Add PACELC theorem

README.md

@@ -104,6 +104,7 @@ Review the [Contributing Guidelines](CONTRIBUTING.md).
 * [Performance vs scalability](#performance-vs-scalability)
 * [Latency vs throughput](#latency-vs-throughput)
 * [Availability vs consistency](#availability-vs-consistency)
+    * [PACELC theorem](#PACELC-theorem)
     * [CAP theorem](#cap-theorem)
         * [CP - consistency and partition tolerance](#cp---consistency-and-partition-tolerance)
         * [AP - availability and partition tolerance](#ap---availability-and-partition-tolerance)
@@ -437,6 +438,22 @@ Generally, you should aim for **maximal throughput** with **acceptable latency**
 
 ## Availability vs consistency
 
+### PACELC theorem
+<p align="center">
+  <img src="images/PACELC.png" width="600" height="300">
+  <br/>
+  <i><a href=https://github.com/Ahmad-alsanie/system-design-examples/blob/main/images/PACELC.png>Source: PACELC theorem </a></i>
+</p>
+
+If there's a partition; a system can trade of between A(availability) and C(Consistency).
+Else the trade of is in between C(consistency) and L(latency)
+
+<p align="center">
+  <img src="images/pacelcDBs.png">
+  <br/>
+  <i><a href=https://en.wikipedia.org/wiki/PACELC_theorem>Source: PACELC theorem </a></i>
+</p>
+
 ### CAP theorem
 
 <p align="center">