Instruction to store data

proposals/0000-publish-arbitrary-data.md

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+---
+Title: Publish arbitrary data instruction
+Number: 0
+Status: Draft
+Version: 4
+Authors:
+ - Francisco Aguirre
+Created: 2023-11-16
+Impact: Low
+Requires:
+Replaces:
+---
+
+## Summary
+
+This RFC proposes a new instruction, `Publish`, to publish some data to another location.
+
+## Motivation
+
+XCM itself is not a good means of storing and retrieving arbitrary data.
+This fixes the storage part, leaving the retrieval to another mechanism.
+
+## Specification
+
+The new instruction looks like this:
+
+```rust
+Publish { data: BoundedVec<Key, Value, Bound> }
+```
+
+`Key` and `Value` are arrays of bits, they could be anything.
+
+The `Bound` value should be configurable.
+
+### Examples
+
+A program like the following might be sent to another location for publishing some data:
+
+```rust
+WithdrawAsset(/* ... */),
+BuyExecution { /* ... */ }
+Publish { data: [(b"My key", b"My value")] }
+RefundSurplus, // <- In case we overpay
+DepositAsset { /* ... */ }
+```
+
+The other location stores this key value pair in any manner appropriate to it, which might allow different types of retrieval afterwards, or none at all.
+
+## Security considerations
+
+Programs that publish data would need to pay a certain amount of funds proportionate to the size of the data.
+
+## Impact
+
+This proposal adds a new instruction, so the impact is low.
+
+## Alternatives
+
+The other alternative to this is to use `Transact`, since it allows sending an encoded blob of bytes.
+This, however, falls short, as it's expected that the blob decodes to a function call on the receiver.
+This new instruction makes it clear that it's talking about data and not a function call, while at the same time allowing for multiple key value pairs to be sent.
+
+## Questions and open Discussions (optional)
+
+- Should we account for the published data's size via Weight?
+- Should we introduce a new instruction for storage deposits or add another operand to the `Publish` instruction specifying the assets used?
+- Should there be a way to delete the published data via XCM?