App Log Rate Limiting

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This topic describes app log rate limiting for apps in Cloud Foundry (Cloud Foundry).

Overview of App Log Rate Limiting

Log rate limits are a feature that limits the number of logs that can be sent to an app.

App log rate limiting is deactivated by default. Cloud Foundry recommends activating this feature to prevent app instances from overloading the Loggregator Agent with logs, so the Loggregator Agent does not drop logs for other app instances on the same Diego Cell.

Allowing app log rate limiting can also do the following:

  • Prevent apps from reporting inaccurate app metrics in the Cloud Foundry Command Line Interface (cf CLI), which can happen if Log Cache evicts metrics from its cache in order to store large volumes of logs.

  • Limit the CPU usage of logging agents on the Diego Cell VM.

You can allow log rate limits in the following ways:

App Log Rate Limiting in Bytes Per Second

In Cloud Foundry, you can limit the number of bytes each app instance can generate per second.

You can configure app log rate limiting in bytes per second on a per-app basis through either the app manifest or the cf CLI. Additionally, you can enforce the log rate limit you configure for all apps that are deployed within a space or org by specifying the log rate limit in the quota plan for the space or org. For more information, see Creating and Modifying Quota Plans.

Global Log Rate Limiting in Lines Per Second

This feature is deprecated in favor of log rate limits in bytes per second. If you have configured a global log rate limit in lines per second, Cloud Foundry recommends that you re-configure your apps to use log rate limits in bytes per second.

In Cloud Foundry, you can limit the number of log lines each app instance can generate per second by configuring the max_log_lines_per_second property in your cf-deployment.yml file. This is a global setting that applies to all apps in your Cloud Foundry deployment.

Determining the Ideal App Log Rate Limit

The ideal app logging rate for a deployment depends on characteristics such as VM sizes and the number and type of apps in Cloud Foundry. Cloud Foundry recommends using at minimum the default limit of 1k bytes per second.

When you allow app log rate limiting, Diego applies the log rate limit to each app instance. For example, if there are five instances of an app running, Diego does not sum the logging rates of all five instances when determining if the log rate limit has been exceeded. Instead, Diego evaluates the logging rate of each individual app instance and only limits instances that exceed the log rate limit.

What Happens When App Instances Exceed the App Log Rate Limit

When an app instance exceeds the log rate limit you configured through the max_log_lines_per_second property in your cf-deployment.yml file, Diego drops the app logs that exceed that limit. When this happens, you see a message indicating that Diego is dropping app logs.

For more information about how Diego rate-limits app logs, see the Go documentation.

How Diego Cells Determine When an App Instance Has Exceeded the App Log Rate Limit

When an app instance exceeds the log rate limit, the Diego Cell containing the app instance emits the AppInstanceExceededLogRateLimitCount counter metric, similar to the following example:

origin:"rep" eventType:CounterEvent timestamp:1582582740243576212 deployment:"cf" job:"diego-cell" index:"0e98fd00-47b2-4589-94f0-385f78b3a04d" ip:"10.0.1.12" tags: tags: counterEvent:

Each Diego Cell in a deployment has a unique AppInstanceExceededLogRateLimitCount counter. The total value of the counter is the sum total of all app instances on that Diego Cell that have exceeded the log rate limit since the creation of the Diego Cell. When there are no app instances exceeding the log rate limit, Diego Cells do not emit the AppInstanceExceededLogRateLimitCount metric.

For example, app-instanceA and app-instanceB are running on one Diego Cell, app-instanceC and app-instanceD are running on a second Diego Cell, and the current total for the AppInstanceExceededLogRateLimitCount is 125 on the first Diego Cell and 43 on the second Diego Cell. If app-instanceD exceeds the log rate limit, the second Diego Cell emits the AppInstanceExceededLogRateLimitCount metric with a incremented total value of 44. However, the first Diego Cell does not emit the AppInstanceExceededLogRateLimitCount metric, and the total value for the AppInstanceExceededLogRateLimitCount metric on the first Diego Cell is still 125.

A Diego Cell emits the AppInstanceExceededLogRateLimitCount metric conditionally when an app instance on that Diego Cell begins to exceed the log rate limit. For example, app-instanceC and app-instanceD are on the same Diego Cell. If app-instanceC exceeds the log rate limit continually over a ten-minute period, and app-instanceD exceeds the log rate limit during the first three minutes of each five-minute interval within that ten-minute period and then stops, the Diego Cell emits the AppInstanceExceededLogRateLimitCount metric three times within that ten-minute period.

Configuring an Alert for the AppInstanceExceededLogRateLimitCount Metric

If you are using a third-party log management service, you can configure an alert for when the aggregated sum of the AppInstanceExceededLogRateLimitCount metric across all the Diego Cells in Cloud Foundry has been incremented more than a certain number of times or over a certain percentage in the last five or more minutes.

When you configure this alert, consider the number of app instances running on Cloud Foundry, the logging rate that you configured in Cloud Foundry, your other Cloud Foundry configuration settings, and so on.

For more information about third-party log management services, see Streaming App Logs to Log Management Services.

Identify Apps That Exceed the App Log Rate Limit

Diego also logs when a noisy app instance exceeds the log rate limit configured in Cloud Foundry. A log message similar to the example below appears in the log stream for the noisy app:

2022-08-22T12:42:18.90-0800 [APP/PROC/WEB/0] OUT app instance exceeded log rate limit (1024 bytes/sec)

To identify which app instances are exceeding the app log rate limit:

Note: The Firehose and Log Cache plugins are developed by the open-source Cloud Foundry community and are not supported by VMware.

  1. In a terminal window, install the Firehose plugin by running:

    cf install-plugin 'Firehose Plugin'
    
  2. Install the Log Cache plugin by running:

    cf install-plugin 'log-cache'
    
  3. Filter your app log messages by running:

    cf nozzle -f LogMessage | grep "app instance exceeded log rate limit"
    

    The above command returns all logs with log messages containing "app instance exceeded log rate limit", similar to the following example:

    origin:"rep" eventType:LogMessage timestamp:1583859621886751670 deployment:"warp-drive" job:"diego-cell" index:"3a574bde-91df-48b8-ae21-1d6913da0908" ip:"10.0.1.33" tags:<key:"app_id" value:"34bcfafc-402b-4bb4-84db-aea5401b79eb" > tags:<key:"app_name" value:"app-2" > tags:<key:"instance_id" value:"0" > tags:<key:"organization_id" value:"a30f39c2-4ff3-48a1-a869-a9ed21812a61" > tags:<key:"organization_name" value:"test" > tags:<key:"process_id" value:"34bcfafc-402b-4bb4-84db-aea5401b79eb" > tags:<key:"process_instance_id" value:"92e2ee78-3a1d-41a6-4933-e47b" > tags:<key:"process_type" value:"web" > tags:<key:"source_id" value:"34bcfafc-402b-4bb4-84db-aea5401b79eb" > tags:<key:"space_id" value:"0e2d2d58-3ef5-43f3-b880-c8a30903a96b" > tags:<key:"space_name" value:"test-2" > logMessage:<message:"app instance exceeded log rate limit (1024 bytes/sec)" message_type:OUT timestamp:1583859621886751670 app_id:"34bcfafc-402b-4bb4-84db-aea5401b79eb" source_type:"APP/PROC/WEB" source_instance:"0" >
    
    You can inspect these logs to identify which app instances are exceeding the log rate limit.

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