What is Lightning Message Service and how does it differ from pub-sub?

When two components on the same Lightning page need to talk but don’t share a parent-child relationship — say a list on the left and a detail panel on the right — CustomEvent won’t reach across. That’s the job of Lightning Message Service (LMS).

What LMS is

LMS is a Salesforce-provided messaging API that lets components communicate across the DOM regardless of where they sit in the page hierarchy or which framework they’re built in. The three pieces:

1. A Message Channel — a .messageChannel-meta.xml metadata file declaring the channel and its fields.
2. The lightning/messageService module — JS APIs for publishing and subscribing.
3. The @wire(MessageContext) adapter — gives the component its identity in the LMS system.

Setting up a channel

<!-- messageChannels/RecordSelected.messageChannel-meta.xml -->
<?xml version="1.0" encoding="UTF-8"?>
<LightningMessageChannel xmlns="http://soap.sforce.com/2006/04/metadata">
    <masterLabel>Record Selected</masterLabel>
    <isExposed>true</isExposed>
    <lightningMessageFields>
        <fieldName>recordId</fieldName>
    </lightningMessageFields>
</LightningMessageChannel>

Publishing a message

import { LightningElement, wire } from 'lwc';
import { publish, MessageContext } from 'lightning/messageService';
import RECORD_SELECTED from '@salesforce/messageChannel/RecordSelected__c';

export default class ContactList extends LightningElement {
    @wire(MessageContext) messageContext;

    handleSelect(event) {
        publish(this.messageContext, RECORD_SELECTED, {
            recordId: event.detail.contactId
        });
    }
}

Subscribing to a message

import { LightningElement, wire } from 'lwc';
import { subscribe, unsubscribe, MessageContext } from 'lightning/messageService';
import RECORD_SELECTED from '@salesforce/messageChannel/RecordSelected__c';

export default class ContactDetail extends LightningElement {
    @wire(MessageContext) messageContext;
    subscription = null;
    selectedId;

    connectedCallback() {
        this.subscription = subscribe(this.messageContext, RECORD_SELECTED,
            (msg) => { this.selectedId = msg.recordId; });
    }

    disconnectedCallback() {
        unsubscribe(this.subscription);
        this.subscription = null;
    }
}

The disconnectedCallback cleanup is not optional. Without it, the subscription survives the component and leaks memory across navigation.

LMS vs the old pub-sub library

Before LMS, the community used a pubsub.js helper from a Salesforce sample app. It worked, but it had three problems:

If you see pubsub.js in a codebase today, treat it as tech debt. Anything new should use LMS.

LMS scopes

subscribe() accepts a third argument that controls how aggressively the listener fires:

subscribe(this.messageContext, CHANNEL, handler, { scope: APPLICATION_SCOPE });

• APPLICATION_SCOPE — receive messages from anywhere in the app, including inactive console tabs.
• Default scope — receive only from the same active page/tab.

In a console app, APPLICATION_SCOPE lets a utility-bar component pick up a message from any open subtab. Without it, the message is scoped to the tab it was published from.

When to use which messaging tool

Don’t use LMS as a shortcut to avoid passing props down a small tree — events that could be CustomEvents are easier to debug than channel messages.

Interview signal

The interviewer wants to hear that you understand LMS replaced pub-sub because pub-sub couldn’t cross frameworks or survive console navigation. If you can explain MessageContext, the schema-driven channel, and the cleanup pattern in disconnectedCallback, you’ve covered everything that matters.

Verified against: LWC Developer Guide — Communicate Across the DOM with Lightning Message Service. Last reviewed 2026-05-17 for Spring ‘26 release.