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<ol class="chapter"><li class="chapter-item expanded affix "><li class="part-title">Introduction</li><li class="chapter-item expanded "><a href="../welcome_and_overview.html">Welcome and Overview</a></li><li class="chapter-item expanded affix "><li class="part-title">Setup</li><li class="chapter-item expanded "><a href="../setup/installation.html">Installation</a></li><li class="chapter-item expanded "><a href="../postgres.html">Using Postgres</a></li><li class="chapter-item expanded "><a href="../reverse_proxy.html">Configuring a Reverse Proxy</a></li><li class="chapter-item expanded "><a href="../setup/forward_proxy.html">Configuring a Forward/Outbound Proxy</a></li><li class="chapter-item expanded "><a href="../turn-howto.html">Configuring a Turn Server</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../setup/turn/coturn.html">coturn TURN server</a></li><li class="chapter-item expanded "><a href="../setup/turn/eturnal.html">eturnal TURN server</a></li></ol></li><li class="chapter-item expanded "><a href="../delegate.html">Delegation</a></li><li class="chapter-item expanded affix "><li class="part-title">Upgrading</li><li class="chapter-item expanded "><a href="../upgrade.html">Upgrading between Synapse Versions</a></li><li class="chapter-item expanded affix "><li class="part-title">Usage</li><li class="chapter-item expanded "><a href="../federate.html">Federation</a></li><li class="chapter-item expanded "><a href="../usage/configuration/index.html">Configuration</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../usage/configuration/config_documentation.html">Configuration Manual</a></li><li class="chapter-item expanded "><a href="../usage/configuration/homeserver_sample_config.html">Homeserver Sample Config File</a></li><li class="chapter-item expanded "><a href="../usage/configuration/logging_sample_config.html">Logging Sample Config File</a></li><li class="chapter-item expanded "><a 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Providers</a></li><li class="chapter-item expanded "><a href="../jwt.html">JSON Web Tokens</a></li><li class="chapter-item expanded "><a href="../usage/configuration/user_authentication/refresh_tokens.html">Refresh Tokens</a></li></ol></li><li class="chapter-item expanded "><a href="../CAPTCHA_SETUP.html">Registration Captcha</a></li><li class="chapter-item expanded "><a href="../application_services.html">Application Services</a></li><li class="chapter-item expanded "><a href="../server_notices.html">Server Notices</a></li><li class="chapter-item expanded "><a href="../consent_tracking.html">Consent Tracking</a></li><li class="chapter-item expanded "><a href="../user_directory.html">User Directory</a></li><li class="chapter-item expanded "><a href="../message_retention_policies.html">Message Retention Policies</a></li><li class="chapter-item expanded "><a href="../modules/index.html">Pluggable Modules</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../modules/writing_a_module.html">Writing a module</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../modules/spam_checker_callbacks.html">Spam checker callbacks</a></li><li class="chapter-item expanded "><a href="../modules/third_party_rules_callbacks.html">Third-party rules callbacks</a></li><li class="chapter-item expanded "><a href="../modules/presence_router_callbacks.html">Presence router callbacks</a></li><li class="chapter-item expanded "><a href="../modules/account_validity_callbacks.html">Account validity callbacks</a></li><li class="chapter-item expanded "><a href="../modules/password_auth_provider_callbacks.html">Password auth provider callbacks</a></li><li class="chapter-item expanded "><a href="../modules/background_update_controller_callbacks.html">Background update controller callbacks</a></li><li class="chapter-item expanded "><a href="../modules/account_data_callbacks.html">Account data callbacks</a></li><li class="chapter-item expanded "><a href="../modules/add_extra_fields_to_client_events_unsigned.html">Add extra fields to client events unsigned section callbacks</a></li><li class="chapter-item expanded "><a href="../modules/porting_legacy_module.html">Porting a legacy module to the new interface</a></li></ol></li></ol></li><li class="chapter-item expanded "><a href="../workers.html">Workers</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../synctl_workers.html">Using synctl with Workers</a></li><li class="chapter-item expanded "><a href="../systemd-with-workers/index.html">Systemd</a></li></ol></li></ol></li><li class="chapter-item expanded "><a href="../usage/administration/index.html">Administration</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../usage/administration/admin_api/index.html">Admin API</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../admin_api/account_validity.html">Account Validity</a></li><li class="chapter-item expanded "><a href="../usage/administration/admin_api/background_updates.html">Background Updates</a></li><li class="chapter-item expanded "><a href="../admin_api/event_reports.html">Event Reports</a></li><li class="chapter-item expanded "><a href="../admin_api/experimental_features.html">Experimental Features</a></li><li class="chapter-item expanded "><a href="../admin_api/media_admin_api.html">Media</a></li><li class="chapter-item expanded "><a href="../admin_api/purge_history_api.html">Purge History</a></li><li class="chapter-item expanded "><a href="../admin_api/register_api.html">Register Users</a></li><li class="chapter-item expanded "><a href="../usage/administration/admin_api/registration_tokens.html">Registration Tokens</a></li><li class="chapter-item expanded "><a href="../admin_api/room_membership.html">Manipulate Room Membership</a></li><li class="chapter-item expanded "><a href="../admin_api/rooms.html">Rooms</a></li><li class="chapter-item expanded 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"><a href="../usage/administration/understanding_synapse_through_grafana_graphs.html">Understanding Synapse Through Grafana Graphs</a></li><li class="chapter-item expanded "><a href="../usage/administration/useful_sql_for_admins.html">Useful SQL for Admins</a></li><li class="chapter-item expanded "><a href="../usage/administration/database_maintenance_tools.html">Database Maintenance Tools</a></li><li class="chapter-item expanded "><a href="../usage/administration/state_groups.html">State Groups</a></li><li class="chapter-item expanded "><a href="../usage/administration/request_log.html">Request log format</a></li><li class="chapter-item expanded "><a href="../usage/administration/admin_faq.html">Admin FAQ</a></li><li class="chapter-item expanded "><div>Scripts</div></li></ol></li><li class="chapter-item expanded "><li class="part-title">Development</li><li class="chapter-item expanded "><a href="../development/contributing_guide.html">Contributing Guide</a></li><li class="chapter-item expanded "><a href="../code_style.html">Code Style</a></li><li class="chapter-item expanded "><a href="../development/reviews.html">Reviewing Code</a></li><li class="chapter-item expanded "><a href="../development/releases.html">Release Cycle</a></li><li class="chapter-item expanded "><a href="../development/git.html">Git Usage</a></li><li class="chapter-item expanded "><div>Testing</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../development/demo.html">Demo scripts</a></li></ol></li><li class="chapter-item expanded "><a href="../opentracing.html">OpenTracing</a></li><li class="chapter-item expanded "><a href="../development/database_schema.html" class="active">Database Schemas</a></li><li class="chapter-item expanded "><a href="../development/experimental_features.html">Experimental features</a></li><li class="chapter-item expanded "><a href="../development/dependencies.html">Dependency management</a></li><li class="chapter-item expanded "><div>Synapse Architecture</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../development/synapse_architecture/cancellation.html">Cancellation</a></li><li class="chapter-item expanded "><a href="../log_contexts.html">Log Contexts</a></li><li class="chapter-item expanded "><a href="../replication.html">Replication</a></li><li class="chapter-item expanded "><a href="../development/synapse_architecture/streams.html">Streams</a></li><li class="chapter-item expanded "><a href="../tcp_replication.html">TCP Replication</a></li><li class="chapter-item expanded "><a href="../development/synapse_architecture/faster_joins.html">Faster remote joins</a></li></ol></li><li class="chapter-item expanded "><a href="../development/internal_documentation/index.html">Internal Documentation</a></li><li><ol class="section"><li class="chapter-item expanded "><div>Single Sign-On</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../development/saml.html">SAML</a></li><li class="chapter-item expanded "><a href="../development/cas.html">CAS</a></li></ol></li><li class="chapter-item expanded "><a href="../development/room-dag-concepts.html">Room DAG concepts</a></li><li class="chapter-item expanded "><div>State Resolution</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../auth_chain_difference_algorithm.html">The Auth Chain Difference Algorithm</a></li></ol></li><li class="chapter-item expanded "><a href="../media_repository.html">Media Repository</a></li><li class="chapter-item expanded "><a href="../room_and_user_statistics.html">Room and User Statistics</a></li></ol></li><li class="chapter-item expanded "><div>Scripts</div></li><li class="chapter-item expanded affix "><li class="part-title">Other</li><li class="chapter-item expanded "><a href="../deprecation_policy.html">Dependency Deprecation Policy</a></li><li class="chapter-item expanded "><a href="../other/running_synapse_on_single_board_computers.html">Running Synapse on a Single-Board Computer</a></li></ol>
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<h1 id="synapse-database-schema-files"><a class="header" href="#synapse-database-schema-files">Synapse database schema files</a></h1>
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<p>Synapse's database schema is stored in the <code>synapse.storage.schema</code> module.</p>
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<h2 id="logical-databases"><a class="header" href="#logical-databases">Logical databases</a></h2>
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<p>Synapse supports splitting its datastore across multiple physical databases (which can
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be useful for large installations), and the schema files are therefore split according
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to the logical database they apply to.</p>
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<p>At the time of writing, the following "logical" databases are supported:</p>
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<ul>
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<li><code>state</code> - used to store Matrix room state (more specifically, <code>state_groups</code>,
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their relationships and contents).</li>
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<li><code>main</code> - stores everything else.</li>
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</ul>
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<p>Additionally, the <code>common</code> directory contains schema files for tables which must be
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present on <em>all</em> physical databases.</p>
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<h2 id="synapse-schema-versions"><a class="header" href="#synapse-schema-versions">Synapse schema versions</a></h2>
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<p>Synapse manages its database schema via "schema versions". These are mainly used to
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help avoid confusion if the Synapse codebase is rolled back after the database is
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updated. They work as follows:</p>
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<ul>
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<li>
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<p>The Synapse codebase defines a constant <code>synapse.storage.schema.SCHEMA_VERSION</code>
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which represents the expectations made about the database by that version. For
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example, as of Synapse v1.36, this is <code>59</code>.</p>
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</li>
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<li>
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<p>The database stores a "compatibility version" in
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<code>schema_compat_version.compat_version</code> which defines the <code>SCHEMA_VERSION</code> of the
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oldest version of Synapse which will work with the database. On startup, if
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<code>compat_version</code> is found to be newer than <code>SCHEMA_VERSION</code>, Synapse will refuse to
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start.</p>
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<p>Synapse automatically updates this field from
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<code>synapse.storage.schema.SCHEMA_COMPAT_VERSION</code>.</p>
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</li>
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<li>
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<p>Whenever a backwards-incompatible change is made to the database format (normally
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via a <code>delta</code> file), <code>synapse.storage.schema.SCHEMA_COMPAT_VERSION</code> is also updated
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so that administrators can not accidentally roll back to a too-old version of Synapse.</p>
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</li>
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</ul>
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<p>Generally, the goal is to maintain compatibility with at least one or two previous
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releases of Synapse, so any substantial change tends to require multiple releases and a
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bit of forward-planning to get right.</p>
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<p>As a worked example: we want to remove the <code>room_stats_historical</code> table. Here is how it
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might pan out.</p>
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<ol>
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<li>
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<p>Replace any code that <em>reads</em> from <code>room_stats_historical</code> with alternative
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implementations, but keep writing to it in case of rollback to an earlier version.
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Also, increase <code>synapse.storage.schema.SCHEMA_VERSION</code>. In this
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instance, there is no existing code which reads from <code>room_stats_historical</code>, so
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our starting point is:</p>
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<p>v1.36.0: <code>SCHEMA_VERSION=59</code>, <code>SCHEMA_COMPAT_VERSION=59</code></p>
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</li>
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<li>
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<p>Next (say in Synapse v1.37.0): remove the code that <em>writes</em> to
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<code>room_stats_historical</code>, but don’t yet remove the table in case of rollback to
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v1.36.0. Again, we increase <code>synapse.storage.schema.SCHEMA_VERSION</code>, but
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because we have not broken compatibility with v1.36, we do not yet update
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<code>SCHEMA_COMPAT_VERSION</code>. We now have:</p>
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<p>v1.37.0: <code>SCHEMA_VERSION=60</code>, <code>SCHEMA_COMPAT_VERSION=59</code>.</p>
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</li>
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<li>
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<p>Later (say in Synapse v1.38.0): we can remove the table altogether. This will
|
||
break compatibility with v1.36.0, so we must update <code>SCHEMA_COMPAT_VERSION</code> accordingly.
|
||
There is no need to update <code>synapse.storage.schema.SCHEMA_VERSION</code>, since there is no
|
||
change to the Synapse codebase here. So we end up with:</p>
|
||
<p>v1.38.0: <code>SCHEMA_VERSION=60</code>, <code>SCHEMA_COMPAT_VERSION=60</code>.</p>
|
||
</li>
|
||
</ol>
|
||
<p>If in doubt about whether to update <code>SCHEMA_VERSION</code> or not, it is generally best to
|
||
lean towards doing so.</p>
|
||
<h2 id="full-schema-dumps"><a class="header" href="#full-schema-dumps">Full schema dumps</a></h2>
|
||
<p>In the <code>full_schemas</code> directories, only the most recently-numbered snapshot is used
|
||
(<code>54</code> at the time of writing). Older snapshots (eg, <code>16</code>) are present for historical
|
||
reference only.</p>
|
||
<h3 id="building-full-schema-dumps"><a class="header" href="#building-full-schema-dumps">Building full schema dumps</a></h3>
|
||
<p>If you want to recreate these schemas, they need to be made from a database that
|
||
has had all background updates run.</p>
|
||
<p>To do so, use <code>scripts-dev/make_full_schema.sh</code>. This will produce new
|
||
<code>full.sql.postgres</code> and <code>full.sql.sqlite</code> files.</p>
|
||
<p>Ensure postgres is installed, then run:</p>
|
||
<pre><code class="language-sh">./scripts-dev/make_full_schema.sh -p postgres_username -o output_dir/
|
||
</code></pre>
|
||
<p>NB at the time of writing, this script predates the split into separate <code>state</code>/<code>main</code>
|
||
databases so will require updates to handle that correctly.</p>
|
||
<h2 id="delta-files"><a class="header" href="#delta-files">Delta files</a></h2>
|
||
<p>Delta files define the steps required to upgrade the database from an earlier version.
|
||
They can be written as either a file containing a series of SQL statements, or a Python
|
||
module.</p>
|
||
<p>Synapse remembers which delta files it has applied to a database (they are stored in the
|
||
<code>applied_schema_deltas</code> table) and will not re-apply them (even if a given file is
|
||
subsequently updated).</p>
|
||
<p>Delta files should be placed in a directory named <code>synapse/storage/schema/<database>/delta/<version>/</code>.
|
||
They are applied in alphanumeric order, so by convention the first two characters
|
||
of the filename should be an integer such as <code>01</code>, to put the file in the right order.</p>
|
||
<h3 id="sql-delta-files"><a class="header" href="#sql-delta-files">SQL delta files</a></h3>
|
||
<p>These should be named <code>*.sql</code>, or — for changes which should only be applied for a
|
||
given database engine — <code>*.sql.posgres</code> or <code>*.sql.sqlite</code>. For example, a delta which
|
||
adds a new column to the <code>foo</code> table might be called <code>01add_bar_to_foo.sql</code>.</p>
|
||
<p>Note that our SQL parser is a bit simple - it understands comments (<code>--</code> and <code>/*...*/</code>),
|
||
but complex statements which require a <code>;</code> in the middle of them (such as <code>CREATE TRIGGER</code>) are beyond it and you'll have to use a Python delta file.</p>
|
||
<h3 id="python-delta-files"><a class="header" href="#python-delta-files">Python delta files</a></h3>
|
||
<p>For more flexibility, a delta file can take the form of a python module. These should
|
||
be named <code>*.py</code>. Note that database-engine-specific modules are not supported here –
|
||
instead you can write <code>if isinstance(database_engine, PostgresEngine)</code> or similar.</p>
|
||
<p>A Python delta module should define either or both of the following functions:</p>
|
||
<pre><code class="language-python">import synapse.config.homeserver
|
||
import synapse.storage.engines
|
||
import synapse.storage.types
|
||
|
||
|
||
def run_create(
|
||
cur: synapse.storage.types.Cursor,
|
||
database_engine: synapse.storage.engines.BaseDatabaseEngine,
|
||
) -> None:
|
||
"""Called whenever an existing or new database is to be upgraded"""
|
||
...
|
||
|
||
def run_upgrade(
|
||
cur: synapse.storage.types.Cursor,
|
||
database_engine: synapse.storage.engines.BaseDatabaseEngine,
|
||
config: synapse.config.homeserver.HomeServerConfig,
|
||
) -> None:
|
||
"""Called whenever an existing database is to be upgraded."""
|
||
...
|
||
</code></pre>
|
||
<h2 id="background-updates"><a class="header" href="#background-updates">Background updates</a></h2>
|
||
<p>It is sometimes appropriate to perform database migrations as part of a background
|
||
process (instead of blocking Synapse until the migration is done). In particular,
|
||
this is useful for migrating data when adding new columns or tables.</p>
|
||
<p>Pending background updates stored in the <code>background_updates</code> table and are denoted
|
||
by a unique name, the current status (stored in JSON), and some dependency information:</p>
|
||
<ul>
|
||
<li>Whether the update requires a previous update to be complete.</li>
|
||
<li>A rough ordering for which to complete updates.</li>
|
||
</ul>
|
||
<p>A new background updates needs to be added to the <code>background_updates</code> table:</p>
|
||
<pre><code class="language-sql">INSERT INTO background_updates (ordering, update_name, depends_on, progress_json) VALUES
|
||
(7706, 'my_background_update', 'a_previous_background_update' '{}');
|
||
</code></pre>
|
||
<p>And then needs an associated handler in the appropriate datastore:</p>
|
||
<pre><code class="language-python">self.db_pool.updates.register_background_update_handler(
|
||
"my_background_update",
|
||
update_handler=self._my_background_update,
|
||
)
|
||
</code></pre>
|
||
<p>There are a few types of updates that can be performed, see the <code>BackgroundUpdater</code>:</p>
|
||
<ul>
|
||
<li><code>register_background_update_handler</code>: A generic handler for custom SQL</li>
|
||
<li><code>register_background_index_update</code>: Create an index in the background</li>
|
||
<li><code>register_background_validate_constraint</code>: Validate a constraint in the background
|
||
(PostgreSQL-only)</li>
|
||
<li><code>register_background_validate_constraint_and_delete_rows</code>: Similar to
|
||
<code>register_background_validate_constraint</code>, but deletes rows which don't fit
|
||
the constraint.</li>
|
||
</ul>
|
||
<p>For <code>register_background_update_handler</code>, the generic handler must track progress
|
||
and then finalize the background update:</p>
|
||
<pre><code class="language-python">async def _my_background_update(self, progress: JsonDict, batch_size: int) -> int:
|
||
def _do_something(txn: LoggingTransaction) -> int:
|
||
...
|
||
self.db_pool.updates._background_update_progress_txn(
|
||
txn, "my_background_update", {"last_processed": last_processed}
|
||
)
|
||
return last_processed - prev_last_processed
|
||
|
||
num_processed = await self.db_pool.runInteraction("_do_something", _do_something)
|
||
await self.db_pool.updates._end_background_update("my_background_update")
|
||
|
||
return num_processed
|
||
</code></pre>
|
||
<p>Synapse will attempt to rate-limit how often background updates are run via the
|
||
given batch-size and the returned number of processed entries (and how long the
|
||
function took to run). See
|
||
<a href="../modules/background_update_controller_callbacks.html">background update controller callbacks</a>.</p>
|
||
<h2 id="boolean-columns"><a class="header" href="#boolean-columns">Boolean columns</a></h2>
|
||
<p>Boolean columns require special treatment, since SQLite treats booleans the
|
||
same as integers.</p>
|
||
<p>Any new boolean column must be added to the <code>BOOLEAN_COLUMNS</code> list in
|
||
<code>synapse/_scripts/synapse_port_db.py</code>. This tells the port script to cast
|
||
the integer value from SQLite to a boolean before writing the value to the
|
||
postgres database.</p>
|
||
<h2 id="event_id-global-uniqueness"><a class="header" href="#event_id-global-uniqueness"><code>event_id</code> global uniqueness</a></h2>
|
||
<p><code>event_id</code>'s can be considered globally unique although there has been a lot of
|
||
debate on this topic in places like
|
||
<a href="https://github.com/matrix-org/matrix-spec-proposals/issues/2779">MSC2779</a> and
|
||
<a href="https://github.com/matrix-org/matrix-spec-proposals/pull/2848">MSC2848</a> which
|
||
has no resolution yet (as of 2022-09-01). There are several places in Synapse
|
||
and even in the Matrix APIs like <a href="https://spec.matrix.org/v1.1/server-server-api/#get_matrixfederationv1eventeventid"><code>GET /_matrix/federation/v1/event/{eventId}</code></a>
|
||
where we assume that event IDs are globally unique.</p>
|
||
<p>When scoping <code>event_id</code> in a database schema, it is often nice to accompany it
|
||
with <code>room_id</code> (<code>PRIMARY KEY (room_id, event_id)</code> and a <code>FOREIGN KEY(room_id) REFERENCES rooms(room_id)</code>) which makes flexible lookups easy. For example it
|
||
makes it very easy to find and clean up everything in a room when it needs to be
|
||
purged (no need to use sub-<code>select</code> query or join from the <code>events</code> table).</p>
|
||
<p>A note on collisions: In room versions <code>1</code> and <code>2</code> it's possible to end up with
|
||
two events with the same <code>event_id</code> (in the same or different rooms). After room
|
||
version <code>3</code>, that can only happen with a hash collision, which we basically hope
|
||
will never happen (SHA256 has a massive big key space).</p>
|
||
<h2 id="worked-examples-of-gradual-migrations"><a class="header" href="#worked-examples-of-gradual-migrations">Worked examples of gradual migrations</a></h2>
|
||
<p>Some migrations need to be performed gradually. A prime example of this is anything
|
||
which would need to do a large table scan — including adding columns, indices or
|
||
<code>NOT NULL</code> constraints to non-empty tables — such a migration should be done as a
|
||
background update where possible, at least on Postgres.
|
||
We can afford to be more relaxed about SQLite databases since they are usually
|
||
used on smaller deployments and SQLite does not support the same concurrent
|
||
DDL operations as Postgres.</p>
|
||
<p>We also typically insist on having at least one Synapse version's worth of
|
||
backwards compatibility, so that administrators can roll back Synapse if an upgrade
|
||
did not go smoothly.</p>
|
||
<p>This sometimes results in having to plan a migration across multiple versions
|
||
of Synapse.</p>
|
||
<p>This section includes an example and may include more in the future.</p>
|
||
<h3 id="transforming-a-column-into-another-one-with-not-null-constraints"><a class="header" href="#transforming-a-column-into-another-one-with-not-null-constraints">Transforming a column into another one, with <code>NOT NULL</code> constraints</a></h3>
|
||
<p>This example illustrates how you would introduce a new column, write data into it
|
||
based on data from an old column and then drop the old column.</p>
|
||
<p>We are aiming for semantic equivalence to:</p>
|
||
<pre><code class="language-sql">ALTER TABLE mytable ADD COLUMN new_column INTEGER;
|
||
UPDATE mytable SET new_column = old_column * 100;
|
||
ALTER TABLE mytable ALTER COLUMN new_column ADD CONSTRAINT NOT NULL;
|
||
ALTER TABLE mytable DROP COLUMN old_column;
|
||
</code></pre>
|
||
<h4 id="synapse-version-n"><a class="header" href="#synapse-version-n">Synapse version <code>N</code></a></h4>
|
||
<pre><code class="language-python">SCHEMA_VERSION = S
|
||
SCHEMA_COMPAT_VERSION = ... # unimportant at this stage
|
||
</code></pre>
|
||
<p><strong>Invariants:</strong></p>
|
||
<ol>
|
||
<li><code>old_column</code> is read by Synapse and written to by Synapse.</li>
|
||
</ol>
|
||
<h4 id="synapse-version-n--1"><a class="header" href="#synapse-version-n--1">Synapse version <code>N + 1</code></a></h4>
|
||
<pre><code class="language-python">SCHEMA_VERSION = S + 1
|
||
SCHEMA_COMPAT_VERSION = ... # unimportant at this stage
|
||
</code></pre>
|
||
<p><strong>Changes:</strong></p>
|
||
<ol>
|
||
<li>
|
||
<pre><code class="language-sql">ALTER TABLE mytable ADD COLUMN new_column INTEGER;
|
||
</code></pre>
|
||
</li>
|
||
</ol>
|
||
<p><strong>Invariants:</strong></p>
|
||
<ol>
|
||
<li><code>old_column</code> is read by Synapse and written to by Synapse.</li>
|
||
<li><code>new_column</code> is written to by Synapse.</li>
|
||
</ol>
|
||
<p><strong>Notes:</strong></p>
|
||
<ol>
|
||
<li><code>new_column</code> can't have a <code>NOT NULL NOT VALID</code> constraint yet, because the previous Synapse version did not write to the new column (since we haven't bumped the <code>SCHEMA_COMPAT_VERSION</code> yet, we still need to be compatible with the previous version).</li>
|
||
</ol>
|
||
<h4 id="synapse-version-n--2"><a class="header" href="#synapse-version-n--2">Synapse version <code>N + 2</code></a></h4>
|
||
<pre><code class="language-python">SCHEMA_VERSION = S + 2
|
||
SCHEMA_COMPAT_VERSION = S + 1 # this signals that we can't roll back to a time before new_column existed
|
||
</code></pre>
|
||
<p><strong>Changes:</strong></p>
|
||
<ol>
|
||
<li>On Postgres, add a <code>NOT VALID</code> constraint to ensure new rows are compliant. <em>SQLite does not have such a construct, but it would be unnecessary anyway since there is no way to concurrently perform this migration on SQLite.</em>
|
||
<pre><code class="language-sql">ALTER TABLE mytable ADD CONSTRAINT CHECK new_column_not_null (new_column IS NOT NULL) NOT VALID;
|
||
</code></pre>
|
||
</li>
|
||
<li>Start a background update to perform migration: it should gradually run e.g.
|
||
<pre><code class="language-sql">UPDATE mytable SET new_column = old_column * 100 WHERE 0 < mytable_id AND mytable_id <= 5;
|
||
</code></pre>
|
||
This background update is technically pointless on SQLite, but you must schedule it anyway so that the <code>portdb</code> script to migrate to Postgres still works.</li>
|
||
<li>Upon completion of the background update, you should run <code>VALIDATE CONSTRAINT</code> on Postgres to turn the <code>NOT VALID</code> constraint into a valid one.
|
||
<pre><code class="language-sql">ALTER TABLE mytable VALIDATE CONSTRAINT new_column_not_null;
|
||
</code></pre>
|
||
This will take some time but does <strong>NOT</strong> hold an exclusive lock over the table.</li>
|
||
</ol>
|
||
<p><strong>Invariants:</strong></p>
|
||
<ol>
|
||
<li><code>old_column</code> is read by Synapse and written to by Synapse.</li>
|
||
<li><code>new_column</code> is written to by Synapse and new rows always have a non-<code>NULL</code> value in this field.</li>
|
||
</ol>
|
||
<p><strong>Notes:</strong></p>
|
||
<ol>
|
||
<li>If you wish, you can convert the <code>CHECK (new_column IS NOT NULL)</code> to a <code>NOT NULL</code> constraint free of charge in Postgres by adding the <code>NOT NULL</code> constraint and then dropping the <code>CHECK</code> constraint, because Postgres can statically verify that the <code>NOT NULL</code> constraint is implied by the <code>CHECK</code> constraint without performing a table scan.</li>
|
||
<li>It might be tempting to make version <code>N + 2</code> redundant by moving the background update to <code>N + 1</code> and delaying adding the <code>NOT NULL</code> constraint to <code>N + 3</code>, but that would mean the constraint would always be validated in the foreground in <code>N + 3</code>. Whereas if the <code>N + 2</code> step is kept, the migration in <code>N + 3</code> would be fast in the happy case.</li>
|
||
</ol>
|
||
<h4 id="synapse-version-n--3"><a class="header" href="#synapse-version-n--3">Synapse version <code>N + 3</code></a></h4>
|
||
<pre><code class="language-python">SCHEMA_VERSION = S + 3
|
||
SCHEMA_COMPAT_VERSION = S + 1 # we can't roll back to a time before new_column existed
|
||
</code></pre>
|
||
<p><strong>Changes:</strong></p>
|
||
<ol>
|
||
<li>(Postgres) Update the table to populate values of <code>new_column</code> in case the background update had not completed. Additionally, <code>VALIDATE CONSTRAINT</code> to make the check fully valid.
|
||
<pre><code class="language-sql">-- you ideally want an index on `new_column` or e.g. `(new_column) WHERE new_column IS NULL` first, or perhaps you can find a way to skip this if the `NOT NULL` constraint has already been validated.
|
||
UPDATE mytable SET new_column = old_column * 100 WHERE new_column IS NULL;
|
||
|
||
-- this is a no-op if it already ran as part of the background update
|
||
ALTER TABLE mytable VALIDATE CONSTRAINT new_column_not_null;
|
||
</code></pre>
|
||
</li>
|
||
<li>(SQLite) Recreate the table by precisely following <a href="https://www.sqlite.org/lang_altertable.html#otheralter">the 12-step procedure for SQLite table schema changes</a>.
|
||
During this table rewrite, you should recreate <code>new_column</code> as <code>NOT NULL</code> and populate any outstanding <code>NULL</code> values at the same time.
|
||
Unfortunately, you can't drop <code>old_column</code> yet because it must be present for compatibility with the Postgres schema, as needed by <code>portdb</code>.
|
||
(Otherwise you could do this all in one go with SQLite!)</li>
|
||
</ol>
|
||
<p><strong>Invariants:</strong></p>
|
||
<ol>
|
||
<li><code>old_column</code> is written to by Synapse (but no longer read by Synapse!).</li>
|
||
<li><code>new_column</code> is read by Synapse and written to by Synapse. Moreover, all rows have a non-<code>NULL</code> value in this field, as guaranteed by a schema constraint.</li>
|
||
</ol>
|
||
<p><strong>Notes:</strong></p>
|
||
<ol>
|
||
<li>We can't drop <code>old_column</code> yet, or even stop writing to it, because that would break a rollback to the previous version of Synapse.</li>
|
||
<li>Application code can now rely on <code>new_column</code> being populated. The remaining steps are only motivated by the wish to clean-up old columns.</li>
|
||
</ol>
|
||
<h4 id="synapse-version-n--4"><a class="header" href="#synapse-version-n--4">Synapse version <code>N + 4</code></a></h4>
|
||
<pre><code class="language-python">SCHEMA_VERSION = S + 4
|
||
SCHEMA_COMPAT_VERSION = S + 3 # we can't roll back to a time before new_column was entirely non-NULL
|
||
</code></pre>
|
||
<p><strong>Invariants:</strong></p>
|
||
<ol>
|
||
<li><code>old_column</code> exists but is not written to or read from by Synapse.</li>
|
||
<li><code>new_column</code> is read by Synapse and written to by Synapse. Moreover, all rows have a non-<code>NULL</code> value in this field, as guaranteed by a schema constraint.</li>
|
||
</ol>
|
||
<p><strong>Notes:</strong></p>
|
||
<ol>
|
||
<li>We can't drop <code>old_column</code> yet because that would break a rollback to the previous version of Synapse. <br />
|
||
<strong>TODO:</strong> It may be possible to relax this and drop the column straight away as long as the previous version of Synapse detected a rollback occurred and stopped attempting to write to the column. This could possibly be done by checking whether the database's schema compatibility version was <code>S + 3</code>.</li>
|
||
</ol>
|
||
<h4 id="synapse-version-n--5"><a class="header" href="#synapse-version-n--5">Synapse version <code>N + 5</code></a></h4>
|
||
<pre><code class="language-python">SCHEMA_VERSION = S + 5
|
||
SCHEMA_COMPAT_VERSION = S + 4 # we can't roll back to a time before old_column was no longer being touched
|
||
</code></pre>
|
||
<p><strong>Changes:</strong></p>
|
||
<ol>
|
||
<li>
|
||
<pre><code class="language-sql">ALTER TABLE mytable DROP COLUMN old_column;
|
||
</code></pre>
|
||
</li>
|
||
</ol>
|
||
|
||
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|
||
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<script type="text/javascript" src="../docs/website_files/table-of-contents.js"></script>
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<script type="text/javascript" src="../docs/website_files/version-picker.js"></script>
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<script type="text/javascript" src="../docs/website_files/version.js"></script>
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</body>
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</html>
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