Simple Json Schemas for Kotlin (#403)

sdk-api-kotlin-gen/src/test/kotlin/dev/restate/sdk/kotlin/CodegenTest.kt

@@ -57,8 +57,8 @@ class CodegenTest : TestDefinitions.TestSuite {
     @Exclusive
     suspend fun complexType(
         context: ObjectContext,
-        request: Map<Output, List<out Input>>
-    ): Map<Input, List<out Output>> {
+        request: Map<String, List<out Input>>
+    ): Map<String, List<out Output>> {
       return mapOf()
     }
   }

sdk-api-kotlin/src/main/kotlin/dev/restate/sdk/kotlin/KtSerdes.kt

@@ -9,14 +9,26 @@
 package dev.restate.sdk.kotlin
 
 import dev.restate.sdk.common.DurablePromiseKey
+import dev.restate.sdk.common.RichSerde
 import dev.restate.sdk.common.Serde
 import dev.restate.sdk.common.StateKey
 import java.nio.ByteBuffer
 import java.nio.charset.StandardCharsets
 import kotlin.reflect.typeOf
+import kotlinx.serialization.ExperimentalSerializationApi
 import kotlinx.serialization.KSerializer
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.builtins.ListSerializer
+import kotlinx.serialization.builtins.serializer
+import kotlinx.serialization.descriptors.PrimitiveKind
+import kotlinx.serialization.descriptors.SerialDescriptor
+import kotlinx.serialization.descriptors.StructureKind
+import kotlinx.serialization.encodeToString
 import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonArray
+import kotlinx.serialization.json.JsonElement
 import kotlinx.serialization.json.JsonNull
+import kotlinx.serialization.json.JsonTransformingSerializer
 import kotlinx.serialization.serializer
 
 object KtStateKey {
@@ -70,12 +82,13 @@ object KtSerdes {
       }
 
   /** Creates a [Serde] implementation using the `kotlinx.serialization` json module. */
-  fun <T : Any?> json(serializer: KSerializer<T>): Serde<T> {
-    return object : Serde<T> {
+  inline fun <reified T : Any?> json(serializer: KSerializer<T>): Serde<T> {
+    return object : RichSerde<T> {
       override fun serialize(value: T?): ByteArray {
         if (value == null) {
           return Json.encodeToString(JsonNull.serializer(), JsonNull).encodeToByteArray()
         }
+
         return Json.encodeToString(serializer, value).encodeToByteArray()
       }
 
@@ -86,6 +99,76 @@ object KtSerdes {
       override fun contentType(): String {
         return "application/json"
       }
+
+      override fun jsonSchema(): String {
+        val schema: JsonSchema = serializer.descriptor.jsonSchema()
+        return Json.encodeToString(schema)
+      }
     }
   }
+
+  @Serializable
+  @PublishedApi
+  internal data class JsonSchema(
+      @Serializable(with = StringListSerializer::class) val type: List<String>? = null,
+      val format: String? = null,
+  ) {
+    companion object {
+      val INT = JsonSchema(type = listOf("number"), format = "int32")
+
+      val LONG = JsonSchema(type = listOf("number"), format = "int64")
+
+      val DOUBLE = JsonSchema(type = listOf("number"), format = "double")
+
+      val FLOAT = JsonSchema(type = listOf("number"), format = "float")
+
+      val STRING = JsonSchema(type = listOf("string"))
+
+      val BOOLEAN = JsonSchema(type = listOf("boolean"))
+
+      val OBJECT = JsonSchema(type = listOf("object"))
+
+      val LIST = JsonSchema(type = listOf("array"))
+
+      val ANY = JsonSchema()
+    }
+  }
+
+  object StringListSerializer :
+      JsonTransformingSerializer<List<String>>(ListSerializer(String.Companion.serializer())) {
+    override fun transformSerialize(element: JsonElement): JsonElement {
+      require(element is JsonArray)
+      return element.singleOrNull() ?: element
+    }
+  }
+
+  /**
+   * Super simplistic json schema generation. We should replace this with an appropriate library.
+   */
+  @OptIn(ExperimentalSerializationApi::class)
+  @PublishedApi
+  internal fun SerialDescriptor.jsonSchema(): JsonSchema {
+    var schema =
+        when (this.kind) {
+          PrimitiveKind.BOOLEAN -> JsonSchema.BOOLEAN
+          PrimitiveKind.BYTE -> JsonSchema.INT
+          PrimitiveKind.CHAR -> JsonSchema.STRING
+          PrimitiveKind.DOUBLE -> JsonSchema.DOUBLE
+          PrimitiveKind.FLOAT -> JsonSchema.FLOAT
+          PrimitiveKind.INT -> JsonSchema.INT
+          PrimitiveKind.LONG -> JsonSchema.LONG
+          PrimitiveKind.SHORT -> JsonSchema.INT
+          PrimitiveKind.STRING -> JsonSchema.STRING
+          StructureKind.LIST -> JsonSchema.LIST
+          StructureKind.MAP -> JsonSchema.OBJECT
+          else -> JsonSchema.ANY
+        }
+
+    // Add nullability constraint
+    if (this.isNullable && schema.type != null) {
+      schema = schema.copy(type = schema.type.plus("null"))
+    }
+
+    return schema
+  }
 }

sdk-common/src/main/java/dev/restate/sdk/common/RichSerde.java

@@ -14,12 +14,15 @@
 /**
  * Richer version of {@link Serde} containing schema information.
  *
+ * <p>This API should be considered unstable to implement.
+ *
  * <p>You can create one using {@link #withSchema(Object, Serde)}.
  */
 public interface RichSerde<T extends @Nullable Object> extends Serde<T> {
 
   /**
-   * @return a Draft 2020-12 Json Schema
+   * @return a Draft 2020-12 Json Schema. It should be self-contained, and MUST not contain refs to
+   *     files. If the schema shouldn't be serialized with Jackson, return a {@link String}
    */
   Object jsonSchema();
 

sdk-core/src/main/java/dev/restate/sdk/core/EndpointManifest.java

@@ -10,6 +10,7 @@
 
 import static dev.restate.sdk.core.ServiceProtocol.*;
 
+import com.fasterxml.jackson.core.JsonProcessingException;
 import dev.restate.sdk.common.HandlerType;
 import dev.restate.sdk.common.RichSerde;
 import dev.restate.sdk.common.ServiceType;
@@ -108,8 +109,17 @@ private static Input convertHandlerInput(HandlerSpecification<?, ?> spec) {
             : new Input().withRequired(true).withContentType(acceptContentType);
 
     if (spec.getRequestSerde() instanceof RichSerde) {
-      input.setJsonSchema(
-          Objects.requireNonNull(((RichSerde<?>) spec.getRequestSerde()).jsonSchema()));
+      Object jsonSchema =
+          Objects.requireNonNull(((RichSerde<?>) spec.getRequestSerde()).jsonSchema());
+      if (jsonSchema instanceof String) {
+        // We need to convert it to databind JSON value
+        try {
+          jsonSchema = MANIFEST_OBJECT_MAPPER.readTree((String) jsonSchema);
+        } catch (JsonProcessingException e) {
+          throw new RuntimeException("The schema generated by RichSerde is not a valid JSON", e);
+        }
+      }
+      input.setJsonSchema(jsonSchema);
     }
     return input;
   }
@@ -123,8 +133,17 @@ private static Output convertHandlerOutput(HandlerSpecification<?, ?> spec) {
                 .withSetContentTypeIfEmpty(false);
 
     if (spec.getResponseSerde() instanceof RichSerde) {
-      output.setJsonSchema(
-          Objects.requireNonNull(((RichSerde<?>) spec.getResponseSerde()).jsonSchema()));
+      Object jsonSchema =
+          Objects.requireNonNull(((RichSerde<?>) spec.getResponseSerde()).jsonSchema());
+      if (jsonSchema instanceof String) {
+        // We need to convert it to databind JSON value
+        try {
+          jsonSchema = MANIFEST_OBJECT_MAPPER.readTree((String) jsonSchema);
+        } catch (JsonProcessingException e) {
+          throw new RuntimeException("The schema generated by RichSerde is not a valid JSON", e);
+        }
+      }
+      output.setJsonSchema(jsonSchema);
     }
 
     return output;

sdk-core/src/main/java/dev/restate/sdk/core/ServiceProtocol.java

@@ -135,7 +135,7 @@ static String serviceDiscoveryProtocolVersionToHeaderValue(
             "Service discovery protocol version '%s' has no header value", version.getNumber()));
   }
 
-  private static final ObjectMapper MANIFEST_OBJECT_MAPPER = new ObjectMapper();
+  static final ObjectMapper MANIFEST_OBJECT_MAPPER = new ObjectMapper();
 
   @JsonFilter("V2FieldsFilter")
   interface V2Mixin {}