AMQP vs. MQTT: 9 Key Differences

The difference between AMQP and MQTT is that the brokers of the former have two parts: queues and exchanges. Both queuing and exchange are bound together.

Within the architecture, client applications are known as publishers or producers. The AMQP server is known as the broker. The client-side generates messages transmitted to the broker, where message queuing and routing occur. Users read messages from the queues, and processing also takes place therein. Client applications are known as consumers or subscribers within the architecture.

Publishers distribute messages in the exchange. Messages feature routing keys used by the exchange module for routing. Messages are held in queues until they are delivered to subscribers.

Several exchange mechanisms exist, including:

• Default exchange, which is an empty-string direct exchange with no name. It is pre-declared by the broker, and the message is routed to a queue. The routing key equals the queue name.

• Direct exchange, which involves message delivery to queues depending on routing keys. Think of routing keys as additional data defined to determine the message’s destination. Direct exchange is typically used for load-balancing tasks in a round-robin way among workers.

• Fanout exchange, which entirely ignores the routing key and transmits messages to all bound queues. Fanout exchanges typically involve message distribution to several clients for purposes similar to notifications. Think sharing of messages (such as chat servers), updates (such as news), and application states (such as configurations).

• Subject exchange, wherein message routing to queues occurs based on the routing key, such as a topic. Here, the routing key matches a pattern.

• Headers exchange involves using additional headers (message attributes) along with messages rather than depending on routing keys for queue routing. Using data types other than routing keys, headers exchange supports differing routing mechanisms with additional possibilities similar to direct exchange via keys.

• Topic exchange is mainly used for publish/subscribe patterns and involves a routing key and the binding of queues to exchanges for matching and sending messages. Whenever specialized consumer involvement is required, such as a single working set for performing a specific action type, topic exchange is useful for distributing messages according to patterns and keys.

Finally, in AMQP, a frame is the fundamental unit of data. Frames initiate, regulate, and terminate message transfers between two peers. The nine AMQP frame bodies are ‘be open’ (the connection), ‘begin’ (the session), ‘affix’ (the link), ‘transfer,’ ‘temperament,’ ‘flow,’ ‘separate’ (the link), ‘final’ (the connection), and ‘final’ (the session).

The MQTT architecture consists of these key parts:

• The MQTT broker (server) is software that receives messages from publishers and routes them to subscribers. Based on the implementation, brokers can simultaneously manage thousands of connected MQTT clients. When choosing an MQTT broker, users must consider factors such as integration and scalability.

• The MQTT client (subscribers and publishers), which can be any application or device — a simple microcontroller, a cloud-hosted application server, or anything in between — establishes a network connection to an MQTT broker and runs an MQTT library. MQTT clients can be a subscriber, a publisher, or even both. The implementation of both functions can take place within one MQTT client.

In the communication phase, a client can perform operations such as connect, disconnect, subscribe, unsubscribe, and publish as required.

MQTT generally relies on the transmission control protocol (TCP) for data transmission between the broker and clients. This allows for the enhancement of communication reliability.

While connection credentials are transmitted in plaintext format, SSL/TLS can be leveraged for encrypting and protecting data in transit. The default unencrypted port for MQTT is 1883 and the encrypted port is 8883.

Depending on the implementation, a publisher transmits messages to a specified exchange while a consumer receives messages from a queue.

However, before this, the connections must be established so publishers and consumers can locate one another. This is typically achieved via the exchange’s name.

Generally, the publisher or the consumer begins the exchange using a provided name, made public later.

After messages are received from clients (publishers), processing occurs at the exchange level, and the message is then routed to one or more queues. The type of routing executed depends on the exchange type: default exchange, direct exchange, fanout exchange, subject exchange, headers exchange, or topic exchange.

Pub/sub delinks the message-sending client (publisher) from the message-receiving clients (subscribers). Publishers and subscribers do not directly contact each other; third parties (brokers) manage the connections between them.

MQTT clients include publishers (clients responsible for publishing messages) and subscribers (clients subscribed for receiving messages). Both functions can be implemented in a single MQTT client.

When a client device must send data to a server (broker), it is known as a ‘publish.’ The reverse operation is known as a ‘subscribe.’ In the pub/sub model, numerous clients can connect to a broker and subscribe to topics per their interests.

If a subscribing client and the broker suffer a broken connection, the broker will buffer messages and transmit them to the subscriber once the connection is back online. If the publishing client and the broker are disconnected abruptly, the broker can close the connection and transmit a cached message with predefined instructions from the publisher to subscribers.

Other patterns supported by AMQP include publish-consume, as-long-as-connected, nobody-is-using-this-queue, and lifetime.

The more comprehensive set of messaging scenarios AMQP supports allows developers to customize their applications according to their requirements.

Pub/sub messaging leverages the concept of ‘topics.’ Any node that subscribes to a particular topic will receive all the messages published with that topic by other nodes.

In the case of AMQP and MQTT, a topic can be defined as a string used by the broker for filtering messages.

Users can consider these points related to framing to assist in the selection process for a suitable messaging protocol for their specific application:

• Size of a single data unit
• Need for fragmentation
• Data flow pattern: stream-oriented vs. buffer-oriented

In the case of AMQP, the protocol features an undefined message size. This means that users can accommodate any amount of data in each packet as required.

AMQP also enables data fragmentation across several packets.

Lastly, the larger header size of AMQP allows for more messaging options.

MQTT also features a smaller header size when compared to AMQP.

Unlike AMQP, MQTT does not support data fragmentation across multiple packets.

Additionally, considering its stream-oriented data flow, MQTT messages effectively exist for a very short time.

Message routing within MQTT actively takes place between simultaneously connected subscribers and publishers. If there are no subscribers, the broker discards the message.

MQTT is well-suited for applications that are optimized for smaller functions and do not require numerous options.

AMQP users can access a larger set of methods for executing different transaction types, including Consume, Publish, Deliver, Select, Get, Delete, Ack, Recover, Nack, Open, Close, and Reject.

AMQP features rich security measures when compared to MQTT. That is a primary reason for its popularity in corporate environments with stringent security requirements.

The AMQP security standard explicitly supports TLS and SASL integration, allows users to leverage extensions such as client authentication and data encryption, and enables separate policies and negotiation for both of these security mechanisms.

Further, when it comes to message discovery, namespaces are useful to find messages.

AMQP gives users numerous approaches to identify messages: spaces such as queues and nodes, users having a choice in how to find messages, users sharing queues, receiving message copies, or message removal from queues.

Additionally, AMQP metadata enables designers to implement innovative methods for message discovery.

In an MQTT network, a network-level firewall secures the broker. At the application level, encryption is used to secure MQTT packets.

Any further enhancements need the protocol and broker to be updated. However, available brokers typically do not support such modifications. In such cases, users must deploy and customize the broker.

Further, regarding message discovery, only a hierarchical ‘topic’ namespace is available in MQTT. All implementations use the topic convention.

In terms of reliability, both AMQP and MQTT are closely comparable. However, AMQP only features two Quality of Service (QoS) levels, while MQTT features three.

QoS0 is the lowest level of reliability (best-effort delivery), while QoS1 is a slightly higher reliability level that can cause message duplication.

Regarding reliability, however, MQTT has three levels of QoS (one greater than AMQP) for the reliable delivery of messages.

QoS2 is the highest available reliability level in MQTT. This protocol can also guarantee message delivery in case of disconnection.

Additionally, the availability of AMQP brokers and libraries in various programming languages is more limited than MQTT. This can potentially increase development costs and implementation time since developers need to invest additional effort to find and adapt suitable resources for a specific platform or programming language.

Regarding compatibility, AMQP is a rather comprehensive protocol that offers options for various infrastructure scenarios.

Its fine-grain control over redirection, load balancing, and other features can help in cases where a change or upgrade is required in the network configuration.

Apart from this, MQTT libraries are more pervasive and available in several popular programming languages, such as Python, Java, JavaScript, and C/C++. Similarly, brokers are available publicly. These factors collectively help increase developer support, reduce costs, and shorten development time.

However, it is worth noting that AMQP and MQTT both have sufficient online support in terms of sample implementations, communities, and documentation.

Regarding compatibility, remember that messaging protocol deployment is a long-term endeavor. As they are communication mechanisms, the ability of these protocols to remain compatible with any upgrades in infrastructure is critical. If a messaging protocol is inflexible, it risks becoming incompatible with infrastructure upgrades and might necessitate changes in the future.

When compared to AMQP, MQTT is less flexible and does not always successfully allow users to account for future infrastructure changes as easily as AMQP.

While AMQP may not be as popular as MQTT in the IoT and social media spaces, its use cases include defense, finance, manufacturing, and telecommunications at the enterprise level. Examples include business messaging, cloud messaging, and cross-datacenter messaging.

Leading enterprises leverage AMQP, including Google, JPMorgan, RedHat, NASA, AT&T, Mozilla, OpenStack, and IBM.

Use cases for MQTT generally include low-bandwidth, low-compute, and high-latency scenarios. Its lower footprint and more efficient protocol allow it to function well in such scenarios.

Due to its efficiency, MQTT is especially prevalent in the world of IoT, with use cases in smart homes, automotive, telecommunications, and manufacturing. Major cloud platforms like Microsoft Azure, IBM Watson, and Amazon Web Services support it along with AMQP.

Additionally, MQTT headers are small and thus allow for optimized network bandwidth. MQTT can also scale up to connect with millions of IoT devices.

MQTT was created for embedded devices; however, this protocol has evolved and is now used for social media applications as well. For instance, both Instagram and Facebook Messenger use MQTT for the chat functionality of their applications.