<commit_before># Changelog

All notable changes to this project will be documented in this
file.  This project adheres to [Semantic Versioning](http://semver.org/).

As this project is pre 1.0, breaking changes may happen for minor version
bumps.  A breaking change will get clearly notified in this log.

## [v0.5.1] - 2016-04-28
<commit_msg>Updates changelog<commit_after># Changelog

All notable changes to this project will be documented in this
file.  This project adheres to [Semantic Versioning](http://semver.org/).

As this project is pre 1.0, breaking changes may happen for minor version
bumps.  A breaking change will get clearly notified in this log.

## [Unreleased]

### Added

- *Elder* ledgers have been introduced:  An elder ledger is the oldest ledger known to a db.  For example, the `core_elder_ledger` attribute on the root endpoint refers to the oldest known ledger stored in the connected stellar-core database.
- Added the `history-retention-count` command line flag, used to specify the amount of historical data to keep in the history db.  This is expressed as a number of ledgers, for example a value of `362880` would retain roughly 6 weeks of data given an average of 10 seconds per ledger.
### Changed

- Horizon no longer needs to begin the ingestion of historical data from ledger sequence 1.  
- Rows in the `history_accounts` table are no longer identified using the "Total Order ID" that other historical records  use, but are rather using a simple auto-incremented id.


## [v0.5.1] - 2016-04-28

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