There is often a need to query and/or process off-chain data before it can be included in the on-chain state. The conventional way of doing this is through oracles. Oracles are external services that typically listen to blockchain events and trigger tasks accordingly. When these tasks complete, their results are submitted back to the blockchain using transactions. While this approach works, it still has several flaws with respect to security, scalability, and infrastructure efficiency.
To make the off-chain data integration secure and more efficient, Substrate provides off-chain workers. The off-chain worker subsystem allows execution of long-running and possibly non- deterministic tasks (e.g. web requests, encryption/decryption and signing of data, random number generation, CPU-intensive computations, enumeration/aggregation of on-chain data, etc.) that could otherwise require longer than the block execution time.
Off-chain workers have their own Wasm execution environment outside of the Substrate runtime. This separation of concerns is to make sure that the block production is not impacted by the long-running tasks. However, as the off-chain workers are declared in the same code as the runtime, they can easily access on-chain state for their computations.
Offchain workers have access to extended APIs for communicating with the external world:
- Ability to submit transactions (either signed or unsigned) to the chain to publish computation results.
- A fully-featured HTTP client allowing the worker to access and fetch data from external services.
- Access to the local keystore to sign and verify statements or transactions.
- An additional, local key-value database shared between all off-chain workers.
- A secure, local entropy source for random number generation.
- Access to the node's precise local time and the ability to sleep and resume work.
Off-chain workers can be initiated from within a special function in your runtime implementation,
fn offchain_worker(block: T::BlockNumber). The function is executed after each block import. To communicate results back to the chain, off-chain workers can submit signed or unsigned transactions to be included in subsequent blocks.
Note that the results from off-chain workers are not subject to regular transaction verification. A verification mechanism (e.g. voting, averaging, checking sender signatures, or simply "trusting") should be implemented to determine what information gets into the chain.
For more information on how to use off-chain workers in your next runtime development project, please refer to our Development Guide.