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EMQ X Broker specifies the limit on access speed and message speed. When the client’s connection request speed exceeds the specified limit, the establishment of a new connection is suspended; when the message reception speed exceeds the specified limit, the reception of messages is suspended.
Rate limit is a backpressure scheme that avoids system overload from the entrance and guarantees system stability and predictable throughput. The rate limit can be configured in etc/emqx.conf :
| Configuration item | Type | Default value | Description |
|---|---|---|---|
| listener.tcp.external.max_conn_rate | Number | 1000 | The maximum allowable connection rate on this node (conn/s) |
| zone.external.publish_limit | Number,Duration | No limit | Maximum allowable publish rate on a single connection (msg/s) |
| listener.tcp.external.rate_limit | Size,Duration | No limit | Maximum allowable packet rate on a single connection (bytes/s) |
- max_conn_rate is the rate limit for connection establishment on a single emqx node.
1000means that 1000 clients can access at most. - publish_limit is the rate limit for receiving PUBLISH packets on a single connection.
100,10smeans that the maximum PUBLISH message rate allowed on each connection is 100 every 10 seconds. - rate_limit is the rate limit for receiving TCP packets on a single connection.
100KB,10smeans that the maximum TCP packet rate allowed on each connection is 100KB every 10 seconds.
publish_limit and rate_limit both provide limits for a single connection. EMQ X Broker currently does not provide a global message rate limit.
Rate limit explanation
EMQ X Broker uses the Token Bucket
(opens new window) algorithm to control all Rate Limits. The logic of the token bucket algorithm is as follows:
- There is a bucket that can hold the maximum burst of the token. The maximum burst is abbreviated as b.
- There is a rate for adding tokens to the bucket per second, abbreviated as r. When the bucket is full, no tokens are added to the bucket.
- Whenever 1 (or N) request arrives, take 1 (or N) token from the bucket. If the token is not enough, it will be blocked and wait for the token to be generated.
It can be seen from this algorithm:
In the long run, the average value of the limited request rate is equal to the value of rate.
When the actual request reaching speed is M, and M> r, then the maximum (peak) rate that can be achieved in actual operation is M = b + r.
It is easy to think that the maximum rate M is the speed that can consume the full state token bucket in 1 unit of time. The consumption rate of token bucket is M-r, so we can see that: b / (M-r) = 1, and we get M = b + r
Application of Token Bucket Algorithm in EMQ X Broker
When the following configuration is used for packet rate limiting:
listener.tcp.external.rate_limit = 100KB,10s
EMQ X Broker will initialize the rate-limit processor of each connection with two values:
- rate = 100 KB / 10s = 10240 B/s
- burst = 100 KB = 102400 B
According to the algorithm in Message Rate Limitation Explanation, it is known:
- In the long run, the allowable average rate is limited to 10240 B/s
- The allowable peak rate is 102400 + 10240 = 112640 B/s
To improve system throughput, the access module of EMQ X Broker does not read packets from the socket one by one, but reads N packets from the socket each time. The timing of the rate-limit check is after receiving these N messages and before preparing to continue to receive the next N messages. Therefore, the actual rate limit will not be as accurate as the algorithm. EMQ X Broker only provides a rough rate limit. The value of N can be configured in etc/emqx.conf :
| Configuration item | Type | Default value | Description |
|---|---|---|---|
| listener.tcp.external.active_n | Number | 100 | how many messages are read from the TCP stack by emqx at a time |
