# 7.1 The Many Shouting Elephants  When the warehouse gets too busy with users asking, "What's in this suitcase?" and "How much does this cost?", the lone elephant can get overwhelmed. Even a lazy elephant has his limits. To solve this, we create **Read Replicas**. ## The Shipping of the Diary As we learned in **[[Postgres/Chapter 4/4.1 - The Pocket Diary (WAL & fsync)|Chapter 4.1]]**, the elephant records everything in his Pocket Diary (WAL) before he does it. To create a Read Replica, we simply hire a dozen more elephants. We don't give them their own filing cabinets; instead, we ship a copy of the **Pocket Diary** to each of them as fast as possible. This is called **Physical Replication**. These replica elephants sitting in their own mini-warehouses read the diary and perform the exact same actions as the primary elephant. They are perfect clones. If the primary elephant changes a row to Red, they change it to Red. ## The Limitation of the Clone However, these replicas are **Read-Only**. If you walk into a replica's warehouse and ask to change a row, the replica will look at you sadly and shake his head. He doesn't have the authority to write in the diary; he is only allowed to read it. ## The Town Crier Revisited Sometimes, you don't want a perfect clone. Maybe you only want the New York elephant to know about "Customer" updates, but not "Internal Employee" updates. For this, we use the **[[Postgres/Chapter 4/4.3 - The Town Crier (Publications & Subscriptions)|Town Crier (Logical Replication)]]**. We filter the news. London only shouts the important bits, and New York only listens for what it needs. This is the foundation of high-scale, globally distributed architectures. --- [[Chapter 7/7.0 - The Cloud Scales|← 7.0 - The Cloud Scales]] | [[Chapter 7/7.0 - The Cloud Scales|↑ 7.0 - The Cloud Scales]] | [[Chapter 7/7.2 - Cloud Storage|7.2 - Cloud Storage →]]