I launched the Promtail agent to send my application logs to the Loki container, which should then be accessible by Grafana. However, I was unable to fetch the application logs from the ECS containers

The issue you're experiencing with retrieving application logs from ECS containers may be related to how container logging is configured in Amazon ECS. By default, logs from containers running in ECS are not stored inside the containers themselves. Instead, the recommended approach for containerized applications is to send their output to STDOUT and STDERR.

For ECS tasks, the standard practice is to configure the CloudWatch log driver to capture these STDOUT and STDERR streams and send them to CloudWatch Logs. This means that your application logs are typically not stored within the container but are instead forwarded to a centralized logging service.

If you're using Promtail to send logs to Loki, you'll need to ensure that your ECS task definition is configured correctly to make the logs accessible to Promtail. Here are a few things to consider:

1. Make sure your application is writing logs to STDOUT/STDERR instead of to internal files.

2. If your application must write to log files, you'll need to mount a volume in your ECS task definition where the application can write its log files.

3. Configure Promtail as a sidecar container in your ECS task definition to access and forward these logs to Loki.

4. Ensure that the necessary permissions are set up for Promtail to read the logs and send them to Loki.

If you've confirmed these configurations and are still unable to retrieve the logs, you may want to check the Promtail configuration and ensure it's correctly set up to scrape logs from the appropriate locations within the ECS task.

Remember, the default behavior in containerized environments is to treat logs as event streams, not as files stored within the container. This approach aligns with best practices for container logging and allows for more efficient log management and analysis.
Sources
Designing and implementing logging and monitoring with CloudWatch FAQ - AWS Prescriptive Guidance
How to push custom application logs to cloudwatch | AWS re:Post