The steward provides workers with the "working", "snapshot", and "content" directories for projects.
Working directories are mutable i.e. files can by created, updated, and deleted within them. The worker service is responsible for these changes in the contents of working directories e.g. via pull and convert jobs. The steward is responsible for ensuring that any changes to a project's working directory are reflected in storage.
Snapshot directories contain a copy of a project's working directory at the time of snapshot. They also include an index.json file that is an executed version of the project's main file, if any. They are read-only ie. immutable. The worker service creates snapshots and may also uses snapshots for session jobs.
Content directories contain content derived from either working or snapshot directories of a project. They act as a cache for HTML, Zip and other files that need to be served for a project e.g. in response to download request. The worker service is responsible for creating those files, if they are not already present.
The steward runs as a process on the same machine as a worker. On a Kubernetes cluster this is achieved by running the steward as a DaemonSet. This allows for caching of content on each machine that has workers and thus reduces latency. For example, multiple sessions for a snapshot can share the same content, rather than it having to be fetched for each session.
The s3fs-fuse package is used to mount a object storage bucket e.g from S3, GCS, or Minio. We chose s3fs-fuse, instead of alternatives such as goofys because of it's built in caching ability.