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MQTT vs HTTP: Which is Better Suited for the Internet of Things (IoT)?

Jun 27, 2024
MQTT vs HTTP: Which is Better Suited for the Internet of Things (IoT)?

Introduction

The rapid evolution of Internet of Things (IoT) technology has led to continuous expansion of its applications. However, at its core, IoT is primarily concerned with data transmission, necessitating a thoughtful and strategic selection of communication protocols for devices.

MQTT and HTTP are two predominant communication protocols, each with distinct strengths and suitable environments. MQTT is specifically designed for IoT, offering a more adaptable approach and numerous features tailored for IoT contexts. In contrast, HTTP, which predates MQTT, is extensively used across a wide array of non-IoT applications. Its users may have more extensive development and operational experience.

This blog aims to provide an in-depth examination of MQTT and HTTP within the IoT landscape, highlighting their unique attributes, suitable scenarios, and their practical deployment performance. By comparing and analyzing them, readers will gain a clearer comprehension of how to select the most fitting communication protocol to enhance the efficiency and dependability of IoT systems.

What is MQTT?

MQTT is a lightweight messaging protocol that operates on a publish/subscribe model, specifically designed to address the intricate and unreliable network conditions of IoT devices, as well as their constrained memory, storage, and processing power. It enables networked devices to receive real-time, reliable messaging services with minimal coding requirements.

In a standard MQTT setup, all clients needing to communicate (typically hardware devices and application services) maintain a continuous TCP connection with a single MQTT server (MQTT Broker). There’s no need for a direct link between the message-sending client (publisher) and the message-receiving client (subscriber); the MQTT server handles the message routing and dissemination.

The cornerstone of this process is the concept of Topic. Topics serve as the basis for MQTT’s message routing and resemble URL paths, using / for hierarchical structuring, such as sensor/1/temperature. Subscribers subscribe to topics they’re interested in, and when a publisher posts a message to that topic, it’s forwarded according to the topic’s structure.

An MQTT topic can be subscribed to by multiple subscribers, and the server relays topic-specific messages to all of them; likewise, a topic can have several publishers, with messages forwarded in the sequence they arrive. A client can function both as a publisher and a subscriber, facilitating communication of messages based on topics, thereby enabling MQTT to support one-to-one, many-to-one, and one-to-many bidirectional communications.

What is HTTP?

HTTP, an application-layer protocol founded on the request/response paradigm, is not only pivotal for traditional client-server models but also plays a crucial role in IoT applications.

For clarity, this blog’s comparison specifically pertains to HTTP in its conventional request/response modality. Extensions of the HTTP protocol, such as WebSocket and Server-Sent Events, are excluded from this comparison.

In standard HTTP practice, clients (typically browsers or web applications) initiate requests to servers for resource retrieval or data submission. Upon request receipt, servers process and respond accordingly, such as storing submitted data for subsequent client access.

HTTP uses URLs to denote resource locations, similar to how MQTT uses Topics. For example, an HTTP request URL like http://example.com/api/sensor has a similar layered format to an MQTT topic such as sensor/1/temperature.

Each communication over HTTP is done through a different request and response process, so it requires additional overhead and lacks real-time performance as the two clients cannot communicate directly with each other.

Resource Consumption Comparison

Both MQTT and HTTP are straightforward protocols supported by many IoT hardware devices and embedded systems. Typically, their resource footprint and runtime memory do not limit their usage. However, MQTT, with its IoT-specific design philosophy and features, tends to be more resource-efficient over prolonged use.

To begin with, MQTT boasts lower overhead in terms of connectivity. It minimizes the protocol’s additional consumption, with message headers that can be as small as 2 bytes. The handshake process for establishing connections is relatively straightforward, ensuring stable operation even in networks with limited bandwidth.

Once a connection is established, it can be maintained for extended periods between the client and server, allowing multiple messages to traverse the same connection. This significantly cuts down the costs associated with frequently establishing and terminating connections. For instance, publishing the message "HelloWorld" to the topic topic/1 would result in the following packet information:

Field Size (Bytes) Description
Fixed Header 1 Set to 0b0011xxxx
Topic Length 2 0x00 0x08
Topic 9 "topic/1"
Message Length 2 Length of "HelloWorld"
Message Content 10 "HelloWorld"
Total: 24

In contrast, HTTP necessitates the establishment and termination of connections for each request-response cycle, which incurs additional server resource consumption. HTTP is comparatively more complex, with larger message headers. Being a stateless protocol, it requires clients to carry extra identity information with each connection, further increasing bandwidth usage.

For example, transmitting the content "HelloWorld" to the URL http://localhost:3000/topic without identity credentials would yield the following packet information:"

Field Size (Bytes) Description
Request Line 17 POST /topic HTTP/1.1
Host 20 Host: localhost:3000
Content-Type 24 Content-Type: text/plain
Content-Length 18 Content-Length: 10
Empty Line 2 Separates headers and body
Request Body 10 "HelloWorld"
Total: 91 Bytes

In summary:

  • MQTT features low connection overhead, straightforward connection establishment, and minimal packet headers, making it ideal for scenarios that demand frequent communication or sustained connections.
  • In contrast, HTTP requires the establishment and termination of connections for each request-response cycle, coupled with larger message headers, potentially exacerbating transmission delays and increasing the load, particularly in bandwidth-constrained environments.

In terms of packet size and connection overhead, MQTT typically outperforms HTTP, especially in IoT contexts that necessitate frequent communication, persistent connections, or operation within bandwidth-constrained environments.

Security Comparison

Both MQTT and HTTP are TCP-based protocols designed with a strong emphasis on security.

SSL/TLS Encryption

Both protocols support secure communication via SSL/TLS encryption:

  • This ensures the confidentiality and integrity of data during transit.
  • It prevents the interception, alteration, or forgery of data.

Varied Authentication and Authorization Mechanisms

  • MQTT provides username/password authentication, extendable support for JWT authentication, and client-server X.509 certificate authentication. Authorization can include topic-based publish/subscribe permission checks, depending on the MQTT server’s capabilities.
  • HTTP offers a broader array of options, including Basic Authentication, Token Authentication, and OAuth. Access to resources can be controlled through application-layer mechanisms, utilizing access tokens, session management, and more for robust access control.

IoT Features Comparison

The MQTT protocol is specifically designed for the IoT and comes with an array of features that cater to IoT scenarios. It facilitates stable and reliable device communication and real-time data transfer, fulfilling the demands of various business contexts.

Reconnection and Persistent Sessions

MQTT supports persistent connections and reconnections, ensuring consistent communication between devices and servers, even amidst unstable network conditions. Clients have the option to establish persistent sessions, which can be restored upon reconnection to prevent message loss.

Quality of Service (QoS) Levels

MQTT offers three QoS levels:

  • QoS 0: Delivers messages at most once, with the possibility of loss.
  • QoS 1: Guarantees at least one delivery, with potential message duplication.
  • QoS 2: Ensures exactly one delivery, with no message loss or duplication.

Clients can choose the appropriate QoS level to match their specific needs for reliable message delivery.

Shared Subscriptions

Multiple clients can subscribe to the same topic, receiving identical messages, which is ideal for scenarios where data sharing or event subscription is needed across multiple devices.

Retained Messages

Servers can retain the latest message for a given topic and dispatch it immediately to new subscribers, ensuring they receive the most current information.

Last Will Message

Clients can set a "last will" message to be published by the server if they disconnect unexpectedly, alerting other subscribers of the disconnection.

Message Expiry Interval

Messages can be assigned an expiry interval, ensuring they are consumed within a specified timeframe and preventing outdated messages from burdening the system.

While HTTP is a widely adopted protocol in web applications, users can implement some IoT-specific features based on a mature toolchain and functional design experience but requires additional development effort. MQTT’s inherent design, which integrates numerous IoT-appropriate features, can reduce development costs and enhance communication efficiency, making it more suitable for IoT applications.

Comparative Analysis

In summary, MQTT and HTTP have significant differences in their communication models and IoT-centric attributes:

  • MQTT operates on a publish-subscribe model, enabling duplex communication, while HTTP follows a request-response model.
  • With MQTT, messages are pushed in real-time, whereas HTTP requires polling to retrieve data updates.
  • MQTT is stateful, maintaining a connection context, while HTTP is stateless.
  • MQTT can handle abnormal disconnections and recover smoothly, unlike HTTP.
  • MQTT comes equipped with a variety of built-in IoT features, whereas HTTP lacks this specific focus in its design.

These variances bear significant implications on their applicability within IoT contexts:

  • Real-time Communication: MQTT excels in scenarios requiring high real-time responsiveness. Its publish-subscribe model allows devices to push messages to servers or other devices instantly without awaiting a request. This makes MQTT ideal for real-time monitoring of sensor data and immediate control of devices, where rapid response is crucial.
  • Lightweight and Frequent Communication: In environments with limited bandwidth and resources, MQTT is typically more efficient than HTTP. MQTT minimizes communication overhead by avoiding frequent connection setups and utilizing small message headers. In contrast, HTTP's synchronous request-response model is less efficient, necessitating complete request and response headers for each interaction, which can waste bandwidth and resources.
  • Scenarios with Network Fluctuations: MQTT maintains persistent connections between clients and servers, allowing it to recover from connection disruptions. Even if the network disconnects, MQTT can resume communication upon reconnection. HTTP, being stateless, handles each communication independently and cannot recover from disconnections in the same manner.

Another Thought: Integrating MQTT with HTTP

We've explored which protocol might be best for IoT devices. In reality, complex IoT applications often involve a mix of hardware, clients, and business processes. MQTT and HTTP, as two of the most widely used protocols in IoT and the broader internet, can complement each other in many scenarios to enhance system efficiency and flexibility.

For instance, in a typical IoV application, HTTP is well-suited for user interactions. Imagine a user controlling a car in the garage via an "open door" button in an app. This action isn't a two-way communication between the app and the server, and using HTTP allows for more complex and flexible security and permission checks. On the other hand, server-to-vehicle communication needs real-time, bidirectional communication: the vehicle must respond to user actions promptly.

Vehicles can periodically report their status via MQTT, which the server records. When the user needs this information, the app retrieves it using HTTP.

The world’s leading MQTT broker, EMQX, enables this process by easily and flexibly integrating the MQTT protocol with the HTTP protocol.

HTTP → MQTT:

The application system can convert HTTP requests into MQTT messages sent to specific devices by calling the API provided by EMQX. This allows the system to send control commands or notifications to the devices.

curl -X POST 'http://localhost:18083/api/v5/publish' \
  -H 'Content-Type: application/json' \
  -u '<appkey>:<secret>'
  -d '{
  "payload_encoding": "plain",
  "topic": "cmd/{CAR_TYPE}/{VIN}",
  "qos": 1,
  "payload": "{ \"oper\": \"unlock\" }",
  "retain": false
}'

MQTT → HTTP:

When a device sends an MQTT message to EMQX, the Webhook feature can forward this message to an HTTP server, instantly transmitting device data to the application system.

MQTT → HTTP

The configuration interface is as follows:

Create webhook

Future versions of EMQX will further enhance this process by saving real-time MQTT messages to an integrated Message Queue and Stream, allowing users to consume these messages via HTTP. This will better support complex IoT scenarios and provide more robust message processing capabilities.

Conclusion

Ultimately, choosing between MQTT and HTTP depends on your specific application needs and scenario characteristics. For real-time, bidirectional communication with low resource consumption, MQTT is ideal. For simple request/response interactions, such as client data collection and submission, actively pulling data from the server, or leveraging existing web infrastructure, HTTP is more appropriate.

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