When getting the format version it matches expected

pom.xml

@@ -23,6 +23,7 @@
         <assertj.version>3.10.0</assertj.version>
         <mon.version>0.4.0</mon.version>
         <mockito.version>2.18.3</mockito.version>
+        <java-semver.version>0.9.0</java-semver.version>
     </properties>
 
     <dependencies>
@@ -36,7 +37,11 @@
             <artifactId>mon</artifactId>
             <version>${mon.version}</version>
         </dependency>
-
+        <dependency>
+            <groupId>com.github.zafarkhaja</groupId>
+            <artifactId>java-semver</artifactId>
+            <version>${java-semver.version}</version>
+        </dependency>
         <dependency>
             <groupId>org.projectlombok</groupId>
             <artifactId>lombok</artifactId>

src/main/java/net/kemitix/gitdb/GitDB.java

@@ -21,6 +21,7 @@
 
 package net.kemitix.gitdb;
 
+import com.github.zafarkhaja.semver.Version;
 import net.kemitix.gitdb.impl.LocalGitDB;
 
 import java.io.IOException;
@@ -34,6 +35,12 @@ import java.util.Optional;
  */
 public interface GitDB {
 
+    int MAJOR = 1;
+    int MINOR = 0;
+    int PATCH = 0;
+
+    Version VERSION = Version.forIntegers(MAJOR, MINOR, PATCH);
+
     /**
      * Initialise a new local gitdb.
      *
@@ -71,4 +78,5 @@ public interface GitDB {
      * @throws IOException if there is an error accessing the branch name
      */
     Optional<GitDBBranch> branch(String name) throws IOException;
+
 }

src/main/java/net/kemitix/gitdb/GitDBBranch.java

@@ -21,6 +21,8 @@
 
 package net.kemitix.gitdb;
 
+import com.github.zafarkhaja.semver.Version;
+
 import java.io.IOException;
 import java.util.Optional;
 
@@ -58,4 +60,14 @@ public interface GitDBBranch {
      * @throws IOException if there was an error removing the key/value
      */
     GitDBBranch remove(String key) throws IOException;
+
+    /**
+     * Returns the GitDB format for the current branch.
+     *
+     * <p>Different branches can have different versions.</p>
+     *
+     * @return the format as per semantic versioning, i.e. "x.y.z" within an Optional
+     * @throws IOException error reading version
+     */
+    Optional<Version> getFormatVersion() throws IOException;
 }

src/main/java/net/kemitix/gitdb/impl/GitDBBranchImpl.java

@@ -21,6 +21,7 @@
 
 package net.kemitix.gitdb.impl;
 
+import com.github.zafarkhaja.semver.Version;
 import lombok.AccessLevel;
 import lombok.RequiredArgsConstructor;
 import net.kemitix.gitdb.GitDBBranch;
@@ -99,6 +100,12 @@ class GitDBBranchImpl implements GitDBBranch {
         return this;
     }
 
+    @Override
+    public Optional<Version> getFormatVersion() throws IOException {
+        return gitDBRepo.readValue(branchRef, "GitDB.Version")
+                .map(Version::valueOf);
+    }
+
     private String commitMessageForAdd(final String key, final String value) {
         return String.format("Add key [%s] = [%s]", key, value);
     }

src/main/java/net/kemitix/gitdb/impl/InitGitDBRepo.java

@@ -21,6 +21,7 @@
 
 package net.kemitix.gitdb.impl;
 
+import net.kemitix.gitdb.GitDB;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.lib.RepositoryCache;
@@ -45,7 +46,7 @@ class InitGitDBRepo {
     private static final String INIT_USER = "GitDB";
     private static final String INIT_EMAIL = "pcampbell@kemitix.net";
     private static final String MASTER = "master";
-    private static final String IS_GIT_DB = "isGitDB";
+    private static final String GIT_DB_VERSION = "GitDB.Version";
     private static final String REFS_HEADS_FORMAT = "refs/heads/%s";
 
     /**
@@ -65,8 +66,9 @@ class InitGitDBRepo {
 
     private void createInitialBranchOnMaster(final Repository repository) throws IOException {
         final GitDBRepo repo = new GitDBRepo(repository);
-        final ObjectId objectId = repo.insertBlob(new byte[0]);
+        final ObjectId objectId =
-        final ObjectId treeId = repo.insertNewTree(IS_GIT_DB, objectId);
+                repo.insertBlob(GitDB.VERSION.toString().getBytes(StandardCharsets.UTF_8));
+        final ObjectId treeId = repo.insertNewTree(GIT_DB_VERSION, objectId);
         final ObjectId commitId = repo.initialCommit(treeId, INIT_MESSAGE, INIT_USER, INIT_EMAIL);
         createBranch(repository, commitId, MASTER);
     }

src/test/java/net/kemitix/gitdb/test/GitDBTest.java

@@ -1,5 +1,6 @@
 package net.kemitix.gitdb.test;
 
+import com.github.zafarkhaja.semver.Version;
 import net.kemitix.gitdb.GitDB;
 import net.kemitix.gitdb.GitDBBranch;
 import net.kemitix.gitdb.InvalidRepositoryException;
@@ -213,6 +214,18 @@ class GitDBTest implements WithAssertions {
         return branchOptional.get();
     }
 
+    // When getting the format version it matches expected
+    @Test
+    void getVersionFormat_thenFormatIsSet() throws IOException {
+        //given
+        final GitDBBranch gitDBBranch = gitDBBranch();
+        //when
+        final Optional<Version> formatVersion = gitDBBranch.getFormatVersion();
+        //then
+        assertThat(formatVersion).contains(GitDB.VERSION);
+        assertThat(formatVersion.get()).isNotSameAs(GitDB.VERSION);
+    }
+
     // When putting a key/value pair then a GitDbBranch is returned
     @Test
     void putValue_thenReturnUpdatedGitDBBranch() throws IOException {