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Rust Clients

Problem​

High-level tests, such as bench-tps, are written in terms of the Client trait. When we execute these tests as part of the test suite, we use the low-level BankClient implementation. When we need to run the same test against a cluster, we use the ThinClient implementation. The problem with that approach is that it means the trait will continually expand to include new utility functions and all implementations of it need to add the new functionality. By separating the user-facing object from the trait that abstracts the network interface, we can expand the user-facing object to include all sorts of useful functionality, such as the "spinner" from RpcClient, without concern for needing to extend the trait and its implementations.

Proposed Solution​

Instead of implementing the Client trait, ThinClient should be constructed with an implementation of it. That way, all utility functions currently in the Client trait can move into ThinClient. ThinClient could then move into openos-sdk since all its network dependencies would be in the implementation of Client. We would then add a new implementation of Client, called ClusterClient, and that would live in the openos-client crate, where ThinClient currently resides.

After this reorg, any code needing a client would be written in terms of ThinClient. In unit tests, the functionality would be invoked with ThinClient<BankClient>, whereas main() functions, benchmarks and integration tests would invoke it with ThinClient<ClusterClient>.

If higher-level components require more functionality than what could be implemented by BankClient, it should be implemented by a second object that implements a second trait, following the same pattern described here.

Error Handling​

The Client should use the existing TransportError enum for errors, except that the Custom(String) field should be changed to Custom(Box<dyn Error>).

Implementation Strategy​

  1. Add new object to openos-sdk, RpcClientTng, where the Tng suffix is temporary and stands for "The Next Generation"
  2. Initialize RpcClientTng with a SyncClient implementation.
  3. Add new object to openos-sdk, ThinClientTng; initialize it with RpcClientTng and an AsyncClient implementation
  4. Move all unit-tests from BankClient to ThinClientTng<BankClient>
  5. Add ClusterClient
  6. Move ThinClient users to ThinClientTng<ClusterClient>
  7. Delete ThinClient and rename ThinClientTng to ThinClient
  8. Move RpcClient users to new ThinClient<ClusterClient>
  9. Delete RpcClient and rename RpcClientTng to RpcClient