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 features:

Off-Chain Worker (OCW) 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 Storage offers storage that is local to a Substrate node that can be accessed both by off-chain workers (both read and write access) and on-chain logic (write access via off-chain indexing but not read access). This is great for different worker threads to communicate to each others and for storing user- / node-specific data that does not require consensus over the whole network.

Off-Chain Indexing allows the runtime, if opted-in, to write directly to the off-chain storage independently from OCWs. This serves as a local/temporary storage for on-chain logic and complement to its on-chain state.

Off-chain features run in their own Wasm execution environment outside of the Substrate runtime. This separation of concerns makes sure that block production is not impacted by long-running off-chain tasks. However, as the off-chain features are declared in the same code as the runtime, they can easily access on-chain state for their computations.

https://substrate.dev/docs/en/knowledgebase/learn-substrate/off-chain-features#off-chain-workers

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