# Persistent Data Overview

The Overleaf Toolkit needs to store persistent data, such as the files required to compile LaTeX projects, and the contents of the MongoDB database. This is achieved by mounting a few directories from the host machine into the docker containers, and writing the data to those directories.


## Data Directories

The Overleaf container requires a directory in which to store data relating to LaTeX compiles. This directory is set with the `OVERLEAF_DATA_PATH` variable in `config/overleaf.rc`. 

The MongoDB container, if it is enabled, requires a directory in which to store it's database files, and the same is true of the Redis container. These directories can also be configured in `config/overleaf.rc`.


## File Permissions

Because docker runs as `root`, the data directories will end up being owned by the `root` user, even if the toolkit is being used by a non-root user. This is not a problem, but is worth being aware of, if you intend to alter the persistent data from outside of the containers.


## Backups

Documentation for creating a backup on data can be found in the [Developer Wiki](https://github.com/overleaf/overleaf/wiki/Backup-of-Data).


## Volumes

If you're running Overleaf on Windows or macOS, the `mongo` service may fail to restart, with an error:

```
Failed to start up WiredTiger under any compatibility version.
Reason: 1: Operation not permitted
```

To avoid this error, the data needs to be stored in a volume rather than a bind mounted directory (see [the `mongo` image documentation for more details](https://github.com/docker-library/docs/blob/master/mongo/content.md#where-to-store-data)).
To store data inside Docker volumes mounted inside the MongoDB and Redis containers, add the following to `config/docker-compose.override.yml` (create this file if it doesn't exist yet):

```yaml
volumes:
  mongo-data:
  redis-data:

services:
  mongo:
    volumes:
      - mongo-data:/data/db

  redis:
    volumes:
      - redis-data:/data
```