# Transaction Logging

> eXtremeDB Transaction Logging extends eXtremeDB with three important capabilities:

> - for eXtremeDB in-memory database recovery in the event of failure
> - for “Data Relay” to facilitate seamless, fine-grained data sharing between eXtremeDB applications and external systems
> - for creating a persistent queue of events

> The following topics explain the general principles of eXtremeDB Transaction Logging application design and implementation:

> Introduction | Introduction to eXtremeDB Transaction Logging
> Transaction Logging Applications | Enabling Transaction Logging in applications
> Data Relay and Persistent Queue of Events | Strategy and Implementation details
> TL with HA and Cluster | Using eXtremeDB Transaction Logging with HA and Cluster applications
> Dynamic DDL | Support for the SQL DDL operations to `create`, `alter` or `drop` tables and indexes
> Optimizations | Techniques and tips for optimizing eXtremeDB Transaction Logging application performance