B4R Question Problem whit mqtt

[Mania]

Hi Erel,

I’ve been debugging MQTT Publish2 behavior in B4R on ESP8266 and found a silent payload size limit.

Using Publish2(Topic As String, Payload As String, Retained As Boolean):

- Short payloads work reliably
- When the combined size of topic + payload exceeds ~85–90 bytes, the message is silently dropped
- No error is returned, and nothing reaches the broker (confirmed with MQTT Explorer)

This happens consistently with retained messages and JSON payloads.
Example:

Topic:
homeassistant/sensor/ph_1/ph/config (35 bytes)

Payload (works):
"{""name"":""pH meter"",""state_topic"":""Esp/ph"",""unique_id"":""ph_1_abcdefghi123456789abc_ph""}"

Payload (does NOT work):
"{""name"":""pH meter"",""state_topic"":""Esp/ph"",""unique_id"":""ph_1_abcdefghi123456789abcde_ph""}"

I verified this by adding/removing characters one by one.
It seems there is a fixed internal buffer limit in the MQTT client for ESP8266.

Questions:
1. Is there a documented payload/topic size limit for Publish2 on ESP8266?
2. Is this a known limitation or a bug?
3. Is there a recommended workaround (larger buffer, Publish instead of Publish2, etc.)?

Thanks,
Mania

 

[peacemaker]

Better to mention what MQTT-lib you use exacly...

 

[peacemaker]

And if you use Erel's lib https://www.b4x.com/android/forum/threads/mqtt.65669/post-463307, so i can see in PubSubClient.h file:

// MQTT_MAX_PACKET_SIZE : Maximum packet size
#ifndef MQTT_MAX_PACKET_SIZE
#define MQTT_MAX_PACKET_SIZE 128
#endif

So, it's not a surprise.

 

[Erel]

There is indeed a limit of 128 the packet size declared in PubSubClient.h.

// MQTT_MAX_PACKET_SIZE : Maximum packet size
#ifndef MQTT_MAX_PACKET_SIZE
#define MQTT_MAX_PACKET_SIZE 128
#endif

You can try to edit it and increase the limit. I haven't tried it myself.

 

[Mania]

Thank you for the suggestions. Increasing the buffer size in the library helped.