**WAL-E needs to be installed on all machines, masters and slaves.**

[How to install WAL-E](https://github.com/batchcom/aws-configs/wiki/Installing-WAL-E)

Only one machine, the master, writes WAL segments via continuous archiving. The configuration for the master `postgresql.conf` is:

```
archive_mode = on
archive_command = 'envdir /etc/wal-e.d/env wal-e wal-push %p'
archive_timeout = 60
```

When a master is first setup, a base backup needs to be performed. As the `postgres` user:

`envdir /etc/wal-e.d/env wal-e backup-push $PGDATA`

This will perform a base backup and push it to S3. The above archiving commands will stream all deltas in realtime as generated by the Postgres master.

The flow for setting up a new Postgres machine slave, which will be based on a base backup.

1. Install Postgres, configure as a slave.
2. Remove the `$PGDATA` dir, if it exists. This will be re-created in step 3.
3. Pull a base backup using WAL-E:

`envdir /etc/wal-e.d/env wal-e backup-fetch $PGDATA LATEST`

4. Implement a `$PGDATA/recovery.conf` which is configured with WAL-E to replay remote segments:

```
standby_mode     = 'on'
primary_conninfo = 'host=$HOST user=$USER password=$PASSWORD'
restore_command  = 'envdir /etc/wal-e.d/env wal-e wal-fetch "%f" "%p"'
trigger_file     = '/data/postgresql/9.1/main/trigger'
```

4b. During recovery, by default WAL-E will attempt to pull and restore all WAL segments available. If you wish to restore to a specific *POINT IN TIME* than specify

```
# restore to 4:38PM on 3/6/2012
recovery_target_time = '2012-03-06 16:38:00'
```


In `recovery.conf` to recover to that specific point in time. This can be helpful it something bad happened at 4:39PM and you want to restore to 4:38PM.

See [Recovery Target Settings](http://www.postgresql.org/docs/9.1/interactive/recovery-target-settings.html) for more details.

5. Ensure proper permissions: 

```
chown -R postgres:postgres $PGDATA
```

6. Start Postgres and tail its log. It should start up and fetch WAL segments from S3, which is being performed by the `restore_command` in the above `recovery.conf`.

### Setup a CRON to perform base backups on a regular schedule
We have WAL-E configured to push a base backup nightly:

`/etc/cron.d/pg_base_backup`:

```
0 8 * * * postgres envdir /etc/wal-e.d/env wal-e backup-push /data/postgresql/9.1/main/
```