Rate limit

EMQX 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 :

• max_conn_rate is the rate limit for connection establishment on a single emqx node. 1000 means that 1000 clients can access at most.
• conn_messages_in is the rate limit for receiving PUBLISH packets on a single connection. 100,10s means that the maximum PUBLISH message rate allowed on each connection is 100 every 10 seconds.
• conn_bytes_in is the rate limit for receiving TCP packets on a single connection. 100KB,10s means that the maximum TCP packet rate allowed on each connection is 100KB every 10 seconds.

conn_messages_in and conn_bytes_in both provide limits for a single connection. EMQX Broker currently does not provide a global message rate limit.

Rate limit explanation

EMQX 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 EMQX Broker

When the following configuration is used for packet rate limiting:

zone.external.rate_limit.conn_bytes_in = 100KB,10s

EMQX 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 EMQX 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. EMQX Broker only provides a rough rate limit. The value of N can be configured in etc/emqx.conf :