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Although it might seem pessimistic, spending some of your limited time preparing for disaster is one of the best investments you can make for the long-term health of your Evergreen system. If one of your servers crashes and burns, you want to be confident that you can get a working system back in place — whether it is your database server that suffers, or an Evergreen application server.
At a minimum, you need to be able to recover your system’s data from your PostgreSQL database server: patron information, circulation transactions, bibliographic records, and the like. If all else fails, you can at least restore that data to a stock Evergreen system to enable your staff and patrons to find and circulate materials while you work on restoring your local customizations such as branding, colors, or additional functionality. This section describes how to back up your data so that you or a colleague can help you recover from various disaster scenarios.
The simplest method to back up your PostgreSQL data is to use the pg_dump utility to create a logical
backup of your database. Logical backups have the advantage of taking up minimal space, as the indexes
derived from the data are not part of the backup. For example, an Evergreen database with 2.25 million
records and 3 years of transactions that takes over 120 GB on disk creates just a 7.0 GB compressed
backup file. The drawback to this method is that you can only recover the data at the exact point in time
at which the backup began; any updates, additions, or deletions of your data since the backup began will
not be captured. In addition, when you restore a logical backup, the database server has to recreate all
of the indexes—so it can take several hours to restore a logical backup of that 2.25 million record
Evergreen database.
As the effort and server space required for logical database backups are minimal, your first step towards preparing for disaster should be to automate regular logical database backups. You should also ensure that the backups are stored in a different physical location, so that if a flood or other disaster strikes your primary server room, you will not lose your logical backup at the same time.
To create a logical dump of your PostgreSQL database:
pg_dump -Fc <database-name> > <backup-filename>. If you
are not running the command as the postgres user on the database server itself, you may need to include
options such as -U <user-name> and -h <hostname> to connect to the database server. You can use a
newer version of the PostgreSQL to run pg_dump against an older version of PostgreSQL if your client
and server operating systems differ. The -Fc option specifies the "custom" format: a compressed format
that gives you a great deal of flexibility at restore time (for example, restoring only one table from
the database instead of the entire schema).
You should establish a routine of nightly logical backups of your database, with older logical backups being automatically deleted after a given interval.
To increase your confidence in the safety of your data, you should regularly test your ability to
restore from a logical backup. Restoring a logical backup that you created using the custom format
requires the use of the pg_restore tool as follows:
Makefile.install prerequisite
installer than came with your version of Evergreen contains an installation target that should
satisfy these requirements. Refer to the installation documentation for more details.
postgres user, create a new database using the createdb command into which you will
restore the data. Base the new database on the template0 template database to enable the
combination of UTF8 encoding and C locale options, and specify the character type and collation
type as "C" using the --lc-ctype and --lc-collate parameters. For example, to create a new
database called "testrestore": createdb --template=template0 --lc-ctype=C --lc-collate=C testrestore
postgres user, restore the logical backup into your newly created database using
the pg_restore command. You can use the -j parameter to use more CPU cores at a time to make
your recovery operation faster. If your target database is hosted on a different server, you can
use the -U <user-name> and -h <hostname> options to connect to that server. For example,
to restore the logical backup from a file named evergreen_20121212.dump into the "testrestore"
database on a system with 2 CPU cores: pg_restore -j 2 -d testrestore evergreen_20171212.dump
While logical database backups require very little space, they also have the disadvantage of taking a great deal of time to restore for anything other than the smallest of Evergreen systems. Physical database backups are little more than a copy of the database file system, meaning that the space required for each physical backup will match the space used by your production database. However, physical backups offer the great advantage of almost instantaneous recovery, because the indexes already exist and simply need to be validated when you begin database recovery. Your backup server should match the configuration of your master server as closely as possible including the version of the operating system and PostgreSQL.
Like logical backups, physical backups also represent a snapshot of the data at the point in time at which you began the backup. However, if you combine physical backups with write-ahead-log (WAL) segment archiving, you can restore a version of your database that represents any point in time between the time the backup began and the time at which the last WAL segment was archived, a feature referred to as point-in-time recovery (PITR). PITR enables you to undo the damage that an accidentally or deliberately harmful UPDATE or DELETE statement could inflict on your production data, so while the recovery process can be complex, it provides fine-grained insurance for the integrity of your data when you run upgrade scripts against your database, deploy new custom functionality, or make global changes to your data.
To set up WAL archiving for your production Evergreen database, you need to modify your PostgreSQL
configuration (typically located on Debian and Ubuntu servers in
/etc/postgresql/<version>/postgresql.conf):
archive_mode to on
%f (representing the
file name of the WAL segment) and %p (representing the complete path name for the WAL segment,
including the file name). You should copy the WAL segments to a remote file system that can be read
by the same server on which you plan to create your physical backups. For example, if /data/wal
represents a remote file system to which your database server can write, a possible value of
archive_command could be: test ! -f /data/wal/%f && cp %p /data/wal/%f, which effectively tests
to see if the destination file already exists, and if it does not, copies the WAL segment to that
location. This command can be and often is much more complex (for example, using scp or rsync
to transfer the file to the remote destination rather than relying on a network share), but you
can start with something simple.
Once you have modified your PostgreSQL configuration, you need to restart the PostgreSQL server before the configuration changes will take hold: . Stop your OpenSRF services. . Restart your PostgreSQL server. . Start your OpenSRF services and restart your Apache HTTPD server.
To create a physical backup of your production Evergreen database:
pg_basebackup -x -D <data-destination-directory> -U <user-name> -h <hostname> <database-name>
command to create a physical backup of database <database-name> on your backup server.
You should establish a process for creating regular physical backups at periodic intervals, bearing in mind that the longer the interval between physical backups, the more WAL segments the backup database will have to replay at recovery time to get back to the most recent changes to the database. For example, to be able to relatively quickly restore the state of your database to any point in time over the past four weeks, you might take physical backups at weekly intervals, keeping the last four physical backups and all of the corresponding WAL segments.
If you have a separate server that you can use to run a replica of your database, consider replicating your database to that server. In the event that your primary database server suffers a hardware failure, having a database replica gives you the ability to fail over to your database replica with very little downtime and little or no data loss. You can also improve the performance of your overall system by directing some read-only operations, such as reporting, to the database replica. In this section, we describe how to replicate your database using PostgreSQL’s streaming replication support.
You need to prepare your master PostgreSQL database server to support streaming replicas with several
configuration changes. The PostgreSQL configuration file is typically located on Debian and Ubuntu
servers at /etc/postgresql/<version>/postgresql.conf. The PostgreSQL host-based authentication
(pg_hba.conf) configuration file is typically located on Debian and Ubuntu servers at
/etc/postgresql/<version>/pg_hba.conf. Perform the following steps on your master database server:
max_wal_senders from the default value of 0 to the number of streaming replicas that you need
to support. Note that these connections count as physical connections for the sake of the
max_connections parameter, so you might need to increase that value at the same time.
postgresql.conf on your production database server,
change wal_keep_segments to a value such as 32 or 64.
postgresql.conf
on your production database server, change checkpoint_segments from its default of 3 to a value such as
16 or 32. This improves the performance of your database at the cost of additional disk space.
Create a database user for the specific purpose of replication. As the postgres user on the production database server, issue the following commands, where replicant represents the name of the new user:
createuser replicant psql -d <database> ALTER ROLE replicant WITH REPLICATION;
Enable your replica database to connect to your master database server as a streaming replica. In
pg_hba.conf on your master database server, add a line to enable the database user replicant to connect
to the master database server from IP address 192.168.0.164:
host replication replicant 192.168.0.164/32 md5
To avoid downtime, you can prepare your master database server for streaming replication at any maintenance interval; then weeks or months later, when your replica server environment is available, you can begin streaming replication. Once you are ready to set up the streaming replica, perform the following steps on your replica server:
Add a recovery.conf file to your replica database configuration directory. This file contains the
information required to begin recovery once you start the replica database:
# turn on standby mode, disabling writes to the database standby_mode = 'on' # assumes WAL segments are available at network share /data/wal restore_command = 'cp /data/wal/%f %p' # connect to the master database to being streaming replication primary_conninfo = 'host=kochab.cs.uoguelph.ca user=replicant password=<password>
wal_keep_segments set on your
master server, the replica should begin streaming replication. Otherwise, it will replay WAL segments
until it catches up enough to begin streaming replication.
Congratulations, you now have a streaming replica database that reflects the latest changes to your Evergreen data! Combined with a routine of regular logical and physical database backups and WAL segment archiving stored on a remote server, you have a significant insurance policy for your system’s data in the event that disaster does strike.
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