com.google.apphosting.vmruntime.VmMetadataCache Maven / Gradle / Ivy

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/**
 * Copyright 2012 Google Inc. All Rights Reserved.
 *
 * Licensed 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 com.google.apphosting.vmruntime;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.HashMap;
import java.util.Map;
import java.util.logging.Logger;

/**
 * A class to retrieve and cache the meta-data of a VM running in Google's Compute Engine.
 */
public class VmMetadataCache {

    private static final Logger logger = Logger.getLogger(VmMetadataCache.class.getCanonicalName());

    /**
     * The meta-data server's URL prefix.
     */
    public static final String DEFAULT_META_DATA_SERVER = "metadata";
    public static final String META_DATA_PATTERN = "http://%s/0.1/meta-data/%s";

    /**
     * Maps paths to their cached values (null if a previous retrieval attempt failed).
     */
    private final Map cache;

    /**
     * Timeout in milliseconds to retrieve data from the server.
     */
    private static final int TIMEOUT_MILLIS = 120 * 1000;

    public VmMetadataCache() {
        cache = new HashMap<>();
    }

    /**
     * Returns the value of the VM's meta-data attribute, or null if retrieval has failed.
     *
     * @param path the meta-data attribute to be retrieved (e.g. "image", "attributes/sshKeys").
     * @return the attribute's string value or null if retrieval has failed.
     */
    public String getMetadata(String path) {
        synchronized (cache) {
            if (cache.containsKey(path)) {
                return cache.get(path);
            }
        }

        String value = null;
        try {
            value = getMetadataFromServer(path);

            synchronized (cache) {
                cache.put(path, value);
            }
        } catch (IOException e) {
            logger.info("Meta-data '" + path + "' path retrieval error: " + e.getMessage());
        }
        return value;
    }

    /**
     * Returns an HTTP URL connection to read the value of the specified attribute.
     * 
     * May be overridden in tests.
     *
     * @param path the meta-data attribute to be retrieved (e.g. "image", "attributes/sshKeys").
     * @return the HTTP URL connection object.
     * @throws IOException if the connection could not be opened.
     */
    protected HttpURLConnection openConnection(String path) throws IOException {
        String server = System.getProperty("metadata_server", DEFAULT_META_DATA_SERVER);
        URL url = new URL(String.format(META_DATA_PATTERN, server, path));
        return (HttpURLConnection) url.openConnection();
    }

    /**
     * Retrieves the specified path from the VM's meta-data server.
     *
     * @param path a path in the meta-data server (e.g. "image").
     * @return the meta-data's string value or null if the attribute is not found.
     * @throws IOException if the connection to the meta-data server fails, or refused.
     */
    protected String getMetadataFromServer(String path) throws IOException {
        BufferedReader reader = null;
        HttpURLConnection connection = null;
        try {
            connection = openConnection(path);
            connection.setConnectTimeout(TIMEOUT_MILLIS);
            connection.setReadTimeout(TIMEOUT_MILLIS);
            reader = new BufferedReader(new InputStreamReader(connection.getInputStream()));
            StringBuffer result = new StringBuffer();
            char[] buffer = new char[4096];
            int read;
            while ((read = reader.read(buffer)) != -1) {
                result.append(buffer, 0, read);
            }
            if (connection.getResponseCode() == HttpURLConnection.HTTP_OK) {
                return result.toString().trim();
            } else if (connection.getResponseCode() == HttpURLConnection.HTTP_NOT_FOUND) {
                return null;
            }
            throw new IOException("Meta-data request for '" + path + "' failed with error: " +
                    connection.getResponseMessage());
        } finally {
            if (reader != null) {
                try {
                    reader.close();
                } catch (IOException e) {
                    logger.info("Error closing connection for " + path + ": " + e.getMessage());
                }
            }
            if (connection != null) {
                connection.disconnect();
            }
        }
    }
}