What are some alternatives?
When comparing twitter.github.io Kestrel and Celery, you can also consider the following products
nginx - A high performance free open source web server powering busiest sites on the Internet.
Kafka - Apache Kafka is publish-subscribe messaging rethought as a distributed commit log.
CloudAMQP - CloudAMQP automates every part of setup, running and scaling of RabbitMQ clusters. Available on all major cloud and application platforms.
Shopify - Shopify is a powerful ecommerce platform that includes everything you need to create an online store and sell online. Try it free for 14 days.
RabbitMQ - RabbitMQ is an open source message broker software.
Airflow - Airflow is a platform to programmaticaly author, schedule and monitor data pipelines.