All you need to know about Altair: The Beacon Chain Upgrade

All you need to know about Altair: The Beacon Chain Upgrade

Altair, the network upgrade on Ethereum Beacon Chain is successfully deployed on the main network today with over 95% validators participation. This is a good indication of validators coming along with the Ethereum upgrade. The main concern of Ethereum developers is a consensus split caused by some logic being hit where two clients disagree. They want to continue observing the chain for the next few days to call it a smooth-running upgrade.

Wednesday, Oct-27-2021 10:56:23 AM +UTC at Epoch 74240

Altair is the 3rd network upgrade for Ethereum ecosystem in 2021, but the first on the Beacon Chain (the POS chain) that went live on Dec 01, 2020 and runs in parallel to the Ethereum POW chain. Berlin & London upgrades were deployed on the Ethereum proof of work chain, earlier this year. The next stop is The Merge.


Image Source: Ethereum Cat Herders

Why is it named "Altair"?

It started as Eth2 protocol upgrade naming theme brainstorming #202 issue on Github. Click on this link to check out the community discussions for naming.

Image Source: Astronomy Trek

Team decided to use star names for the upgrade after community discussions and emoji voting. Altair was chosen as the result. Altair is the brightest star in the Aquila Constellation and 12th brightest star in the night sky.

Altair - Key Features


Let's have a look into the key features of Altair upgrade.

Sync Committees

  • Developers will now be able to create light clients on the beacon chain, rather than using full node software for verifying blocks and the transactions.
  • Light Client or light node is software that connects to full nodes to interact with the blockchain without a need to continuously sync with the whole blockchain. This is a much simpler method to interact with the network.


Image Source: Kouohhashi

Inactivity Leaks

  • The main task of validators is to finalize the blockchain by proposing and attesting blocks.
  • Before Altair: If the chain stops being finalized, in the case when a large percentage of validators go offline simultaneously. Then, fully inactive validators will lose 11.8% of their balance, while validators that are active 75% of the time will lose 3.1%.
  • After Altair: Now fully inactive validator's loss would be 15.4 percent, whereas the 75 percent active validator's loss would be just 0.3 percent.
  • This makes inactivity leaks more efficient to Active validators.
  • In Altair; Case 1: If validators who successfully participate more than 80% of their epochs, then their penalties will approximately be equal to zero. Case 2: If validators who miss more than 20% of the time will have their inactivity status grow but penalties will be far lower for validators who miss 20-40% or all of their epochs.
  • In the graph below we can see the example of with five validators showing different behavior:


Image Source: GitHub

  • Now If we look at the second graph below, we can see the example of penalties on the same five validators:


Image Source: GitHub

  • In the above graph we can see that the offline validator's losses are larger as compared to the others. Also 90%-online validator's losses are barely visible on the graph. After the chain resumes finality, 70%-online validator's losses are noticeable and offline validator's losses are still growing.

Validator Duties and Rewards

In Altair upgrade, the validator's duties include: TIMELY_HEAD, TIMELY_TARGET, TIMELY_SOURCE, Sync Committee Participation and Proposing. To know more about these duties. Checkout the Official Github Repository.

Altair upgrade redesigns how validators are rewarded in order for fulfilling above duties to a more systematic and simpler approach.

Total theoretical long-run average reward per epoch that a validator can get is determined by the get_base_reward function, but this is not the maximum reward in any given epoch. Rewards for submitting sync committee signatures and for proposing are larger than the base reward, but validators can only get those rewards when they are assigned to those tasks during infrequent epochs.

  • Here we can see the distribution of base reward:


Image Source: GitHub

A duty assigned as share in the above pie does not guarantee that a validator who completes that duty will receive his fraction times the base reward. This is because actual rewards can diverge due to following reasons;

  1. When validators complete the duty imperfectly. As they are offline. Also if they cannot get their attestation or sync committee signature included within a slot as the next proposer is offline.
  2. Validator Rewards for completing an attestation duty are proportional to the percentage of all validators who complete that duty i.e if 70% of validators complete their duties then the reward for each validator is multiplied by 0.7.
  3. Validator Rewards can diverge due to random variance from randomly assigned duties i.e. proposal and sync committee participation which means that if there are 100 validators, then there is a 1/100 chance to propose per slot.


This upgrade has been rigorously tested many times.

  • Bug Bounty Program: To find loopholes in the upgrade, all bounties for bugs in the upgrade are increased now. Critical bugs in both clients and specifications are now valued at up to $100,000. Main aim is at identifying flaws in the basic Eth2 Beacon Chain as well as the Lighthouse, Nimbus, Teku, and Prysm client implementations.

Upgrade Client Nodes

Node operators who are already running their beacon chain node, must upgrade to a version that is compatible with the Altair upgrade.

  • If node operators don't upgrade, then their client will sync to a pre-fork beacon chain and will be stuck on an incompatible chain with old rules.
  • If node operators want to do this upgrade, then they should checkout their client's communication channels for more information.
Client Link
Lighthouse v2.0.1
Lodestar v0.31.0
Nimbus v1.5.2
Prysm v2.0.2
Teku 21.10.1

Table Source : Ethereum Foundation

Frequently Asked Question

Q. What will happen to the user who couldn't upgrade the node before the upgrade but decides to upgrade after a week and catch up?

A. Essentially, the user will be on the minority chain with the old rules and the new chain is expected to be finalizing greater than 2/3rd. In the time period, the user stands to lose about as much as he could have made. It could be something around 0.5% of the earnings but if you jump on to the new chain, slowly you can earn that.

On the earning chart, a user may go negative for a month, then even out and then go positive. It's not catastrophic for hobbyist validators, but the best thing would be to upgrade. Because you will still be doing attestation but not making as much as you could with the new chain.

Q. Will a user be slashed for late upgrading?

A. A user wouldn't get slashed for being late in upgrading client node(s), said Danny Ryan in the Altair Ethereum Upgrade Livestream

Q. Will there be any effect on validators?

A. Upgrade is mandatory for everyone who wants to continue running their validators nodes.
If they don't upgrade, then they will be unable to participate in the updated consensus and will instead operate on the post-upgrade network with the penalties imposed.

  • Validators will need to download or update to the latest version of the software.
  • According to the Ethereum Foundation, this upgrade has been well tested, but they have validators advised to remain online throughout the upgrade in case any difficulties arise.

Q. Will there be any effect on Ethereum users or ETH holders?

A. This upgrade will only affect the beacon chain and there will be no effect on users' transaction and accounts.

Check out this stats at Node Watch to know the status of nodes which are ready for Altair Upgrade.


Image Source : Node Watch


Altair upgrade will bring changes to reward and penalties' structure of the current beacon chain along with introducing sync committees duties for the light client, it is another significant step toward Ethereum's future. It is the first stage in preparing developers and client teams for the Merge which will transform the original POW Ethereum blockchain to POS. This is probably the only upgrade before the Merge. As after the Merge with a following upgrade in 2022, validators may be able to withdraw funds. The Ethereum network is growing and we hope to continue sharing the latest update & upgrade with the community.

Also Read

  1. An overview of Account Abstraction in Ethereum blockchain
  2. Ethereum Layer 2 projects: An Overview
  3. All About The Validator In Ethereum 2.0 Beacon Chain
  4. How To Become Eth2 Validator On Windows/Linux/Mac
  5. Ethereum 2.0 Basic Terminology

Reference Links:

  1. EF Blog
  2. PEEPanEIP video
  3. Anotated specs

Cover Image Source: Astronomy Trek


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