How to Use the Command `docker update` (with Examples)

The docker update command is a versatile tool in the Docker ecosystem that allows users to modify the configuration of running Docker containers. It provides the flexibility to adjust various settings on-the-fly without stopping the container. This command proves indispensable when managing resource constraints and behavior for containers, ensuring they run efficiently and align with desired operational policies. Notably, the docker update command is not supported for Windows containers. A range of parameters can be modified using this command, and this article explores several practical use cases with examples.

Use case 1: Update Restart Policy to Apply When a Specific Container Exits

Code:

docker update --restart=always container_name

Motivation:

Updating the restart policy for a Docker container is crucial for ensuring container availability and resilience. By setting the restart policy to always, the container will automatically restart whenever it exits, which is helpful for maintaining service uptime without manual intervention, especially for essential services that need to be up continuously.

Explanation:

• --restart=always: This flag changes the restart policy of the container to always. It implies that Docker will always attempt to restart the container if it stops, regardless of the exit status.

Example Output:

When the command is executed successfully, there will be no output. You can verify the update by running docker inspect container_name and checking the RestartPolicy attribute within the configuration.

Use case 2: Update the Policy to Restart Up to Three Times a Specific Container When It Exits with Non-Zero Exit Status

Code:

docker update --restart=on-failure:3 container_name

Motivation:

Setting a restart policy that limits the number of restart attempts is vital for scenarios where further investigation is necessary before additional retries. The on-failure:3 option is optimal for situations where transient issues might cause non-zero exits and a few retries might resolve the problem without intervention but prevents endless restart loops that may indicate deeper issues.

Explanation:

• --restart=on-failure:3: Specifies that Docker should restart the container only when it exits with a non-zero exit status and limits it to a maximum of three attempts. This is a strategic approach to managing resource use while still allowing for automatic recovery from temporary issues.

Example Output:

As with other docker update commands, successful execution provides no output. Validate the setting by inspecting the container for the updated RestartPolicy.

Use case 3: Update the Number of CPUs Available to a Specific Container

Code:

docker update --cpus 2 container_name

Motivation:

Allocating the appropriate amount of CPU resources to a container is crucial for performance optimization and efficient resource management. Increasing or decreasing CPU allocation dynamically helps balance workloads across containers, ensuring that high-priority services receive sufficient resources while optimizing overall CPU usage.

Explanation:

• --cpus 2: This argument adjusts the CPU allocation for the specified container to two CPUs. It effectively controls and limits the processing power available to the container, which impacts the speed and efficiency of operations within the container.

Example Output:

No direct output will be visible upon executing this command. Verify the resources allocated by checking the container’s configuration using docker inspect.

Use case 4: Update the Memory Limit in Megabytes for a Specific Container

Code:

docker update --memory 512M container_name

Motivation:

Managing memory resources is imperative in container environments to prevent any single container from using excessive memory, potentially destabilizing the system. Setting memory limits can prevent runaway memory use that can impact other containers’ performance, ensuring predictable and stable operation of applications.

Explanation:

• --memory 512M: Sets the memory limit for the container to 512 Megabytes. This ensures that the container will not exceed this limit, which can prevent out-of-memory errors and other related issues by controlling resource usage.

Example Output:

Although no output will be directly shown after the execution, the update can be confirmed by inspecting the container’s resource configurations.

Use case 5: Update the Maximum Number of Process IDs Allowed Inside a Specific Container

Code:

docker update --pids-limit 100 container_name

Motivation:

Imposing a limit on the number of process IDs (PIDs) within a container is a crucial security and resource management measure. Constraining PIDs helps protect against fork bombs or misbehaving applications that might spawn an excessive number of processes, which could deplete system resources and affect other containers.

Explanation:

• --pids-limit 100: Sets the maximum number of process IDs to 100 within the container. By defining this limit, the system can prevent scenarios where an application might consume all available PIDs, ensuring balanced distribution of system resources.

Example Output:

The execution of this command does not produce visible output. To confirm the limit, one can inspect the container’s settings using relevant Docker commands.

Use case 6: Update the Amount of Memory in Megabytes a Specific Container Can Swap to Disk

Code:

docker update --memory-swap 1024M container_name

Motivation:

Adjusting the swap memory limit for a container is critical for scenarios where containers might temporarily exceed their memory limits. Allowing swap space helps provide a buffer for such occasions, potentially safeguarding containers from being terminated due to excessive memory use while optimizing the use of physical memory.

Explanation:

• --memory-swap 1024M: This flag sets the total amount of memory including swap to 1024 Megabytes. It determines the maximal swap usage for this particular container, beyond which the host will not permit any more swapping. Thus, it’s effective in managing memory overflow situations.

Example Output:

Upon running the command, there’s no direct output. Validation of the applied setting can be done using a container inspection to review the updated memory parameters.

Conclusion:

The docker update command is a powerful utility for those managing Docker containers. It allows for real-time adjustment and optimization of container settings without the need for downtime, ensuring operational efficiency, stability, and the optimal use of system resources. By understanding and implementing these configurations, users can strategically manage workloads in a Dockerized environment, tailoring container behavior to meet specific needs and performance goals.