Eclipse Zenoh

The Eclipse Zenoh: Zero Overhead Pub/sub, Store/Query and Compute.

Zenoh (pronounce /zeno/) unifies data in motion, data at rest and computations. It carefully blends traditional pub/sub with geo-distributed storages, queries and computations, while retaining a level of time and space efficiency that is well beyond any of the mainstream stacks.

Check the website zenoh.io and the roadmap for more detailed information.

File system backend

In zenoh a backend is a storage technology (such as DBMS, time-series database, file system...) alowing to store the keys/values publications made via zenoh and return them on queries. See the zenoh documentation for more details.

This backend relies on the host's file system to implement the storages. Its library name (without OS specific prefix and extension) that zenoh will rely on to find it and load it is zenoh_backend_fs.

👉 Install latest release: see below

👉 Build "master" branch: see below

Examples of usage

Prerequisites:

You have a zenoh router (zenohd) installed, and the zenoh_backend_fs library file is available in ~/.zenoh/lib.
Declare the ZENOH_BACKEND_FS_ROOT environment variable to the directory where you want the files to be stored (or exposed from). If you don't declare it, the ~/.zenoh/zenoh_backend_fs directory will be used.

You can setup storages either at zenoh router startup via a configuration file, either at runtime via the zenoh admin space, using for instance the REST API.

Setup via a JSON5 configuration file

Create a zenoh.json5 configuration file containing:

{
  plugins: {
    // configuration of "storage-manager" plugin:
    storage_manager: {
      volumes: {
        // configuration of a "fs" volume (the "zenoh_backend_fs" backend library will be loaded at startup)
        fs: {},
      },
      storages: {
        // configuration of a "demo" storage using the "fs" volume
        demo: {
          // the key expression this storage will subscribes to
          key_expr: "demo/example/**",
          // this prefix will be stripped from the received key when converting to file path
          // this argument is optional.
          strip_prefix: "demo/example",
          volume: {
            id: "fs",
            // the key/values will be stored as files within this directory (relative to ${ZENOH_BACKEND_FS_ROOT})
            dir: "example"
          }
        }
      }
    },
    // Optionally, add the REST plugin
    rest: { http_port: 8000 }
  }
}

Run the zenoh router with:
zenohd -c zenoh.json5

Setup at runtime via `curl` commands on the admin space

Run the zenoh router, with write permissions to its admin space: zenohd --adminspace-permissions rw
Add the "fs" backend (the "zenoh_backend_fs" library will be loaded):
curl -X PUT -H 'content-type:application/json' -d '{}' http://localhost:8000/@/router/local/config/plugins/storage_manager/volumes/fs
Add the "demo" storage using the "fs" backend:
curl -X PUT -H 'content-type:application/json' -d '{key_expr:"demo/example/**",strip_prefix:"demo/example", volume: {id: "fs", dir:"example"}}' http://localhost:8000/@/router/local/config/plugins/storage_manager/storages/demo

Tests using the REST API

Using curl to publish and query keys/values, you can:

# Put values that will be stored under ${ZENOH_BACKEND_FS_ROOT}/example
curl -X PUT -d "TEST-1" http://localhost:8000/demo/example/test-1
curl -X PUT -d "B" http://localhost:8000/demo/example/a/b

# Retrive the values
curl http://localhost:8000/demo/example/**

Configuration

Extra configuration for filesystem-backed volumes

Volumes using the fs backend don't need any extra configuration at the volume level. Any volume can use the fs backend by specifying the value "fs" for the backend configuration key. A volume named fs will automatically be backed by the fs backend if no other backend is specified.

Storage-level configuration for filesystem-backed volumes

Storages relying on a fs backed volume must/can specify additional configuration specific to that volume, as shown in the example above:

dir (required, string) : The directory that will be used to store data.
read_only (optional, boolean) : the storage will only answer to GET queries. It will not accept any PUT or DELETE message, and won't write any file. false by default.
on_closure (optional, string) : the strategy to use when the Storage is removed. There are 2 options:
- "do_nothing": the storage's directory remains untouched (this is the default behaviour)
- "delete_all": the storage's directory is deleted with all its content.
follow_links (optional, boolean) : If set to true the storage will follow the symbolic links. The default value is false.
keep_mime_types (optional, boolean) : When replying to a GET query with a file for which the zenoh encoding is not known, the storage guess its mime-type according to the file extension. If the mime-type doesn't correspond to a supported zenoh encoding, this option will drive the returned value:
- true (default value): a Custom value is returned with the description set to the mime-type.
- false: a Raw value with APP_OCTET_STREAM encoding is returned.

Behaviour of the backend

Mapping to file system

Each storage will map to a directory with path: ${ZENOH_BACKEND_FS_ROOT}/<dir>, where:

${ZENOH_BACKEND_FS_ROOT} is an environment variable that could be specified before zenoh router startup. If this variable is not specified ${ZENOH_HOME}/zenoh_backend_fs will be used (where the default value of ${ZENOH_HOME} is ~/.zenoh).
<dir> is the "dir" property specified at storage creation. Each zenoh key/value put into the storage will map to a file within the storage's directory where:
the file path will be ${ZENOH_BACKEND_FS_ROOT}/<dir>/<relative_zenoh_key>, where <relative_zenoh_key> will be the zenoh key, stripped from the "strip_prefix" property specified at storage creation.
the content of the file will be the value written as a RawValue. I.e. the same bytes buffer that has been transported by zenoh. For UTF-8 compatible formats (StringUTF8, JSon, Integer, Float...) it means the file will be readable as a text format.
the encoding and the timestamp of the key/value will be stored in a RocksDB database stored in the storage directory.

Behaviour on deletion

On deletion of a key, the corresponding file is removed. An entry with deletion timestamp is inserted in the RocksDB database (to avoid re-insertion of points with an older timestamp in case of un-ordered messages).
At regular interval, a task cleans-up the RocksDB database from entries with old timestamps that don't have a corresponding existing file.

Behaviour on GET

On GET operations, the storage searches for matching and existing files, and return their raw content as a reply. For each, the encoding and timestamp are retrieved from the RocksDB database. But if no entry is found in the database for a file (e.g. for files created without zenoh), the encoding is deduced from the file's extension (using mime_guess), and the timestamp is deduced from the file's modification time.

How to install it

To install the latest release of this backend library, you can do as follows:

Manual installation (all platforms)

All release packages can be downloaded from:

https://download.eclipse.org/zenoh/zenoh-backend-filesystem/latest/

Each subdirectory has the name of the Rust target. See the platforms each target corresponds to on https://doc.rust-lang.org/stable/rustc/platform-support.html

Choose your platform and download the .zip file.
Unzip it in the same directory than zenohd or to any directory where it can find the backend library (e.g. /usr/lib or ~/.zenoh/lib)

Linux Debian

Add Eclipse Zenoh private repository to the sources list, and install the zenoh-backend-filesystem package:

echo "deb [trusted=yes] https://download.eclipse.org/zenoh/debian-repo/ /" | sudo tee -a /etc/apt/sources.list.d/zenoh.list > /dev/null
sudo apt update
sudo apt install zenoh-backend-filesystem

How to build it

⚠️ WARNING ⚠️ : Zenoh and its ecosystem are under active development. When you build from git, make sure you also build from git any other Zenoh repository you plan to use (e.g. binding, plugin, backend, etc.). It may happen that some changes in git are not compatible with the most recent packaged Zenoh release (e.g. deb, docker, pip). We put particular effort in mantaining compatibility between the various git repositories in the Zenoh project.

At first, install Clang and Cargo and Rust. If you already have the Rust toolchain installed, make sure it is up-to-date with:

$ rustup update

⚠️ WARNING ⚠️ : As Rust doesn't have a stable ABI, the backend library should be built with the exact same Rust version than zenohd, and using for zenoh dependency the same version (or commit number) than 'zenohd'. Otherwise, incompatibilities in memory mapping of shared types between zenohd and the library can lead to a "SIGSEV" crash.

To know the Rust version you're zenohd has been built with, use the --version option.
Example:

$ zenohd --version
The zenoh router v0.6.0-beta.1 built with rustc 1.64.0 (a55dd71d5 2022-09-19)

Here, zenohd has been built with the rustc version 1.64.0.
Install and use this toolchain with the following command:

$ rustup default 1.64.0

And zenohd version corresponds to an un-released commit with id 1f20c86. Update the zenoh dependency in Cargo.lock with this command:

$ cargo update -p zenoh --precise 1f20c86

Then build the backend with:

$ cargo build --release --all-targets

p-avital/zenoh-backend-filesystem