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People are calling Polygon both a layer-2 and a sidechain even though these are 2 different concepts. Which one is it?

4 Answers 4

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A blockchain doesn't exclusively have to be one or the other - it can be both a sidechain and a layer-2. They aren't opposites.

A sidechain refers to a blockchain which has it's own consensus mechanism and doesn't depend on Ethereum for security.

A layer-2 refers to a blockchain that is improving upon Ethereum by adding scalability features (usually lower gas fees).

There's no reason why a blockchain can't be a side chain and a layer-2 at the same time, which is what Polygon is

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Short Answer:

Polygon PoS can be considered as a combination of sidechain and layer2 solution because it is a separate chain that has its own consensus mechanism but also uses the main Ethereum layer for some features in its consensus mechanism to improve security.

Long Answer:

Polygon is a platform (framework) that offers many different scaling solutions (most of them are still in development phase), so we can't have a general answer for all of them.

Normally when people are talking about Polygon, they reference "Polygon PoS " which is the most widely used and adopted Polygon solution, so I suppose the question is for this specific chain.

So is "Polygon PoS" a sidechain or a layer2?

Let's first discuss what these terms mean.
Based on the Ethereum docs:

Sidechain is:

A sidechain is a separate blockchain that runs independent of Ethereum and is connected to Ethereum Mainnet by a two-way bridge. Sidechains can have separate block parameters and consensus algorithms, which are often designed for efficient processing of transactions. Using a sidechain involves trade-offs, though, as they do not inherit Ethereum's security properties. Unlike layer 2 scaling solutions, sidechains do not post state changes and transaction data back to Ethereum Mainnet

Layer 2 is:

Layer 2 (L2) is a collective term to describe a specific set of Ethereum scaling solutions. A layer 2 is a separate blockchain that extends Ethereum and inherits the security guarantees of Ethereum.

On the Polygon official site, the "Polygon PoS is called "An EVM enabled sidechain" which sounds reasonable because it is a separate chain with a separate mechanism, but also its consensus mechanism has a lot of extra features that rely on the security of the main Ethereum layer.
In this video Finematics explains how the Polygon PoS consensus mechanism works.
The combination of a sidechain that uses the main layer for some additional security features is called a "Commit Chain". I didn't find an official uses of this term.

So the final answer must be:

Polygon PoS can be considered as a combination of sidechain and layer2 solution because it is a separate chain that has its own consensus mechanism but also uses the main Ethereum layer for some features in its consensus mechanism to improve security.

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    Does polygon categorize as an optimistic roll-up or a zk roll-up?
    – user610620
    Commented Dec 16, 2022 at 2:02
  • @user610620 also interested in this Commented Jan 12, 2023 at 19:36
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    @user610620 Polygon Ecosystem is building roll-up solutions, but they are still in the development phase. Polygon PoS is NOT a roll-up solution. it is actually based on Geth nodes (excluding the consensus mechanism). Commented Jan 17, 2023 at 7:55
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A significant difference between layer 2 and sidechain -

  1. Sidechains have security mechanisms of their own.
  2. Layer 2 solutions derive their security from their parent blockchain while using their own tools to address scalability issue of parent blockchains.

Polygon is a Layer-2 scaling sidechain solution because it has an ecosystem of its own (POS Chain, Plasma Chains, ZK-rollups, Optimistic rollups, etc). And while it might be possible to classify these individual solutions in either of the two categories, polygon as a whole cannot of classified in either of them simply because Polygon's scope is much broader. Hence, a lot of times for Polygon the terms L2 and sidechains are used interchangably.

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  • could you edit your answer to describe what sidechains are, besides what it has
    – user610620
    Commented Mar 31, 2022 at 14:48
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Sidechain is another blockchain that is connected to Ethereum by a two-way peg which allows users to transfer funds from the main chain to the sidechain and from the sidechain to the main chain. So polygon is a sidechain.

Layer-2 refers to a network or technology that operates on top of an underlying blockchain protocol to improve its scalability and efficiency.

So it is also layer-two.

From here

Polygon is a layer-two (L2) blockchain for Ethereum that uses different scaling technologies to improve the scalability of the Ethereum mainnet. It will connect Ethereum-compatible blockchains in an internet of blockchains, similarly to the mechanism used by Polkadot. It uses a modular “security-as-a-service” approach that builds on Ethereum’s security, or uses its own pool of professional validators.

Polygon is working on different scaling solutions for Ethereum, namely:

  • Polygon PoS: its Commit Chain, a proof-of-stake EVM-compatible sidechain already live.
  • Polygon Miden: a zero-knowledge rollup based on STARK.
  • Polygon Hermez: an open-source zk-rollup.
  • Polygon Avail: a standalone chain focused on data availability
  • Polygon Zero: another zk-rollup chain.
  • Polygon Nightfall: a privacy-focused rollup chain.
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  • what is a "two-way peg"? Can't understand what was copy and pasted. could you walk through it. And what is your answer. Both a layer-2 and sidechain? Edit your answer then. How can it be both, I thought they're two different things, given the other person's answer.
    – user610620
    Commented Oct 23, 2022 at 0:43
  • why cannot it be both? each feature is not opposite each other
    – Yilmaz
    Commented Oct 23, 2022 at 1:05
  • Explain how they are not mutually exclusive. That was the point of the question really.
    – user610620
    Commented Oct 24, 2022 at 7:30

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