Give modules access to a device's local storage

Applies to: yes icon IoT Edge 1.1 yes icon IoT Edge 1.2

In addition to storing data using Azure storage services or in your device's container storage, you can also dedicate storage on the host IoT Edge device itself for improved reliability, especially when operating offline.

To enable a link from module storage to the storage on the host system, create an environment variable for your module that points to a storage folder in the container. Then, use the create options to bind that storage folder to a folder on the host machine.

For example, if you wanted to enable the IoT Edge hub to store messages in your device's local storage and retrieve them later, you can configure the environment variables and the create options in the Azure portal in the Runtime Settings section.

  1. For both IoT Edge hub and IoT Edge agent, add an environment variable called storageFolder that points to a directory in the module.

  2. For both IoT Edge hub and IoT Edge agent, add binds to connect a local directory on the host machine to a directory in the module. For example:

    Add create options and environment variables for local storage

Or, you can configure the local storage directly in the deployment manifest. For example:

"systemModules": {
    "edgeAgent": {
        "settings": {
            "image": "mcr.microsoft.com/azureiotedge-agent:1.1",
            "createOptions": {
                "HostConfig": {
                    "Binds":["<HostStoragePath>:<ModuleStoragePath>"]
                }
            }
        },
        "type": "docker",
        "env": {
            "storageFolder": {
                "value": "<ModuleStoragePath>"
            }
        }
    },
    "edgeHub": {
        "settings": {
            "image": "mcr.microsoft.com/azureiotedge-hub:1.1",
            "createOptions": {
                "HostConfig": {
                    "Binds":["<HostStoragePath>:<ModuleStoragePath>"],
                    "PortBindings":{"5671/tcp":[{"HostPort":"5671"}],"8883/tcp":[{"HostPort":"8883"}],"443/tcp":[{"HostPort":"443"}]}}}
        },
        "type": "docker",
        "env": {
            "storageFolder": {
                "value": "<ModuleStoragePath>"
            }
        },
        "status": "running",
        "restartPolicy": "always"
    }
}

Replace <HostStoragePath> and <ModuleStoragePath> with your host and module storage path; both values must be an absolute path.

For example, on a Linux system, "Binds":["/etc/iotedge/storage/:/iotedge/storage/"] means the directory /etc/iotedge/storage on your host system is mapped to the directory /iotedge/storage/ in the container. On a Windows system, as another example, "Binds":["C:\\temp:C:\\contemp"] means the directory C:\temp on your host system is mapped to the directory C:\contemp in the container.

Additionally, on Linux devices, make sure that the user profile for your module has the required read, write, and execute permissions to the host system directory. Returning to the earlier example of enabling IoT Edge hub to store messages in your device's local storage, you need to grant permissions to its user profile, UID 1000. (The IoT Edge agent operates as root, so it doesn't need additional permissions.) There are several ways to manage directory permissions on Linux systems, including using chown to change the directory owner and then chmod to change the permissions, such as:

sudo chown 1000 <HostStoragePath>
sudo chmod 700 <HostStoragePath>

You can find more details about create options from docker docs.

Encrypted data in module storage

When modules invoke the IoT Edge daemon's workload API to encrypt data, the encryption key is derived using the module ID and module's generation ID. A generation ID is used to protect secrets if a module is removed from the deployment and then another module with the same module ID is later deployed to the same device. You can view a module's generation id using the Azure CLI command az iot hub module-identity show.

If you want to share files between modules across generations, they must not contain any secrets or they will fail to be decrypted.

Next steps

For an additional example of accessing host storage from a module, see Store data at the edge with Azure Blob Storage on IoT Edge.