Excellent question. This is a critical concept for anyone working with PostgreSQL in production.
**”PG license slots” is a bit of a misnomer.** Unlike proprietary databases (Oracle, SQL Server), **PostgreSQL itself does not have a licensing fee or a concept of “slots” for its core functionality.** It is released under the **liberal PostgreSQL License**, which is a permissive open-source license similar to the MIT or BSD licenses.
However, the term “slots” is commonly used in two specific contexts within the PostgreSQL ecosystem:
### 1. Connection Slots (`max_connections`)
This is the most common meaning of “slots” in a PostgreSQL context. It refers to the maximum number of concurrent client connections the database server can handle.
* **What it is:** A configuration parameter set in `postgresql.conf` (e.g., `max_connections = 100`). Each connected client (application, user, tool) consumes one “slot.”
* **Why it matters:** If all slots are used, new connection attempts will fail with an error like **”sorry, too many clients already.”**
* **Management:** You can monitor connections using queries like `SELECT * FROM pg_stat_activity;`. To handle more clients than your `max_connections` setting, you must use a **connection pooler** (like PgBouncer or pgpool-II), which maintains a smaller pool of actual database connections and multiplexes client requests over them.
### 2. Replication Slots
These are a feature that ensures a standby server (or a logical replication subscriber) does not fall behind, even if it is disconnected or offline.
* **What it is:** A replication slot is a persistent record on the primary server that tracks how much WAL (Write-Ahead Log) data a replica has consumed.
* **Why it matters:** Without a replication slot, the primary can delete old WAL files once all *connected* replicas have received them. If a replica disconnects, it could miss data when it reconnects. A replication slot **guarantees** that the primary retains all necessary WAL files until the slot is advanced, preventing data loss for that replica.
* **Danger:** If a replica is permanently removed, you **must** manually drop its replication slot on the primary. Orphaned slots will cause the primary’s disk to fill up with WAL files, leading to a crash.
### 3. “License” Context from Cloud Providers & Commercial Distributors
This is where the term “license slots” might appear in a commercial sense.
* **Cloud Databases (AWS RDS, Azure, GCP Cloud SQL):** You pay for the DB instance itself (vCPU, memory, storage). There’s no separate “slot” fee, but your `max_connections` is often limited by your instance size (memory/CPU).
* **Commercial PostgreSQL Distributors (EDB, Percona):** These companies sell **enterprise-grade support, management tools, and advanced features** (e.g., advanced security, Oracle compatibility, better management consoles). Their pricing models might be based on:
* **Per Instance:** Fee per database server.
* **Per Core/Socket:** Fee based on the server’s CPU.
* **”Per Node” or “Per Deployment”:** Similar to per instance.
* They do **not** sell “connection licenses,” but they sell the **software and support license** for their enhanced PostgreSQL distribution.
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### Summary Table
| Term | What It Is | Purpose | Key Consideration |
| :— | :— | :— | :— |
| **Connection Slots** (`max_connections`) | Configuration limit for concurrent client sessions. | Prevents server overload from too many connections. | Use a **connection pooler** for scalability. Monitor `pg_stat_activity`. |
| **Replication Slots** | A persistence mechanism for WAL retention on the primary for a replica. | Guarantees data stream for replicas, even if they are down. | **CRITICAL:** Manually drop unused slots to prevent disk fill. |
| **Commercial “License”** | Fee for supported enterprise distributions (EDB, Percona) or managed cloud services. | Provides support, tools, and enhanced features. | You are paying for the **service/tooling**, not for the right to connect users to the core PostgreSQL engine. |
**Bottom Line:** When you hear “PG license slots,” the speaker is most likely referring to **connection limits (`max_connections`)**, which are a **technical configuration**, not a **licensing cost**. The core PostgreSQL database is and always will be free from licensing fees.
