This post was written by Diggory Briercliffe, Senior IoT Architect.

When implementing a task queue, you can use Amazon SQS standard or FIFO (First-In-First-Out) queue types. Both queue types give priority to tasks created earlier over tasks that are created later. However, there are use cases where you need a LIFO (Last-In-First-Out) queue.

This post shows how to implement a serverless LIFO task queue. This uses AWS Lambda, Amazon DynamoDB, AWS Serverless Application Model (AWS SAM), and other AWS Serverless technologies.

The LIFO task queue gives priority to newer queue tasks over earlier tasks. Under heavy load, earlier tasks are deprioritized and eventually removed. This is useful when your workload must communicate with a system that is throughput-constrained and newer tasks should have priority.

To help understand the approach, consider the following use case. As part of optimizing the responsiveness of a mobile application, an IoT application validates device IP addresses after connecting to AWS IoT Core. Users open the application soon after the device connects so the most recent connection events should take priority for the validation work.

If the validation work is not done at connection time, it can be done later. A legacy system validates the IP addresses, but its throughput capacity cannot match the peak connection rate of the IoT devices. A LIFO queue can manage this load, by prioritizing validation of newer connection events. It can buffer or load shed earlier connection event validation.

For a more detailed discussion around insurmountable queue backlogs and queuing theory, read “Avoiding insurmountable queue backlogs” in the Amazon Builders’ Library.

Example application

An example application implementing the LIFO queue approach is available at https://github.com/aws-samples/serverless-lifo-queue-demonstration.

The application uses AWS SAM and the Lambda functions are written in Node.js. The AWS SAM template describes AWS resources required by the application. These include a DynamoDB table, Lambda functions, and Amazon SNS topics.

The README file contains instructions on deploying and testing the application, with detailed information on how it works.

Overview

The example application has the following queue characteristics:

  1. Newer queue tasks are prioritized over earlier tasks.
  2. Queue tasks are buffered if they cannot be processed.
  3. Queue tasks are eventually deleted if they are never processed, such as when the queue is under insurmountable load.
  4. Correct queue task state transition is maintained (such as PENDING to TAKEN, but not PENDING to SUCCESS).

A DynamoDB table stores queue task items. It uses the following DynamoDB features:

  • A global secondary index (GSI) sorts queue task items by a created timestamp, in reverse chronological (LIFO) order.
  • Update expressions and condition expressions provide atomic and exclusive queue task item updates. This prevents duplicate processing of queue tasks and ensures that the queue task state transitions are valid.
  • Time to live (TTL) deletes queue task items once they expire. Under insurmountable load, this ensures that tasks are deleted if they are never processed from the queue. It also deletes queue task items once they have been processed.
  • DynamoDB Streams invoke a Lambda function when new queue task items are inserted into the table and must be processed.

The application consists of the following resources defined in the AWS SAM template:

  • QueueTable: A DynamoDB table containing queue task items, which is configured for DynamoDB Streams to invoke a TriggerFunction.
  • TriggerFunction: A Lambda function, which governs triggering of queue task processing. Source code: app/trigger.js
  • ProcessTasksFunction: A Lambda function, which processes queue tasks and ensures consistent queue task state flow. Source code: app/process_tasks.js
  • CreateTasksFunction: A Lambda function, which inserts queue task items into the QueueTable. Source code: app/create_tasks.js
  • TriggerTopic: An SNS topic which TriggerFunction subscribes to.
  • ProcessTasksTopic: An SNS topic which ProcessTasksFunction subscribes to.

The following diagram illustrates how those resources interact to implement the LIFO queue.

LIFO Architecture diagram

LIFO Architecture diagram

  1. CreateTasksFunction inserts queue task items into QueueTable with PENDING state.
  2. A DynamoDB stream invokes TriggerFunction for all queue task item activity in QueueTable.
  3. TriggerFunction publishes a notification on ProcessTasksTopic if queue tasks should be processed.
  4. ProcessTasksFunction subscribes to ProcessTasksTopic.
  5. ProcessTasksFunction queries for PENDING queue task items in QueueTable for up to 1 minute, or until no PENDING queue task items remain.
  6. ProcessTasksFunction processes each PENDING queue task by calling the throughput constrained legacy system.
  7. ProcessTasksFunction updates each queue task item during processing to reflect state (first to TAKEN, and then to SUCCESS, FAILURE, or PENDING).
  8. ProcessTasksFunction publishes an SNS notification on TriggerTopic if PENDING tasks remain in the queue.
  9. TriggerFunction subscribes to TriggerTasksTopic.

Application activity continues while DynamoDB Streams receives QueueTable events (2) or TriggerTasksTopic receives notifications (9).

LIFO queue DynamoDB table

A DynamoDB table stores the LIFO queue task items. The AWS SAM template defines this resource (named QueueTable):

  • Each item in the table represents a queue task. It has the item attributes taskId (hash key), taskStatus, taskCreated, and taskUpdated.
  • The table has a single global secondary index (GSI) with taskStatus as the hash key and taskCreated as the range key. This GSI is fundamental to LIFO queue characteristics. It allows you to query for PENDING queue tasks, in reverse chronological order, so that the newest tasks can be processed first.
  • The DynamoDB TTL attribute causes earlier queue tasks to expire and be deleted. This prevents the queue from growing indefinitely if there is insurmountable load.
  • DynamoDB Streams invokes the TriggerFunction Lambda function for all changes in QueueTable.

Triggering queue task processing

The application continuously processes all PENDING queue tasks until there is none remaining. With no PENDING queue tasks, the application will be idle.

As the application is serverless, task processing is triggered by events. If a single Lambda function cannot process the volume of PENDING tasks, the application notifies itself so that processing can continue in another invocation. This is a tail call, which is an SNS notification sent by ProcessTasksFunction to TriggerTopic.

The Lambda functions, which collaborate on managing the LIFO queue are:

  • TriggerFunction is a proxy to ProcessTasksFunction and decides if task processing should be triggered. This function is invoked by DynamoDB Streams events on item changes in QueueTable or by a tail call SNS notification received from TriggerTopic.
  • ProcessTasksFunction performs the processing of queue tasks and implements the LIFO queue behavior. An SNS notification published on ProcessTasksTopic invokes this function.

Processing queue task items

The ProcessTasksFunction function processes queue tasks:

  1. The function is invoked by an SNS notification on ProcessTasksTopic.
  2. While the function runs, it polls QueueTable for PENDING queue tasks.
  3. The function processes each queue task and then updates the item.
  4. The function stops polling after 1 minute or if there are no PENDING queue tasks remaining.
  5. If there are more PENDING tasks in the queue, the function triggers another task. It sends a tail call SNS notification to TriggerTopic.

This uses DynamoDB expressions to ensure that tasks are not processed more than once during periods of concurrent function invocations. To prevent higher concurrency, the reserved concurrent executions attribute is set to 1.

Before processing a queue task, the taskStatus item attribute is transitioned from PENDING to TAKEN. Following queue task processing, the taskStatus item attribute is transitioned from TAKEN to SUCCESS or FAILURE.

If a queue task cannot be processed (for example, an external system has reached capacity), the item taskStatus attribute is set to PENDING again. Any aging PENDING queue tasks that cannot be processed are buffered. They are eventually deleted once they expire, due to the TTL configuration.

Querying for queue task items

To get the most recently created PENDING queue tasks, query the task-status-created-index GSI. The following shows the DynamoDB query action request parameters for the task-status-created-index. By using a Limit of 10 and setting ScanIndexForward to false, it retrieves the 10 most recently created queue task items:

{ "TableName": "QueueTable", "IndexName": "task-status-created-index", "ExpressionAttributeValues": { ":taskStatus": { "S": "PENDING" } }, "KeyConditionExpression": "taskStatus = :taskStatus", "Limit": 10, "ScanIndexForward": false
}

Updating queue tasks items

The following code shows request parameters for the DynamoDB UpdateItem action. This sets the taskStatus attribute of a queue task item (to TAKEN from PENDING). The update expression and condition expression ensure that the taskStatus is set (to TAKEN) only if the current value is as expected (from PENDING). It also ensures that the update is atomic. This prevents more-than-once processing of a queue task.

{ "TableName": "QueueTable", "Key": { "taskId": { "S": "task-123" } }, "UpdateExpression": "set taskStatus = :toTaskStatus, taskUpdated = :taskUpdated", "ConditionExpression": "taskStatus = :fromTaskStatus", "ExpressionAttributeValues": { ":fromTaskStatus": { "S": "PENDING" }, ":toTaskStatus": { "S": "TAKEN" }, ":taskUpdated": { "N": "1623241938151" } }
}

Conclusion

This post describes how to implement a LIFO queue with AWS Serverless technologies, using an example application as an example. Newer tasks in the queue are prioritized over earlier tasks. Tasks that cannot be processed are buffered and eventually load shed. This helps for use cases with heavy load and where newer queue tasks must take priority.

For more serverless learning resources, visit Serverless Land.