synapse/docs/usage/administration/backups.md

How to back up a Synapse homeserver

It is critical to maintain good backups of your server, to guard against hardware failure as well as potential corruption due to bugs or administrator error.

This page documents the things you will need to consider backing up as part of a Synapse installation.

Configuration files

Keep a copy of your configuration file (homeserver.yaml), as well as any auxiliary config files it refers to such as the log_config file, app_service_config_files. Often, all such config files will be kept in a single directory such as /etc/synapse, which will make this easier.

Server signing key

Your server has a signing key which it uses to sign events and outgoing federation requests. It is easiest to back it up with your configuration files, but an alternative is to have Synapse create a new signing key if you have to restore.

If you do decide to replace the signing key, you should add the old public key to old_signing_keys.

Database

Synapse's support for SQLite is only suitable for testing purposes, so for the purposes of this document, we'll assume you are using PostgreSQL.

A full discussion of backup strategies for PostgreSQL is out of scope for this document; see the PostgreSQL documentation for detailed information.

Synapse-specfic details

• Be very careful not to restore into a database that already has tables present. At best, this will error; at worst, it will lead to subtle database inconsistencies.

• The e2e_one_time_keys_json table should not be backed up, or if it is backed up, should be TRUNCATEd after restoring the database before Synapse is started.

  [Background: restoring the database to an older backup can cause used one-time-keys to be re-issued, causing subsequent message decryption errors. Clearing all one-time-keys from the database ensures that this cannot happen, and will prompt clients to generate and upload new one-time-keys.]

Quick and easy database backup and restore

Typically, the easiest solution is to use pg_dump to take a copy of the whole database. We recommend pg_dump's custom dump format, as it produces significantly smaller backup files.

sudo -u postgres pg_dump -Fc --exclude-table-data e2e_one_time_keys_json synapse > synapse.dump

There is no need to stop Postgres or Synapse while pg_dump is running: it will take a consistent snapshot of the databse.

To restore, you will need to recreate the database as described in Using Postgres, then load the dump into it with pg_restore:

sudo -u postgres createdb --encoding=UTF8 --locale=C --template=template0 --owner=synapse_user synapse
sudo -u postgres pg_restore -d synapse < synapse.dump

(If you forgot to exclude e2e_one_time_keys_json during pg_dump, remember to connect to the new database and TRUNCATE e2e_one_time_keys_json; before starting Synapse.)

To reiterate: do not restore a dump over an existing database.

Again, if you plan to run your homeserver at any sort of production level, we recommend studying the PostgreSQL documentation on backup options.

Media store

Synapse keeps a copy of media uploaded by users, including avatars and message attachments, in its Media store.

It is a directory on the local disk, containing the following directories:

• local_content: this is content uploaded by your local users. As a general rule, you should back this up: it may represent the only copy of those media files anywhere in the federation, and if they are lost, users will see errors when viewing user or room avatars, and messages with attachments.

• local_thumbnails: "thumbnails" of images uploaded by your users. If dynamic_thumbnails is enabled, these will be regenerated if they are removed from the disk, and there is therefore no need to back them up.

  If dynamic_thumbnails is not enabled (the default): although this can theoretically be regenerated from local_content, there is no tooling to do so. We recommend that these are backed up too.

• remote_content: this is a cache of content that was uploaded by a user on another server, and has since been requested by a user on your own server.

  Typically there is no need to back up this directory: if a file in this directory is removed, Synapse will attempt to fetch it again from the remote server.

• remote_thumbnails: thumbnails of images uploaded by users on other servers. As with remote_content, there is normally no need to back this up.

• url_cache, url_cache_thumbnails: temporary caches of files downloaded by the URL previews feature. These do not need to be backed up.