Using Amazon Kinesis with the AWS.NET API Part 6: storage

Introduction

In the previous post we added some validation to our demo message handling application. Validation adds some sanity checks to our logic so that bogus inputs are discarded.

In this post, which will be the last in the series on Amazon Kinesis, we’ll be looking at storage. We’ll save the data on disk, which in itself is not too interesting, but we’ll also discuss some formats that are suitable for further processing.

Formats

It is seldom that we’re saving data just to fill up a data store. This is true in our case as well. We’re getting the messages from the Kinesis stream and we’ll be soon saving them. However, we’ll certainly want to perform some actions on the data, such as data aggregations:

• Calculate the average response time for http://www.bbc.co.uk/africa between 12:15 to 12:30 on 13 January 2015 for users with Firefox 11
• Calculate max response time in week 45 2014 for the domain cnn.com for users located in Seattle
• Calculate the 99th percentile of the response time for http://www.twitter.com for February 2014

…etc. Regardless of where you’re planning to save the data, such as a traditional relational DB like MS SQL or a NoSql DB such as MongoDb, you’ll need to plan on the storage format i.e. what tables, collections, columns and datatypes you’ll need. As the next Amazon component we’ll take up on this blog is the blob storage S3 we’ll be concentrating on storing the raw data points in a text file. At first this may seem like a very bad idea but S3 is a very efficient, durable and scalable storage. However, don’t assume that this is a must for your Big Data system to work, you can save your data the way you want. Here we’re just paving the way for the next step.

As mentioned before in this series I have another, higher-level set of posts dedicated to Amazon architecture available here. I took up a similar topic there about message formats, I’ll re-use some of those explanations below.

The format will most likely depend on the mechanism that will eventually pull data from the raw data store. Data mining and analysis solutions such as Amazon RedShift or Elastic MapReduce (EMR) – which we’ll take up later on – will all need to work with the raw data. So at this stage you’ll need to do some forward thinking:

• A: What mechanism will need to read from the raw data store for aggregation?
• B: How can we easily – or relatively easily – read the raw data visually by just opening a raw data file?

B is important for debugging purposes if you want to verify the calculations. It’s also important if some customer is interested in viewing the raw data for some time period. For B you might want to store the raw data as it is, i.e. as JSON. E.g. you can have a text file with the following data points:

{"CustomerId": "abc123", "DateUnixMs": 1416603010000, "Activity": "buy", "DurationMs": 43253}
{"CustomerId": "abc123", "DateUnixMs": 1416603020000, "Activity": "buy", "DurationMs": 53253}
{"CustomerId": "abc123", "DateUnixMs": 1416603030000, "Activity": "buy", "DurationMs": 63253}
{"CustomerId": "abc123", "DateUnixMs": 1416603040000, "Activity": "buy", "DurationMs": 73253}

…i.e. with one data point per line.

However, this format is not really suitable for point A above. Other mechanisms will have a hard time understanding this data format. For RedShift and EMR to work most efficiently we’ll need to store the raw data in some delimited fields such as CSV or tab delimited fields. So the above data points will then be stored as follows in a tab-delimited file:

abc123     1416603010000    buy    43253
abc123     1416603020000    buy    53253
abc123     1416603030000    buy    63253
abc123     1416603040000    buy    73253

This is probably OK for point B above as well. It’s not too hard on your eyes to understand this data structure so we’ll settle for that. You might ask why we didn’t select some other delimiter, such as a pipe ‘|’ or a comma ‘,’. The answer is that our demo system is based on URLs and URLs can have pipes and commas in them making them difficult to split. Tabs will work better but you are free to choose whatever fits your system best.

Implementation

This time we’ll hide the implementation of the storage mechanism behind an interface. It will be a forward-looking solution where we’ll be able to easily switch between the concrete implementations. Open the demo C# application we’ve been working on so far and locate the WebTransaction object in the AmazonKinesisConsumer application. We’ll add a method to create a tab-delimited string out of its properties:

public string ToTabDelimitedString()
{
	StringBuilder sb = new StringBuilder();
	sb.Append(CustomerName)
		.Append("\t")
		.Append(Url)
		.Append("\t")
		.Append(WebMethod)
		.Append("\t")
		.Append(ResponseTimeMs)
		.Append("\t")
		.Append(UtcDateUnixMs);
	return sb.ToString();
}

Create a text file on your hard drive, like c:\raw-data\storage.txt. Add the following interface to AmazonKinesisConsumer:

public interface IRawDataStorage
{
	void Save(IEnumerable<WebTransaction> webTransactions);
}

…and also the following file based implementation:

public class FileBasedDataStorage : IRawDataStorage
{
	private readonly FileInfo _fileName;

	public FileBasedDataStorage(string fileFullPath)
	{
		if (string.IsNullOrEmpty(fileFullPath)) throw new ArgumentNullException("File full path");
		_fileName = new FileInfo(fileFullPath);
		if (!_fileName.Exists)
		{
			throw new ArgumentException(string.Concat("Provided file path ", fileFullPath, " does not exist."));
		}			
	}
		
	public void Save(IEnumerable<WebTransaction> webTransactions)
	{
		StringBuilder stringBuilder = new StringBuilder();
		foreach (WebTransaction wt in webTransactions)
		{
			stringBuilder.Append(wt.ToTabDelimitedString()).Append(Environment.NewLine);
		}

		using (StreamWriter sw = File.AppendText(_fileName.FullName))
		{
			sw.Write(stringBuilder.ToString());
		}
	}
}

The implementation of the Save method should be quite straightforward. We build a string with the tab delimited representation of the WebTransaction object which is then appended to the source file.

Here comes the updated ReadFromStream() method:

private static void ReadFromStream()
{
	IRawDataStorage rawDataStorage = new FileBasedDataStorage(@"c:\raw-data\storage.txt");
	AmazonKinesisConfig config = new AmazonKinesisConfig();
	config.RegionEndpoint = Amazon.RegionEndpoint.EUWest1;
	AmazonKinesisClient kinesisClient = new AmazonKinesisClient(config);
	String kinesisStreamName = ConfigurationManager.AppSettings["KinesisStreamName"];

	DescribeStreamRequest describeRequest = new DescribeStreamRequest();
	describeRequest.StreamName = kinesisStreamName;

	DescribeStreamResponse describeResponse = kinesisClient.DescribeStream(describeRequest);
	List<Shard> shards = describeResponse.StreamDescription.Shards;

	foreach (Shard shard in shards)
	{
		GetShardIteratorRequest iteratorRequest = new GetShardIteratorRequest();
		iteratorRequest.StreamName = kinesisStreamName;
		iteratorRequest.ShardId = shard.ShardId;
		iteratorRequest.ShardIteratorType = ShardIteratorType.TRIM_HORIZON;

		GetShardIteratorResponse iteratorResponse = kinesisClient.GetShardIterator(iteratorRequest);
		string iteratorId = iteratorResponse.ShardIterator;

		while (!string.IsNullOrEmpty(iteratorId))
		{
			GetRecordsRequest getRequest = new GetRecordsRequest();
			getRequest.Limit = 1000;
			getRequest.ShardIterator = iteratorId;

			GetRecordsResponse getResponse = kinesisClient.GetRecords(getRequest);
			string nextIterator = getResponse.NextShardIterator;
			List<Record> records = getResponse.Records;

			if (records.Count > 0)
			{
				Console.WriteLine("Received {0} records. ", records.Count);
				List<WebTransaction> newWebTransactions = new List<WebTransaction>();
				foreach (Record record in records)
				{
					string json = Encoding.UTF8.GetString(record.Data.ToArray());
					try
					{
						JToken token = JContainer.Parse(json);
						try
						{									
							WebTransaction wt = JsonConvert.DeserializeObject<WebTransaction>(json);
							List<string> validationErrors = wt.Validate();
							if (!validationErrors.Any())
							{
								Console.WriteLine("Valid entity: {0}", json);
								newWebTransactions.Add(wt);
							}
							else
							{
								StringBuilder exceptionBuilder = new StringBuilder();
								exceptionBuilder.Append("Invalid WebTransaction object from JSON: ")
									.Append(Environment.NewLine).Append(json)
									.Append(Environment.NewLine).Append("Validation errors: ")
									.Append(Environment.NewLine);
								foreach (string error in validationErrors)
								{
									exceptionBuilder.Append(error).Append(Environment.NewLine);																										
								}
								Console.WriteLine(exceptionBuilder.ToString());
							}									
						}
						catch (Exception ex)
						{
							//simulate logging
							Console.WriteLine("Could not parse the following message to a WebTransaction object: {0}", json);
						}
					}
					catch (Exception ex)
					{
						//simulate logging
						Console.WriteLine("Could not parse the following message, invalid json: {0}", json);
					}
				}

				if (newWebTransactions.Any())
				{
					try
					{
						rawDataStorage.Save(newWebTransactions);
						Console.WriteLine("Saved all new web transactions to the data store.");
					}
					catch (Exception ex)
					{
						Console.WriteLine("Failed to save the web transactions to file: {0}", ex.Message);
					}
				}
			}

			iteratorId = nextIterator;
		}
	}
}

Run both the consumer and producer applications and send a couple of web transactions to Kinesis. You should end up with the tab delimited observations in the storage file. In my case I have the following:

yahoo http://www.yahoo.com GET 432 1417556120657
google http://www.google.com POST 532 1417556133322
bbc http://www.bbc.co.uk GET 543 1417556148276
twitter http://www.twitter.com GET 623 1417556264008
wiki http://www.wikipedia.org POST 864 1417556302529
facebook http://www.facebook.com DELETE 820 1417556319381

This concludes our discussion of Amazon Kinesis. We’ve also set the path for the next series where we’ll be looking into Amazon S3. If you’re interested in a full Big Data chain using cloud-based Amazon components then you’re more than welcome to read on.

View all posts related to Amazon Web Services and Big Data here.

Using Amazon Kinesis with the AWS.NET API Part 5: validation

Introduction

In the previous post we got as far as having a simple but functioning messaging system. The producer and client apps are both console based and the message handler is the ready-to-use Amazon Kinesis. We have a system that we can built upon and scale up as the message load increases. Kinesis streams can be scaled to handle virtually unlimited amounts of messages.

This post on Kinesis will discuss message validation.

You’ll need to handle the incoming messages from the stream. Normally they should follow the specified format, such as JSON or XML with the predefined property names and casing. However, this is not always guaranteed as Kinesis does not itself validate any incoming message. Also, your system might be subject to fake data. So you’ll almost always need to have some message validation in place and log messages that cannot be processed or are somehow invalid.

Open the demo application we’ve been working on so far and let’s get to it.

Validation

We ended up with the following bit of code in AmazonKinesisConsumer:

if (records.Count > 0)
{
	Console.WriteLine("Received {0} records. ", records.Count);
	foreach (Record record in records)
	{
		string json = Encoding.UTF8.GetString(record.Data.ToArray());
		Console.WriteLine("Json string: " + json);
	}
}

We’ll build up the new code step by step and present the new version of the ReadFromStream() method at the end.

Our first task is to check if “json” is in fact valid JSON. There’s no dedicated method for that in JSON.NET so we’ll just see if the string can be parsed into a generic JToken:

string json = Encoding.UTF8.GetString(record.Data.ToArray());
try
{
        JToken token = JContainer.Parse(json);
}
catch (Exception ex)
{
        //simulate logging
	Console.WriteLine("Could not parse the following message, invalid json: {0}", json);
}

Normally every message that cannot be parsed should be logged and analysed. Here we just print the unparseable message to the console. If you’re interested in logging you can check out the posts on this blog here and here.

Next we want to parse the JSON into a WebTransaction object:

try
{
	JToken token = JContainer.Parse(json);
        try
	{
		WebTransaction wt = JsonConvert.DeserializeObject<WebTransaction>(json);
	}
	catch (Exception ex)
	{
		//simulate logging
		Console.WriteLine("Could not parse the following message to a WebTransaction object: {0}", json);
	}
}
catch (Exception ex)
{
	//simulate logging
	Console.WriteLine("Could not parse the following message, invalid json: {0}", json);
}

Next we can perform some validation on the object itself. We’ll make up some arbitrary rules:

• The web method can only be one of the following: GET, POST, PUT, HEAD, DELETE, OPTIONS, TRACE, CONNECT
• Acceptable range for response times: 0-30000 ms, probably not wide enough, but it’s OK for now
• We only accept valid URLs using a validator function I’ve found here. It might not be perfect but at least we can filter out useless inputs like “this is spam” or “you’ve been hacked”

We’ll add the validation rules to WebTransaction.cs of the AmazonKinesisConsumer app:

public class WebTransaction
{
	private string[] _validMethods = { "get", "post", "put", "delete", "head", "options", "trace", "connect" };
	private int _minResponseTimeMs = 0;
	private int _maxResponseTimeMs = 30000;

        public long UtcDateUnixMs { get; set; }
	public string CustomerName { get; set; }
	public string Url { get; set; }
	public string WebMethod { get; set; }
	public int ResponseTimeMs { get; set; }

	public List<string> Validate()
	{
		List<string> brokenRules = new List<string>();
		if (!IsWebMethodValid())
		{
			brokenRules.Add(string.Format("Invalid web method: {0}", WebMethod));
		}
		if (!IsResponseTimeValid())
		{
			brokenRules.Add(string.Format("Response time outside acceptable limits: {0}", ResponseTimeMs));
		}
		if (!IsValidUrl())
		{
			brokenRules.Add(string.Format("Invalid URL: {0}", Url));
		}
		return brokenRules;
	}

	private bool IsWebMethodValid()
	{
		return _validMethods.Contains(WebMethod.ToLower());
	}

	private bool IsResponseTimeValid()
	{
		if (ResponseTimeMs < _minResponseTimeMs
			|| ResponseTimeMs > _maxResponseTimeMs)
		{
			return false;
		}
        	return true;
	}

	private bool IsValidUrl()
	{
		Uri uri;
		string urlToValidate = Url;
		if (!urlToValidate.Contains(Uri.SchemeDelimiter)) urlToValidate = string.Concat(Uri.UriSchemeHttp, Uri.SchemeDelimiter, urlToValidate);
		if (Uri.TryCreate(urlToValidate, UriKind.RelativeOrAbsolute, out uri))
		{
			try
			{
				if (Dns.GetHostAddresses(uri.DnsSafeHost).Length > 0)
				{
					return true;
				}
			}
			catch
			{
				return false;
			}
		}

		return false; 
	}

}

The Validate method will collect all validation errors. IsWebMethodValid() and IsResponseTimeValid() should be quite straightforward. If you don’t understand the IsValidUrl function check out the StackOverflow link referred to above.

We can use the Validate method from within the ReadFromStream() method as follows:

List<WebTransaction> newWebTransactions = new List<WebTransaction>();
foreach (Record record in records)
{
	string json = Encoding.UTF8.GetString(record.Data.ToArray());
	try
	{
        	JToken token = JContainer.Parse(json);
		try
		{									
			WebTransaction wt = JsonConvert.DeserializeObject<WebTransaction>(json);
          		List<string> validationErrors = wt.Validate();
			if (!validationErrors.Any())
			{
				Console.WriteLine("Valid entity: {0}", json);
				newWebTransactions.Add(wt);
			}
			else
			{
				StringBuilder exceptionBuilder = new StringBuilder();
				exceptionBuilder.Append("Invalid WebTransaction object from JSON: ")
				.Append(Environment.NewLine).Append(json)
				.Append(Environment.NewLine).Append("Validation errors: ")
				.Append(Environment.NewLine);
				foreach (string error in validationErrors)
				{
					exceptionBuilder.Append(error).Append(Environment.NewLine);																										
				}
				Console.WriteLine(exceptionBuilder.ToString());
			}									
		}
        	catch (Exception ex)
		{
			//simulate logging
			Console.WriteLine("Could not parse the following message to a WebTransaction object: {0}", json);
		}
	}
	catch (Exception ex)
	{
		//simulate logging
		Console.WriteLine("Could not parse the following message, invalid json: {0}", json);
	}
}

As you can see we’re also collecting all valid WebTransaction objects into a list. That’s a preparation for the next post where we’ll store the valid objects on disk.

Here’s the current version of the ReadFromStream method:

private static void ReadFromStream()
{
	AmazonKinesisConfig config = new AmazonKinesisConfig();
	config.RegionEndpoint = Amazon.RegionEndpoint.EUWest1;
	AmazonKinesisClient kinesisClient = new AmazonKinesisClient(config);
	String kinesisStreamName = ConfigurationManager.AppSettings["KinesisStreamName"];

	DescribeStreamRequest describeRequest = new DescribeStreamRequest();
	describeRequest.StreamName = kinesisStreamName;

	DescribeStreamResponse describeResponse = kinesisClient.DescribeStream(describeRequest);
	List<Shard> shards = describeResponse.StreamDescription.Shards;

	foreach (Shard shard in shards)
	{
		GetShardIteratorRequest iteratorRequest = new GetShardIteratorRequest();
		iteratorRequest.StreamName = kinesisStreamName;
		iteratorRequest.ShardId = shard.ShardId;
		iteratorRequest.ShardIteratorType = ShardIteratorType.TRIM_HORIZON;

		GetShardIteratorResponse iteratorResponse = kinesisClient.GetShardIterator(iteratorRequest);
		string iteratorId = iteratorResponse.ShardIterator;

		while (!string.IsNullOrEmpty(iteratorId))
		{
			GetRecordsRequest getRequest = new GetRecordsRequest();
			getRequest.Limit = 1000;
			getRequest.ShardIterator = iteratorId;

			GetRecordsResponse getResponse = kinesisClient.GetRecords(getRequest);
			string nextIterator = getResponse.NextShardIterator;
			List<Record> records = getResponse.Records;

			if (records.Count > 0)
			{
				Console.WriteLine("Received {0} records. ", records.Count);
				List<WebTransaction> newWebTransactions = new List<WebTransaction>();
				foreach (Record record in records)
				{
					string json = Encoding.UTF8.GetString(record.Data.ToArray());
					try
					{
						JToken token = JContainer.Parse(json);
						try
						{									
							WebTransaction wt = JsonConvert.DeserializeObject<WebTransaction>(json);
							List<string> validationErrors = wt.Validate();
							if (!validationErrors.Any())
							{
								Console.WriteLine("Valid entity: {0}", json);
								newWebTransactions.Add(wt);
							}
							else
							{
								StringBuilder exceptionBuilder = new StringBuilder();
								exceptionBuilder.Append("Invalid WebTransaction object from JSON: ")
									.Append(Environment.NewLine).Append(json)
									.Append(Environment.NewLine).Append("Validation errors: ")
									.Append(Environment.NewLine);
								foreach (string error in validationErrors)
								{
									exceptionBuilder.Append(error).Append(Environment.NewLine);																										
								}
								Console.WriteLine(exceptionBuilder.ToString());
							}									
						}
						catch (Exception ex)
						{
							//simulate logging
							Console.WriteLine("Could not parse the following message to a WebTransaction object: {0}", json);
						}
					}
					catch (Exception ex)
					{
						//simulate logging
						Console.WriteLine("Could not parse the following message, invalid json: {0}", json);
					}
				}
			}

			iteratorId = nextIterator;
		}
	}
}

Run the application with F5. This will start the project that is set as the start-up project. You can start the other one using the technique we saw in the previous post: right-click, Debug, Start new instance. You’ll have two console windows running. If you had some messages left in the Kinesis stream then they should be validated now. I can see the following output:

Let’s now send some new messages to Kinesis:

Great, we have some basic validation logic in place.

We’ll discuss storing the messages in the next post which will finish the series on Amazon Kinesis.

View all posts related to Amazon Web Services and Big Data here.

Big Data: using Amazon Kinesis with the AWS.NET API Part 4: reading from the stream

Introduction

In the previous post of this series on Amazon Kinesis we looked at how to publish messages to a Kinesis stream. In this post we’ll see how to extract them. We’ll create a Kinesis Client application.

It’s necessary to extract the messages from the stream as it only stores them for 24 hours. Also, a client application can filter, sort and validate the incoming messages according to some pre-defined rules.

Our demo client will be a completely separate application. We’ll see some duplication of code but that has a good reason. We’ll want to simulate a scenario where the producers are completely different applications, such as a bit of JavaScript on a web page, a Java web service, an iOS app or some other smart device. Our Kinesis producer is good for demo purposes but in reality the producer can be any software that can send HTTP requests. However, if both your producer and client apps are of the same platform then of course go ahead and introduce a common layer in the project.

Open the demo app we’ve been working on and let’s get to it.

The Kinesis client

Add a new C# console application called AmazonKinesisConsumer. Add the same NuGet packages as before:

Add a reference to the System.Configuration library already now. Also, add the same configurations to app.config:

<appSettings>
        <add key="AWSProfileName" value="demo-aws-profile"/>
	<add key="KinesisStreamName" value="test-stream"/>
</appSettings>

Insert the same WebTransaction object again:

public class WebTransaction
{
	public long UtcDateUnixMs { get; set; }
	public string CustomerName { get; set; }
	public string Url { get; set; }
	public string WebMethod { get; set; }
	public int ResponseTimeMs { get; set; }
}

We’ll make it easy for us here and re-use the same WebTransaction object as we know that we’ll be able to parse the incoming JSON string. However, as mentioned in the first post of this series, be prepared for different message formats and property names. If you can, always aim for some well accepted standard such as JSON or XML, they are easy to handle in code. E.g. if the incoming JSON has different names – including variations in casing – then you can use the JSON library to match the property names:

public class WebTransaction
{
	[JsonProperty(PropertyName="dateUtc")]
	public long UtcDateUnixMs { get; set; }
	[JsonProperty(PropertyName = "cust")]
	public string CustomerName { get; set; }
	[JsonProperty(PropertyName = "url")]
	public string Url { get; set; }
	[JsonProperty(PropertyName = "method")]
	public string WebMethod { get; set; }
	[JsonProperty(PropertyName = "responseTime")]
	public int ResponseTimeMs { get; set; }
}

In any case you can assume that the messages will come in as strings – or bytes that can be converted to strings to be exact.

Do not assume anything about the ordering of the messages. Messages in Kinesis are handled in parallel and they will be extracted in batches by a Kinesis client. So for best performance and consistency aim for short, independent and self-contained messages. If ordering matters or if the total message is too large for Kinesis then you can send extra properties with the messages such as “Index” and “Total” to indicate the order like “1 of 10”, “2 of 10” etc. so that the client can collect and sort them.

The shard iterator

Insert the following private method to Program.cs:

private static void ReadFromStream()
{
	AmazonKinesisConfig config = new AmazonKinesisConfig();
	config.RegionEndpoint = Amazon.RegionEndpoint.EUWest1;
	AmazonKinesisClient kinesisClient = new AmazonKinesisClient(config);
	String kinesisStreamName = ConfigurationManager.AppSettings["KinesisStreamName"];

	DescribeStreamRequest describeRequest = new DescribeStreamRequest();
	describeRequest.StreamName = kinesisStreamName;

	DescribeStreamResponse describeResponse = kinesisClient.DescribeStream(describeRequest);
	List<Shard> shards = describeResponse.StreamDescription.Shards;

	foreach (Shard shard in shards)
	{
		GetShardIteratorRequest iteratorRequest = new GetShardIteratorRequest();
		iteratorRequest.StreamName = kinesisStreamName;
		iteratorRequest.ShardId = shard.ShardId;
		iteratorRequest.ShardIteratorType = ShardIteratorType.TRIM_HORIZON;

		GetShardIteratorResponse iteratorResponse = kinesisClient.GetShardIterator(iteratorRequest);
		string iteratorId = iteratorResponse.ShardIterator;

		while (!string.IsNullOrEmpty(iteratorId))
		{
			GetRecordsRequest getRequest = new GetRecordsRequest();
			getRequest.Limit = 1000;
			getRequest.ShardIterator = iteratorId;

			GetRecordsResponse getResponse = kinesisClient.GetRecords(getRequest);
			string nextIterator = getResponse.NextShardIterator;
			List<Record> records = getResponse.Records;

			if (records.Count > 0)
			{
				Console.WriteLine("Received {0} records. ", records.Count);
				foreach (Record record in records)
				{
					string json = Encoding.UTF8.GetString(record.Data.ToArray());
					Console.WriteLine("Json string: " + json);
				}
			}
			iteratorId = nextIterator;
		}
	}
}

Let’s see what’s going on here. The first 4 lines are identical to what we had in the Kinesis producer: we simply configure the access to Kinesis. We use the Kinesis client object to describe the Kinesis stream referred to by its name in the DescribeStreamRequest object. We then extract the available shards in the stream.

We then iterate through the shards. For each shard – we have only one – we need to request a shard iterator. A shard iterator will help us iterate through the messages in the shard. We specify where we want to start using the ShardIteratorType enumeration. TRIM_HORIZON means that we want to start with the oldest message first and work our way up from there. This is like a first-in-first-out collection and is probably the most common way to extract the messages. Other enumeration values are the following:

• AT_SEQUENCE_NUMBER: read from the position indicated by a sequence number
• AFTER_SEQUENCE_NUMBER: start right after the sequence number
• LATEST: always read the most recent data in the shard

If you recall from the previous post a sequence number is an ID attached to each message.

Once we get the iterator we extract its ID which is used in the GetRecordsRequest object. Note that we enter a while loop and check if the iterator ID is null or empty. The GetRecordsResponse will also include an iterator ID which is a handle to read any subsequent messages. This will normally be an endless loop allowing us to always listen to messages from the stream. If there are any records returned by the iterator we print the number of records and the pure string data of each record. We expect to see some JSON messages. We don’t yet parse them to our WebTransaction messages, we’ll continue with processing the raw data in the next post.

Call this method from Main:

static void Main(string[] args)
{
	ReadFromStream();

	Console.WriteLine("Main done...");
	Console.ReadKey();
}

Test

Let’s see this in action. Make AmazonKinesisCustomer the start-up project of the solution and start the application. If you followed the previous post of this series within 24 hours of completing this post then you should see the messages you sent to the Kinesis stream before – recall that Kinesis keeps the messages for 24 hours. I can see the following JSON messages:

Keep the application running. You’ll see that the loop just continues to run and the application doesn’t stop – we’re effectively waiting for new messages from the sream. Back in VS right-click AmazonKinesisProducer, select Debug, Start new instance. You’ll have two console windows up and running:

Enter a couple of new web transactions into the producer and send it to Kinesis. The client should fetch them in a couple of seconds:

Great, we have now a highly efficient cloud-based message handler in form of Amazon Kinesis, a Kinesis client and a Kinesis producer. We’ve also seen that although the stream is located in the cloud, the producers and clients can be virtually any platforms that are able to handle HTTP messages. Therefore don’t get bogged down by the thought that you have to use Amazon components with Kinesis.

In the next post we’ll add some validation to the incoming messages.

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