I'm off-chain indexing some pallet events into my off-chain local storage using an OCW. Now, I want to be able to retrieve this data via RPC from an external service. Is this possible? Does it require the OCW to access the local-storage as part of the RPC query?

Do Custom RPC work for the off-chain runtime? https://docs.substrate.io/v3/runtime/custom-rpcs/

Any example will be appreciate it. TIA, Cheers!

NB: I'm not after an Oracle, I need the opposite case.

1 Answer 1


Yes, Custom RPC work for the off-chain runtime. Custom Rpc Doc For example, you can define custom rpc in the following way:

public trait SomeOcsApi {
   fn some_method(params...) {

And, in the implementation, just read the OCS storage

struct SomeOcsApiImpl<T: OffchainStorage> {
   storage: Arc<<RwLock<T>>>,
impl SomeOcsApi for SomeOcsApiImpl {
   value = self
                &*derive_storage_key(key.as_bytes(), &did),


Surely, in your OC indexer should write event to this storage also, share this reference with OC indexer.

  • Hi @ironman, thanks for your reply. There is a missing step in you answer that I could not figure it out yet, which is: How to extend the RPC into the rpc.rs file. There is a lot of doc, about this when you are trying to reach out the runtime storage, but in this case I want the offchain storage, something like the following snip: // Add a silly RPC that returns constant values io.extend_with(SomeOcsApi::to_delegate(SomeOcsApiImpl<T> ?????? ))); TIA Mar 28, 2022 at 4:43
  • 2
    @PabloBuitrago as my answer says: 1. annotate your rpc trait with #[rpc] 2. annotate your rpc method with #[rpc] 3. impl the rpc trait, with the Off-Chain Storage as a field. 4. in your node/rpc.rs, config the delegate to your defined rpc. For step 1-3, you can take our rpc as reference; For step4, you cant take our node rpc as reference. Mar 29, 2022 at 10:36
  • Thanks @ironman. Congrats! or Parami. Looks promising. Mar 29, 2022 at 15:02
  • 1
    A common suggestion is when you encounter how-to-use problem, alway go to substrate/Polkadot/Acala/Parami's repository, search a example. Mar 30, 2022 at 2:48
  • Great suggestion! Once more, thank you @ironman. Mar 30, 2022 at 21:37

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