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Amazon EC2 Metadata
We use the following class to obtain Metadata from a running Amazon instance. This code also demonstrates a way set a timeout variable when using the WebClient object.
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| public class AwsHelper | |
| { | |
| private const string LOG_PREFIX = "AwsHelper/"; | |
| /* A class to return Metadata from a running Amazon instance. | |
| Requires: using System.Net; | |
| We call these methods in the startup routine of our windows service. | |
| Here is an example: | |
| // If called from a non AWS instance, each request would take 5 seconds (the timeout), | |
| // so we wrap them to only be called when an AvailabilityZone is returned. | |
| string avZone = AwsHelper.AvailabilityZone; | |
| if (!string.IsNullOrEmpty(avZone)) | |
| { | |
| Logging.LogInfo(logMsg + " Amazon Availability Zone: " + avZone); | |
| Logging.LogInfo(logMsg + " AmiId: " + AwsHelper.AmiId); | |
| Logging.LogInfo(logMsg + " PublicIpv4: " + AwsHelper.PublicIpv4); | |
| Logging.LogInfo(logMsg + " InstanceId: " + AwsHelper.InstanceId); | |
| Logging.LogInfo(logMsg + " InstanceType: " + AwsHelper.InstanceType); | |
| Logging.LogInfo(logMsg + " SecurityGroups: " + AwsHelper.SecurityGroups); | |
| } | |
| Here is a list of other meta data that could be returned. This list is | |
| obtained by calling: http://169.254.169.254/latest/meta-data/ | |
| ami-id | |
| ami-launch-index | |
| ami-manifest-path | |
| block-device-mapping/ | |
| hostname | |
| instance-action | |
| instance-id | |
| instance-type | |
| local-hostname | |
| local-ipv4 | |
| mac | |
| metrics/ | |
| network/ | |
| placement/ | |
| profile | |
| public-hostname | |
| public-ipv4 | |
| public-keys/ | |
| reservation-id | |
| security-groups | |
| services/ | |
| */ | |
| public static string AvailabilityZone | |
| { | |
| // Returns Region + AvailabilityZone, Example: eu-west-1c | |
| get { return GetMetaData("placement/availability-zone"); } | |
| } | |
| public static string PublicIpv4 | |
| { | |
| get { return GetMetaData("public-ipv4"); } | |
| } | |
| public static string AmiId | |
| { | |
| get { return GetMetaData("ami-id"); } | |
| } | |
| public static string InstanceId | |
| { | |
| get { return GetMetaData("instance-id"); } | |
| } | |
| public static string InstanceType | |
| { | |
| get { return GetMetaData("instance-type"); } | |
| } | |
| public static string SecurityGroups | |
| { | |
| get { return GetMetaData("security-groups"); } | |
| } | |
| public static string GetMetaData(string urlSuffix) | |
| { | |
| string logMsg = LOG_PREFIX + "GetMetaData"; | |
| string retVal = ""; | |
| const string URL_PREFIX = "http://169.254.169.254/latest/meta-data/"; | |
| const int WAIT_TIMEOUT_MILLISECONDS = 5000; | |
| try | |
| { | |
| Logging.LogTrace(logMsg + string.Format(", for urlSuffix: {0}", urlSuffix)); | |
| // Create the Full Url we will need to find the data. | |
| string fullUrl = URL_PREFIX; | |
| if (!urlSuffix.StartsWith("/")) | |
| { fullUrl += "/"; } | |
| fullUrl += urlSuffix; | |
| WebClientWithTimeout webClient = new WebClientWithTimeout(WAIT_TIMEOUT_MILLISECONDS); | |
| retVal = webClient.DownloadString(fullUrl); | |
| } | |
| catch (WebException ex) | |
| { | |
| if (ex.Message.Contains("The operation has timed out")) | |
| { | |
| // This "EXCEPTION" will normally be thrown when we try to use this | |
| // method from a computer that is not running on an Amazon server. | |
| Logging.LogTrace(logMsg + ", Property called from a non Amazon server."); | |
| } | |
| else | |
| { Logging.LogError(logMsg + ", WEBEXCEPTION: " + ex.Message, ex); } | |
| } | |
| catch (Exception ex) | |
| { Logging.LogError(logMsg + ", EXCEPTION: " + ex.Message, ex); } | |
| return retVal; | |
| } | |
| private class WebClientWithTimeout : WebClient | |
| { | |
| // The reason we are not using the default WebClient class is because we need to specify | |
| // a timeout. The default Timeout for the WebClient is at least 20 seconds (Some people | |
| // say 100 seconds). When we are running on an Amazon server, the request returns almost | |
| // instantaneously, so the timeout doesn't matter. | |
| // However, when we call this from a non Amazon computer, the code will just "hang" until | |
| // the timeout has elapsed. | |
| // http://stackoverflow.com/questions/1789627/how-to-change-the-timeout-on-a-net-webclient-object | |
| private int _timeoutMilliSeconds = 5000; | |
| public WebClientWithTimeout() { } | |
| public WebClientWithTimeout(int timeoutMilliSeconds) | |
| { | |
| _timeoutMilliSeconds = timeoutMilliSeconds; | |
| } | |
| protected override WebRequest GetWebRequest(Uri uri) | |
| { | |
| WebRequest webRequest = base.GetWebRequest(uri); | |
| webRequest.Timeout = _timeoutMilliSeconds; | |
| return webRequest; | |
| } | |
| } | |
| } |
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Technology
- C#
- WebClient (with timeout)