Isolation Sessions

Isolation sessions run a long-lived bash inside a bubblewrap namespace, so one sandbox pod can host many mutually isolated task runs without spinning up a new container. Each session gets its own PID, mount, tmpfs, and env namespaces; startup is sub-millisecond.

Requirements
Overview
How It Works
Quick Start
Session Lifecycle
Workspace Modes
Bind Mounts and Allowlist
Profiles and Defaults
Environment, UID, and Networking
Filesystem Proxy
Capabilities and Probing
Server Configuration
Limitations
See Also

Requirements

Component versions needed for the features covered by this guide:

execd >= 1.0.20 for base isolation session support; >= 1.0.21 recommended for binds, List sessions, uid_mode: "userns", and the default writable allowlist (/workspace, /mnt, /media, /data)
opensandbox-server >= 0.2.1 — the server injects CAP_SYS_ADMIN, apparmor=unconfined, and the tmpfs mount required by bwrap when the execd image declares bootstrap.execd.isolation
Python SDK >= 0.1.14 (isolation.run_once / isolation.session context manager); >= 0.1.13 for the generated isolation client only
JavaScript / TypeScript SDK >= 0.1.10 (isolation.runOnce / isolation.withSession)
Kotlin / Java SDK >= 1.0.16 (isolation.runOnce() / isolation.withSession { ... }); >= 1.0.15 for the generated isolation client only
C# SDK >= 0.1.4 (RunOnceAsync / WithSessionAsync)
Go SDK >= 1.0.4 (IsolationRunOnce / IsolationWithSession)

The setpriv_available / userns_available fields on /capabilities were added after execd v1.0.21 and are only present on execd builds from main at the time of writing; older execd omits both fields (clients must tolerate their absence).

Host requirements (bwrap binary, CAP_SYS_ADMIN, overlayfs, etc.) are listed under Server Configuration → Host Requirements.

The published execd image includes the fail-closed native workload gate used during session startup. For a Linux source build, make build requires a C compiler plus static libc and produces bin/opensandbox-session-gate; run make build-session-gate and then sudo make install-session-gate before starting execd to install the helper at /opt/opensandbox/opensandbox-session-gate. Keep that path and its parent directory root-owned and not group- or world-writable.

Overview

Concept	Boundary	Startup	Typical Use
Isolation session	bubblewrap namespaces inside one sandbox	~100 ms to create; subsequent `run` in an existing session is near-zero overhead	Many short, mutually isolated task runs reusing one long-lived session
Bash session (`/session`)	bash process, no extra namespaces	ms	Interactive REPL-style command sequences
Sandbox	container or pod	100s of ms to seconds	Tenant, workspace, or user boundary
Secure runtime (gVisor / Kata)	user-space kernel or VM per sandbox	10–500 ms	Hardware-level protection against container escape

Session creation vs. per-run cost

Creating a session takes about 100 ms because execd waits briefly after starting bwrap to detect an immediately-exiting child. Once the session exists, each POST /run reuses the same bash process, so per-run overhead is negligible. Design workloads that amortize the create cost across many run calls in one session.

Good fits: RL rollouts, batch code grading, multi-tool agent runs — one sandbox per worker, many isolated tasks inside.

Not a fit: cross-language kernels (use /code), interactive REPLs (use /session), or hard trust boundaries against kernel exploits (use gVisor / Kata; see Secure Container Runtime).

How It Works

execd forks one bwrap child per session; bwrap sets up Linux namespaces, then execs a long-lived bash inside them.

Isolation session runtime layout

Two things the diagram doesn't show:

bash is long-lived, so export X=1 in one run is visible to the next run in the same session (never in another session).
The FS proxy reads and writes the merged workspace view from outside the namespace, so uploads and downloads work while bash is busy.

`run` request flow

Isolation session run flow

Non-happy paths:

Event	What happens
Run `timeout_seconds` elapsed	execd cancels the run context, which sends `SIGINT` to the bwrap process group and emits an `IsolatedError` SSE event; the session itself stays alive.
`bwrap` process exited	The next `run` returns an `IsolatedError` SSE event with `session process has exited`; only `ErrContextNotFound` (session ID unknown) becomes an HTTP-level error. `DELETE` + recreate.
Idle timeout reached	GC runs the same teardown as `DELETE`.
Client disconnects mid-SSE	The Gin request context is cancelled and execd sends `SIGINT` to the running command; the run does not continue in the background.

Quick Start

curl

bash

# Probe.
curl -s http://localhost:44772/v1/isolated/capabilities

# Create.
SESSION=$(curl -s -X POST http://localhost:44772/v1/isolated/session \
  -H "Content-Type: application/json" \
  -d '{
    "profile": "strict",
    "workspace": {"path": "/workspace", "mode": "overlay"},
    "idle_timeout_seconds": 300
  }' | jq -r .session_id)

# Run (SSE: stdout / error / complete).
curl -N -X POST "http://localhost:44772/v1/isolated/session/$SESSION/run" \
  -H "Content-Type: application/json" \
  -d '{"code": "export X=1; echo $X", "timeout_seconds": 30}'

# Second run reuses shell state.
curl -N -X POST "http://localhost:44772/v1/isolated/session/$SESSION/run" \
  -H "Content-Type: application/json" \
  -d '{"code": "echo $X"}'  # prints 1

# Delete.
curl -X DELETE "http://localhost:44772/v1/isolated/session/$SESSION"

Python SDK

python

from opensandbox import Sandbox
from opensandbox.models.isolated import (
    CreateIsolatedSessionRequest, IsolatedWorkspaceSpec, IsolatedRunOpts,
)

# Sandbox.create is an async classmethod, so await it before entering
# the async context manager.
async with (await Sandbox.create("python:3.11")) as sandbox:
    # One-shot.
    await sandbox.isolation.run_once(
        "python -c 'print(42)'", workspace="/workspace", profile="strict",
    )

    # Persistent session.
    async with sandbox.isolation.session(
        CreateIsolatedSessionRequest(
            workspace=IsolatedWorkspaceSpec(path="/workspace", mode="overlay"),
            profile="strict",
            idle_timeout_seconds=300,
        )
    ) as session:
        await session.run("export STAGE=train")
        await session.run("python train.py", opts=IsolatedRunOpts(timeout_seconds=600))
        await session.files.write_file("/workspace/data.csv", data_bytes)

    # Reattach after a client restart.
    handle = await sandbox.isolation.attach(known_session_id)

JavaScript / TypeScript SDK

await sandbox.isolation.runOnce("node -e 'console.log(1)'", "/workspace", {
  profile: "strict",
});

await sandbox.isolation.withSession(
  { profile: "strict", workspace: { path: "/workspace", mode: "overlay" } },
  async (session) => { await session.run("npm test"); },
);

Kotlin SDK

kotlin

sandbox.isolation().runOnce(code = "python -c 'print(1)'", workspace = "/workspace")

sandbox.isolation().withSession(request) { session ->
    session.run("ls /workspace")
}

Session Lifecycle

Under /v1/isolated/ on execd. Use X-EXECD-ACCESS-TOKEN when execd has an access token configured.

Method	Path	Purpose
`POST`	`/session`	Create; returns `{session_id, created_at}`.
`GET`	`/sessions`	List active sessions.
`GET`	`/session/{id}`	Full state; echoes creation params so a stateless client can rebuild the handle.
`POST`	`/session/{id}/run`	SSE stream: `stdout` / `error` / `complete`. Runs on the same session are serialized.
`DELETE`	`/session/{id}`	Destroy.
`GET`	`/capabilities`	Probe.

idle_timeout_seconds > 0 destroys idle sessions automatically; set to 0 to disable idle GC and always DELETE explicitly.

Workspace Modes

Mode	Semantics
`rw`	Bind-mount read-write; writes persist on the host.
`overlay` (default)	Copy-on-write via overlayfs; writes go to a per-session upper dir and vanish on `DELETE`.
`ro`	Bind-mount read-only; writes fail with `EROFS`.

Overlay upper dirs live under upper_root (default /var/lib/execd/isolation).

json

{ "workspace": { "path": "/workspace", "mode": "overlay" } }

Bind Mounts and Allowlist

extra_writable — paths bind-mounted read-write at the same path (source == destination).
binds — explicit source → dest mappings, optionally readonly. source must already exist; dest must already exist inside the namespace (bake it into the sandbox image).

json

{
  "workspace": { "path": "/workspace", "mode": "rw" },
  "extra_writable": ["/data/scratch"],
  "binds": [
    { "source": "/data/in",  "dest": "/mnt/in",  "readonly": true },
    { "source": "/data/out", "dest": "/mnt/out" }
  ]
}

Every source is checked against the operator-configured allowed_writable allowlist after symlink resolution, so symlinks cannot escape it. Default allowlist: /workspace, /mnt, /media, /data (subpaths allowed). Empty allowlist rejects all extra binds.

Profiles and Defaults

The profile field currently controls only how /tmp is exposed inside the namespace:

Profile	`/tmp`
`strict` (default)	Private tmpfs (`--tmpfs /tmp`)
`balanced`	Shared with the sandbox (`--bind /tmp /tmp`)

workspace.mode and env_passthrough are independent of the profile: when they are omitted, execd normalizes workspace.mode to overlay and env_passthrough.mode to deny regardless of which profile you pick. Set those fields explicitly if you want persistent workspace writes ("rw") or host env passthrough ("allow").

Environment, UID, and Networking

env_passthrough — mode: "allow" + keys whitelists host env vars; default deny. Per-run overrides go in IsolatedRunRequest.envs.
uid / gid with uid_mode: "setpriv" (default, real setuid/setgid drop) or "userns" (user namespace remap). Check setpriv_available / userns_available from /capabilities before requesting a mode.
share_net: true shares the sandbox's network namespace. Sandbox-level egress and Credential Vault policies still apply.

Filesystem Proxy

Reads and writes the session's merged workspace view from outside the namespace — this is how SDKs upload, download, and list without spawning a shell. All paths are per session under /v1/isolated/session/{id}/:

Method	Path
`GET`	`files/info?path=...`
`GET`	`files/download?path=...` (supports `Range` and `offset`/`limit`)
`POST`	`files/upload` (multipart: `metadata` + `file`)
`DELETE`	`files?path=...`
`POST`	`files/mv`
`POST`	`files/permissions`
`POST`	`files/replace?verbose=true`
`GET`	`files/search?path=...&pattern=...`
`GET`	`directories/list?path=...&depth=N`
`POST`	`directories`
`DELETE`	`directories?path=...`

Writes on a ro workspace fail; writes on overlay land in the upper dir and vanish on DELETE.

Capabilities and Probing

bash

curl -s http://localhost:44772/v1/isolated/capabilities

json

{
  "available": true,
  "isolator": "bwrap",
  "version": "0.9.0",
  "setpriv_available": true,
  "userns_available": false,
  "commit_supported": false,
  "diff_supported": false
}

available: false — the trusted native workload gate is missing or untrusted, bubblewrap is missing, or the host can't create the required namespaces (missing CAP_SYS_ADMIN, restricted user-ns sysctl, etc.).
setpriv_available / userns_available — whether sessions with uid_mode: "setpriv" or "userns" can be created. setpriv_available reflects only execd's default UID/GID; a session that requests a different UID/GID may still return 503 NOT_SUPPORTED when identity switching is unavailable.
commit_supported / diff_supported — Phase 2 stubs, currently return 503.

Server Configuration

Point execd at an optional TOML file:

Flag	Env
`--isolation-config`	`EXECD_ISOLATION_CONFIG`

toml

# Parent directory for per-session overlay upper dirs.
upper_root = "/var/lib/execd/isolation"

# Hard limit on total upper directory size across all sessions (bytes).
# Default: 8 GiB. Set to 0 only if you want to disable the quota entirely.
upper_max_bytes = 8589934592  # 8 GiB

# Sources allowed for extra_writable / binds (symlink-resolved).
# Default: ["/workspace", "/mnt", "/media", "/data"]. Empty = reject all.
allowed_writable = ["/workspace", "/mnt", "/media", "/data"]

Example: components/execd/configs/isolation.example.toml.

Host requirements: bwrap and the trusted native workload gate in the execd image; CAP_SYS_ADMIN (and kernel.unprivileged_userns_clone=1 for uid_mode: "userns"); overlayfs in the kernel for overlay workspaces. The published image installs the gate automatically. Linux source builds must run make build-session-gate and then sudo make install-session-gate from components/execd before starting execd.

Note: /capabilities reports available: false when the native workload gate cannot be opened as a trusted executable or when bwrap itself cannot be started (missing binary or missing namespace capabilities). A missing overlayfs does not flip available — the overlay probe only influences Phase 2 commit/diff support, and default overlay-mode session creation can still fail at runtime on such hosts. If you rely on workspace.mode: "overlay", verify overlayfs support directly on the host.

Limitations

diff / commit are Phase 2 stubs, currently return 503. Tracked in OSEP-0013.
No hardware-level guarantee. Namespaces + seccomp only; pair with a secure runtime for kernel-exploit defense.
Linux only. Non-Linux builds return available: false.
Serialized runs per session — create multiple sessions for parallelism.
Bind destinations must pre-exist in the sandbox image.

Isolation Sessions ​

Table of Contents ​

Requirements ​

Overview ​

How It Works ​

run request flow ​

Quick Start ​

curl ​

Python SDK ​

JavaScript / TypeScript SDK ​

Kotlin SDK ​

Session Lifecycle ​

Workspace Modes ​

Bind Mounts and Allowlist ​

Profiles and Defaults ​

Environment, UID, and Networking ​

Filesystem Proxy ​

Capabilities and Probing ​

Server Configuration ​

Limitations ​

See Also ​

Isolation Sessions

Table of Contents

Requirements

Overview

How It Works

`run` request flow

Quick Start

curl

Python SDK

JavaScript / TypeScript SDK

Kotlin SDK

Session Lifecycle

Workspace Modes

Bind Mounts and Allowlist

Profiles and Defaults

Environment, UID, and Networking

Filesystem Proxy

Capabilities and Probing

Server Configuration

Limitations

See Also