WAL-E

Continuous archiving for Postgres

WAL-E is a program designed to perform continuous archiving of PostgreSQL WAL files and base backups.

To correspond on using WAL-E or to collaborate on its development, do not hesitate to send mail to the mailing list at [email protected] (archives and subscription settings). Github issues are also currently being used to track known problems, so please feel free to submit those.

Installation

If no up-to-date packages are available to you via a package manager, this command can work on most operating systems:

sudo python3 -m pip install wal-e[aws,azure,google,swift]

You can omit storage services you do not wish to use from the above list.

Primary Commands

WAL-E has these key commands:

• backup-fetch
• backup-push
• wal-fetch
• wal-push
• delete

All of these operators work in a context of several environment variables that WAL-E reads. The variables set depend on the storage provider being used, and are detailed below.

WAL-E's organizing concept is the PREFIX. Prefixes must be set uniquely for each writing database, and prefix all objects stored for a given database. For example: s3://bucket/databasename.

Of these, the "push" operators send backup data to storage and "fetch" operators get backup data from storage.

wal commands are called by Postgres's archive_command and restore_command to fetch or pull write ahead log, and backup commands are used to fetch or push a hot backup of the base database that WAL segments can be applied to. Finally, the delete command is used to prune the archives as to retain a finite number of backups.

AWS S3 and Work-alikes

• WALE_S3_PREFIX (e.g. s3://bucket/path/optionallymorepath)
• AWS_ACCESS_KEY_ID
• AWS_SECRET_ACCESS_KEY
• AWS_REGION (e.g. us-east-1)

Optional:

• WALE_S3_ENDPOINT: See Manually specifying the S3 Endpoint
• WALE_S3_STORAGE_CLASS: One of: STANDARD (default), REDUCED_REDUNDANCY, GLACIER, STANDARD_IA, ONEZONE_IA, INTELLIGENT_TIERING, DEEP_ARCHIVE
• AWS_SECURITY_TOKEN: When using AWS STS
• Pass --aws-instance-profile to gather credentials from the Instance Profile. See Using AWS IAM Instance Profiles.

Azure Blob Store

Example below is based on the following blob storage in Azure in the resource group resgroup : https://store1.blob.core.windows.net/container1/nextpath

• WALE_WABS_PREFIX (e.g. wabs://container1/nextpath)
• WABS_ACCOUNT_NAME (e.g. store1)
• WABS_ACCESS_KEY (Use key1 from running azure storage account keys list store1 --resource-group resgroup You will need to have the Azure CLI installed for this to work.)
• WABS_SAS_TOKEN (You only need this if you have not provided an WABS_ACCESS_KEY)

Google Storage

• WALE_GS_PREFIX (e.g. gs://bucket/path/optionallymorepath)
• GOOGLE_APPLICATION_CREDENTIALS

Swift

• WALE_SWIFT_PREFIX (e.g. swift://container/path/optionallymorepath)
• SWIFT_AUTHURL
• SWIFT_TENANT
• SWIFT_USER
• SWIFT_PASSWORD

Optional Variables:

• SWIFT_AUTH_VERSION which defaults to 2. Some object stores such as Softlayer require version 1.
• SWIFT_ENDPOINT_TYPE defaults to publicURL, this may be set to internalURL on object stores like Rackspace Cloud Files in order to use the internal network.

File System

• WALE_FILE_PREFIX (e.g. file://localhost/backups/pg)

Dependencies

• python (>= 3.4)
• lzop
• psql (>= 8.4)
• pv

This software also has Python dependencies: installing with pip will attempt to resolve them:

• gevent>=1.1.1
• boto>=2.40.0
• azure==3.0.0
• google-cloud-storage>=1.4.0
• python-swiftclient>=3.0.0
• python-keystoneclient>=3.0.0

It is possible to use WAL-E without the dependencies of back-end storage one does not use installed: the imports for those are only performed if the storage configuration demands their use.

Examples

Pushing a base backup to S3:

$ AWS_SECRET_ACCESS_KEY=... wal-e                     \
  -k AWS_ACCESS_KEY_ID                                \
  --s3-prefix=s3://some-bucket/directory/or/whatever  \
  backup-push /var/lib/my/database

Sending a WAL segment to WABS:

$ WABS_ACCESS_KEY=... wal-e                                   \
  -a WABS_ACCOUNT_NAME                                        \
  --wabs-prefix=wabs://some-bucket/directory/or/whatever      \
  wal-push /var/lib/my/database/pg_xlog/WAL_SEGMENT_LONG_HEX

Push a base backup to Swift:

$ WALE_SWIFT_PREFIX="swift://my_container_name"              \
  SWIFT_AUTHURL="http://my_keystone_url/v2.0/"               \
  SWIFT_TENANT="my_tennant"                                  \
  SWIFT_USER="my_user"                                       \
  SWIFT_PASSWORD="my_password" wal-e                         \
  backup-push /var/lib/my/database

Push a base backup to Google Cloud Storage:

$ WALE_GS_PREFIX="gs://some-bucket/directory-or-whatever"     \
  GOOGLE_APPLICATION_CREDENTIALS=...                          \
  wal-e backup-push /var/lib/my/database

It is generally recommended that one use some sort of environment variable management with WAL-E: working with it this way is less verbose, less prone to error, and less likely to expose secret information in logs.

envdir, part of the daemontools package is one recommended approach to setting environment variables. One can prepare an envdir-compatible directory like so:

# Assumption: the group is trusted to read secret information
# S3 Setup
$ umask u=rwx,g=rx,o=
$ mkdir -p /etc/wal-e.d/env
$ echo "secret-key-content" > /etc/wal-e.d/env/AWS_SECRET_ACCESS_KEY
$ echo "access-key" > /etc/wal-e.d/env/AWS_ACCESS_KEY_ID
$ echo 's3://some-bucket/directory/or/whatever' > \
  /etc/wal-e.d/env/WALE_S3_PREFIX
$ chown -R root:postgres /etc/wal-e.d


# Assumption: the group is trusted to read secret information
# WABS Setup
$ umask u=rwx,g=rx,o=
$ mkdir -p /etc/wal-e.d/env
$ echo "secret-key-content" > /etc/wal-e.d/env/WABS_ACCESS_KEY
$ echo "access-key" > /etc/wal-e.d/env/WABS_ACCOUNT_NAME
$ echo 'wabs://some-container/directory/or/whatever' > \
  /etc/wal-e.d/env/WALE_WABS_PREFIX
$ chown -R root:postgres /etc/wal-e.d

After having done this preparation, it is possible to run WAL-E commands much more simply, with less risk of accidentally using incorrect values:

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

envdir is conveniently combined with the archive_command functionality used by PostgreSQL to enable continuous archiving. To enable continuous archiving, one needs to edit postgresql.conf and restart the server. The important settings to enable continuous archiving are related here:

wal_level = archive # hot_standby and logical in 9.x is also acceptable
archive_mode = on
archive_command = 'envdir /etc/wal-e.d/env wal-e wal-push %p'
archive_timeout = 60

Every segment archived will be noted in the PostgreSQL log.

A base backup (via backup-push) can be uploaded at any time, but this must be done at least once in order to perform a restoration. It must be done again if you decided to skip archiving any WAL segments: replication will not be able to continue if there are any gaps in the stored WAL segments.

Primary Commands

backup-push, backup-fetch, wal-push, wal-fetch represent the primary functionality of WAL-E and must reside on the database machine. Unlike wal-push and wal-fetch commands, which function as described above, the backup-push and backup-fetch require a little additional explanation.

backup-push

By default backup-push will include all user defined tablespaces in the database backup. please see the backup-fetch section below for WAL-E's tablespace restoration behavior.

backup-fetch

Use backup-fetch to restore a base backup from storage.

This command makes use of the LATEST pseudo-backup-name to find a backup to download:

$ envdir /etc/wal-e.d/fetch-env wal-e               \
--s3-prefix=s3://some-bucket/directory/or/whatever  \
backup-fetch /var/lib/my/database LATEST

Also allowed is naming a backup specifically as seen in backup-list, which can be useful for restoring older backups for the purposes of point in time recovery:

$ envdir /etc/wal-e.d/fetch-env wal-e               \
--s3-prefix=s3://some-bucket/directory/or/whatever  \
backup-fetch                                        \
/var/lib/my/database base_LONGWALNUMBER_POSITION_NUMBER

One will need to provide a recovery.conf file to recover WAL segments associated with the backup. In short, recovery.conf needs to be created in the Postgres's data directory with content like:

restore_command = 'envdir /etc/wal-e.d/env wal-e wal-fetch %f %p'
standby_mode = on

A database with such a recovery.conf set will poll WAL-E storage for WAL indefinitely. You can exit recovery by running pg_ctl promote.

If you wish to perform Point In Time Recovery (PITR) can add recovery targets to recovery.conf, looking like this:

recovery_target_time = '2017-02-01 19:58:55'

There are several other ways to specify recovery target, e.g. transaction id.

Regardless of recovery target, the result by default is Postgres will pause recovery at this time, allowing inspection before promotion. See recovery targets for details on how to customize what happens when the target criterion is reached.

Tablespace Support

If and only if you are using Tablespaces, you will need to consider additional issues on how run backup-fetch. The options are:

• User-directed Restore

  WAL-E expects that tablespace symlinks will be in place prior to a backup-fetch run. This means prepare your target path by insuring ${PG_CLUSTER_DIRECTORY}/pg_tblspc contains all required symlinks before restoration time. If any expected symlink does not exist backup-fetch will fail.

• Blind Restore

  If you are unable to reproduce tablespace storage structures prior to running backup-fetch you can set the option flag --blind-restore. This will direct WAL-E to skip the symlink verification process and place all data directly in the ${PG_CLUSTER_DIRECTORY}/pg_tblspc path.

• Restoration Specification

  You can provide a restoration specification file to WAL-E using the backup-fetch --restore-spec RESTORE_SPEC option. This spec must be valid JSON and contain all contained tablespaces as well as the target storage path they require, and the symlink postgres expects for the tablespace. Here is an example for a cluster with a single tablespace:

  {
      "12345": {
          "loc": "/data/postgres/tablespaces/tblspc001/",
          "link": "pg_tblspc/12345"
      },
      "tablespaces": [
          "12345"
      ],
  }
  

  Given this information WAL-E will create the data storage directory and symlink it appropriately in ${PG_CLUSTER_DIRECTORY}/pg_tblspc.

Auxiliary Commands

These are commands that are not used expressly for backup or WAL pushing and fetching, but are important to the monitoring or maintenance of WAL-E archived databases. Unlike the critical four operators for taking and restoring backups (backup-push, backup-fetch, wal-push, wal-fetch) that must reside on the database machine, these commands can be productively run from any computer with the appropriate _PREFIX set and the necessary credentials to manipulate or read data there.

backup-list

backup-list is useful for listing base backups that are complete for a given WAL-E context. Some fields are only filled in when the --detail option is passed to backup-list [1].

Firstly, the fields that are filled in regardless of if --detail is passed or not:

Secondly, the fields that are filled in only when --detail is passed:

delete

delete contains additional subcommands that are used for deleting data from storage for various reasons. These commands are organized separately because the delete subcommand itself takes options that apply to any subcommand that does deletion, such as --confirm.

All deletions are designed to be reentrant and idempotent: there are no negative consequences if one runs several deletions at once or if one resubmits the same deletion command several times, with or without canceling other deletions that may be concurrent.

These commands have a dry-run mode that is the default. The command is basically optimized for not deleting data except in a very specific circumstance to avoid operator error. Should a dry-run be performed, wal-e will instead simply report every key it would otherwise delete if it was not running in dry-run mode, along with prominent HINT-lines for every key noting that nothing was actually deleted from the blob store.

To actually delete any data, one must pass --confirm to wal-e delete. If one passes both --dry-run and --confirm, a dry run will be performed, regardless of the order of options passed.

Currently, these kinds of deletions are supported. Examples omit environment variable configuration for clarity:

• before: Delete all backups and wal segment files before the given base-backup name. This does not include the base backup passed: it will remain a viable backup.

  Example:

  $ wal-e delete [--confirm] before base_00000004000002DF000000A6_03626144
  

• retain: Leave the given number of backups in place, and delete all base backups and wal segment files older than them.

  Example:

  $ wal-e delete [--confirm] retain 5
  

• old-versions: Delete all backups and wal file segments with an older format. This is only intended to be run after a major WAL-E version upgrade and the subsequent base-backup. If no base backup is successfully performed first, one is more exposed to data loss until one does perform a base backup.

  Example:

  $ wal-e delete [--confirm] old-versions
  

• everything: Delete all backups and wal file segments in the context. This is appropriate if one is decommissioning a database and has no need for its archives.

  Example:

  $ wal-e delete [--confirm] everything
  

Compression and Temporary Files

All assets pushed to storage are run through the program "lzop" which compresses the object using the very fast lzo compression algorithm. It takes roughly 2 CPU seconds to compress a gigabyte, which when sending things to storage at about 25MB/s occupies about 5% CPU time. Compression ratios are expected to make file sizes 50% or less of the original file size in most cases, making backups and restorations considerably faster.

Because storage services generally require the Content-Length header of a stored object to be set up-front, it is necessary to completely finish compressing an entire input file and storing the compressed output in a temporary file. Thus, the temporary file directory needs to be big enough and fast enough to support this, although this tool is designed to avoid calling fsync(), so some memory can be leveraged.

Base backups first have their files consolidated into disjoint tar files of limited length to avoid the relatively large per-file transfer overhead. This has the effect of making base backups and restores much faster when many small relations and ancillary files are involved.

Other Options

Encryption

To encrypt backups as well as compress them, first generate a key pair using gpg --gen-key. You don't need the private key on the machine to back up, but you will need it to restore. The private key may have a password, but to restore, the password should be present in GPG agent. WAL-E does not support entering GPG passwords via a tty device.

Once this is done, set the WALE_GPG_KEY_ID environment variable or the --gpg-key-id command line option to the ID of the secret key for backup and restore commands.

Here's an example of how you can restore with a private key that has a password, by forcing decryption of an arbitrary file with the correct key to unlock the GPG keychain:

# This assumes you have "keychain" gpg-agent installed.
eval $( keychain --eval --agents gpg )

# If you want default gpg-agent, use this instead
# eval $( gpg-agent --daemon )

# Force storing the private key password in the agent.  Here you
# will need to enter the key password.
export TEMPFILE=`tempfile`
gpg --recipient "$WALE_GPG_KEY_ID" --encrypt "$TEMPFILE"
gpg --decrypt "$TEMPFILE".gpg || exit 1

rm "$TEMPFILE" "$TEMPFILE".gpg
unset TEMPFILE

# Now use wal-e to fetch the backup.
wal-e backup-fetch [...]

# If you have WAL segments encrypted, don't forget to add
# restore_command to recovery.conf, e.g.
#
# restore_command = 'wal-e wal-fetch "%f" "%p"'

# Start the restoration postgres server in a context where you have
# gpg-agent's environment variables initialized, such as the current
# shell.
pg_ctl -D [...] start

Controlling the I/O of a Base Backup

To reduce the read load on base backups, they are sent through the tool pv first. To use this rate-limited-read mode, use the option --cluster-read-rate-limit as seen in wal-e backup-push.

Logging

WAL-E supports logging configuration with following environment variables:

• WALE_LOG_DESTINATION comma separated values, syslog and stderr are supported. The default is equivalent to: syslog,stderr.
• WALE_SYSLOG_FACILITY from LOCAL0 to LOCAL7 and USER.

To restrict log statements to warnings and errors, use the --terse option.

Increasing throughput of wal-push

In certain situations, the wal-push process can take long enough that it can't keep up with WAL segments being produced by Postgres, which can lead to unbounded disk usage and an eventual crash of the database.

One can instruct WAL-E to pool WAL segments together and send them in groups by passing the --pool-size parameter to wal-push. This can increase throughput significantly.

As of version 1.x, --pool-size defaults to 32.

Note: You can also use this parameter when calling backup-fetch and backup-push (it defaults to 4).

Using AWS IAM Instance Profiles

Storing credentials on AWS EC2 instances has usability and security drawbacks. When using WAL-E with AWS S3 and AWS EC2, most uses of WAL-E would benefit from use with the AWS Instance Profile feature, which automatically generates and rotates credentials on behalf of an instance.

To instruct WAL-E to use these credentials for access to S3, pass the --aws-instance-profile flag.

Instance profiles may not be preferred in more complex scenarios when one has multiple AWS IAM policies written for multiple programs run on an instance, or an existing key management infrastructure.

Manually specifying the S3 Endpoint

If one wishes to target WAL-E against an alternate S3 endpoint (e.g. Ceph RADOS), one can set the WALE_S3_ENDPOINT environment variable. This can also be used take fine-grained control over endpoints and calling conventions with AWS.

The format is that of:

protocol+convention://hostname:port

Where valid protocols are http and https, and conventions are path, virtualhost, and subdomain.

Example:

# Turns off encryption and specifies us-west-1 endpoint.
WALE_S3_ENDPOINT=http+path://s3-us-west-1.amazonaws.com:80

# For radosgw.
WALE_S3_ENDPOINT=http+path://hostname

# As seen when using Deis, which uses radosgw.
WALE_S3_ENDPOINT=http+path://deis-store-gateway:8888

Development

Development is heavily reliant on the tool tox being existent within the development environment. All additional dependencies of WAL-E are managed by tox. In addition, the coding conventions are checked by the tox configuration included with WAL-E.

To run the tests, run:

$ tox -e py35

To run a somewhat more lengthy suite of integration tests that communicate with a real blob store account, one might run tox like this:

$ WALE_S3_INTEGRATION_TESTS=TRUE      \
  AWS_ACCESS_KEY_ID=[AKIA...]         \
  AWS_SECRET_ACCESS_KEY=[...]         \
  WALE_WABS_INTEGRATION_TESTS=TRUE    \
  WABS_ACCOUNT_NAME=[...]             \
  WABS_ACCESS_KEY=[...]               \
  WALE_GS_INTEGRATION_TESTS=TRUE      \
  GOOGLE_APPLICATION_CREDENTIALS=[~/my-credentials.json] \
  tox -e py35 -- -n 8

Looking carefully at the above, notice the -n 8 added the tox invocation. This -n 8 is after a -- that indicates to tox that the subsequent arguments are for the underlying test program pytest.

This is to enable parallel test execution, which makes the integration tests complete a small fraction of the time it would take otherwise. It is a design requirement of new tests that parallel execution not be sacrificed.

Coverage testing can be used by combining any of these using pytest-cov, e.g.: tox -- --cov wal_e and tox -- --cov wal_e --cov-report html; see htmlcov/index.html.