Container image pull behavior for the EC2 and external launch types for Amazon ECS

The time that it takes a container to start up varies, based on the underlying container image. For example, a fatter image (full versions of Debian, Ubuntu, and Amazon1/2) might take longer to start up because there are a more services that run in the containers compared to their respective slim versions (Debian-slim, Ubuntu-slim, and Amazon-slim) or smaller base images (Alpine).

When the Amazon ECS agent starts a task, it pulls the Docker image from its remote registry, and then caches a local copy. When you use a new image tag for each release of your application, this behavior is unnecessary.

The ECS_IMAGE_PULL_BEHAVIOR agent parameter determines the image pull behavior. The following options are available:

• ECS_IMAGE_PULL_BEHAVIOR: default

  The image will be pulled remotely. If the pull fails, the cached image in the instance is used.

• ECS_IMAGE_PULL_BEHAVIOR: always

  The image will be pulled remotely. If the pull fails, the task fails.

To speed up deployment, set the Amazon ECS agent parameter to one of the following values:

• ECS_IMAGE_PULL_BEHAVIOR: once

  The image is pulled remotely only if it wasn't pulled by a previous task on the same container instance or if the cached image was removed by the automated image cleanup process. Otherwise, the cached image on the instance is used. This ensures that no unnecessary image pulls are attempted.

• ECS_IMAGE_PULL_BEHAVIOR: prefer-cached

  The image is pulled remotely if there is no cached image. Otherwise, the cached image on the instance is used. Automated image cleanup is turned off for the container to ensure that the cached image isn't removed.

Setting the ECS_IMAGE_PULL_BEHAVIOR parameter to either of the preceding values can save time because the Amazon ECS agent uses the existing downloaded image. For larger Docker images, the download time might take 10-20 seconds to pull over the network.