Shared volumes

Modal lets you create writeable volumes that can be simultaneously attached to multiple Modal functions. These are helpful for use cases such as:

1. Caching model checkpoints
2. Storing datasets
3. Keeping a shared cache for expensive computations

Basic example

The modal.SharedVolume constructor initializes an empty volume. This can be mounted within a function by providing a mapping between mount paths and SharedVolume objects. For example, to use a SharedVolume to initialize a shared shelve disk cache:

import shelve
import modal

volume = modal.SharedVolume()

@stub.function(shared_volumes={"/root/cache": volume})
def expensive_computation(key: str):
    with shelve.open("/root/cache/shelve") as cache:
        cached_val = cache.get(key)

    if cached_val is not None:
        return cached_val

    # cache miss; populate value
    ...

The above implements basic disk caching, but be aware that shelve does not guarantee correctness in the event of concurrent read/write operations. To protect against concurrent write conflicts, the flufl.lock package is useful. An example of that library’s usage is in the Datasette example.

Persisting volumes

By default, a modal.SharedVolume lives as long as the app it’s defined in, just like any other Modal object. However in many situations you might want to persist the cache between runs of the app. To do this, you can use the persist method on the SharedVolume object. For example, to avoid re-downloading a HuggingFace model checkpoint every time you run your function:

import modal

volume = modal.SharedVolume().persist("model-cache-vol")

stub = modal.Stub()

CACHE_DIR = "/cache"

@stub.function(
    shared_volumes={CACHE_DIR: volume},
    # Set the transformers cache directory to the volume we created above.
    # For details, see https://huggingface.co/transformers/v4.0.1/installation.html#caching-models
    secret=modal.Secret.from_dict({"TRANSFORMERS_CACHE": CACHE_DIR})
)
def run_inference():
    ...

Deleting volumes

To remove a persisted shared volume, deleting all its data, you must stop the volume. This can be done via the volume’s dashboard app page or the CLI.

For example, a volume with the name my-vol that lives in the e-corp workspace could be stopped (i.e. deleted) by going to its dashboard page at https://modal.com/apps/e-corp/my-vol and clicking the trash icon. Alternatively, you can use the volume’s app ID with modal app stop.

(Volumes are currently a specialized app type within Modal, which is why deleting a volume is done by stopping an app.)

Further examples

• The Modal Podcast Transcribe uses a persisted volume to durably store raw audio, metadata, and finished transcriptions.
• News Article Summarizer uses a persisted volume to store pretrained models.