---
source_url: https://www.pubnub.com/docs/sdks/lua/api-reference/publish-and-subscribe
title: Publish and Subscribe API for Lua SDK
updated_at: 2026-06-15T12:14:21.235Z
---

> Documentation Index
> For a curated overview of PubNub documentation, see: https://www.pubnub.com/docs/llms.txt
> For the full list of all documentation pages, see: https://www.pubnub.com/docs/llms-full.txt


# Publish and Subscribe API for Lua SDK

## Publish

The `publish()` function is used to send a message to all subscribers of a channel. To publish a message you must first specify a valid `publish_key` at initialization. A successfully published message is replicated across the PubNub Real-Time Network and sent simultaneously to all subscribed clients on a channel.

Messages in transit can be secured from potential eavesdroppers with SSL/TLS by setting ssl to true during initialization.

##### Publish anytime

It's not required to be subscribed to a channel in order to publish to that channel.

##### Message data

The message argument can contain any JSON serializable data, including: Objects, Arrays, Ints and Strings. `data` should not contain special Lua classes or functions as these will not serialize. String content can include any single-byte or multi-byte UTF-8 character.

##### Message size

The maximum number of characters per message is 32 KiB by default. The maximum message size is based on the final escaped character count, including the channel name. An ideal message size is under 1800 bytes which allows a message to be compressed and sent using single IP datagram (1.5 KiB) providing optimal network performance.

If the message you publish exceeds the configured size, you will receive the following message:

##### Message too large error

```lua
["PUBLISHED",[0,"Message Too Large","13524237335750949"]]
```

For further details, check the [support article](https://support.pubnub.com/hc/en-us/articles/360051495932-Calculating-Message-Payload-Size-Before-Publish).

:::tip Need larger messages?
Our platform is optimized for payloads up to 32 KiB. PubNub supports larger messages, but increasing the limit requires a verification of compatibility with your use case.
Talk to [our team](https://www.pubnub.com/company/contact-sales/) to discuss increasing the message size limit for your use case.
:::

##### Message publish rate

Messages can be published as fast as bandwidth conditions will allow. There is a soft limit based on max throughput since messages will be discarded if the subscriber can't keep pace with the publisher.

For example, if 200 messages are published simultaneously before a subscriber has had a chance to receive any messages, the subscriber may not receive the first 100 messages because the message queue has a limit of only 100 messages stored in memory.

##### Publishing to multiple channels

It is not possible to publish a message to multiple channels simultaneously. The message must be published to one channel at a time.

##### Publishing messages reliably

There are some best practices to ensure messages are delivered when publishing to a channel:

* Publish to any given channel in a serial manner (not concurrently).
* Check that the return code is success (for exmple, `[1,"Sent","136074940..."]`)
* Publish the next message only after receiving a success return code.
* If a failure code is returned (`[0,"blah","<timetoken>"]`), retry the publish.
* Avoid exceeding the in-memory queue's capacity of 100 messages. An overflow situation (aka missed messages) can occur if slow subscribers fail to keep up with the publish pace in a given period of time.
* Throttle publish bursts in accordance with your app's latency needs, for example, Publish no faster than 5 msgs per second to any one channel.

### Method(s)

To `Publish a message` you can use the following method(s) in the Lua SDK:

```lua
pubnub_obj:publish(params)
```

| Parameter | Type | Required | Default | Description |
| --- | --- | --- | --- | --- |
| params | table | Yes |  | Table of publish parameters. See [Publish Parameters](#publish-parameters) for more details. |

#### Publish parameters

| Parameter | Description |
| --- | --- |
| `channel` *Type: stringDefault: `none` | The channel to publish to. |
| `message` *Type: stringDefault: `none` | The message to publish. |
| `callback`Type: function(r)Default: `function(r) end` | The function to call on success with result. |
| `error`Type: function(r)Default: `function(r) end` | The function to call on failure, with result. |

### Sample code

#### Publish a message to a channel

```lua
pubnub_obj:publish({
    channel = channel,
    message = text,
    callback = function(r) --textout(r)
    end,
    error = function(r) textout(r)
    end
})
```

:::note Subscribe to the channel
Before running the above publish example, either using the [Debug Console](https://www.pubnub.com/docs/console/) or in a separate script running in a separate terminal window, [subscribe to the same channel](#subscribe) that is being published to.
:::

## Subscribe

This function causes the client to create an open TCP socket to the PubNub Real-Time Network and begin listening for messages on a specified `channel`. To subscribe to a `channel` the client must send the appropriate `subscribe_key` at initialization. By default a newly subscribed client will only receive messages published to the channel after the `subscribe()` call completes.

:::warning Unsubscribing from all channels
**Unsubscribing** from all channels, and then **subscribing** to a new channel Y is not the same as subscribing to channel Y and then unsubscribing from the previously-subscribed channel(s). Unsubscribing from all channels resets the last-received `timetoken` and thus, there could be some gaps in the subscription that may lead to message loss.
:::

### Method(s)

To `Subscribe to a channel` you can use the following method(s) in the Lua SDK:

```lua
pubnub_obj:subscribe(params)
```

| Parameter | Description |
| --- | --- |
| `params` *Type: table | Table of subscribe parameters. See [Subscribe Parameters](#subscribe-parameters) for more details. |

#### Subscribe parameters

| Parameter | Description |
| --- | --- |
| `channel` *Type: stringDefault: `none` | The channel to subscribe to - can be a comma-separated list of channels. |
| `callback` *Type: function(msg)Default: `no` | The function to call on each received message. |
| `error`Type: function(r)Default: `function(r) end` | The function to call on failure, with result. |
| `connect`Type: function()Default: `function() end` | The function to call on connecting to PubNub (before first message). |
| `reconnect`Type: function()Default: `funcition() end` | Call if connection lost and then re-established. |
| `presence`Type: booleanDefault: `false` | Whether to subscribe to the presence channel(s) for given channel(s). |
| `timeout`Type: integerDefault: `SUB_TIMEOUT (310)` | The number of seconds to wait for a response to subscribe. |

### Sample code

Subscribe to a channel:

```lua
pubnub_obj:subscribe({
    channel = "demo",
    connect = function()
        textout('Connected to channel ')
        textout(channel)
    end,
    callback = function(message)
        textout(message)
    end,
    error = function()
        textout("Oh no!!! Dropped Connection!")
    end
})
```

## Unsubscribe

When subscribed to a single channel, this function causes the client to issue a `leave` from the `channel` and close any open socket to the PubNub Network. For multiplexed channels, the specified `channel`(s) will be removed and the socket remains open until there are no more channels remaining in the list.

:::warning Unsubscribing from all channels
**Unsubscribing** from all channels, and then **subscribing** to a new channel Y is not the same as subscribing to channel Y and then unsubscribing from the previously-subscribed channel(s). Unsubscribing from all channels resets the last-received `timetoken` and thus, there could be some gaps in the subscription that may lead to message loss.
:::

### Method(s)

To `Unsubscribe from a channel` you can use the following method(s) in the Lua SDK:

```lua
pubnub_obj:unsubscribe(params)
```

| Parameter | Description |
| --- | --- |
| `params` *Type: table | Table of `unsubscribe` parameters - channel(s) to unsubscribe from. See [Unsubscribe Parameters](#unsubscribe-parameters) for more details. |

#### Unsubscribe parameters

| Parameter | Description |
| --- | --- |
| `channel` *Type: stringDefault: `none` | The channel to `unsubscribe` from - can be a comma-separated list of channels. |

### Sample code

Unsubscribe from a channel:

```lua
pubnub_obj:unsubscribe({
    channel = "demo",
})
```