Fix ListExchangeSteps to not require a filter.

* Expose the party of an ExchangeStep.
* Fix documentation of fields in ListExchangeStepsRequest.Filter.
* Drop fields from ListExchangeStepsRequest.Filter that do not make sense.

src/main/proto/wfa/measurement/api/v2alpha/exchange.proto

@@ -24,7 +24,8 @@ option java_package = "org.wfanet.measurement.api.v2alpha";
 option java_multiple_files = true;
 option java_outer_classname = "ExchangeProto";
 
-// Represents a RecurringExchange on a specific date.
+// Resource that represents an instance of a `RecurringExchange` for a single
+// date.
 message Exchange {
   option (google.api.resource) = {
     type: "halo.wfanet.org/Exchange"
@@ -38,8 +39,8 @@ message Exchange {
   // Resource name.
   string name = 1;
 
-  // Must be a complete date (no field can be unset/zero).
-  google.type.Date date = 2;
+  // Date of the Exchange. Must be a complete date (no field can be unset/zero).
+  google.type.Date date = 2 [(google.api.field_behavior) = REQUIRED];
 
   // State of an `Exchange`.
   enum State {

src/main/proto/wfa/measurement/api/v2alpha/exchange_step.proto

@@ -38,18 +38,42 @@ message ExchangeStep {
   // Resource name.
   string name = 1;
 
-  // State of this `ExchangeStep`.
-  State state = 2 [(google.api.field_behavior) = OUTPUT_ONLY];
-
-  // Serialized ExchangeWorkflow that this ExchangeStep corresponds to.
-  bytes serialized_exchange_workflow = 3;
-
-  // Denormalized Exchange date. Must be a complete date (no field can be
-  // unset/zero).
-  google.type.Date exchange_date = 4;
+  // Denormalized `date` from the parent `Exchange`.
+  //
+  // Must be a complete date (no field can be unset/zero).
+  google.type.Date exchange_date = 4 [
+    (google.api.field_behavior) = OUTPUT_ONLY,
+    (google.api.field_behavior) = IMMUTABLE
+  ];
 
   // Current index of the step inside the serialized_exchange_workflow.
-  int32 step_index = 5;
+  int32 step_index = 5 [
+    (google.api.field_behavior) = REQUIRED,
+    (google.api.field_behavior) = IMMUTABLE
+  ];
+
+  // The party that executes this step. Required.
+  //
+  // This must be one of the parties on the ancestor `RecurringExchange`.
+  oneof party {
+    // Resource name of the `DataProvider` that executes this step.
+    string data_provider = 6 [
+      (google.api.resource_reference).type = "halo.wfanet.org/DataProvider",
+      (google.api.field_behavior) = IMMUTABLE
+    ];
+    // Resource name of the `ModelProvider` that executes this step.
+    string model_provider = 7 [
+      (google.api.resource_reference).type = "halo.wfanet.org/ModelProvider",
+      (google.api.field_behavior) = IMMUTABLE
+    ];
+  }
+
+  // Serialized denormalized `exchange_workflow` field from the ancestor
+  // `RecurringExchange`.
+  bytes serialized_exchange_workflow = 3 [
+    (google.api.field_behavior) = OUTPUT_ONLY,
+    (google.api.field_behavior) = IMMUTABLE
+  ];
 
   // State of an `ExchangeStep`.
   enum State {
@@ -81,4 +105,6 @@ message ExchangeStep {
     // associated ExchangeStepAttempt is in state `FAILED_STEP`.
     FAILED = 6;
   }
+  // State of this `ExchangeStep`.
+  State state = 2 [(google.api.field_behavior) = OUTPUT_ONLY];
 }

src/main/proto/wfa/measurement/api/v2alpha/exchange_step_attempt.proto

@@ -29,6 +29,8 @@ message ExchangeStepAttempt {
   option (google.api.resource) = {
     type: "halo.wfanet.org/ExchangeStepAttempt"
     pattern: "recurringExchanges/{recurring_exchange}/exchanges/{exchange}/steps/{exchange_step}/attempts/{exchange_step_attempt}"
+    pattern: "dataProviders/{data_provider}/recurringExchanges/{recurring_exchange}/exchanges/{exchange}/steps/{exchange_step}/attempts/{exchange_step_attempt}"
+    pattern: "modelProviders/{data_provider}/recurringExchanges/{recurring_exchange}/exchanges/{exchange}/steps/{exchange_step}/attempts/{exchange_step_attempt}"
     singular: "exchangeStepAttempt"
     plural: "exchangeStepAttempts"
   };

src/main/proto/wfa/measurement/api/v2alpha/exchange_steps_service.proto

@@ -37,7 +37,7 @@ service ExchangeSteps {
   rpc ClaimReadyExchangeStep(ClaimReadyExchangeStepRequest)
       returns (ClaimReadyExchangeStepResponse);
 
-  // Lists `ExchangeStep`s.
+  // Lists `ExchangeStep` resources.
   rpc ListExchangeSteps(ListExchangeStepsRequest)
       returns (ListExchangeStepsResponse) {
     option (google.api.method_signature) = "parent";
@@ -72,6 +72,9 @@ message ClaimReadyExchangeStepResponse {
 // Request message for `ListExchangeSteps` method.
 message ListExchangeStepsRequest {
   // Resource name of the parent `Exchange`.
+  //
+  // The wildcard ID (`-`) may be used in place of the `Exchange` ID to list
+  // across every `Exchange` in the ancestor `RecurringExchange`.
   string parent = 1 [
     (google.api.resource_reference).type = "halo.wfanet.org/Exchange",
     (google.api.field_behavior) = REQUIRED
@@ -94,34 +97,22 @@ message ListExchangeStepsRequest {
   // Filter criteria. Repeated fields are treated as logical ORs, and multiple
   // fields specified as logical ANDs.
   message Filter {
+    reserved 3, 4;
+
     // Matches against the `exchange_date` field.
     repeated google.type.Date exchange_dates = 1;
     // Matches against the `state` field.
     //
     // (-- api-linter: core::0216::state-field-output-only=disabled
     //     aip.dev/not-precedent: This is not a resource state field. --)
     repeated ExchangeStep.State states = 2;
-    // Matches against the `event_data_provider` field of the ancestor
-    // `RecurringExchange`.
-    repeated string recurring_exchange_data_providers = 3
+
+    // Matches against the `data_provider` field.
+    string data_provider = 5
         [(google.api.resource_reference).type = "halo.wfanet.org/DataProvider"];
-    // Matches against the `model_provider` field of the ancestor
-    // `RecurringExchange`.
-    repeated string recurring_exchange_model_providers = 4
-        [(google.api.resource_reference).type =
-             "halo.wfanet.org/ModelProvider"];
-
-    // Party name.
-    oneof party {
-      // Matches against the `event_data_provider` field of the ancestor
-      // `RecurringExchange`.
-      string data_provider = 5 [(google.api.resource_reference).type =
-                                    "halo.wfanet.org/DataProvider"];
-      // Matches against the `model_provider` field of the ancestor
-      // `RecurringExchange`.
-      string model_provider = 6 [(google.api.resource_reference).type =
-                                     "halo.wfanet.org/ModelProvider"];
-    }
+    // Matches against the `model_provider` field.
+    string model_provider = 6 [(google.api.resource_reference).type =
+                                   "halo.wfanet.org/ModelProvider"];
   }
   // Filter criteria for this request.
   //