Warning: Previously, Diego placed logs into a buffer and released them in accordance with the log line limit that was set. If the limit was exceeded, logs would be delayed. If the buffer overflowed, logs would be dropped. The new behavior is to immediately output logs without placing them into a buffer. When the limit is reached, logs are dropped. This change makes the timestamps of logs match when they were actually output by the app.

This topic describes app log rate limiting for apps in VMware Tanzu Application Service for VMs (TAS for VMs).

Overview of App Log Rate Limiting

In TAS for VMs, you can limit the number of log lines each app instance can generate per second by configuring the App log rate limit section in the App Containers pane of the TAS for VMs tile.

App log rate limiting is disabled by default. VMware recommends enabling 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. Enabling this feature can also 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 the cache in order to store large volumes of logs. It also can limit the CPU usage of logging agents on the Diego Cell VM.

To configure app log rate limits, see Configure App Containers in Configuring TAS for VMs.

Note: In TAS for VMs, this rate limit is applied globally across all your apps. If only some of your apps require this setting, see Configure App Containers in Installing Isolation Segment.

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 TAS for VMs. VMware recommends using at minimum the default limit of 100 lines/second.

When you enable app log rate limiting, Diego applies the 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 rate limit has been exceeded. Instead, Diego evaluates the logging rate of each individual app instance and only limits instances that exceed the rate limit.

What Happens When App Instances Exceed the App Log Rate Limit

When an app instance exceeds the configured rate limit, Diego drops the app logs that exceed the per-second rate you configured through the App log rate limit section in the App Containers pane of the TAS for VMs tile. When this happens, you see a message indicating that logs are being dropped.

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

The Diego Cell containing the app instance emits the AppInstanceExceededLogRateLimitCount counter metric when it exceeds the rate limit, similar to the following example:

origin:"rep" eventType:CounterEvent timestamp:1582582740243576212 deployment:"cf" job:"diego-cell" index:"0e98fd00-47b2-4589-94f0-385f78b3a04d" ip:"" tags:<key:"instance_id" value:"0e98fd00-47b2-4589-94f0-385f78b3a04d" > tags:<key:"source_id" value:"rep" > counterEvent:<name:"AppInstanceExceededLogRateLimitCount" delta:1 total:206 >

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 rate limit since the creation of the Diego Cell. When there are no app instances exceeding the 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 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 rate limit. For example, app-instanceC and app-instanceD are on the same Diego Cell. If app-instanceC exceeds the rate limit continually over a ten-minute period, and app-instanceD exceeds the 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.

Configure 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 on TAS for VMs 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 TAS for VMs, the logging rate that you configured in TAS for VMs, your other TAS for VMs 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 rate limit set in TAS for VMs. A log message similar to the example below appears in the log stream for the noisy app:

2020-02-24T12:42:18.90-0800 [APP/PROC/WEB/0] OUT app instance exceeded log rate limit (100 log-lines/sec) set by platform operator

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

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

  1. 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 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:"" 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 (100 log-lines/sec) set by platform operator" 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 the app instances that are exceeding the app log rate limit.
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