Use cases, implementation features, changelog for flespi MQTT broker .

Rejected messages counter was added to the session log record upon disconnection. Message can be rejected by one of these reasons: Permissions denied for publisher token. Message has reserved prefix flespi/ Access denied to specified subaccount passed through cid property.

[Changelog] flespi MQTT broker

shal

We are migrating from 32-bit to 64-bit IDs of various broker elements. Broker was just upgraded with new binaries capable or working with both types of entities and in a hour we will upgrade the database schema. We do not expect any problems to the broker, may be short service interruptions in a second or two.

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Latest broker API changes:
'recursive' parameter added to GET mqtt/messages method. Parameter is optional and has value 'true' by default. If you want to fetch your own messages, i.e. skip subaccount's messages, just set parameter value to 'false'.

Method parameters, behavior and returning result are compatible with previous implementation.

adsa

We introduced a new rule for processing overflowed sessions.
Broker keeps unacknowledged messages in the internal buffer wich has limit up to 1Gb per each MQTT session.
If session buffer is overflowed, the connection will be disconnected and undelivered messages will be deleted.

shal

In flespi we are using timestamp user property automatically set by the broker as the time of message appearance.
We had some messages, like telematics messages telemetry, with expiry interval set that were outdated twice earlier then expected (twice shorter messages life). This was because broker, before delivering retained message marked expiry interval in seconds from current time, but at the same time it checked for message actuality starting from this timestamp time.

Now we fixed this behavior - broker will not decrement expiry interval based on current time, but will always count it from message timestamp. This not conforms to standard MQTT protocol and can be counted as flespi broker specifics.

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Rejected messages counter was added to the session log record upon disconnection.
Message can be rejected by one of these reasons:

Permissions denied for publisher token.
Message has reserved prefix flespi/
Access denied to specified subaccount passed through cid property.

shal

One of our users has reported a security issue in flespi broker that allowed to use will message for publishing into topics protected by ACL (all topics except flespi/ were affected). We have installed a fix that prohibits session connections with invalid for given token at this moment of time will message topic.

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A bunch of internal updates have been deployed. Please contact us if you notice an incorrect broker behavior.

adsa

Update: please refer the post below

Some hardware MQTT implementations have limitations for username/password field length of authorization packet. The typical limitation is 32 bytes per field.
Thus regular authorization via 64-byte token is not possible.

So we have implemented an alternative way to authorize such MQTT connections.
To do it, use token id as username and at least 32 first bytes of the token key as a password. The token MUST be of ACL type.

Example:
username=123
password=at-least-32-first-bytes-of-token-key

shal

It is now possible to specify for ACL tokens MQTT topics using flespi comma-separated topic selectors.

Imagine, you have this topics:

path/1/field1
path/1/field2
path/1/field3

If you want to limit publish/subscribe access to to field1 and field2 only, leaving field3 protected you need to specify this in ACL section of ACL token.

Before today update you need to specify each topic separately, e.g. define two mqtt sections in the token:
path/1/field1 and path/1/field2.

After today update you may use flespi special topic selector for providing correct topics and have everything in just one rule:
path/1/field1,field2

Same applies if you have multiple identifiers in path, e.g. 1,2,3,4,5,6. You may describe full ACL with just one rule: path/1,2,3,4,5,6/field1,field2 Previously you need to define 12 MQTT ACL rules for this construction or use topic wildcards.

This is applicable for both publish and subscribe sections of mqtt ACL rule.

adsa

Dear flespi users!

The latest broker update, which proposed an alternative way to authorize the MQTT connection, also closed the security vulnerability. It was possible to log in with any string as a username and a valid token as a password.

Please refer Authorization section on the broker description page: https://flespi.com/mqtt-broker and review your connection settings and correct it if necessary.

adsa

Two more properties were added to the MQTT session state.

Pending messages queue size in bytes:
flespi/state/mqtt/sessions/+/bytes_out_queue

In-flight, i.e. not ACK'ed QoS(1,2) messages count:
flespi/state/mqtt/sessions/+/puback_queue

The growth of these properties usually indicates insufficient bandwidth of the client network or the bottleneck of ACK processing.

kial

Some hardware MQTT implementations have limitations for username/password field length of authorization packet (CONNECT). Sometimes limitation is set to 16 bytes.
Thus regular authorization via 64-byte token is not possible.

So we have changed previously implemented way to authorize such MQTT connections.
To do it, use token id as username and at least 16 first bytes of the token key as a password. The token MUST be of ACL type.

Example:
username=123
password=at-least-16-first-bytes-of-token-key

adsa

The parameter set for the POST https://flespi.io/mqtt/messages method has been extended with a full set of MQTT v5.0 message parameters such as user properties, correlation data, etc.

adsa

Broker REST API has moved to a new engine. This should not affect the functionality of the external API.
The sticky shared subscriptions have also been changed to make the messaging more balanced.

If you have experienced any issues, please contact us and we will fix them immediately.

adsa

The REST API GET mqtt/messages method has been extended with the optional "fields" parameter, which allows you to customize the format of response messages.

adsa

We are constantly trying to provide the flespi broker with new, useful features. Without breaking the standard, we have implemented additional methods for filtering messages.

Subscription filter format:
$filter/cid={cid filter}&modified_since={timestamp}&payload={expression}/{filter topic}

cid - message filter from the specified subaccount
modified_since - message filter by publication time
payload - message filter by content. The content of the message must comply with the JSON specification - be a number, a double-quoted string, null, true/false, JSON object, JSON array. Expression format: https://flespi.com/kb/expressions-the-way-to-analyze-message
Filters are listed with '&', the filter value must be urlencoded.

Payload filter added as an optional parameter to the REST API method GET /mqtt/messages.

Example 1, subscription to temperature sensors:
topic: sensor/1/temperature, payload: 21, filter: $>20
Subscription filter:
$filter/payload=%24%3E20/sensor/+/temperature

Example 2, subscription to active alarm sensors
topic: alarm/2/status, payload: true, fitler: $=true
Subscription filter:
$filter/payload=%24%3Dtrue/alarm/+/status

Example 3, telemetry subscription for generators with voltages outside the specified range
topic: generator/3, payload: {"name": "test", "voltage": 12}, filter: voltage<10||voltage>=12
Subscription filter:
$filter/payload=voltage%3C10%7C%7Cvoltage%3E12/generator/+

shal

We removed special processing of user property with name seqno. Previously this user property was used by the MQTT Broker and its retained storage in order to select latest message if timestamps are equal. We were able to get rid of it after modifying the way we are generate message timestamp. Now upon processing and caching messages on client side it is possible to compare timestamp property of message and ignore message with earlier timestamp.

adsa

We are going to limit the maximum supported QoS for Flespi MQTT Broker subscriptions to QoS 1. This will increase the message throughput for both the clients and the broker. All existing subscriptions with QoS=2 will continue to operate as is, but new subscriptions will be downgraded to 1 upon subscription. We will set the Maximum QoS property in the CONNACK response. Publishing topics with QoS=2 is allowed. All of these steps should ensure that existing customers are not affected.
The update is scheduled for June 16th.

shal

For REST API calls over MQTT (very cool feature when using microservices architecture) additional user properties available in the response message:

client_host - contains IP address of MQTT connection that initiated request
client_id - contains client-id of MQTT connection that initiated request

Additionally each such REST API call towards flespi is enhanced with "x-flespi-app" header with urlencoded value of MQTT client-id which allow you to identify the source of such calls.

adsa

We have added a new feature unique to the flespi broker. You can get the number of subscribers for a topic you have published. Just add a count_subscribers user property with 1 payload and publish a QoS(1) message. You can find the subscribers_count user property in the received PUBACK.