Amazon CloudWatch Concepts

A metric is the fundamental concept for Amazon CloudWatch and represents a time-ordered set of data points. Either you or AWS products publish metric data points into Amazon CloudWatch and you retrieve statistics about those data points as an ordered set of time-series data.

You can think of a metric as a variable that you want to monitor. The data points represent the values of that variable over time. For example, the CPU usage of a particular Amazon EC2 instance is one metric, and the latency of an elastic load balancer is another.

The data points themselves can come from any application or business activity from which you collect data, not just Amazon Web Services products and applications. For example, a metric might be the CPU usage of a particular Amazon EC2 instance or the temperature in a refrigeration facility.

Metrics are uniquely defined by a name, a namespace, and one or more dimensions. Each data point has a time stamp, and (optionally) a unit of measure. When you request statistics, the returned data stream is identified by namespace, metric name, dimension, and (optionally) the unit.

You can use the PutMetricData API (or the mon-put-data command) to create a custom metric and publish data points for it. You can add the data points in any order, and at any rate you choose. Amazon CloudWatch aggregates data points that are fully identical (duplicate values, time stamps, and units) when you request statistics on them.

Amazon CloudWatch stores your metric data for two weeks. You can publish metric data from multiple sources, such as incoming network traffic from dozens of different Amazon EC2 instances, or requested page views from several different web applications. You can request statistics on metric data points that occur within a specified time window.

Amazon CloudWatch namespaces are conceptual containers for metrics. Metrics in different namespaces are isolated from each other, so that metrics from different applications are not mistakenly aggregated into the same statistics.

Namespace names are strings you define when you create a metric. The names must be valid XML characters, typically containing the alphanumeric characters "0-9A-Za-z" plus "."(period), "-" (hyphen), "_" (underscore), "/" (slash), "#" (hash), and ":" (colon). AWS namespaces all follow the convention AWS/<service>, such as AWS/EC2 and AWS/ELB.

There is no default namespace. You must specify a namespace for each data element you put into Amazon CloudWatch.

A dimension is a name/value pair that helps you to uniquely identify a metric. Every metric has specific characteristics that describe it, and you can think of dimensions as categories for those characteristics. Dimensions help you design a conceptual structure for your statistics plan. Because dimensions are part of the unique identifier for a metric, whenever you add a unique name/value pair to one of your metrics, you are creating a new metric.

You specify dimensions when you create a metric with the PutMetricData action (or its command line equivalent mon-put-data). AWS products that feed data to Amazon CloudWatch also attach dimensions to each metric. You can use dimensions to filter result sets that Amazon CloudWatch queries return.

For example, you can get statistics for a specific Amazon EC2 instance by calling GetMetricStatistics with the InstanceID dimension set to a specific Amazon EC2 instance ID.

For metrics produced by certain AWS products such as Amazon EC2, Amazon CloudWatch can aggregate data across dimensions. For example, if you call GetMetricStatistics for a metric in the AWS/EC2 namespace and do not specify any dimensions, Amazon CloudWatch aggregates all data for the specified metric to create the statistic that you requested. However, Amazon CloudWatch does not aggregate across dimensions for metrics that you create with PutMetricData or mon-put-data.

In the figure at the end of this section, the four calls to mon-put-data create four distinct metrics. If you make only those four calls, you could retrieve statistics for these four dimension combinations:

You could not retrieve statistics using combinations of dimensions that you did not specifically create. For example, you could not retrieve statistics for any of the following combinations of dimensions unless you create new metrics that specify these combinations with additional calls to mon-put-data:

With Amazon CloudWatch, each metric data point must be marked with a time stamp. The time stamp can be up to two weeks in the past and up to two hours into the future. If you do not provide a time stamp, Amazon CloudWatch creates a time stamp for you based on the time the data element was received.

The time stamp you use in the request must be a dateTime object, with the complete date plus hours, minutes, and seconds (for more information, go to http://www.w3.org/TR/xmlschema-2/#dateTime). For example: 2007-01-31T23:59:59Z. Although it is not required, we recommend you provide the time stamp in the Coordinated Universal Time (UTC or Greenwich Mean Time) time zone. When you retrieve your statistics from Amazon CloudWatch, all times reflect the UTC time zone.

Units represent your statistic's unit of measure. For example, the units for the Amazon EC2 NetworkIn metric is Bytes because NetworkIn tracks the number of bytes that an instance receives on all network interfaces.

You can also specify a unit when you create a custom metric. Units help provide conceptual meaning to your data. Metric data points you create that specify a unit of measure, such as Percent, will be aggregated separately. The following list provides some of the more common units that Amazon CloudWatch supports:

For a complete list, go to the MetricDatum data type in the Amazon CloudWatch API Reference.

Though Amazon CloudWatch attaches no significance to a unit internally, other applications can derive semantic information based on the unit you choose. When you publish data without specifying a unit, Amazon CloudWatch associates it with the None unit. When you get statistics without specifying a unit, Amazon CloudWatch aggregates all data points of the same unit together. If you have two otherwise identical metrics with different units, two separate data streams will be returned, one for each unit.

Statistics are metric data aggregations over specified periods of time. Amazon CloudWatch provides statistics based on the metric data points you or AWS products have provided to Amazon CloudWatch. Aggregations are made using the namespace, metric name, dimensions, and the data point unit of measure, within the time period you specify. The following table describes the available statistics.

You use the GetMetricStatistics API (mon-get-stats) to retrieve statistics, specifying the same values used for the namespace, metric name, and dimension parameters used when the metric values were created. You also specify the encompassing period, or start and end times that Amazon CloudWatch will use for the aggregation. The starting and ending points can be as close together as 60 seconds, and as far apart as two weeks.

Amazon CloudWatch allows you to add pre-calculated statistics using the PutMetricData API (mon-put-data command) using the StatisticValues (statistic-values) parameter. Instead of data point values, you specify values for SampleCount, Minimum, Maximum, and Sum (Amazon CloudWatch calculates the average for you). The values you add in this way are aggregated with any other values associated with the matching metric.

A period is the length of time associated with a specific Amazon CloudWatch statistic. Each statistic represents an aggregation of the metrics data collected for a specified period of time. You can adjust how the data is aggregated by varying the length of the period. A period can be as short as one minute (60 seconds) or as long as two weeks (1,209,600 seconds).

When you call GetMetricStatistics, you can specify the period length with the Period parameter. Two related parameters, StartTime and EndTime, determine the overall length of time associated with the statistics. The default value for the Period parameter is 60 seconds, whereas the default values for StartTime and EndTime give you the last hour's worth of statistics.

The values you select for the StartTime and EndTime parameters determine how many periods GetMetricStatistics will return. For example, calling GetMetricStatistics with the default values for the Period, EndTime, and StartTime parameters returns an aggregated set of statistics for each minute of the previous hour. If you prefer statistics aggregated into ten-minute blocks, set Period to 600. For statistics aggregated over the entire hour, use a Period value of 3600.

Periods are also an important part of the Amazon CloudWatch Alarms feature. When you create an alarm to monitor a specific metric, you are asking Amazon CloudWatch to compare that metric to the threshold value that you supplied. You have extensive control over how Amazon CloudWatch makes that comparison. Not only can you specify the period over which the comparison is made, but you can also specify how many consecutive periods the threshold must be breached before you are notified. For more information on Alarms, see Alarms.

Amazon CloudWatch aggregates statistics according to the period length that you specify in calls to GetMetricStatistics. You can publish as many data points as you want with the same or similar time stamps. Amazon CloudWatch aggregates them by period length when you get statistics about those data points with GetMetricStatistics.

You can publish data points for a metric that share not only the same time stamp, but also the same namespace and dimensions. Subsequent calls to GetMetricStatistics will return aggregated statistics about those data points. You can even do this in one PutMetricData request. Amazon CloudWatch accepts multiple data points in the same PutMetricData call with the same time stamp. You can also publish multiple data points for the same or different metrics, with any time stamp. The size of a PutMetricData request, however, is limited to 8KB for HTTP GET requests and 40KB for HTTP POST requests. You can include a maximum of 20 data points in one PutMetricData request.

For large data sets that would make the use of PutMetricData impractical, Amazon CloudWatch allows for the insertion of a pre-aggregated data set called a StatisticSet. With StatisticSets you give Amazon CloudWatch the Min, Max, Sum, and SampleCount of a number of data points. A common use case for StatisticSets is when you are collecting data many times in a minute. For example, let’s say you have a metric for the request latency of a web page. It doesn’t make sense to do a PutMetricData request with every web page hit. We suggest you collect the latency of all hits to that web page, aggregate them together once a minute and send that StatisticSet to Amazon CloudWatch.

Amazon CloudWatch doesn't differentiate the source of a metric. If you publish a metric with the same namespace and dimensions from different sources, Amazon CloudWatch treats this as a single metric. This can be useful for service metrics in a distributed, scaled system. For example, all the hosts in a web server application could publish identical metrics representing the latency of requests they are processing. Amazon CloudWatch treats these as a single metric, allowing you to get the statistics for minimum, maximum, average, and sum of all requests across your application.

Amazon CloudWatch is especially useful because it helps you make decisions and take immediate, automatic actions based on your metric data. Alarms can automatically initiate actions on your behalf, based on parameters you specify. An alarm watches a single metric over a time period you specify, and performs one or more actions based on the value of the metric relative to a given threshold over a number of time periods. The action is a notification sent to an Amazon SNS topic or Auto Scaling policy. Alarms invoke actions for sustained state changes only. Amazon CloudWatch alarms will not invoke actions simply because they are in a particular state, the state must have changed and been maintained for a specified number of periods.

When creating an alarm, select a period that is greater than or equal to the frequency of the metric to be monitored. For example, Basic Monitoring for Amazon EC2 instances provides metrics every 5 minutes; when setting an alarm on a Basic Monitoring metric, select a period of at least 300 seconds (5 minutes). Detailed Monitoring for Amazon EC2 instances provides metrics every 1 minute; when setting an alarm on a Detailed Monitoring metric, select a period of at least 60 seconds (1 minute).

For examples of setting up Amazon CloudWatch alarms that invoke an Auto Scaling policy and SNS topic, see Creating Amazon CloudWatch Alarms.

Amazon cloud computing resources are housed in highly available data center facilities. To provide additional scalability and reliability, each data center facility is located in a specific geographical area, known as a Region. Regions are large and widely dispersed geographic locations.

Each Amazon Region is designed to be completely isolated from the other Amazon Regions. This achieves the greatest possible failure independence and stability, and it makes the locality of each Amazon resource unambiguous. Amazon CloudWatch does not aggregate data across Regions. Therefore, metrics are completely separate between Regions.

For a list of endpoints that represent each region, go to Regions and Endpoints in the Amazon Web Services General Reference.