GraphQL Error Handling and Partial Responses

In GraphQL, field errors do not necessarily cancel the entire request. Servers frequently return both "data" (containing resolved fields) and "errors" in the same response.

ESP32-HTTP-Client preserves all successfully resolved data bindings while making error objects available for inspection.

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Accessing Errors and Partial Data

int userId = 0;
String userName;
GraphQLError error;

client.graphql("/graphql")
    .query("query { user { id name restrictedProfile } }")
    .getData("user.id", &userId)
    .getData("user.name", &userName)
    .getError(&error)
    .onGraphQLError([](const std::vector<GraphQLError>& errors) {
      for (const auto& err : errors) {
        Serial.printf("Error: %s\n", err.message.c_str());
        for (const auto& loc : err.locations) {
          Serial.printf("  at line %d, col %d\n", loc.line, loc.column);
        }
        if (!err.extensions.isEmpty()) {
          Serial.printf("  extensions: %s\n", err.extensions.c_str());
        }
      }
    });

// Partial data is retained and populated!
if (userId > 0) {
  Serial.printf("User ID: %d, Name: %s\n", userId, userName.c_str());
}

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HTTP Status Code Handling

Under the GraphQL over HTTP specification, validation or syntax errors return HTTP 400 with a GraphQL response body containing "errors". GraphQLRequest parses errors regardless of 4xx/5xx status codes:

String errorMessage;

client.graphql("/graphql")
    .query("query { syntaxError ")
    .getErrorMessage(&errorMessage);

if (!errorMessage.isEmpty()) {
  Serial.printf("Parsed error: %s\n", errorMessage.c_str());
}