<!DOCTYPE HTML> <html lang="en" class="sidebar-visible no-js light"> <head> <!-- Book generated using mdBook --> <meta charset="UTF-8"> <title>Database Schemas - Synapse</title> <!-- Custom HTML head --> <meta content="text/html; charset=utf-8" http-equiv="Content-Type"> <meta name="description" content=""> <meta name="viewport" content="width=device-width, initial-scale=1"> <meta name="theme-color" content="#ffffff" /> <link rel="icon" href="../favicon.svg"> <link rel="shortcut icon" href="../favicon.png"> <link rel="stylesheet" href="../css/variables.css"> <link rel="stylesheet" href="../css/general.css"> <link rel="stylesheet" href="../css/chrome.css"> <link rel="stylesheet" href="../css/print.css" media="print"> <!-- Fonts --> <link rel="stylesheet" href="../FontAwesome/css/font-awesome.css"> <link rel="stylesheet" href="../fonts/fonts.css"> <!-- Highlight.js Stylesheets --> <link rel="stylesheet" href="../highlight.css"> <link rel="stylesheet" href="../tomorrow-night.css"> <link rel="stylesheet" href="../ayu-highlight.css"> <!-- Custom theme stylesheets --> <link rel="stylesheet" href="../docs/website_files/table-of-contents.css"> <link rel="stylesheet" href="../docs/website_files/remove-nav-buttons.css"> <link rel="stylesheet" href="../docs/website_files/indent-section-headers.css"> <link rel="stylesheet" href="../docs/website_files/version-picker.css"> </head> <body> <!-- Provide site root to javascript --> <script type="text/javascript"> var path_to_root = "../"; var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? 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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 href="../structured_logging.html">Structured Logging</a></li><li class="chapter-item expanded "><a href="../templates.html">Templates</a></li><li class="chapter-item expanded "><a href="../usage/configuration/user_authentication/index.html">User Authentication</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../usage/configuration/user_authentication/single_sign_on/index.html">Single-Sign On</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../openid.html">OpenID Connect</a></li><li class="chapter-item expanded "><a href="../usage/configuration/user_authentication/single_sign_on/saml.html">SAML</a></li><li class="chapter-item expanded "><a href="../usage/configuration/user_authentication/single_sign_on/cas.html">CAS</a></li><li class="chapter-item expanded "><a href="../sso_mapping_providers.html">SSO Mapping Providers</a></li></ol></li><li class="chapter-item expanded "><a href="../password_auth_providers.html">Password Auth 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 "><a href="../admin_api/server_notices.html">Server Notices</a></li><li class="chapter-item expanded "><a href="../admin_api/statistics.html">Statistics</a></li><li class="chapter-item expanded "><a href="../admin_api/user_admin_api.html">Users</a></li><li class="chapter-item expanded "><a href="../admin_api/version_api.html">Server Version</a></li><li class="chapter-item expanded "><a href="../usage/administration/admin_api/federation.html">Federation</a></li></ol></li><li class="chapter-item expanded "><a href="../manhole.html">Manhole</a></li><li class="chapter-item expanded "><a href="../metrics-howto.html">Monitoring</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../usage/administration/monitoring/reporting_homeserver_usage_statistics.html">Reporting Homeserver Usage Statistics</a></li></ol></li><li class="chapter-item expanded "><a href="../usage/administration/monthly_active_users.html">Monthly Active Users</a></li><li class="chapter-item expanded "><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 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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 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(Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar"> <i class="fa fa-search"></i> </button> <div class="version-picker"> <div class="dropdown"> <div class="select"> <span></span> <i class="fa fa-chevron-down"></i> </div> <input type="hidden" name="version"> <ul class="dropdown-menu"> <!-- Versions will be added dynamically in version-picker.js --> </ul> </div> </div> </div> <h1 class="menu-title">Synapse</h1> <div class="right-buttons"> <a href="../print.html" title="Print this book" aria-label="Print this book"> <i id="print-button" class="fa fa-print"></i> </a> <a href="https://github.com/matrix-org/synapse" title="Git repository" aria-label="Git repository"> <i id="git-repository-button" class="fa fa-github"></i> </a> <a href="https://github.com/matrix-org/synapse/edit/develop/docs/development/database_schema.md" title="Suggest an edit" aria-label="Suggest an edit"> <i id="git-edit-button" class="fa fa-edit"></i> </a> </div> </div> <div id="search-wrapper" class="hidden"> <form id="searchbar-outer" class="searchbar-outer"> <input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header"> </form> <div id="searchresults-outer" class="searchresults-outer hidden"> <div id="searchresults-header" class="searchresults-header"></div> <ul id="searchresults"> </ul> </div> </div> <!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM --> <script type="text/javascript"> document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible'); document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible'); Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) { link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1); }); </script> <div id="content" class="content"> <main> <!-- Page table of contents --> <div class="sidetoc"> <nav class="pagetoc"></nav> </div> <h1 id="synapse-database-schema-files"><a class="header" href="#synapse-database-schema-files">Synapse database schema files</a></h1> <p>Synapse's database schema is stored in the <code>synapse.storage.schema</code> module.</p> <h2 id="logical-databases"><a class="header" href="#logical-databases">Logical databases</a></h2> <p>Synapse supports splitting its datastore across multiple physical databases (which can be useful for large installations), and the schema files are therefore split according to the logical database they apply to.</p> <p>At the time of writing, the following "logical" databases are supported:</p> <ul> <li><code>state</code> - used to store Matrix room state (more specifically, <code>state_groups</code>, their relationships and contents).</li> <li><code>main</code> - stores everything else.</li> </ul> <p>Additionally, the <code>common</code> directory contains schema files for tables which must be present on <em>all</em> physical databases.</p> <h2 id="synapse-schema-versions"><a class="header" href="#synapse-schema-versions">Synapse schema versions</a></h2> <p>Synapse manages its database schema via "schema versions". These are mainly used to help avoid confusion if the Synapse codebase is rolled back after the database is updated. They work as follows:</p> <ul> <li> <p>The Synapse codebase defines a constant <code>synapse.storage.schema.SCHEMA_VERSION</code> which represents the expectations made about the database by that version. For example, as of Synapse v1.36, this is <code>59</code>.</p> </li> <li> <p>The database stores a "compatibility version" in <code>schema_compat_version.compat_version</code> which defines the <code>SCHEMA_VERSION</code> of the oldest version of Synapse which will work with the database. On startup, if <code>compat_version</code> is found to be newer than <code>SCHEMA_VERSION</code>, Synapse will refuse to start.</p> <p>Synapse automatically updates this field from <code>synapse.storage.schema.SCHEMA_COMPAT_VERSION</code>.</p> </li> <li> <p>Whenever a backwards-incompatible change is made to the database format (normally via a <code>delta</code> file), <code>synapse.storage.schema.SCHEMA_COMPAT_VERSION</code> is also updated so that administrators can not accidentally roll back to a too-old version of Synapse.</p> </li> </ul> <p>Generally, the goal is to maintain compatibility with at least one or two previous releases of Synapse, so any substantial change tends to require multiple releases and a bit of forward-planning to get right.</p> <p>As a worked example: we want to remove the <code>room_stats_historical</code> table. Here is how it might pan out.</p> <ol> <li> <p>Replace any code that <em>reads</em> from <code>room_stats_historical</code> with alternative implementations, but keep writing to it in case of rollback to an earlier version. Also, increase <code>synapse.storage.schema.SCHEMA_VERSION</code>. In this instance, there is no existing code which reads from <code>room_stats_historical</code>, so our starting point is:</p> <p>v1.36.0: <code>SCHEMA_VERSION=59</code>, <code>SCHEMA_COMPAT_VERSION=59</code></p> </li> <li> <p>Next (say in Synapse v1.37.0): remove the code that <em>writes</em> to <code>room_stats_historical</code>, but don’t yet remove the table in case of rollback to v1.36.0. Again, we increase <code>synapse.storage.schema.SCHEMA_VERSION</code>, but because we have not broken compatibility with v1.36, we do not yet update <code>SCHEMA_COMPAT_VERSION</code>. We now have:</p> <p>v1.37.0: <code>SCHEMA_VERSION=60</code>, <code>SCHEMA_COMPAT_VERSION=59</code>.</p> </li> <li> <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> </main> <nav class="nav-wrapper" aria-label="Page navigation"> <!-- Mobile navigation buttons --> <a rel="prev" href="../opentracing.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> <i class="fa fa-angle-left"></i> </a> <a rel="next" href="../development/experimental_features.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> <i class="fa fa-angle-right"></i> </a> <div style="clear: both"></div> </nav> </div> </div> <nav class="nav-wide-wrapper" aria-label="Page navigation"> <a rel="prev" href="../opentracing.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> <i class="fa fa-angle-left"></i> </a> <a rel="next" href="../development/experimental_features.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> <i class="fa fa-angle-right"></i> </a> </nav> </div> <script type="text/javascript"> window.playground_copyable = true; </script> <script src="../elasticlunr.min.js" type="text/javascript" charset="utf-8"></script> <script src="../mark.min.js" type="text/javascript" charset="utf-8"></script> <script src="../searcher.js" type="text/javascript" charset="utf-8"></script> <script src="../clipboard.min.js" type="text/javascript" charset="utf-8"></script> <script src="../highlight.js" type="text/javascript" charset="utf-8"></script> <script src="../book.js" type="text/javascript" charset="utf-8"></script> <!-- Custom JS scripts --> <script type="text/javascript" src="../docs/website_files/table-of-contents.js"></script> <script type="text/javascript" src="../docs/website_files/version-picker.js"></script> <script type="text/javascript" src="../docs/website_files/version.js"></script> </body> </html>