MQTT vs CoAP: Choosing the Right Protocol for IoT Development

Recently at Exadel we faced an interesting challenge for IoT developers. Because IoT applications have gained so much momentum, there are increasingly many choices on how to develop them. For device communication, two competing protocols have emerged: Message Queuing Telemetry Transport (MQTT) and Constrained Application Protocol (CoAP). Both are designed to be lightweight and make careful use of scarce network resources. Both are appropriate in the right environment, but the problem is that, given the relative youth of IoT, many people do not know what these protocols are or when to use them.

These are not standard web protocols used by everyone.

Given our internal discussions, I decided to help explain them. First, let's look at what these protocols are.

What is MQTT?

For laypeople, MQTT is like Twitter. It is a publish‑subscribe protocol. You can subscribe to topics and publish on others. You receive messages for topics you subscribe to, and those who subscribe to topics you publish will receive those messages. There are differences, for example you can configure the protocol for guaranteed delivery to make it more reliable. The publish/subscribe system uses a broker; analogous to how Twitter filters messages based on your subscription preferences.

What is CoAP?

CoAP is more like a traditional website‑based service such as Amazon. You request resources (pages and search results in the Amazon example) and occasionally submit your own data (to make a purchase). CoAP is designed to look compatible with HTTP and can leverage most of the Internet as we know it. CoAP can use proxy servers to translate to HTTP, or communicate directly with specialized servers designed for CoAP when environmental constraints apply.

When should you use them?

You might be asking, "If they are so similar, when should I choose one over the other?"

Because of its publish/subscribe architecture with a broker intermediary, MQTT is ideal for communication between devices over wide‑area networks (WAN, Internet). It is most useful when bandwidth is limited, such as remote field sites or areas lacking robust networks. MQTT is part of Azure and Amazon service offerings, so it has many established architectures that make it easy for developers to adopt.

In the case of CoAP, the strongest use case is HTTP compatibility. If you have an existing web‑service‑based system, adding CoAP is a good choice. It is built on the User Datagram Protocol (UDP), which is useful in some resource‑constrained environments. Because UDP allows broadcasting and multicasting, you can potentially transmit to multiple hosts with less bandwidth, making it well‑suited for local network environments where devices need rapid communication, typical of many M2M settings.

If IoT developers are using devices that will leverage existing web‑server architecture, they will use CoAP. However, if developers are building something that merely "reports"—i.e., it is dropped onto the network and only needs to send data back to a server—MQTT will be better. Other scenarios, such as cloud architectures, may be best served by MQTT.

The future of MQTT and CoAP

Over time, adoption of other protocols tends to shift toward more open and inclusive platforms unless a non‑inclusive platform proves superior. Both MQTT and CoAP are open standards that anyone can implement. CoAP was initiated by standards bodies, whereas MQTT was designed by private companies (including IBM). CoAP was designed to handle resource‑limited environments, which may make it the winner, but currently MQTT appears to be ahead. MQTT has significant momentum—major cloud players have embraced it, and many commercial use cases require MQTT’s capabilities (store‑and‑forward, centralized host). However, a possible scenario is that software development standardized around HTTP (e.g., mobile app development) could start using CoAP to handle peripheral devices and communicate with back‑ends, helping reduce poor connection bandwidth.

Ultimately, these protocols can be effectively deployed over the Internet in different applications. We know there are specific use cases where each is best, and we also know that IoT and IoT devices will continue to evolve in complexity and prevalence. For developers, understanding the key differences enables better initial deployments and lays a solid foundation for future development work.