The Fiber Architecture: Powering React's Efficient Rendering

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4 min read

If you’ve been working with React for a while, you’re likely familiar with the occasional performance hiccups that can occur, especially in large-scale applications with deeply nested component trees. Despite React’s innovative virtual DOM and diffing algorithm, the rendering process could become a bottleneck when a frontend application scales. That’s where React Fibers come in — a complete architectural overhaul introduced in React 16 that fundamentally changes how rendering and updates are handled.

The Fiber Architecture

The fiber architecture, in simple terms, can be described as new, better and more efficient way for React to perform reconciliation. We will dig deeper into the architecture in a bit.

Wait, what does reconciliation even mean?

Reconciliation is the process of determining what changes need to be made to the DOM tree when there is a state change or update in the component tree. It’s a critical part of React’s rendering process, and it’s how React achieves efficient updates without rerendering the entire UI on every change.

A deeper look into the working reconciliation

React performs reconciliation through the following steps:

  1. Virtual DOM: React creates an in-memory representation of the UI called the Virtual DOM, which is a lightweight JavaScript tree that mirrors the component tree.

  2. Diffing: When state or props change, React creates a new Virtual DOM tree. It then “diffs” or compares the new tree with the previous one to identify the differences.

  3. Batching: React batches multiple state updates into a single re-render for performance optimization.

  4. Reconciliation Algorithm: React traverses the new and old Virtual DOM trees, comparing each node (component) and its children. It determines which nodes need to be updated, removed, or inserted in the actual DOM.

  5. DOM Updates: React generates a minimal set of operations required to update the actual DOM to match the new Virtual DOM representation, applying these changes in a batch.

Now, back to The Fiber Architecture

Traditionally, React performs reconciliation synchronously, which blocks the main thread while calculating the changes and updates the DOM. This is why we notice the occasional performance dips in React applications.

The Fiber architecture aims to make the whole rendering process more efficient by performing reconciliation asynchronously. This is done by splitting the work into smaller chunks or more formally known as “fibers”. These fibers can scheduled, processed and prioritized independently.

Fibers are set out to achieve:

  • Pause, resume, and restart work as needed (incremental)

  • Split work into chunks and prioritize (asynchronous)

  • Reuse previously completed work (memoization)

Introducing the Fiber Tree and Fibers

The core part of this architecture resides in the fiber tree and it’s fibers.

The Fiber tree is similar to a DOM tree, which represents the virtual DOM that allows the framework to keep track of the work that needs to be done in a more granular way. The fiber tree is made of fibers.

A Fiber can be thought of as a unit of work that needs to be performed, such as updating a component, rendering a new element, updating props, etc.

When a component’s state changes, React creates a new Fiber for that component and adds it to the Fiber tree. React then schedules the Fiber for processing, adding it to a queue of Fibers that need to be worked on. React then begins processing the queue, one Fiber at a time.

The inner workings of Fiber

Each fiber in the queue of fibers is processed, by executing the following steps:

  1. When a component’s state is updated, it triggers a request to schedule an update. The update is scheduled by appending a new fiber tree to a queue of pending work.

  2. The next step is reconciliation. This walks the fiber tree and calls shouldComponentUpdate and render to build up the new tree. Diffing algorithms are applied to only update nodes needing changes.

  3. Work is picked up from the queue and looped on until no more work remains. Each iteration is called a unit of work. Units of work are interrupted at intervals to check for higher-priority updates.

  4. Once a fiber sub-tree is complete, effects are fired. Effects include layout updates or DOM writes among others.

  5. Work continues until the commit phase. Here the changes are flushed to the DOM in a batched update.

Benefits of the Fiber Architecture

  • Pause and resume work at different intervals, which helps avoid blocking the main thread.

  • Prioritizing some work over others ensures that the UI stays responsive.

  • Reuse previously completed work which skip redundancies through memoization.

  • Split work into chunks to be more efficient.

  • Easily change or degrade priorities to handle edge case problems gracefully.

Conclusion

The Fiber Architecture brought a huge upgrade to how React works under the hood. By rethinking rendering and updates, Fibers make even the most complex apps feel blazing fast. Incremental rendering, better parallelization, and a smarter reconciliation process mean silky smooth user interfaces that never miss a frame.

While the inner workings are advanced, the impact is clear — incredibly fluid experiences without sacrificing React’s easy developer model. As richer web and mobile apps become the norm, Fibers ensure React stays ahead of the game for building top-notch, high-performance UI.

References

React: What is Fiber Architecture and How Does it Work?
Understanding React Fiber — How it Works and Why it Matters
React Fiber Architecture Documentation