RabbitMQ 3.7 connection limits

[Dmitry Andrianov]

Hello.

A question regarding per-vhost limits ( http://www.rabbitmq.com/vhosts.html ).

So in 3.7 we can limit total number of concurrent connections for a broker. But what exactly happens when the limit is reached?

Does RabbitMQ stop accepting more? Or does it accept and drop them immediately? Or something else?

How does it look from client's perspective? Does it get a failure quickly or the connection attempt just times out?

Put differently - is it a good replacement for ipfilter/conntrack based connection limiter?

If RabbitMQ keeps accepting new connections and does anything at all to them - that probably means it keeps using more resources (especially if these are SSL connections) and so it cannot be used as a reliable mechanism of preventing a possible overload and crash?

Thanks

[Michael Klishin]

You can limit the max number of connections per vhost. Note that tests that connect in a tight loop

can exceed that value somewhat but in practice it won't be a problem.

When the limit is reached, no more connections will be accepted with appropriate messages logged by the node.

If your goal is to limit the number of connections to the entire node, I'd definitely stick to conntrack or proxy limits or similar.

This feature was introduced primarily for limiting the number of connections in a specific vhost, which is important

when a RabbitMQ node/cluster is offered as a multi-tenant service to N parties.

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MK

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[Dmitry Andrianov]

Understood, will stick to conntrack, thanks.

Out of curiosity - do you have any plans for the global limit? As a resource exhaustion prevention.

Cheers

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[Michael Klishin]

There's already plenty of tools (including a setting in Ranch) that can be used to limit the total number of connections.

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[Dmitry Andrianov]

Do you have an example by any chance how to limit it in ranch via RabbitMQ config?

Cheers

[Michael Klishin]

It's not a documented feature and up until a few days ago I wasn't aware of it myself.

It's `rabbit.connection_max` [1].

We are not yet sure whether it should be exposed to the users and what values should plugins such as Web MQTT and Web STOMP use [2].

1. https://github.com/rabbitmq/rabbitmq-server/blob/08700282b7176d62cb262a66eceb576610d66c4d/src/tcp_listener_sup.erl#L52

2. https://github.com/rabbitmq/rabbitmq-web-mqtt/issues/28

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