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What you will learn
Prerequisite
App requirements
What I need to do?
An iExec DOracle needs two parts: Off-chain dockerized application, and on-chain smart contract.
1. Dockerized application (off-chain) You need to write a logic that does the following steps:
Then, push you docker image on Dockerhub, and deploy it on iExec using the SDK.
2. Smart contract (on-chain)
Going further: About Decentralized Oracles (optional read)
Programmable blockchains and smart contracts are a big deal, but they live in siloed environments and as such have very limited access to the outside world (i.e. the Internet). The iExec V3 makes it trivial for anyone to deploy his own decentralized oracle, hence allowing developers to leverage the terabytes of data available on the Web 2.0 to build a whole new range of useful and impactful dapps without compromising on the security.
The iExec DOracle uses iExec to execute the oracle logic off-chain and return certified results to the blockchain. Certification can be achieved using a combination of hardware security (TEE - Trusted Execution Environment) and on-chain consensus (iExec’s PoCo). These two mechanism provides tools to ensure results of a DOracle execution can be trusted for the on-chain settlement of off-chain events.
Documentation of the iExec platform, including details about de PoCo protocol can be found at https://docs.iex.ec.
Components of an iExec DOracle.
In order to run, an iExec DOracle needs three essential components:
The app & the output
The dockerized oracle app contains the logic that will be executed and verified by the iExec platform when a call is requested. This logic can be implemented in any language and access any data as long as it can be dockerized and it follows below simple guidelines:
The iExec middleware will perform the consensus on the determinism.iexec file, and will store the callback.iexec file on Ethereum.
The receiving smart contract
The receiving smart contract will be the on-chain receiver that will consume the output. This contract can contain any logic, but it should inherit from the IexecDoracle (available on Github) which provides a routine to retrieve and verify the result of an oracle execution. The PriceOracle shows a good example of such contract.
Security considerations
The iExec framework proposes all the tools to build a secure decentralized oracle. However, simply using the iExec framework won’t prevent issues in the oracle design from threatening the security and the decentralization of the oracle. Here are some points that should be considered:
External resources (such as Web 2.0 APIs) are centralized point of failure. DOracles can forward data from such APIs to the blockchain in a trustless manner, but the data reported on-chain is still subject to manipulation by the API administrator.
If trying to report random values from an off-chain source to the blockchain, be aware that the requester of the call might be able to control the seed. You may want to use additional proof of origin or use commit reveal pattern that only accept oracle calls for which the request has been registered on-chain before the actual value was predictable. In general, providing cryptographically secure randomness to a blockchain is difficult, so don’t try it unless you really understand the different attack vectors.
Design your receiver smart contract to be trustless and censorship resistant. If only a single actor can update it then your oracle might not be properly designed. Anyone with an interest on updating a value must be able to pay for an oracle call that will provided an updated value to the blockchain which nobody can argue against.
Need help?
Again, the documentation is here: https://docs.iex.ec. Feel free to ask for more information on our Slack.