/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.flink.runtime.webmonitor.metrics;

import com.fasterxml.jackson.core.JsonGenerator;
import org.apache.flink.runtime.instance.ActorGateway;
import org.apache.flink.runtime.webmonitor.handlers.AbstractJsonRequestHandler;
import org.apache.flink.runtime.webmonitor.handlers.JsonFactory;
import org.apache.flink.util.Preconditions;

import java.io.IOException;
import java.io.StringWriter;
import java.util.Map;

/**
 * Abstract request handler that returns a list of all available metrics or the values for a set of metrics.
 *
 * If the query parameters do not contain a "get" parameter the list of all metrics is returned.
 * {@code [ { "id" : "X" } ] }
 *
 * If the query parameters do contain a "get" parameter a comma-separate list of metric names is expected as a value.
 * {@code /get?X,Y}
 * The handler will then return a list containing the values of the requested metrics.
 * {@code [ { "id" : "X", "value" : "S" }, { "id" : "Y", "value" : "T" } ] }
 */
public abstract class AbstractMetricsHandler extends AbstractJsonRequestHandler {
	private final MetricFetcher fetcher;

	public AbstractMetricsHandler(MetricFetcher fetcher) {
		this.fetcher = Preconditions.checkNotNull(fetcher);
	}

	@Override
	public String handleJsonRequest(Map<String, String> pathParams, Map<String, String> queryParams, ActorGateway jobManager) throws Exception {
		fetcher.update();
		String requestedMetricsList = queryParams.get("get");
		return requestedMetricsList != null
			? getMetricsValues(pathParams, requestedMetricsList)
			: getAvailableMetricsList(pathParams);
	}

	/**
	 * Returns a Map containing the metrics belonging to the entity pointed to by the path parameters.
	 *
	 * @param pathParams REST path parameters
	 * @param metrics MetricStore containing all metrics
	 * @return Map containing metrics, or null if no metric exists
	 */
	protected abstract Map<String, String> getMapFor(Map<String, String> pathParams, MetricStore metrics);

	private String getMetricsValues(Map<String, String> pathParams, String requestedMetricsList) throws IOException {
		if (requestedMetricsList.isEmpty()) {
			/**
			 * The WebInterface doesn't check whether the list of available metrics was empty. This can lead to a 
			 * request for which the "get" parameter is an empty string.
			 */
			return "";
		}
		MetricStore metricStore = fetcher.getMetricStore();
		synchronized (metricStore) {
			Map<String, String> metrics = getMapFor(pathParams, metricStore);
			if (metrics == null) {
				return "";
			}
			String[] requestedMetrics = requestedMetricsList.split(",");

			StringWriter writer = new StringWriter();
			JsonGenerator gen = JsonFactory.jacksonFactory.createGenerator(writer);

			gen.writeStartArray();
			for (String requestedMetric : requestedMetrics) {
				Object metricValue = metrics.get(requestedMetric);
				if (metricValue != null) {
					gen.writeStartObject();
					gen.writeStringField("id", requestedMetric);
					gen.writeStringField("value", metricValue.toString());
					gen.writeEndObject();
				}
			}
			gen.writeEndArray();

			gen.close();
			return writer.toString();
		}
	}

	private String getAvailableMetricsList(Map<String, String> pathParams) throws IOException {
		MetricStore metricStore = fetcher.getMetricStore();
		synchronized (metricStore) {
			Map<String, String> metrics = getMapFor(pathParams, metricStore);
			if (metrics == null) {
				return "";
			}
			StringWriter writer = new StringWriter();
			JsonGenerator gen = JsonFactory.jacksonFactory.createGenerator(writer);

			gen.writeStartArray();
			for (String m : metrics.keySet()) {
				gen.writeStartObject();
				gen.writeStringField("id", m);
				gen.writeEndObject();
			}
			gen.writeEndArray();

			gen.close();
			return writer.toString();
		}
	}
}