CryptoKitties (2017): The Game That Clogged Ethereum
CryptoKitties launched in November 2017 and filled Ethereum with transactions. See how a collectible game exposed scaling limits and popularised NFT standards.
In late 2017 a game about collecting and breeding digital cats became one of the first applications to push a public blockchain to its limits. CryptoKitties launched on 28 November 2017 and, within weeks, was credited with slowing down Ethereum. The episode is a clear, early demonstration of the scaling problem.
The game
CryptoKitties was created by Axiom Zen, a Canadian studio in Vancouver, whose blockchain team later became Dapper Labs. Players bought virtual cats, each one a unique token on Ethereum with its own appearance determined by a code. Two cats could be bred to produce a new kitten that inherited traits from its parents. Cats were bought and sold for ether. New generation-zero cats were sold by the company in auctions where the price declined over time until a buyer accepted.
The key point is that each cat was a distinct item with an owner recorded on the blockchain, not a copy of a shared file. This was a different kind of token from the interchangeable ones used in most fundraising.
Why the network struggled
Every action in the game, such as buying, selling, breeding or birthing a kitten, was a transaction that needed to be processed by Ethereum. Ethereum had a limit on the amount of computation that each block could hold, and a block arrived about every fifteen seconds. When many people played at the same time, the number of waiting transactions rose.
In early December 2017 the number of pending transactions on the network increased sharply, and users experienced slower confirmations and higher fees. Because fees work like an auction, people who wanted fast confirmation bid more, which pushed costs up for everybody, including people who had nothing to do with the game.
The effect was serious enough that the game's creators adjusted the fees for certain actions. Reports at the time linked CryptoKitties to a visible portion of network activity, although the exact share varied by day and by source.
The ERC-721 standard
The game popularised a token standard for unique items. ERC-721 was proposed by Dieter Shirley of Axiom Zen in late 2017 and was later finalised. Unlike the ERC-20 standard for interchangeable tokens, each ERC-721 token has an individual ID. This is the technical basis of what became widely known as non-fungible tokens, or NFTs.
| Feature | ERC-20 | ERC-721 |
|---|---|---|
| Token type | Fungible, each unit identical | Non-fungible, each token unique |
| Typical use | Currencies, fundraising tokens | Collectibles, tickets, game items |
| Ownership record | Balance per address | Owner per token ID |
What the episode revealed
- Blockchains have fixed capacity. Public networks share limited resources, and demand can quickly exceed supply.
- Fees are a market. When a network is congested, fees rise for all users.
- One popular application can affect the whole chain. This is a concern for any platform that hosts many unrelated applications.
Where the cats actually lived
Each cat's genes were stored on the blockchain as a 256-bit number, which the game used to decide its appearance. The artwork itself, however, was served from conventional web servers run by the company. That design is common in the NFT world: the token records ownership and a pointer, while the image or file sits elsewhere. If that web hosting disappears, the token may point to nothing. It is a reminder that on-chain ownership and off-chain content are different things, and that ordinary web hosting still matters. You can compare reliable options on our hosting page.
Breeding was limited by a cooldown period for each cat, which meant the game could not be used at unlimited speed. Even so, the cumulative demand was enough to strain the network.
The responses that followed
The developer community took several routes to improve capacity:
- Layer-2 networks. These process transactions off the main chain and post compressed results to it, reducing load and fees. Rollups became the main form of this approach.
- Alternative blockchains. Dapper Labs built its own chain, Flow, designed for consumer applications, and other projects emphasised throughput.
- Protocol changes. Ethereum's roadmap shifted towards scaling through rollups, and later upgrades added cheaper data storage for them.
It is fair to note the trade-offs. Layer-2 networks add complexity, and moving funds between networks introduces risk, a point that later bridge hacks illustrated.
A short note on the market
CryptoKitties attracted attention for the prices some rare cats fetched. Those prices were set by buyers in a small market, and the activity declined after the initial excitement. The significance of the game lies in the technology and the lessons about capacity, not in any individual sale.
A practical lesson for site owners
CryptoKitties is a story about traffic. A sudden surge can overwhelm a system that was not sized for it, whether that is a blockchain or a web server. If you expect spikes, consider a CDN to absorb static requests, or a VPS with room to grow. See the CDN page to learn how caching can reduce the load on your origin server.
