# Redis

EMQX supports integration with Redis so you can save client messages and events to Redis. With Redis data bridge, you can use Redis for message caching and statistics of published/subscribed/discarded messages.

TIP

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Prerequisites

Knowledge about EMQX data integration rules
Knowledge about data bridge

# Feature List

[Connection pool](./data-bridges.md#Connection pool)
[Async mode](./data-bridges.md#Async mode)
[Batch mode](./data-bridges.md#Batch mode)
Buffer queue

# Quick Start

In this section, we will illustrate how to leverage Redis to:

Cache the last message of every client;
Collect the message discard statistics.

# Install Redis

Install and run Redis via Docker:

# Start a Redis container and set the password to public
docker run --name redis -p 6379:6379 -d redis --requirepass "public"

# Access the container
docker exec -it redis bash

# Access the Redis server, use the AUTH command for authentication
redis-cli
127.0.0.1:6379> AUTH public
OK

# Verify the installation
127.0.0.1:6379> set emqx "Hello World"
OK
127.0.0.1:6379> get emqx
"Hello World"

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Now you have successfully installed Redis and verified the installation with the SET and GET commands. For more Redis commands, see Redis Commands (opens new window).

# Connect to Redis

We will create two Redis data bridges to illustrate the messaging caching and statistics features. The connection configurations for both data bridges are the same.

Go to EMQX Dashboard, click Data Integration -> Data Bridge.
Click Create on the top right corner of the page.
In the Create Data Bridge page, click to select Redis, and then click Next.
Input a name for the data bridge. Note: It should be a combination of upper/lower case letters and numbers.
Set Redis Mode as the business needs, for example, single.
Input the connection information. Input 127.0.0.1:6379 as the Server Host, public as the Password, and 0 for Database ID.

Now we have configured the connection information, for the Redis Command template, the setting differs a little depending on the feature to use.

# Message Caching

This section will illustrate how to use Redis to cache the last message from every client.

Finish the Redis connection configuration.
Configure Redis Command Template: Use the Redis HSET (opens new window) command and hash data structure to store messages, the data format uses clientid as the key, and stores fields such as username, payload, and timestamp. To distinguish it from other keys in Redis, we add an emqx_messages prefix to the message and separate it with :

# HSET key filed value [field value...]
HSET emqx_messages:${clientid} username ${username} payload ${payload} timestamp ${timestamp}

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Advanced settings (optional): Choose whether to use sync or async query mode as needed. For details, see Configuration parameters.
Then click Create to finish the creation of the data bridge. A confirmation dialog will appear and ask if you like to create a rule using this data bridge, you can click Create Rule to continue creating rules to specify the data to be saved into Redis. You can also access the Rule page by clicking Data Integration -> **Rules **on EMQX dashboard.

# Create Rules

Click Create on the top right corner of the page.
Input cache_to_redis as the rule ID, and set the rules in the SQL Editor. Here we want to save the MQTT messages under topic t/# to PostgreSQL, we can use the SQL syntax below. Note: If you are testing with your SQL, please ensure you have included all required fields in the SELECT part.

SELECT
  *
FROM
  "t/#"

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Then click the Add Action button, select Forwarding with Data Bridge from the dropdown list and then select the data bridge we just created under Data bridge. Then, click the Add button.
Then click the Create button to finish the setup.

Now we have successfully finished the configuration for message caching.

# Message Discard Statistics

This section will illustrate how to use Redis for message discard statistics.

Note: Except for the Redis Command template and rules, the other settings are the same as the Message caching section.

Redis Command template

Use HINCRBY (opens new window) command below to collect the discarded messages under every topic.

# HINCRBY key field increment
HINCRBY emqx_message_dropped_count ${topic} 1

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Each time the command is executed, the corresponding counter is incremented by 1.

Rule SQL

EMQX rules define 2 message discarding events, through which the rules can be triggered and recorded in Redis:

Event	Topic	Parameter
Messages are discarded during forwarding	$events/message_dropped	$events/message_dropped
Messages are discarded during delivery	$events/delivery_dropped	$events/delivery_dropped

The corresponding SQL is as follows:

SELECT
  *
FROM
  "$events/message_dropped", "$events/delivery_dropped"

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# Test

Use MQTT X to send a message to topic t/1 to trigger a message caching event. If topic t/1 does not have any subscribers, the message will be discarded and trigger the message discard rule.

mqttx pub -i emqx_c -u emqx_u -t t/1 -m '{ "msg": "hello Redis" }'

Check the running status of the two data bridges, there should be one new Matched and one Sent Successfully message.

Check whether the message is cached.

127.0.0.1:6379> HGETALL emqx_messages:emqx_c
1) "username"
2) "emqx_u"
3) "payload"
4) "{ \"msg\": \"hello Redis\" }"
5) "timestamp"
6) "1675263885119"

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Rerun the test, the timestamp field should be updated.

Check whether the discarded messages are collected:

127.0.0.1:6379> HGETALL emqx_message_dropped_count
1) "t/1"
2) "1"

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Repeat the test, the number on the counter corresponding to t/1 should also increase.

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