Preventing log loss with non-blocking mode in the AWSLogs container log driver


For improved observability and troubleshooting, it is recommended to ship container logs from the compute platform to a container running on to a centralized logging server. In the real world, the logging server may occasionally be unreachable or unable to accept logs. There is an architectural tradeoff when designing for log server failures. Service owners must choose from the following considerations:

  1. Should the application stop responding to traffic (or performing work) and wait for the centralized logging server to be restored? (i.e., is an accurate audit log higher priority than service availability?)
  2. Should the application continue to serve traffic while buffering logs in the hope that the logging server comes back before the buffer is full. Should you accept the risk of log loss in the rare case when the log destination is unavailable?

In container logging drivers, this tradeoff is implemented with a configuration parameter blocking for the first consideration and non-blocking for second. In the AWS post, Choosing container logging options to avoid backpressure, Rob Charlton explored this tradeoff and explained how to test how your application behaves in the default blocking mode for the AWSLogs container log driver.

In this post, we’ll dive into non-blocking, and show the results of log loss testing with the AWSLogs logging driver.

Solution overview

AWSLogs driver modes

In Amazon Elastic Container Service (Amazon ECS), the AWSLogs logging driver captures logs from container’s stdout and stderr, and then uploads them to Amazon CloudWatch Logs via the PutLogEvents API. The log driver supports a mode setting, which can be configured as follows:

  • blocking (default): When logs cannot be immediately sent to Amazon CloudWatch, calls from container code to write to stdout or stderr will block and halt execution of the code. The logging thread in the application will block, which may prevent the application from functioning and lead to health check failures and task termination. Container startup fails if the required log group or log stream cannot be created.
  • non-blocking: When logs cannot be immediately sent to Amazon CloudWatch, they are stored in an in-memory buffer configured with the max-buffer-size setting. When the buffer fills up, logs are lost. Calls to write to stdout or stderr from container code won’t block and will immediately return. With Amazon ECS on Amazon Elastic Compute Cloud (Amazon EC2), container startup won’t fail if the required log group or log stream cannot be created. With Amazon ECS on AWS Fargate, container startup always fails if the log group or log stream cannot be created regardless of the mode configured.

Diagram which shows an application writing logs to the stdout pipe, which are sent to CloudWatch Logs by the AWSLogs driver. CloudWatch Logs is unreachable, consequently nothing moves through the stdout pipe and the application blocks.

Should I switch to the non-default non-blocking mode?

Due to the application availability risk of the default blocking mode, service owners may consider switching to the non-blocking mode instead. This raises these questions:

  • How should you choose the max-buffer-size? Can the default 1 MB size prevent log loss?
  • Will the non-blocking mode lead to log loss for applications that log at a high rate?

To answer these questions, the AWS team ran log ingestion tests at scale on the AWSLogs driver in non-blocking mode.

What value for max-buffer-size is recommended?

If you choose non-blocking mode, then the recommended Amazon ECS Task Definition settings from this testing are the following:

"logConfiguration": {
    "logDriver": "awslogs",
    "options": {
        "mode": "non-blocking",
        "max-buffer-size": "25m",

Which variables determine how large the buffer should be?

The main variable that influences the maximum buffer size needed is how frequently the application outputs data and the log throughput.

Use the IncomingBytes metric in CloudWatch Metrics to track the ingestion rate to your log group(s). Assuming that all containers send at roughly the same rate, you can then divide the log group ingestion rate by the number of containers. Then you have the rate for each individual container.

It is recommended to over-estimate the log throughput from each container; log output may spike occasionally, especially during incidents. If possible, calculate your throughput during a load test or recent incident. Use the peak log output rate over a time interval of a minute or less, to account for bursts in throughput.

What did the tests find?

Please be aware that the results discussed in this post don’t represent a guarantee of performance. We are simply sharing the results of tests that we ran.

Here are the key findings when the central logging server is available and healthy.

  • max-buffer-size of >= 4MB doesn’t show any log loss for <= 2 MB/s log output rate from the container.
  • max-buffer-size of >= 25 MB doesn’t show any log loss for <= 5MB/s log output rate from the container.
  • Above 6 MB/s, the performance of the AWSLogs driver is less predictable and consistent. For example, there was an outlier test failure with a 100 MB buffer and 7 MB/s. If you log at 6+ MB/s (sustained or burst), it may not be possible to prevent occasional log loss.
  • The results are similar for Amazon ECS on Amazon EC2 launch type compared with the AWS Fargate launch type.

This document presents a simple summary of the test results. The full benchmark results, analysis, and data broken down by launch type and log size, can be found on GitHub.

How were tests run?

The code used for benchmarking can be found on GitHub. Amazon EC2 tests were performed on Docker version v20.10.25. AWS Fargate tests were performed on platform version 1.4.

Each log loss test run was an Amazon ECS Task that sends 1 GB of log data to Amazon CloudWatch Logs with the AWSLogs Driver. The task then queries Amazon CloudWatch Logs to get back all log events and checks how many were received. Each log message has a unique ID that is a predictable sequence number. Tests were run with 1 KB and 250 KB-sized single log messages.

Several thousand test runs were executed to acquire sufficient data for meaningful statistically analysis of log loss.

How do I know if the buffer is full and logs are lost?

Unfortunately, with the AWSLogs logging driver there is no visibility into logs lost by the non-blocking mode buffer. There is no log statement or metric emitted by the Docker Daemon when loss occurs. Please comment on the proposal for log loss metrics on GitHub.

How the does the buffer size effect the memory available to my application?

The max-buffer-size setting controls the byte size of messages in a go slice. It doesn’t directly constrain the memory usage, because Go is a garbage collected language. One test suite noted that the real size of the queue on average is fairly small and generally less than 500 KB. The buffer size climbs to the limit occasionally during periods of latency or increased log throughput. This means that the memory used by the buffer varies significantly from moment to moment and the real memory usage may exceed the configured size, due to Go garbage collection.

Does the compute platform effect the buffer size?

In our testing, we found that the results are similar with Amazon ECS Tasks launched on both Amazon EC2 and AWS Fargate.

Is the non-blocking mode safe when send logs across Regions?

AWSLogs driver can upload consistently at a much higher rate when sending logs to the Amazon CloudWatch API in the same Region as the test task, due to lower latency connections to CloudWatch. Cross-region log upload is less reliable. Moreover, it violates the best practice of Region isolation. Cross-region log push also incurs higher network cost.

Test results

Please be aware that the results discussed in this post don’t represent a guarantee of performance. We are simply sharing the results of tests that we ran. Please see GitHub for the full data tables broken down by dimensions such as compute platform (AWS Fargate versus Amazon EC2) and log message size.

Summary of in-region test runs

Below is a heat-map summary of approximately 17,000 in-region test runs. The percent annotation inside the shaded box is the percent log loss in the worst test run. The darker the red shade, the more log loss was observed. Notice that there was no log loss for all test runs with log output rate less than 2 MB/s.

Heat map image which summarizes the test results. The heat map shows that log loss is very likely with a less than 12 MB buffer and more than 2 MB/s log output rate from the container.

Summary of cross-region test runs

Tasks were run in us-west-2 uploading to Amazon CloudWatch in us-east-1.

The results show that cross-region log uploads are less reliable and require a much larger buffer size to prevent log loss.

Heat map image showing test results for cross-region test runs. The heat map shows log loss for most combinations of buffer size and log output rate from the container. A 40MB or larger buffer and less than 2 MB/s log output rate from the container is required for low risk of log loss.


In this post, you learned:

  • The tradeoff between application availability and log loss with container log drivers blocking and non-blocking.
  • How the AWSLogs driver performs in non-blocking mode with different values of max-buffer-size.
  • Cross-region log upload is not recommended and has a much higher risk of log loss with non-blocking.
  • How to find your own log output rate per container.
  • It isn’t possible to monitor for log loss with AWSLogs driver in non-blocking.

When considering the tradeoff between application availability and log loss, you should decide whether your use case requires blocking or non-blocking mode. If you choose the application availability side of the trade-off, then should you choose AWSLogs driver in non-blocking mode or another log collection solution? Most other log collection solutions, such as Fluent Bit with FireLens, only support the store logs in a finite buffer but do not block the application side of the trade-off. However, other solutions may be easier to tune and monitor to prevent log loss. If you choose AWSLogs driver in non-blocking mode, then given your log output rate per container, which value of max-buffer-size fits your risk tolerance? We recommend carefully reviewing the full test results on GitHub. Given the results, we recommend 25m for max-buffer-size; and to ensure that all log uploads are within region, as cross-region log push is very unreliable.

Wesley Pettit

Wesley Pettit

Wesley Pettit is a software engineer for Amazon ECS, focused on Observability. He is a maintainer of fluent/fluent-bit, a CNCF log routing project.