New test suite deps/rabbitmq_mqtt/test/shared_SUITE contains tests that
are executed against both MQTT and Web MQTT.
This has two major advantages:
1. Eliminates test code duplication between rabbitmq_mqtt and
rabbitmq_web_mqtt making the tests easier to maintain and to understand.
2. Increases test coverage of Web MQTT.
It's acceptable to add a **test** dependency from rabbitmq_mqtt to
rabbitmq_web_mqtt. Obviously, there should be no such dependency
for non-test code.
Traditionally, queue types implement flow control by keeping state in
both sending and receiving Erlang processes (for example credit based flow
control keeps the number of credits within the process dictionary).
The rabbit_mqtt_qos0_queue cannot keep such state in sending or receiving
Erlang process because such state would consume a large amount of memory
in case of large fan-ins or large fan-outs.
The whole idea of the rabbit_mqtt_qos_queue type is to not keep any
state in the rabbit_queue_type client. This makes this new queue
type scale so well.
Therefore the new queue type cannot (easily) implement flow control
throttling individual senders.
In this commit, we take a different approach:
Instead of implementing flow control throttling individual senders,
the receiving MQTT connection process drops QoS 0 messages from the
rabbit_mqtt_qos_queue if it is overflowed with messages AND its MQTT
client is not capable of receiving messages fast enough.
This is a simple and sufficient solution because it's better to drop QoS
0 (at most once) messages instead of causing cluster-wide memory alarms.
The user can opt out of dropping messages by setting the new env setting
mailbox_soft_limit to 0.
Additionally, we reduce the send_timeout from 30 seconds default in
Ranch to 15 seconds default in MQTT. This will detect hanging MQTT
clients faster causing the MQTT connection to be closed.
Record #state{} is purely local to rabbit_mqtt_reader.
Record #proc_state{} is purely local to rabbit_mqtt_processor.
Therefore, move these record definitions to the defining module.
This avoids unnecessarily exposing internal information.
Now, that #proc_state{} is defined in rabbit_mqtt_processor,
rename #proc_state to #state{}.
- subscriptions information can be retrieved directly from mnesia
- when unsubscribe, we check if there is binding between topic name
and queue (check for both qos0 queue name and qos1 queue name) to
unbind
- added a boolean value has_subs in proc state which will indicate
if connection has any active subscriptions. Used for setting consumer
global counter
Some users use classic mirrored queues for MQTT queues by
applying a policy.
Given that classic mirrored queues are deprecated, but still supported
in RabbitMQ 3.x, native MQTT must support classic mirrored queues.
Instead of tracking {Vhost, ClientId} to ConnectionPid mappings in our
custom process registry, i.e. custom local ETS table with a custom
gen_server process managing that ETS table, this commit uses the pg module
because pg is better tested.
To save memory with millions of MQTT client connections, we want to save
the mappings only locally on the node where the connection resides and
therfore not be replicated across all nodes.
According to Maxim Fedorov:
"The easy way to provide per-node unique pg scope is to start it like
pg:start_link(node()). At least that's what we've been doing to have
node-local scopes. It will still try to discover scopes on nodeup from
nodes joining the cluster, but since you cannot have nodes with the
same name in one cluster, using node() for local-only scopes worked
well for us."
So that's what we're doing in this commit.
Chunyi Lyu
Chunyi Lyu
David Ansari