# Slow Subscriptions
Typically, EMQX will finish the message transmission within milliseconds, affected mainly by the network. However, there are cases where the latency of subscription messages is very high on the client side. To solve this problem, EMQX provides a slow subscriptions feature.
# How It Works
EMQX will start calculating the time (delay) it takes for the message to be processed and transmitted since messages arrive at EMQX.
If the delay exceeds the specified threshold, EMQX will insert the corresponding subscriber and topic into the slow subscriptions list or update the existing record.
In the slow subscriptions list:
- The record data are presented in descending order by the time delay and can save up to 1000 records.
- The list records the subscriber-topic data rather than the messages that exceed the threshold.
- When a record is generated, it will be inserted into the list if it is a new record. If it is an existing record, the occurrence time will be updated. In both cases, the entire list will be re-ranked.
- After the record is generated, it will be removed from the list if it is not triggered again within the effective period (default: 300 seconds).
The definition of message transfer complete varies with the message QoS level:
- QoS 0: The message is sent successfully.
- QoS 1: EMQX receives the PUBACK packet from the client.
- QoS 2: EMQX receives the PUBCOMP packet from the client.
Factors affecting message latency:
- The network between the publisher and EMQX is slow (will be available in future releases).
- The execution speed of the Hooks is slow, which blocks message publishing, such as ACL checks, ExHooks, rule engines, etc.
- Too many messages accumulate in the queue, resulting in significant latency. For example, PUBLISH and SUBSCRIBE share the same connection, and many PUBLISH messages are accumulated in the queue.
- The receiving speed of the subscribers is too slow.
# Enable and Configure Slow Subscriptions
On EMQX Dashboard, click Diagnose -> Slow Subscriptions from the left navigation menu. On the Slow Subscriptions page, click the Enable button.
Follow the instruction below for configuration:
- Stats Threshold: Latency threshold for statistics, only messages information with latency exceeding the value will be collected. Minimum value: 100ms Maximum Number of Statistics: Maximum number of records in the slow subscription statistics record table. Maximum value: 1,000
- Eviction Time of Record: Eviction time of the record; will start the counting since creation of the record, and the records that are not triggered again within the specified period will be removed from the list. Default: 300s
- Stats Type: Calculation methods of the latency, which are
- whole: From the time the message arrives at EMQX until the message transmission completes
- internal: From when the message arrives at EMQX until when EMQX starts delivering the message
- response : From the time EMQX starts delivering the message until the message transmission completes
# Check Slow Subscriptions List
The Slow Subscriptions list contains the following fields:
Client ID: Client with the slow subscriptions issue
Topic: Topic with the slow subscriptions issue
Duration: Message latency
Node: Node with the slow subscriptions issue
Updated: Creation/update time of the record
You can click on the Client ID to view the details and troubleshoot the issue.