# 4.3 The Town Crier  So far, we’ve talked about a single elephant in a single warehouse. But what if the warehouse is too small? What if you need an elephant in New York to know exactly what the elephant in London is doing? ## The Crier's Scroll Postgres has a way of sharing its Pocket Diary with the world. This is called **Logical Replication**. Instead of just keeping his notes to himself, the London elephant becomes a **Publication**. He hires a **Town Crier** to stand on the roof of the warehouse. Every time a new scribble is committed to the diary, the Crier shouts it out across the ocean. ## The Listener's Notebook In New York, another elephant is acting as a **Subscription**. He has a dedicated worker sitting on his roof with a telescope, listening for the London Crier. Every time the worker hears a change—_"London just added a new customer!"_—he writes it down and hands it to the New York elephant, who immediately performs the same action in his own warehouse. ## Why Shouting is Better than Shipping Before the Town Crier, if you wanted the New York warehouse to match London, you had to ship the **entire filing cabinet** (Physical Replication) over the ocean every time. It was heavy, slow, and you couldn't change anything in New York without breaking the connection. With **Publications and Subscriptions**, you are only shipping the _meaning_ of the changes. New York can have a completely different filing cabinet layout, or even run a different version of Postgres, and it can still keep up with the news from London. This "Logical" connection is the secret sauce for moving data between different systems, upgrading your database without downtime, and eventually building the massive, interconnected networks we'll explore in **[[Postgres/Chapter 7 - The Cloud Scales|Chapter 7: The Cloud Scales]]**. --- [[Chapter 4/4.2 - The Recovery Parade|← 4.2 - The Recovery Parade]] | [[Chapter 4/4.0 - Safety Without Sweating|↑ 4.0 - Safety Without Sweating]] | [[Chapter 4/4.4 - Transactions|4.4 - Transactions →]]