Mutagen-based file synchronizationEstimated reading time: 6 minutes
Docker Desktop for Mac on Edge has a new file sharing feature which performs a continuous two-way sync of files between the host and containers using Mutagen. This feature is ideal for app development where:
- the source code tree is quite large
- the source is edited on Mac
- the source is compiled and run interactively inside Linux containers.
This page contains an example to show how the Mutagen feature should be used to sync files between the host and containers. It also gives some best practices to maximize performance.
After completing the synchronizing process, you must delete and re-create any containers which will make use of the synchronized directories.
A simple React app
The following example bootstraps a simple React app with
npx and configures
Docker Desktop to sync the source code between the host and a
First, create a directory which will contain the app:
$ mkdir ~/workspace/my-app
Next, enable the two-way file sync feature in Docker Desktop:
From the Docker Desktop menu, click Preferences > Resources > File sharing.
Type the new directory name in the bottom of the list and press the enter key.
Click on the toggle next to the directory name.
Click Apply & Restart for the changes to take effect.
When Docker Desktop has restarted, the File sharing page looks like this:
Run the following command to start a container and bootstrap the app with
$ docker run -it -v ~/workspace/my-app:/my-app -w /my-app -p 3000:3000 node:lts bash root@95441305251a:/my-app# npx create-react-app app root@95441305251a:/my-app# cd app root@95441305251a:/my-app# npm start
Once the development webserver has started, open https://localhost:3000/ in your browser and observe the app is running.
Return to the File sharing page in the UI and observe the status of the synchronize toggle located next to the directory name. The status will be updated as file changes are detected and then synchronized between the host and the containers.
Wait until the text says Ready and then open the source code in your IDE on
the host. Edit the file
src/App.js, save the changes and observe the change
on the webserver.
As you edit code on the host, the changes are detected and transferred to the container for testing. Changes inside the container (for example, the creation of build artifacts) are detected and transferred back to the host.
Adding synchronized directories through the CLI
As of Docker Desktop Edge 188.8.131.52 the
:delegated flag on a shared volume will
automatically enable synchronization. This is particularly useful if you want to share
the synchronization setting with other developers in your team, through your source code.
Many projects have scripts which modify files in a container and then read them back on
the host, or the other way around. To support these, Docker Desktop automatically flushes
file changes around
docker run calls. Therefore, the following will work as expected:
docker run -v ~/foo:/foo:delegated touch /foo/new-file-in-container stat ~/foo/new-file-in-container
touch ~/foo/new-file-on-host docker run -v ~/foo:/foo:delegated stat /foo/new-file-on-host
Note that changes are not flushed around
docker exec calls, so this will not work as
docker exec existing-container touch /foo/new-file-from-existing-container stat ~/foo/new-file-from-existing-container # file will not be found
Bypassing a two-way sync for a volume
By default, if a directory has two-way sync enabled, then shared volumes will use the
sync. However, if you want to avoid using a two-way file synchronization, use
:consistent: this will bypass both file caching and the two-way sync. This is
backwards compatible with previous Docker versions.
Avoiding synchronizing a subdirectory from a container to the host
Although two-way file sync is suitable for many types of files, sometimes containers can generate lots of data which doesn’t require copying to the host, for example, debug logs.
If your project has a subdirectory that doesn’t need to be continuously copied back to the host, then use a named docker volume to bypass the sync.
First create a volume using:
$ docker volume create donotsyncme donotsyncme
Use the volume for the subdirectory you want to avoid syncing:
$ docker run -it -v ~/workspace/my-app:/my-app -v donotsyncme:/my-app/dontsyncme -w /my-app -p 3000:3000 node:lts bash
Docker Desktop will sync all changes written by the app to
the host, except changes written to
/my-app/dontsyncme which will be written
to the named volume instead.
Avoiding synchronizing a subdirectory in both directions
Although the example in the previous section successfully prevents changes written in
the container from being copied back to the host, changes written on the host will
still be written to the container (although hidden by the
To completely avoid synchronization in both directions, create a
global mutagen config file.
For example, create a file
sync: defaults: ignore: paths: - /path1/to/ignore - /path2/to/ignore - ... - /pathN/to/ignore
To achieve maximum performance when you enable two-way file sync:
Avoid wasting disk space and CPU by minimising the size of the synchronized directories. For example, synchronize a project directory like
~/my-app, but never sync a large directory like
Remember that the files will be copied inside the container and therefore must fit within the
For every volume you want to sync in
docker run -vor in
docker-compose.yml, ensure either the directory itself or a parent/grandparent/... directory is listed in Preferences > Resources > File sharing.
Note in particular that if only a child directory is listed on the File sharing page, then the whole
docker run -vmay bypass the two-way sync and be slower.
Avoid changing the same files from both the host and containers. If changes are detected on both sides, the host takes precedence and the changes in the containers will be discarded.
After completing the caching process, you must delete and re-create any containers which will make use of the synchronized directories.
Your feedback is very important to us. Let us know your feedback by creating an issue in the Docker Desktop for Mac GitHub repository with the Mutagen label.mac, mutagen, volumes, file sync