# Backends

# MQTT Message Persistence

# One-to-one message Persistence

1. PUB publishes a message;
2. Backend records this message in DB;
3. SUB subscribes to a topic;
4. Backend retrieves the messages of this topic from DB;
5. Messages are sent to SUB;
6. Once the SUB acknowledged / received the message, backend removes the message from DB.

# Many-to-many message Persistence

1. PUB publishes a message;
2. Backend records the message in DB;
3. SUB1 and SUB2 subscribe to a topic;
4. Backend retrieves the messages of this topic;
5. Messages are sent to SUB1 and SUB2;
6. Backend records the read position of SUB1 and SUB2, the next message’s retrieval starts from this position.

# Client Connection State

EMQX supports retaining the client's connection state in Redis or DB.

# Client Subscription by Broker

EMQX Persistence supports subscription by broker. When a client goes online, the persistence module loads the subscriptions of the client from Redis or Databases.

# List of Persistence Plugins

EMQX allowes storing messages in Redis, MySQL, PostgreSQL, MongoDB, Cassandra, DynamoDB, InfluxDB, OpenTSDB and Timescale:

Persistence Plugins	Config File	Description
emqx_backend_redis	emqx_backend_redis.conf	Redis Message Persistence
emqx_backend_mysql	emqx_backend_mysql.conf	MySQL Message Persistence
emqx_backend_pgsql	emqx_backend_pgsql.conf	PostgreSQL Message Persistence
emqx_backend_mongo	emqx_backend_mongo.conf	MongoDB Message Persistence
emqx_backend_cassa	emqx_backend_cassa.conf	Cassandra Message Persistence
emqx_backend_dynamo	emqx_backend_dynamo.conf	DynamoDB Message Persistence
emqx_backend_influxdb	emqx_backend_influxdb.conf	InfluxDB Message Persistence
emqx_backend_opentsdb	emqx_backend_opentsdb.conf	OpenTSDB Message Persistence
emqx_backend_timescale	emqx_backend_timescale.conf	Timescale Message Persistence