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MQTT Durable Sessions

EMQX includes a durable sessions feature, which allows MQTT sessions and messages to be persistently stored on disk, providing high-availability replicas to ensure data redundancy and consistency. With session durability, effective failover and recovery mechanisms can be implemented, ensuring service continuity and availability, thereby improving system reliability.

This page introduces the concepts, principles, and usage of session persistence in EMQX.

Important Notice

This feature is available starting from EMQX v5.7.0. However, it does not yet support the persistence of shared subscription sessions, which is planned to be implemented in future versions.

Basic Concepts

Before learning the durable sessions feature in EMQX, it's essential to understand some basic concepts about EMQX.

Sessions and Durable Storage

Session: A session is a lightweight process within EMQX created for every client connection. Sessions implement behaviors prescribed to the broker by MQTT Standard, including initial connection, subscribing and unsubscribing to topics, and message dispatching.

Durable Storage: Durable storage is an internal database within EMQX. Sessions may use it to save their state and MQTT messages sent to the topics. Database engine powering durable storage uses RocksDB to save the data on disk, and Raft algorithm to consistently replicate data across the cluster. It is important not to confuse durable storage with Durable Sessions.

Session Expiry Interval

According to the MQTT standard, client sessions facilitate the management of client connections and states within the MQTT broker. Expiry Interval is a property of a session that controls how long the broker keeps the session state after the client connection terminates. This property plays a crucial role in the context of this document.

Sessions configured with an expiry interval of 0 exist only during the client's connection to EMQX. When the client disconnects, all session information, including subscriptions and undelivered messages, is immediately discarded. For sessions with a non-zero expiry interval, the broker keeps them even after the client's connection terminates. They can be resumed if the client reconnects to the broker within the session expiry interval. Messages sent to the topics while the client was offline are delivered.

  • Clients using the MQTT 5 protocol can explicitly specify session expiry interval using the Session Expiry Interval property of CONNECT or DISCONNECT packets.

  • For the clients using MQTT 3.* protocol, EMQX derives the session expiry interval according to the following rule: if the Clean Session flag is true, then the session expiry interval is set to 0. Otherwise, the value of mqtt.session_expiry_interval configuration parameter is used.

Session Implementations in EMQX

EMQX provides 2 different client session implementations, each optimized for specific use cases:

  • Regular sessions: Sessions that keep their state in the memory of a running EMQX node. Their state is lost when the EMQX node restarts.
  • Durable sessions: Sessions that back up their state and received messages in the durable storage. They can be resumed after restart of the EMQX node.

The choice of session implementation depends on the session expiry interval and the durable_sessions.enable configuration parameter, which can be set globally or per zone. The implementation can be selected based on the following criteria:

durable_sessions.enableSession Expiry Interval = 0Session Expiry Interval > 0
falseRegularRegular
trueRegularDurable

EMQX uses a unique approach to manage message durability, allowing regular and durable sessions to coexist while minimizing storage costs.

Comparison of the Session Implementations

The management strategy for client sessions is a crucial factor in ensuring service stability and reliability. This section provides a comparative analysis of the characteristics of the 2 session implementations. It aims to help developers better understand their respective features and applicable scenarios, enabling more precise deployment decisions.

Regular Sessions

This session implementation is the default and has been used in all EMQX releases before version 5.7. State of regular sessions is maintained entirely in RAM of the running EMQX node.

Advantages of regular sessions include:

  • Very high throughput and low latency.
  • Immediate message dispatch to clients.

However, there are some drawbacks:

  • Session data is lost when the EMQX node hosting the session stops or restarts.
  • Undelivered messages are stored in the session's memory queue, increasing memory footprint of the broker.
  • EMQX imposes a limit on the size of the memory queue to prevent memory exhaustion. New messages are discarded when this limit is reached, leading to potential loss of undelivered messages.

Durable Sessions

Introduced in EMQX v5.7.0, the durable session implementation stores session state and messages routed to the durable sessions on disk. This feature is disabled by default and can be enabled by setting the durable_sessions.enable configuration parameter to true.

When a durable session subscribes to a topic filter, EMQX marks the topics matching that filter as "durable." This ensures that, in addition to routing MQTT PUBLISH messages from these topics to regular sessions, the broker also saves these messages to durable storage called messages.

It is important to note that message dispatch protocol depends on the durability of the subscriber's session rather than the publisher's.

Each durable MQTT message is stored exactly once on each replica, regardless of the number of subscribing durable sessions or their connection status. This ensures efficient message fan-out and minimizes disk writes.

Durable storage provides robust durability and high availability by consistently replicating session metadata and MQTT messages across multiple nodes within an EMQX cluster. The configurable replication factor determines the number of replicas for each message or session, enabling users to customize the balance between durability and performance to meet their specific requirements.

Advantages of durable sessions include:

  • Sessions can be resumed after EMQX nodes are restarted or stopped.
  • MQTT messages are stored in a shared, replicated, durable storage instead of a memory queue, reducing RAM usage for both online and offline sessions.
  • There is no upper limit on the number of undelivered messages, and undelivered messages are never discarded due to memory queue overrun.

However, there are some disadvantages:

  • Storing messages on disk results in lower overall system throughput.
  • Durable sessions have higher latency compared to regular sessions because both writing and reading MQTT messages are performed in batches. While batching improves throughput, it also increases end-to-end latency (the delay before clients see the published messages).

Quick Start with Durable Sessions

This section will help you quickly understand how to use the durable sessions on EMQX and MQTT clients, and introduce a simple workflow of durable sessions.

Note

Even if durable sessions are not enabled, following steps 2-4 will still retain the session and messages will be saved in the client queue. The difference lies in whether the session is persistently stored and whether the session can be restored after the node restarts in step 5.

  1. Enable the durable sessions feature on EMQX.

    By default, EMQX does not enable durable sessions. You need to modify the etc/emqx.conf file and add the following configuration to enable this feature:

    bash
    durable_sessions {
      enable = true
    }

    Restart EMQX to apply the configuration.

  2. Adjust MQTT client connection parameters to activate session durability.

    Using MQTTX CLI as an example, which defaults to using MQTT 5.0 protocol, add the --no-clean option to set Clean Start = false, and specify the client ID as emqx_c. Connect to EMQX and subscribe to the t/1 topic:

    bash
    mqttx sub -t t/1 -i emqx_c --no-clean
  3. Disconnect the client, and the session will be retained.

    Disconnect the client from step 2. Open the EMQX Dashboard, go to the Monitoring -> Clients page, and you still can see the client status as Disconnected, indicating that the session has been retained.

    MQTT persistent session

  4. Send messages to the client, and messages will be saved in the client queue.

    Using MQTTX CLI again, use the bench command to repeatedly publish messages to the t/1 topic with one client:

    bash
    mqttx bench pub -t t/1 -c 1

    According to the MQTT protocol, even if the emqx_c client is offline, the messages for the t/1 topic it subscribed to will be saved in the client queue and will be delivered when it reconnects.

  5. Restart the EMQX node, the session and messages will be restored from the durable storage.

    Restart the EMQX node. Without making any client connection operations, open the EMQX Dashboard and go to the Monitoring -> Clients page to see the client with status Disconnected, indicating that the session has been restored.

    Try connecting to EMQX with the same client ID emqx_c and using the --no-clean option to set Clean Start = false:

    bash
    mqttx sub -t t/1 -i emqx_c --no-clean

    The messages received during the offline period will be delivered to the current client:

    bash
    ...
    [2024-5-22] [16:14:14] › …  Connecting...
    [2024-5-22] [16:14:14] › ✔  Connected
    [2024-5-22] [16:14:14] › …  Subscribing to t/1...
    [2024-5-22] [16:14:14] › ✔  Subscribed to t/1
    [2024-5-22] [16:14:14] › payload: Hello From MQTTX CLI
    ...

    Note

    • You must use the same client ID emqx_c and specify the --no-clean option to set Clean Start to false. These two requirements must be met to restore the persistent session.
    • Since the previous subscription information is already saved in the session, messages will be delivered to the client even if it does not resubscribe to the t/1 topic upon reconnection.

Durable Storage Architecture

The database engine powering EMQX's builtin durability facilities organizes data into a hierarchical structure comprising storages, shards, generations, and streams.

Diagram of EMQX durable storage sharding

Storage

Storage encapsulates all data of a certain type, such as MQTT messages or MQTT sessions.

Shard

Messages are segregated by client and stored in shards based on the publisher's client ID. The number of shards is determined by n_shards configuration parameter during the initial startup of EMQX. A shard is also a unit of replication. Each shard is consistently replicated the number of times specified by durable_storage.messages.replication_factor across different nodes, ensuring identical message sets in each replica.

Generation

Messages within a shard are segmented into generations corresponding to specific time frames. New messages are written to the current generation, while previous generations are read-only. EMQX cleans up old MQTT messages by deleting old generations in their entirety. The retention period for old MQTT messages is determined by the durable_sessions.message_retention_period parameter.

Generations can organize data differently according to the storage layout specification. Currently, only one layout is supported, optimized for high throughput of wildcard and single-topic subscriptions. Future updates will introduce layouts optimized for different workloads.

The storage layout for new generations is configured by the durable_storage.messages.layout parameter, with each layout engine defining its own configuration parameters.

Stream

Messages in each shard and generation are split into streams. Streams serve as units of message serialization in EMQX. Streams can contain messages from multiple topics. Various storage layouts can employ different strategies for mapping topics into streams.

Durable sessions fetch messages in batches from the streams, with batch size adjustable via the durable_sessions.batch_size parameter.

Durable Storages Across Cluster

Each node within an EMQX cluster is assigned a unique Site ID, which serves as a stable identifier, independent of the Erlang node name (emqx@...). Site IDs are persistent, and they are randomly generated at the first startup of the node. This stability maintains the integrity of the data, especially in scenarios where nodes might undergo name modifications or reconfigurations.

Administrators can manage and monitor durable storage status across the cluster by using the emqx_ctl ds info CLI command to view the status of different sites.

Hardware Requirements for Durable Sessions

When session durability is enabled, EMQX saves the metadata of durable sessions and MQTT messages sent to the durable sessions on disk. Therefore, EMQX must be deployed on a server with sufficiently large storage capacity. To achieve the best throughput, it is recommended to use Solid State Drive (SSD) storage.

The disk space requirements can be estimated according to the following guidelines:

  • Message Storage: The space required for storing messages on each replica is proportional to the rate of incoming messages multiplied by the duration specified by the durable_sessions.message_retention_period parameter. This parameter dictates how long messages are retained, influencing the total storage needed.
  • Session Metadata Storage: The amount of storage for session metadata is proportional to the number of sessions multiplied by the number of streams to which they are subscribed.
  • Stream Calculation: The number of streams is proportional to the number of shards. It also depends (in a non-linear fashion) on the number of topics. EMQX automatically combines topics that have a similar structure into the same stream, ensuring that the number of streams doesn't grow too fast with the number of topics, minimizing the volume of metadata stored per session.

Next Steps

To learn how to configure and manage the durable sessions feature, as well as how to initially set up and modify durable sessions in an EMQX cluster, please refer to the following pages: