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API Behavior

This page documents the behavior of a hydra-node at the API layer. That is, how the system behaves given ClientInputs and what ServerOutputs are produced in response to it. See also the API reference for more details about individual API messages. The only discrepancy is http POST /commit action which is not a state transition but a user action that submits a commit transaction which should produce Committed output.

The formalism uses UML statechart language where transitions are labeled: input [condition] / output. When two outputs (e.g. A and B) are expected we write A,B, while {A,B} denotes mutual exclusiveness of outputs.

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Not pictured is the CommandFailed output, which is implicit emitted whenever an input is used when no transition below applies. Also non-state-changing or life-cycle relevant inputs like GetUTxO are not mentioned, as well as outputs like Greetings, InvalidInput, PeerConnected, PeerDisconnected and GetUTxOResponse.

API configuration

There are some options for API clients to control the server outputs. Server outputs are controlled using the following query parameters:

  • history=no -> Prevents historical outputs display. All server outputs are recorded and when a client re-connects these outputs are replayed unless history=no query param is used.
  • tx-output=cbor -> Outputs transaction fields encoded as CBOR instead of default JSON.
  • snapshot-utxo=no -> In case of a SnapshotConfirmed message the utxo field in the inner Snapshot will be omitted.

Replay of past server outputs

When a hydra-node restarts, by default it will load its history from persistence and replay previous server outputs to enable clients to re-establish their state upon re-connection. If that happens, obviously some of these outputs are not relevant anymore. One example of this is the PeerConnected and PeerDisconnected. To make it possible to determine the end of replayed history, client applications can use the Greetings, which will be emitted on every hydra-node start. See the hydra-tui example client for how this is handled.

Clients can optionally decide to skip history outputs and receive only the Greetings and following ones. In order to do that they can use query param history=no.

For example if the client wants to connect to a local hydra-node and doesn't want to view the server history but also wants to have the transactions encoded as CBOR (base16) and prevent utxo display in SnapshotConfirmed messages, they would connect using default port 4001 and the full path ws://localhost:4001/?history=no&tx-output=cbor.