@@ -584,6 +584,14 @@ double-quotes and respecting backslash escapes. E.g., the value
 	repositories will be set to this value. The default is "files".
 	See `--ref-format` in linkgit:git-init[1].
 
+`GIT_REF_URI`::
+    Specify which reference backend to be used along with its URI. Reference
+    backends like the files, reftable backend use the $GIT_DIR as their URI.
++
+Expects the format `<ref_backend>://<URI-for-resource>`, where the
+_<ref_backend>_ specifies the reference backend and the _<URI-for-resource>_
+specifies the URI used by the backend.
+
 Git Commits
 ~~~~~~~~~~~
 `GIT_AUTHOR_NAME`::

@@ -42,6 +42,7 @@
 #define GIT_OPTIONAL_LOCKS_ENVIRONMENT "GIT_OPTIONAL_LOCKS"
 #define GIT_TEXT_DOMAIN_DIR_ENVIRONMENT "GIT_TEXTDOMAINDIR"
 #define GIT_ATTR_SOURCE_ENVIRONMENT "GIT_ATTR_SOURCE"
+#define GIT_REF_URI_ENVIRONMENT "GIT_REF_URI"
 
 /*
  * Environment variable used to propagate the --no-advice global option to the

@@ -2186,15 +2186,73 @@ static struct ref_store *get_ref_store_for_dir(struct repository *r,
 	return maybe_debug_wrap_ref_store(dir, ref_store);
 }
 
+static struct ref_store *get_ref_store_from_uri(struct repository *repo,
+						const char *uri)
+{
+	struct string_list ref_backend_info = STRING_LIST_INIT_DUP;
+	enum ref_storage_format format;
+	struct ref_store *store = NULL;
+	char *format_string;
+	char *dir;
+
+	if (!uri || !uri[0]) {
+		error("reference backend uri is empty");
+		goto cleanup;
+	}
+
+	if (string_list_split(&ref_backend_info, uri, ":", 2) != 2) {
+		error("invalid reference backend uri format '%s'", uri);
+		goto cleanup;
+	}
+
+	format_string = ref_backend_info.items[0].string;
+	if (!starts_with(ref_backend_info.items[1].string, "//")) {
+		error("invalid reference backend uri format '%s'", uri);
+		goto cleanup;
+	}
+	dir = ref_backend_info.items[1].string + 2;
+
+	format_string = ref_backend_info.items[0].string;
+	dir = ref_backend_info.items[1].string + 2;
+
+	if (!dir || !dir[0]) {
+		error("invalid path in uri '%s'", uri);
+		goto cleanup;
+	}
+
+	format = ref_storage_format_by_name(format_string);
+	if (format == REF_STORAGE_FORMAT_UNKNOWN) {
+		error("unknown reference backend '%s'", format_string);
+		goto cleanup;
+	}
+
+	store = get_ref_store_for_dir(repo, dir, format);
+
+cleanup:
+	string_list_clear(&ref_backend_info, 0);
+	return store;
+}
+
 struct ref_store *get_main_ref_store(struct repository *r)
 {
+	char *ref_uri;
+
 	if (r->refs_private)
 		return r->refs_private;
 
 	if (!r->gitdir)
 		BUG("attempting to get main_ref_store outside of repository");
 
-	r->refs_private = get_ref_store_for_dir(r, r->gitdir, r->ref_storage_format);
+	ref_uri = getenv(GIT_REF_URI_ENVIRONMENT);
+	if (ref_uri) {
+		r->refs_private = get_ref_store_from_uri(r, ref_uri);
+		if (!r->refs_private)
+			die("failed to initialize ref store from URI: %s", ref_uri);
+
+	} else {
+		r->refs_private = get_ref_store_for_dir(r, r->gitdir,
+							r->ref_storage_format);
+	}
 	return r->refs_private;
 }
 

@@ -208,6 +208,7 @@ integration_tests = [
   't1420-lost-found.sh',
   't1421-reflog-write.sh',
   't1422-show-ref-exists.sh',
+  't1423-ref-backend.sh',
   't1430-bad-ref-name.sh',
   't1450-fsck.sh',
   't1451-fsck-buffer.sh',

@@ -0,0 +1,121 @@
+#!/bin/sh
+
+test_description='Test different reference backend URIs'
+
+. ./test-lib.sh
+
+test_expect_success 'empty uri provided' '
+	test_when_finished "rm -rf repo" &&
+	git init --ref-format=files repo &&
+	(
+		cd repo &&
+		GIT_REF_URI="" &&
+		export GIT_REF_URI &&
+		test_must_fail git refs list 2>err &&
+		test_grep "reference backend uri is empty" err
+	)
+'
+
+test_expect_success 'invalid uri provided' '
+	test_when_finished "rm -rf repo" &&
+	git init --ref-format=files repo &&
+	(
+		cd repo &&
+		GIT_REF_URI="reftable@/home/reftable" &&
+		export GIT_REF_URI &&
+		test_must_fail git refs list 2>err &&
+		test_grep "invalid reference backend uri format" err
+	)
+'
+
+test_expect_success 'empty path in uri' '
+	test_when_finished "rm -rf repo" &&
+	git init --ref-format=files repo &&
+	(
+		cd repo &&
+		GIT_REF_URI="reftable://" &&
+		export GIT_REF_URI &&
+		test_must_fail git refs list 2>err &&
+		test_grep "invalid path in uri" err
+	)
+'
+
+test_expect_success 'uri ends at colon' '
+	test_when_finished "rm -rf repo" &&
+	git init --ref-format=files repo &&
+	(
+		cd repo &&
+		GIT_REF_URI="reftable:" &&
+		export GIT_REF_URI &&
+		test_must_fail git refs list 2>err &&
+		test_grep "invalid reference backend uri format" err
+	)
+'
+
+test_expect_success 'unknown reference backend' '
+	test_when_finished "rm -rf repo" &&
+	git init --ref-format=files repo &&
+	(
+		cd repo &&
+		GIT_REF_URI="db://.git" &&
+		export GIT_REF_URI &&
+		test_must_fail git refs list 2>err &&
+		test_grep "unknown reference backend" err
+	)
+'
+
+ref_formats="files reftable"
+for from_format in $ref_formats
+do
+	for to_format in $ref_formats
+	do
+		if test "$from_format" = "$to_format"
+		then
+			continue
+		fi
+
+		test_expect_success "read from $to_format backend" '
+			test_when_finished "rm -rf repo" &&
+			git init --ref-format=$from_format repo &&
+			(
+				cd repo &&
+				test_commit 1 &&
+				test_commit 2 &&
+				test_commit 3 &&
+
+				git refs migrate --dry-run --ref-format=$to_format >out &&
+				BACKEND_PATH=$(cat out | sed "s/.* ${SQ}\(.*\)${SQ}/\1/") &&
+				git refs list >expect &&
+				GIT_REF_URI="$to_format://$BACKEND_PATH" git refs list >actual &&
+				test_cmp expect actual
+			)
+		'
+
+		test_expect_success "write to $to_format backend" '
+			test_when_finished "rm -rf repo" &&
+			git init --ref-format=$from_format repo &&
+			(
+				cd repo &&
+				test_commit 1 &&
+				test_commit 2 &&
+				test_commit 3 &&
+
+				git refs migrate --dry-run --ref-format=$to_format >out &&
+				git refs list >expect &&
+
+				BACKEND_PATH=$(cat out | sed "s/.* ${SQ}\(.*\)${SQ}/\1/") &&
+				GIT_REF_URI="$to_format://$BACKEND_PATH" git tag -d 1 &&
+
+				git refs list >actual &&
+				test_cmp expect actual &&
+
+				GIT_REF_URI="$to_format://$BACKEND_PATH" git refs list >expect &&
+				git refs list >out &&
+				cat out | grep -v "refs/tags/1" >actual &&
+				test_cmp expect actual
+			)
+		'
+	done
+done
+
+test_done