Hermes SDK - Next Generation IBC Relayer Framework
Work in Progress
Hermes SDK is work-in-progress. At the current stage, a minimal implementation of Cosmos-to-Cosmos relaying has been implemented, however this is not yet ready for production use. We are also experimenting or actively working to support relaying for Starknet and Sovereign SDK at the moment.
For any questions or concerns, please open an issue in this repo, or send an email to hermes@informal.systems.
Overview
Hermes SDK is a next-generation IBC relayer framework for building high performance and modular IBC relayers. At its core, Hermes SDK makes use of context-generic programming, a new programming paradigm in Rust, to implement the IBC relayer as highly modular components that can be customized and reused easily.
Inter-Blockchain Communication (IBC) and Relaying
For readers who are new to the project, IBC is a protocol that enables secure communication between two blockchains in a permissionless way. IBC mirrors the concepts of networking protocols, with each message sent from one chain to another being represented as an IBC packet. At a high level, we can think of blockchains as pure state machines that have no access to external I/O. To facilitate communication between two blockchains, a relayer is used to deliver the IBC packets from a source chain to a destination chain.
We can think of IBC relaying similar to mail delivery in the real life. Consider the case which Alice wants to send a letter to Bob. She would first put her letter in an envelop, and write down the sender and recipient address. The letter is placed in a mailbox at Alice's home, which is picked up by a mailman who delivers it to Bob's home. In the case of IBC, Alice and Bob would be two chains A and B, and the mailman would be an IBC relayer. The envelop would be an IBC packet, and the mailboxes would be provable storage locations on the respective chains.
Although the concept of IBC relaying is relatively simple, similar to real world mail delivery, complexity arises when there are many packets need to be delivered. There are many cross-cutting concerns in IBC relaying, including latency (time for a packet to be delivered), throughput (number of packets delivered per timeframe), reliability (failure recovery), efficiency (avoid delivering the same packet multiple times), and cost (batch delivery to reduce transaction cost). Instead of choosing a specific strategy, Hermes SDK allows different relaying strategies to be implemented to balance different trade offs.
Context-Generic Programming (CGP)
Hermes SDK makes heavy use of a new programming paradigm, context-generic programming,
which is developed by us to implement the relayer as a collection of loosely-coupled
components. At a high level, CGP allows code to be written to be generic over a
context type, i.e. the type that is used as Self. With that, we can easily define
multiple context types that re-use the same context-generic code through minimal wiring.
For readers with object-oriented programming (OOP) background, CGP shares some similarities with advanced OOP concepts, such as mixins and dependency injection. The main difference is that CGP offers polymorphism at compile time, with no dynamic dispatch involved at runtime. With the use of associated types, CGP also allows strongly-typed relations to be established between different types, thus achieving type safety without any use of upcast or downcast operation.
In Hermes SDK, we make use of the cgp
crate to define context-generic components. To learn more about CGP, readers are
encouraged to read the in-progress book,
Context-Generic Programming Patterns.
Heterogeneous Chain Relaying
Compared to alternative IBC relayers, Hermes SDK is built with first-class support for IBC relaying between different implementations of blockchains. When IBC was first developed, the initial focus was on supporting communication between blockchains in the Cosmos ecosystem, which are implemented using CometBFT and CosmosSDK. As a result, many early IBC relayer implementations are strongly coupled with Cosmos-specific assumptions on how the chain is implemented.
As IBC gains traction within Cosmos, the use of IBC is now expanding beyond the Cosmos ecosystem, to other blockchain ecosystems such as Starknet. A main challenge in supporting non-Cosmos blockchains with existing IBC relayers is that these blockchains may have entirely different ways of how to interact with the chain, including how queries are made, how messages and events are formatted, how transactions are signed and submitted, how to track the consensus finality of chains, etc.
To expand beyond Cosmos, the core relaying logic of Hermes SDK is fully
abstract, and makes no assumption about how to interact with a chain.
All chain types in Hermes SDK, including height, message, event,
transaction, and errors, are defined as abstract associated types
that can be instantiated with any concrete chain-specific types.
Hermes SDK provides abstract interfaces such as the MessageSender
trait, with very little restriction on how chains can implement these
interfaces.
At the moment, we have two sub-projects that make use of Hermes SDK to implement Cosmos to non-Cosmos relaying for Starknet and Sovereign SDK.
Fully Abstract Core Relayer Logic
The core relaying logic of Hermes SDK is implemented in the
hermes-relayer-components crate.
When inspecting the Cargo.toml manifest,
you may notice that aside from the core CGP components definition, there
is no use of any implementation-specific crates, such as std and tokio.
This shows that the core logic of Hermes SDK is fully abstract, and can be
instantiated with any concrete implementation.
Hermes SDK not only abstract over the concrete chain implementation, but also everything else, including error handling, logging, and async runtime. This means that one can easily build a custom IBC relayer that runs in restricted environment, such as on Wasm web applications, or in sandboxed test environments such as Kani.
Type-Safe Relaying
Hermes SDK makes heavy use of associated types to make use of types abstractly. This allows the core relaying logic to differentiate values coming from different chains as different abstract types, even when they have the same underlying concrete type. Because of that, we can make use of the strong type system in Rust to ensure that the core relaying logic is implemented correctly.
For a simplified demonstraction, consider the implementation of the mailman delivery example that we covered earlier. After the mailman collects Alice's letter, it is supposed read the recipient's address on the envelop, and deliver it to Bob. However, we may make a mistake in the mailman's program, and read the sender's address. When using common programming techniques, such an error would only be caught during runtime, when the mailman knocks Alice's door and try to deliver the letter she sent to Bob. But with the techniques used in Hermes SDK, the sender's address would have a different abstract type than the recipient's address, and hence it would result in a compilation error in the mailman's program.
The actual logic for IBC relaying is more complicated than the simplified mailman delivery example, so there are many more places to potentially make such mistakes. Furthermore, it can be difficult to track where the mistake happens, because errors may happen only after the mistaken value has flowed to other parts of the code base.
By making use of CGP and the strong type system provided by Rust, Hermes SDK offers unique advantage over other IBC relaying implementations that the core relaying logic is not only correct now, but that it will always remain correct in the future. We do not need to rely on an expert developer to carefully audit the relayer code to ensure that it is correct. Instead, the Rust compiler helps to the job, and will raise an error any time a developer mistakenly mixes up values coming from different chains.
Context-Generic Chain Clients
The primary use case for Hermes SDK is to implement IBC relaying. But to do that, we first need to implement clients for communicating with a chain full node. It turns out that there are sufficiently many client-side features required, that we have implemented fairly rich chain client libraries for Hermes SDK.
Most of the chain-specific client implementations in Hermes SDK are implemented as context-generic components. This means that one can potentially reuse some of the chain components to build other kinds of client-side blockchain applications. Compared to other chain client libraries, the chain clients provided by Hermes SDK is more extensible and customizable through CGP.
Build Your Own Relayer
Hermes SDK provides concrete relay contexts that are ready to use, without requiring the end user to understand CGP. However, the default relay context may not satisfy the need for all potential use cases. For example, a chain may have implemented some new features which require custom behavior from the chain client.
Most commonly, users would submit feature request to the relayer's project to add custom behavior to specific chains. However, doing so repeatedly can bloat the relayer's code base, and make it more challenging to reason about the special case behavior. In our case, we instead offer Hermes SDK as a library, which users can make use of to build their own custom relayer.
With CGP, developers can reuse chain components provided by Hermes SDK, without having to fork the entire code base. At the same time, any chain component is fully replaceable with customized implementations.
Reproducible Test Framework
Hermes SDK offers a comprehensive test framework for testing the relayer. The Hermes SDK IBC test suite is written as abstract tests that can work with any concrete chain and relay contexts.
Hermes SDK also provides chain bootstrapping implementations, which would bootstrap new blockchain instances to be used for each test case. The bootstrapping helps ensure that each test is reproducible, and simplifies the need to manually setup blockchain instances before running the tests.
Requirements
The crates in this project require Rust with version 1.79.0 or later.
Hermes Guide
We have a comprehensive guide at hermes.informal.systems.
Contributing
See CONTRIBUTING.md.
Versioning
We follow Semantic Versioning, though APIs are still under active development.
Resources
License
Copyright © 2023 Informal Systems Inc. and Hermes authors.
Licensed under the Apache License, Version 2.0 (the "License"); you may not use the files in this repository except in compliance with the License. You may obtain a copy of the License at
https://www.apache.org/licenses/LICENSE-2.0Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.