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+
+                        <h1 id="using-postgres"><a class="header" href="#using-postgres">Using Postgres</a></h1>
+<p>The minimum supported version of PostgreSQL is determined by the <a href="deprecation_policy.html">Dependency
+Deprecation Policy</a>.</p>
+<h2 id="install-postgres-client-libraries"><a class="header" href="#install-postgres-client-libraries">Install postgres client libraries</a></h2>
+<p>Synapse will require the python postgres client library in order to
+connect to a postgres database.</p>
+<ul>
+<li>
+<p>If you are using the <a href="setup/installation.html#matrixorg-packages">matrix.org debian/ubuntu
+packages</a>, the necessary python
+library will already be installed, but you will need to ensure the
+low-level postgres library is installed, which you can do with
+<code>apt install libpq5</code>.</p>
+</li>
+<li>
+<p>For other pre-built packages, please consult the documentation from
+the relevant package.</p>
+</li>
+<li>
+<p>If you installed synapse <a href="setup/installation.html#installing-as-a-python-module-from-pypi">in a
+virtualenv</a>, you can install
+the library with:</p>
+<pre><code>~/synapse/env/bin/pip install &quot;matrix-synapse[postgres]&quot;
+</code></pre>
+<p>(substituting the path to your virtualenv for <code>~/synapse/env</code>, if
+you used a different path). You will require the postgres
+development files. These are in the <code>libpq-dev</code> package on
+Debian-derived distributions.</p>
+</li>
+</ul>
+<h2 id="set-up-database"><a class="header" href="#set-up-database">Set up database</a></h2>
+<p>Assuming your PostgreSQL database user is called <code>postgres</code>, first authenticate as the database user with:</p>
+<pre><code class="language-sh">su - postgres
+# Or, if your system uses sudo to get administrative rights
+sudo -u postgres bash
+</code></pre>
+<p>Then, create a postgres user and a database with:</p>
+<pre><code class="language-sh"># this will prompt for a password for the new user
+createuser --pwprompt synapse_user
+
+createdb --encoding=UTF8 --locale=C --template=template0 --owner=synapse_user synapse
+</code></pre>
+<p>The above will create a user called <code>synapse_user</code>, and a database called
+<code>synapse</code>.</p>
+<p>Note that the PostgreSQL database <em>must</em> have the correct encoding set
+(as shown above), otherwise it will not be able to store UTF8 strings.</p>
+<p>You may need to enable password authentication so <code>synapse_user</code> can
+connect to the database. See
+<a href="https://www.postgresql.org/docs/current/auth-pg-hba-conf.html">https://www.postgresql.org/docs/current/auth-pg-hba-conf.html</a>.</p>
+<h2 id="synapse-config"><a class="header" href="#synapse-config">Synapse config</a></h2>
+<p>When you are ready to start using PostgreSQL, edit the <code>database</code>
+section in your config file to match the following lines:</p>
+<pre><code class="language-yaml">database:
+  name: psycopg2
+  args:
+    user: &lt;user&gt;
+    password: &lt;pass&gt;
+    database: &lt;db&gt;
+    host: &lt;host&gt;
+    cp_min: 5
+    cp_max: 10
+</code></pre>
+<p>All key, values in <code>args</code> are passed to the <code>psycopg2.connect(..)</code>
+function, except keys beginning with <code>cp_</code>, which are consumed by the
+twisted adbapi connection pool. See the <a href="https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-PARAMKEYWORDS">libpq
+documentation</a>
+for a list of options which can be passed.</p>
+<p>You should consider tuning the <code>args.keepalives_*</code> options if there is any danger of
+the connection between your homeserver and database dropping, otherwise Synapse
+may block for an extended period while it waits for a response from the
+database server. Example values might be:</p>
+<pre><code class="language-yaml">database:
+  args:
+    # ... as above
+
+    # seconds of inactivity after which TCP should send a keepalive message to the server
+    keepalives_idle: 10
+
+    # the number of seconds after which a TCP keepalive message that is not
+    # acknowledged by the server should be retransmitted
+    keepalives_interval: 10
+
+    # the number of TCP keepalives that can be lost before the client's connection
+    # to the server is considered dead
+    keepalives_count: 3
+</code></pre>
+<h2 id="tuning-postgres"><a class="header" href="#tuning-postgres">Tuning Postgres</a></h2>
+<p>The default settings should be fine for most deployments. For larger
+scale deployments tuning some of the settings is recommended, details of
+which can be found at
+<a href="https://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server">https://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server</a>.</p>
+<p>In particular, we've found tuning the following values helpful for
+performance:</p>
+<ul>
+<li><code>shared_buffers</code></li>
+<li><code>effective_cache_size</code></li>
+<li><code>work_mem</code></li>
+<li><code>maintenance_work_mem</code></li>
+<li><code>autovacuum_work_mem</code></li>
+</ul>
+<p>Note that the appropriate values for those fields depend on the amount
+of free memory the database host has available.</p>
+<p>Additionally, admins of large deployments might want to consider using huge pages
+to help manage memory, especially when using large values of <code>shared_buffers</code>. You
+can read more about that <a href="https://www.postgresql.org/docs/10/kernel-resources.html#LINUX-HUGE-PAGES">here</a>.</p>
+<h2 id="porting-from-sqlite"><a class="header" href="#porting-from-sqlite">Porting from SQLite</a></h2>
+<h3 id="overview"><a class="header" href="#overview">Overview</a></h3>
+<p>The script <code>synapse_port_db</code> allows porting an existing synapse server
+backed by SQLite to using PostgreSQL. This is done in as a two phase
+process:</p>
+<ol>
+<li>Copy the existing SQLite database to a separate location and run
+the port script against that offline database.</li>
+<li>Shut down the server. Rerun the port script to port any data that
+has come in since taking the first snapshot. Restart server against
+the PostgreSQL database.</li>
+</ol>
+<p>The port script is designed to be run repeatedly against newer snapshots
+of the SQLite database file. This makes it safe to repeat step 1 if
+there was a delay between taking the previous snapshot and being ready
+to do step 2.</p>
+<p>It is safe to at any time kill the port script and restart it.</p>
+<p>However, under no circumstances should the SQLite database be <code>VACUUM</code>ed between
+multiple runs of the script. Doing so can lead to an inconsistent copy of your database
+into Postgres.
+To avoid accidental error, the script will check that SQLite's <code>auto_vacuum</code> mechanism
+is disabled, but the script is not able to protect against a manual <code>VACUUM</code> operation
+performed either by the administrator or by any automated task that the administrator
+may have configured.</p>
+<p>Note that the database may take up significantly more (25% - 100% more)
+space on disk after porting to Postgres.</p>
+<h3 id="using-the-port-script"><a class="header" href="#using-the-port-script">Using the port script</a></h3>
+<p>Firstly, shut down the currently running synapse server and copy its
+database file (typically <code>homeserver.db</code>) to another location. Once the
+copy is complete, restart synapse. For instance:</p>
+<pre><code class="language-sh">synctl stop
+cp homeserver.db homeserver.db.snapshot
+synctl start
+</code></pre>
+<p>Copy the old config file into a new config file:</p>
+<pre><code class="language-sh">cp homeserver.yaml homeserver-postgres.yaml
+</code></pre>
+<p>Edit the database section as described in the section <em>Synapse config</em>
+above and with the SQLite snapshot located at <code>homeserver.db.snapshot</code>
+simply run:</p>
+<pre><code class="language-sh">synapse_port_db --sqlite-database homeserver.db.snapshot \
+    --postgres-config homeserver-postgres.yaml
+</code></pre>
+<p>The flag <code>--curses</code> displays a coloured curses progress UI.</p>
+<p>If the script took a long time to complete, or time has otherwise passed
+since the original snapshot was taken, repeat the previous steps with a
+newer snapshot.</p>
+<p>To complete the conversion shut down the synapse server and run the port
+script one last time, e.g. if the SQLite database is at <code>homeserver.db</code>
+run:</p>
+<pre><code class="language-sh">synapse_port_db --sqlite-database homeserver.db \
+    --postgres-config homeserver-postgres.yaml
+</code></pre>
+<p>Once that has completed, change the synapse config to point at the
+PostgreSQL database configuration file <code>homeserver-postgres.yaml</code>:</p>
+<pre><code class="language-sh">synctl stop
+mv homeserver.yaml homeserver-old-sqlite.yaml
+mv homeserver-postgres.yaml homeserver.yaml
+synctl start
+</code></pre>
+<p>Synapse should now be running against PostgreSQL.</p>
+<h2 id="troubleshooting"><a class="header" href="#troubleshooting">Troubleshooting</a></h2>
+<h3 id="alternative-auth-methods"><a class="header" href="#alternative-auth-methods">Alternative auth methods</a></h3>
+<p>If you get an error along the lines of <code>FATAL:  Ident authentication failed for user &quot;synapse_user&quot;</code>, you may need to use an authentication method other than
+<code>ident</code>:</p>
+<ul>
+<li>
+<p>If the <code>synapse_user</code> user has a password, add the password to the <code>database:</code>
+section of <code>homeserver.yaml</code>. Then add the following to <code>pg_hba.conf</code>:</p>
+<pre><code>host    synapse     synapse_user    ::1/128     md5  # or `scram-sha-256` instead of `md5` if you use that
+</code></pre>
+</li>
+<li>
+<p>If the <code>synapse_user</code> user does not have a password, then a password doesn't
+have to be added to <code>homeserver.yaml</code>. But the following does need to be added
+to <code>pg_hba.conf</code>:</p>
+<pre><code>host    synapse     synapse_user    ::1/128     trust
+</code></pre>
+</li>
+</ul>
+<p>Note that line order matters in <code>pg_hba.conf</code>, so make sure that if you do add a
+new line, it is inserted before:</p>
+<pre><code>host    all         all             ::1/128     ident
+</code></pre>
+<h3 id="fixing-incorrect-collate-or-ctype"><a class="header" href="#fixing-incorrect-collate-or-ctype">Fixing incorrect <code>COLLATE</code> or <code>CTYPE</code></a></h3>
+<p>Synapse will refuse to set up a new database if it has the wrong values of
+<code>COLLATE</code> and <code>CTYPE</code> set. Synapse will also refuse to start an existing database with incorrect values
+of <code>COLLATE</code> and <code>CTYPE</code> unless the config flag <code>allow_unsafe_locale</code>, found in the 
+<code>database</code> section of the config, is set to true. Using different locales can cause issues if the locale library is updated from
+underneath the database, or if a different version of the locale is used on any
+replicas.</p>
+<p>If you have a database with an unsafe locale, the safest way to fix the issue is to dump the database and recreate it with
+the correct locale parameter (as shown above). It is also possible to change the
+parameters on a live database and run a <code>REINDEX</code> on the entire database,
+however extreme care must be taken to avoid database corruption.</p>
+<p>Note that the above may fail with an error about duplicate rows if corruption
+has already occurred, and such duplicate rows will need to be manually removed.</p>
+<h3 id="fixing-inconsistent-sequences-error"><a class="header" href="#fixing-inconsistent-sequences-error">Fixing inconsistent sequences error</a></h3>
+<p>Synapse uses Postgres sequences to generate IDs for various tables. A sequence
+and associated table can get out of sync if, for example, Synapse has been
+downgraded and then upgraded again.</p>
+<p>To fix the issue shut down Synapse (including any and all workers) and run the
+SQL command included in the error message. Once done Synapse should start
+successfully.</p>
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