# Controllers

Istio has a variety of [controllers](https://kubernetes.io/docs/concepts/architecture/controller/), which basically watch some inputs and do something.
This can be reading from Kubernetes and writing other objects back, writing to proxies over XDS, etc.

Unfortunately, writing controllers is very error prone, even for seemingly simple cases.
To work around this, Istio has a variety of abstractions meant to make writing controllers easier.

## Clients

      // checkNotNull for GWT (do not optimize)
      array[i] = ((Number) checkNotNull(boxedArray[i])).doubleValue();
    }
    return array;
  }

  /**
   * Returns a fixed-size list backed by the specified array, similar to {@link
   * Arrays#asList(Object[])}. The list supports {@link List#set(int, Object)}, but any attempt to
   * set a value to {@code null} will result in a {@link NullPointerException}.
   *

                                </table>
                            </div>
                            <div class="card-footer">
                                <button type="submit" class="btn btn-default" name="list" value="back">
                                    <em class="fa fa-arrow-circle-left">
                                    <la:message key="labels.crud_button_back"/>
                                </button>

  // Status is filled in by the server with the user attributes.
  optional SelfSubjectReviewStatus status = 2;
}

// SelfSubjectReviewStatus is filled by the kube-apiserver and sent back to a user.
message SelfSubjectReviewStatus {
  // User attributes of the user making this request.
  // +optional
  optional k8s.io.api.authentication.v1.UserInfo userInfo = 1;
}

                    () -> asListHtml(form.dictId));
        });
        saveToken();
        if (form.crudMode.intValue() == CrudMode.EDIT) {
            // back
            form.crudMode = CrudMode.DETAILS;
            return asDetailsHtml();
        }
        form.crudMode = CrudMode.EDIT;
        return asEditHtml();
    }

der stattfindet, wenn Ihre API-Anwendung die *externe API* aufruft, wird als „Callback“ („Rückruf“) bezeichnet. Denn die Software, die der externe Entwickler geschrieben hat, sendet einen Request an Ihre API und dann *ruft Ihre API zurück* (*calls back*) und sendet einen Request an eine *externe API* (die wahrscheinlich vom selben Entwickler erstellt wurde).

In diesem Fall möchten Sie möglicherweise dokumentieren, wie diese externe API aussehen *sollte*. Welche *Pfadoperation* sie haben sollte,...

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	// Lock log message printing to prevent log messages from different CNI
	// processes interleave.
	l.mu.Lock()
	defer l.mu.Unlock()
	for _, m := range messages {
		logger := pluginLog
		// For any k/v pairs, add them back
		for k, v := range m.Arbitrary {
			logger = logger.WithLabels(k, v)
		}
		// There is no fatal log from CNI plugin
		switch m.Level {
		case "debug":
			logger.LogWithTime(istiolog.DebugLevel, m.Msg, m.Time)

            searchPaging(data, form);
        });
    }

    @Execute
    @Secured({ ROLE, ROLE + VIEW })
    public HtmlResponse back(final SearchForm form) {
        saveToken();
        return asHtml(path_AdminCrawlinginfo_AdminCrawlinginfoJsp).renderWith(data -> {
            searchPaging(data, form);
        });
    }

        testDir = testFile.getParentFile(); // the testdata directory
      } catch (Exception ignore) {
        // probably URISyntaxException or IllegalArgumentException
        // fall back to copying URLs to files in the testDir == null block below
      }
    }

    if (testDir == null) {
      // testdata resources aren't file:// urls, so create a directory to store them in and then