RFC-0092 - Sessions

RFC-0092: Sessions
  • Sessions

Defines the role of the session component.

Gerrit change
Date submitted (year-month-day)2021-04-08
Date reviewed (year-month-day)2021-05-05


This document describes the role and responsibilities of a Fuchsia session.

A session is a component. Each Fuchsia product builds its user experience within a session. For example, the workstation session instantiates components to handle input, set up the Scenic scene graph, and to render its UI.

The session component is offered all capabilities required to create the user experience of a product, and is thus often the most privileged non-platform component in a product. As such, the session component serves as a boundary between the Fuchsia platform and a product built with Fuchsia. The session_manager is the parent component of the session, and exposes debug protocols that developers can use to control the session. Product variants (e.g., product_eng, product_userdebug) may result in different flavors of the same session.

The intention of this document is to ratify design decisions that have been made since the Modular Framework was deprecated.


The Fuchsia platform aims to support many products. These products are composed of product-specific and platform components. The product-specific components must live somewhere in the component instance tree. The session is the root of the product-specific component instance tree that drives the user experience.

The session serves as a boundary between the Fuchsia platform and the product. As such, the session can be used to improve the product development cycle. For example, the session can be destroyed and recreated to "restart" the product's user experience.


This section outlines the current design for Fuchsia sessions, and highlights where the current design is not the intended long-term design. The alternatives section describes some potential long-term approaches at a high level.


core is a non-executable component that provisions capabilities that are consistent across products. core is the parent of session_manager. session_manager is instantiated in response to startup connecting to the fuchsia.sessionmanager/Startup protocol exposed by session_manager.

session_manager reads the initial boot session from a configuration file located in its /config/data. It then instantiates the session as a child in a dynamic collection. Although the session lives in a collection, at most one session component exists in this collection. This means that all sessions receive the same set of capabilities, as the set of offered capabilities doesn't change from product to product. See Session Manager Configuration for further discussion.

The session then instantiates its own, product-specific, component hierarchy. Even for simple products, the session ends up with a lot of responsibilities. For example, it configures the scene graph, wires up input, integrates accessibility, and so on.

Capabilities and components

The session uses the capabilities it is offered to instantiate a tree of components required for the desired product experience. The session must be offered all platform capabilities used within the session's component hierarchy.

The Fuchsia platform must consider the capabilities offered to the session carefully even when the capabilities are specific to a single class of products. The goal is to provide products the control and flexibility they need while maintaining a trusted platform that is consistent across products.

For example, consider products with displays. The Fuchsia platform may decide to offer the session the capability to interact with the display and then let the session be responsible for instantiating a graphics subsystem (e.g., Scenic). This gives the session a lot of flexibility and control, but it also makes it more difficult for the Fuchsia platform to evolve: if the Fuchsia platform offers the session the capabilities it needs to instantiate Scenic, it also gives the session the ability to create its own Scenic replacement. If enough products define their own graphics protocols, a "graphical component" will only run on systems with the matching graphical protocols. In addition, the platform can no longer provide guarantees about the scene graph, since any product has the ability to circumvent them.

Scenic is currently instantiated as a "v1" component under appmgr. When Scenic is migrated to "v2" (component_manager) component hierarchy, a decision will need to be made about where in the component hierarchy it is instantiated.

To determine whether or not a component should be instantiated within the session, consider the question: "should interaction with this capability be consistent across products"? For example, a product should be able to choose whether or not it needs graphics, but if it needs graphics it should use Scenic. Similarly, a product should be able to decide which types of input devices it supports, but it should use the input pipeline to receive events from said devices. This implies that both Scenic and the input pipeline should be product configurable but be instantiated outside the session, because the capabilities those components require are privileged.

In the current architecture some of these components are planned to live within the session for practical reasons (e.g., lifecycle management, lack of .cml configurability, etc.). There are several ongoing efforts across Fuchsia which introduce other mechanisms for platform configurability that will allow many of these platform components to be instantiated outside the session again:

  • Drivers are already instantiated outside of the session yet are not static across all Fuchsia products.
  • Features defined in the core realm need to be toggled depending on security context, like enabling component instance ID enforcement for storage on user builds.
  • The temperature-logger component exists in the core realm today, but is only included on certain boards via a mechanism defined in RFC-0089.

Some examples of components that should clearly be instantiated within the session:

  • Graphical presentation ("shell") components.
  • Components that contribute directly to the user experience (e.g., video player, terminal, etc.).


The session_manager component has been implemented and can be found in the session_manager directory.

Some simple example sessions can be found in examples.

The session used by reference Fuchsia products can be found at tiles-session.


The session is the first product owned component that gets instantiated, but many platform components are instantiated before the session component.

Security considerations

The session_manager offers the session all the capabilities it needs to instantiate a product's user experience. All sessions receive the same set of capabilities from session_manager. This set contains a wide range of capabilities. However, the set of capabilities available to the session is smaller, and more auditable, than the set that is offered to components that run on product configurations that have not been migrated to use a session (i.e., products that use the Modular Framework).

It is important that the capabilities offered to the session go through security review. It unlikely that the Fuchsia team will be able to perform thorough security reviews of all sessions in the future. Thus the security of the session, from the platform's perspective, relies heavily on the set of capabilities offered to the session.

Privacy considerations

session_manager only stores the URL of the current session, which may be logged. This is an improvement over the `Modular Framework, which manages components that are launched by the user.

Each session will require its own comprehensive privacy review.


Product owners need tools to test and debug their sessions.

A testing framework that focuses on the needs of session developers will be created. This framework will allow developers to, for example, test the setup of their session's component hierarchy and verify the interactions between the components in the session.

Developers will be able to interact with a running session via ffx commands. For example, ffx session restart instructs the session_manager to destroy and recreate the session.


Per RFC-194, conceptual documentation has been removed.

Each individual binary and library in //src/session also contains a README explaining its purpose.

Drawbacks, alternatives, and unknowns

The platform as a child of the session

The relationship between the session and core could be reversed, such that core is a child of the session. This would give the product full control over how the system is configured at the cost of a stable platform foundation.

Session manager configuration

The session_manager configuration currently only toggles which session component to instantiate during boot. There is no variance in session_manager.cml, and thus every session is offered the same set of capabilities. Ideally the session manager configuration would minimize the number of offered protocols.

Prior art and references