wildcard(3)

🌸 NAME 🌸

wildcard — Subscription wildcards 💖

okay bestie, gather round, because this one is lowkey the whole personality of MQTT and you're gonna be obsessed ✨

Every single MQTT message carries a topic, and that topic is basically the message's little name tag — it's what classifies it. MQTT servers use topics to figure out which subscribers should get a message that got published to the server. no topic, no vibe check, no delivery 💅

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🦋 so like… imagine a bunch of sensors 🦋

Picture the server getting messages from several environmental sensors. Each sensor publishes its measurement data as a message with an associated topic. And subscribing applications need to know which sensor originally published each received message. So a unique topic is used to identify each sensor and each measurement type. Topics like SENSOR1TEMP, SENSOR1HUMIDITY, SENSOR2TEMP and so on totally achieve this — but they are not very flexible, and that's the problem. 😭 If additional sensors get added to the system at a later date, subscribing applications must be modified to receive them. it's giving maintenance nightmare fr fr.

🌈 the glow-up: hierarchical topics 🌈

To provide more flexibility, MQTT supports a hierarchical topic namespace. This lets application designers organize topics to simplify their management. Levels in the hierarchy are delimited by the '/' character, such as SENSOR/1/HUMIDITY. Publishers and subscribers use these hierarchical topics as already described. see? structure. elegance. main character energy 👑

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✨ the two wildcard characters ✨

For subscriptions, two wildcard characters are supported — and you're gonna stan both:

• The '#' character represents a complete sub-tree of the hierarchy, and thus must be the last character in a subscription topic string, such as SENSOR/#. This will match any topic starting with SENSOR/, such as SENSOR/1/TEMP and SENSOR/2/HUMIDITY. it's giving "everything under here, thank you" 🫶
• The '+' character represents a single level of the hierarchy and is used between delimiters. For example, SENSOR/+/TEMP will match SENSOR/1/TEMP and SENSOR/2/TEMP. one level only, no more no less, periodt 💅

🚨 and here's the rule that will humble you: Publishers are not allowed to use the wildcard characters in their topic names. that's a subscribers-only thing, iykyk 🙈

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Deciding on your topic hierarchy is an important step in your system design. like genuinely, don't skip it, it's the blueprint of the whole thing 🎀

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mwah 💋 that was wildcard(3), brought to you by the Paho MQTT C Client Library, dated Tue Jun 2 2026 00:00:00. go forth and structure your topics, bestie 🌟