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diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/notedb/NotesMigrationState.java b/gerrit-server/src/main/java/com/google/gerrit/server/notedb/NotesMigrationState.java
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+// Copyright (C) 2017 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.notedb;
+
+import com.google.gerrit.server.notedb.NoteDbChangeState.PrimaryStorage;
+import com.google.gerrit.server.notedb.NotesMigration.Snapshot;
+import java.util.Optional;
+import java.util.stream.Stream;
+import org.eclipse.jgit.lib.Config;
+
+/**
+ * Possible high-level states of the NoteDb migration for changes.
+ *
+ * <p>This class describes the series of states required to migrate a site from ReviewDb-only to
+ * NoteDb-only. This process has several steps, and covers only a small subset of the theoretically
+ * possible combinations of {@link NotesMigration} return values.
+ *
+ * <p>These states are ordered: a one-way migration from ReviewDb to NoteDb will pass through states
+ * in the order in which they are defined.
+ */
+public enum NotesMigrationState {
+  REVIEW_DB(false, false, false, PrimaryStorage.REVIEW_DB, false),
+
+  WRITE(false, true, false, PrimaryStorage.REVIEW_DB, false),
+
+  READ_WRITE_NO_SEQUENCE(true, true, false, PrimaryStorage.REVIEW_DB, false),
+
+  READ_WRITE_WITH_SEQUENCE_REVIEW_DB_PRIMARY(true, true, true, PrimaryStorage.REVIEW_DB, false),
+
+  READ_WRITE_WITH_SEQUENCE_NOTE_DB_PRIMARY(true, true, true, PrimaryStorage.NOTE_DB, false),
+
+  NOTE_DB(true, true, true, PrimaryStorage.NOTE_DB, true);
+
+  public static final NotesMigrationState FINAL = NOTE_DB;
+
+  public static Optional<NotesMigrationState> forConfig(Config cfg) {
+    return forSnapshot(Snapshot.create(cfg));
+  }
+
+  public static Optional<NotesMigrationState> forNotesMigration(NotesMigration migration) {
+    return forSnapshot(migration.snapshot());
+  }
+
+  private static Optional<NotesMigrationState> forSnapshot(Snapshot s) {
+    return Stream.of(values()).filter(v -> v.snapshot.equals(s)).findFirst();
+  }
+
+  private final Snapshot snapshot;
+
+  NotesMigrationState(
+      // Arguments match abstract methods in NotesMigration.
+      boolean readChanges,
+      boolean rawWriteChangesSetting,
+      boolean readChangeSequence,
+      PrimaryStorage changePrimaryStorage,
+      boolean disableChangeReviewDb) {
+    this.snapshot =
+        Snapshot.builder()
+            .setReadChanges(readChanges)
+            .setWriteChanges(rawWriteChangesSetting)
+            .setReadChangeSequence(readChangeSequence)
+            .setChangePrimaryStorage(changePrimaryStorage)
+            .setDisableChangeReviewDb(disableChangeReviewDb)
+            .build();
+  }
+
+  public void setConfigValues(Config cfg) {
+    snapshot.setConfigValues(cfg);
+  }
+
+  public String toText() {
+    Config cfg = new Config();
+    setConfigValues(cfg);
+    return cfg.toText();
+  }
+
+  Snapshot snapshot() {
+    return snapshot;
+  }
+}