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I read that there is a cost that should be paid when deploying a smart contact to the etherum. Is there is a cost for an end user to interact with a certain smart contact ?? For example : if I build a contract for a renting company , id end users who are looking for certain rental offers who need to interact with that contact, do they need to pay ethers ??
And if yes, that not make sense to let users who need to use smart contact to pay fees. How it can be avoided ??
All interactions between users and smart contracts happen through transactions.
Every transactions has a cost, which is measured in two parts:
Thus you can see that all transactions require some amount of ether to complete. This system is in place to prevent denial of service attacks, infinite loops, and other resource heavy computations.
In general, there is no way to avoid paying these fees per transaction. There are systems you can set up where if you want your customers to not pay fees, you can pay for fees on behalf of the user. But these systems go around the underlying authentication processes in place for Ethereum, and require you to own a service which has a high trust with a smart contract.
You can learn more here: Transactions
Ethereum uses the "Ethereum Virtual Machine (EVM)" to execute the code of smart contracts. This EVM is quasi-turing complete. A machine is turing complete when it is able to solve any calculable problem given enough space and time. But why is it only "quasi" turing complete? Because besides enough space and time, the EVM requires enough GAS (Ether). Every instruction, besides the instructions STOP, RETURN and REVERT, requires GAS to be executed in the EVM. Example: If you want to add two values from memory you have to pay 3 GAS for the addition. You can inform yourself about the cost per instruction in the yellowpaper, Appendix G and H (https://ethereum.github.io/yellowpaper/paper.pdf). If a user wants to execute a smart contract and sends a transaction to the ethereum network that contains the order to do so, the user has to pay gas. So to answer your first question, every user has to pay gas if they send an order to execute a smart contract via a transaction. Nevertheless, there is a special case. If you only want to read data using a smart contract, so that you don't change the state of the storage, you can do so without paying gas. This is achieved by running the EVM and executing the smart contract only locally on your computer (or any other computer which offers this service).
How can you avoid this? If you want to protect the users of your smart contract from paying those costs, you have to take the gas from another source. This means that you or somebody else has to pay the gas for the user before they use the smart contract. For example, you could put 1 ether into your smart contract to cover those costs. This is risky though, since you have to protect your smart contract from a "ddos" attack which consumes all the ether you have reserved for gas. Further, you have to watch that you smart contract does not run out of gas for your users, otherwise your services will be unavailable for the users and they are losing their money they have to pay to call your smart contract (every transaction costs 21000 GAS at least).
