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Why choose to enable fast withdrawals for your Arbitrum chain

Enabling fast withdrawals refers to configuring the chain to use a fast confirmation mechanism that allows transactions and state assertions to achieve finality much quicker than the standard multi-day challenge period in optimistic rollups. This is achieved by routing assertions through a permissioned committee of validators (often overlapping with the Data Availability Committee or DAC in AnyTrust mode) that unanimously confirms them via a multisig wallet, bypassing the full fraud-proof window. As a result, users can withdraw assets or access finalized states in as little as 15 minutes for L2 chains or 15 seconds for L3 chains, significantly enhancing user experience by reducing wait times and enabling faster cross-chain interactions.

Enabling this feature shifts the chain from relying solely on time-based fraud proofs to a committee-driven confirmation process, where validators (recommended to be at least three for reduced trust risks) must agree unanimously. It's primarily recommended for AnyTrust chains, as it leverages the existing DAC trust model without adding new assumptions—ideally, DAC members run the validators.

For pure Rollup chains, enabling it effectively introduces a committee trust layer, making it less "trust-minimized" than standard mode. Once enabled, the chain processes a potential backlog of unconfirmed assertions before operating at full speed, and confirmations may slightly delay under high load (e.g., >1 Mgas/s throughput), but security remains intact.

Key Concepts​

  • Standard Withdrawal Process: In optimistic rollups like Arbitrum, assertions about the chain's state (e.g., transaction batches) are posted to the parent chain (e.g., Ethereum or Arbitrum One) and remain challengeable for a configurable period (default ~6.4-7 days) to allow fraud proofs. Unchallenged assertions confirm, enabling secure withdrawals, but this delays user access to funds.
  • Fast Withdrawals Mechanism: When enabled, a committee (e.g., an n/n multisig Safe wallet) of validators processes and unanimously signs assertions at a set frequency. This provides immediate confirmation, with finality tied to the parent chain's block times rather than the full challenge period. For example, L2 chains on Ethereum can confirm in ~15 minutes (above Ethereum's ~12.8-minute finality), while L3 chains on Arbitrum One can leverage "soft finality" (~250ms sequencer confirmations) for near-instant speeds.
  • Related Parameters: Often paired with adjusting the minimumAssertionPeriod (e.g., to 1 block for frequent assertions) and node flags to control posting intervals.

Compatibility​

  • DA Modes: Optimized for AnyTrust (leverages DAC); possible but not recommended for Rollup or Alt-DA, as it adds trust assumptions.
  • Validation: Works with BoLD (permissionless) or pre-BoLD (permissioned), but the committee must be allowlisted.
  • Gas Tokens: No restrictions; compatible with native ETH or custom ERC-20s.
  • Parent Chain: Frequency must exceed the parent's finality time (e.g., >12.8 minutes for Ethereum L2s).

Pros​

  • Significantly faster withdrawals and finality: Reduces withdrawal times from days to minutes, improving user experience and enabling quicker asset movement across chains. This is particularly beneficial for cross-chain dApps, as they can read the finalized state at the same accelerated rate.
  • Enhanced capital efficiency: Shorter wait times minimize opportunity costs for users, allowing faster re-deployment of funds and better liquidity in DeFi or other applications.
  • Improved performance for AnyTrust: When combined with AnyTrust mode, Fast Withdrawals leverage existing infrastructure without introducing entirely new trust elements, enabling near-instant withdrawals for non-fungible assets.
  • Flexibility for Layer 3 setups: For chains settling to Arbitrum One or similar, finality can be configured down to 15 seconds, supporting high-throughput applications.

Cons​

  • Introduced trust assumptions: Shifts security from the fully decentralized optimistic Rollup model to reliance on a validator committee. For Rollup-mode chains, this means it's no longer a "true" Rollup, as users must trust the committee for fast confirmations. In AnyTrust mode, it assumes at least two honest members in the Data Availability Committee (DAC), which could be a risk if the committee is compromised.
  • Potential delays under high load: High network throughput can cause confirmation delays beyond the configured rate, with a practical minimum of 1-2 minutes in busy scenarios. Initial setup may also slow the chain temporarily due to backlog processing.
  • Setup complexity and requirements: Requires upgrading to specific versions, creating a multi-signature Safe wallet for the committee, and running at least three validators. This adds operational overhead and potential costs.
  • Security trade-offs compared to standard withdrawals: While standard withdrawals maintain full Ethereum-level security via the challenge period, fast withdrawals prioritize speed over decentralization, potentially exposing users to risks if the committee fails or is attacked.

Impact on User Experience​

From a UX perspective, enabling fast withdrawals transforms slow, multi-day processes into near-real-time ones, making the chain feel more responsive. Users get quicker access to withdrawn funds (e.g., 15 minutes vs. 6.4 days), reducing frustration and capital lockup. dApps benefit from faster cross-chain state reads, enabling seamless interactions like oracle updates or multi-chain DeFi. However, it assumes users trust the committee (e.g., for high-value transfers), and any load-based delays could still impact predictability. Overall, it's ideal for consumer-facing apps like gaming or social, where speed is key.

Examples​

  • AnyTrust L2 Chain: Configure for 15-minute withdrawals on Ethereum, using DAC members as validators for trust alignment.
  • L3 on Arbitrum One: Set to 15 seconds using soft finality, suitable for high-throughput gaming appchains.

How to configure​

  1. Upgrade Contracts and Nodes: Ensure nitro-contracts v2.1.0+ (update RollupAdminLogic and RollupUserLogic) and node software to nitro v3.1.2+. Use the Orbit versioner script to check and upgrade.
  2. Run Configuration Scripts: Use the Orbit SDK (e.g., setup-fast-withdrawal example) or Orbit actions repo (e.g., fast-confirm script with Foundry's cast CLI) to create a Safe multisig for the committee, add validators to the allowlist, set the Safe as the anytrustFastConfirmer, and optionally adjust minimumAssertionPeriod (defaults to 75 blocks ~15 minutes, measured in the first non-Arbitrum ancestor chain's blocks).
  3. Configure Nodes:
    • Batch Poster: Set --node.batch-poster.max-delay to match the assertion period (e.g., 15m).
    • Validators (BoLD mode): Enable --node.bold.enable-fast-confirmation=true and set --node.bold.assertion-posting-interval to the period.
    • Validators (pre-BoLD): Similar flags like --node.staker.enable-fast-confirmation=true.

This feature aligns with Arbitrum's focus on scalable, user-friendly chains while maintaining core security. For setup, consult the official docs, or your RaaS; a list of RaaSes is on the Third-party providers page.