This image claims you can build "decentralized apps", and each computer "contributes a little to process it". Is that claim correct?

Question

This image claims that you can put decentralized apps (say, a messaging program) on the Ethereum network. It also claims that each computer on the network contributes a little bit to the processing of that app. That claim does not seem correct. Specifically,

1. I believed computation was expensive and anything but very lightweight logics (such as, if condition is met, pay bob $10) could realistically be put on the blockchain. Not whole complex apps. Is that wrong?

2. I believed that every computer on the network performed all the historical computation of all apps, which opposes the view that each computer does a little bit of the work. If that didn't happen, what would stop a participant to lie about the result of a computation?

Answer 1

This image is a bit misleading.

1. Computation is not that expensive. According to the stats page, one gas is about 50 gwei, or 5x10^-8 ETH. A simple transaction is 21k gas, or 0.00105 ETH. A relatively complex application like Augur can easily function at these prices. Storage is more expensive, but most heavy-duty storage can be done off-chain, in ipfs.

2. You are correct, every node does every computation. The infographic is simply incorrect. In addition, only miners are rewarded, not every full node. There are paralellization schemes, in theory, but none have been implemented.