(memoryStore, persistentStore);
+            var writeThroughStore = new WriteThroughStore(memoryStore, persistentStore,
+                dataSystemConfiguration.PersistentDataStoreMode);
 
             // TODO: When a persistent store is available we monitor it, is this a consistent choice.
             // TODO: Update the responses data store monitoring?

pkgs/sdk/server/src/Internal/DataSystem/WriteThroughStore.cs

@@ -1,4 +1,3 @@
-using System;
 using LaunchDarkly.Sdk.Internal.Concurrent;
 using LaunchDarkly.Sdk.Server.Subsystems;
 
@@ -26,15 +25,18 @@ internal class WriteThroughStore : IDataStore, ITransactionalDataStore
         /// </para>
         /// </summary>
         private readonly AtomicBoolean _hasReceivedAnInitializingPayload = new AtomicBoolean(false);
+        
+        private readonly DataSystemConfiguration.DataStoreMode _persistenceMode;
 
-        public WriteThroughStore(IDataStore memoryStore, IDataStore persistentStore)
+        public WriteThroughStore(IDataStore memoryStore, IDataStore persistentStore, DataSystemConfiguration.DataStoreMode persistenceMode)
         {
             _memoryStore = memoryStore;
             _txMemoryStore = (ITransactionalDataStore)_memoryStore;
             _persistentStore = persistentStore;
             _hasPersistence = persistentStore != null;
             // During initializations read will happen from the persistent store.
             _activeReadStore = _hasPersistence ? _persistentStore : _memoryStore;
+            _persistenceMode = persistenceMode;
         }
 
         public void Dispose()
@@ -56,7 +58,11 @@ private void Dispose(bool disposing)
         public void Init(DataStoreTypes.FullDataSet<DataStoreTypes.ItemDescriptor> allData)
         {
             _memoryStore.Init(allData);
-            _persistentStore?.Init(allData);
+            if (_persistenceMode == DataSystemConfiguration.DataStoreMode.ReadWrite)
+            {
+                _persistentStore?.Init(allData);
+            }
+
             MaybeSwitchStore();
         }
 
@@ -73,7 +79,7 @@ public void Init(DataStoreTypes.FullDataSet<DataStoreTypes.ItemDescriptor> allDa
         public bool Upsert(DataStoreTypes.DataKind kind, string key, DataStoreTypes.ItemDescriptor item)
         {
             var result = _memoryStore.Upsert(kind, key, item);
-            if (_hasPersistence)
+            if (_hasPersistence && _persistenceMode == DataSystemConfiguration.DataStoreMode.ReadWrite)
             {
                 result &= _persistentStore.Upsert(kind, key, item);
             }
@@ -93,7 +99,7 @@ public void Apply(DataStoreTypes.ChangeSet<DataStoreTypes.ItemDescriptor> change
         {
             _txMemoryStore.Apply(changeSet);
 
-            if (_hasPersistence)
+            if (_hasPersistence && _persistenceMode == DataSystemConfiguration.DataStoreMode.ReadWrite)
             {
                 if (_persistentStore is ITransactionalDataStore txPersistentStore)
                 {