# Using Postgres The minimum supported version of PostgreSQL is determined by the [Dependency Deprecation Policy](deprecation_policy.md). ## Install postgres client libraries Synapse will require the python postgres client library in order to connect to a postgres database. - If you are using the [matrix.org debian/ubuntu packages](setup/installation.md#matrixorg-packages), 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 `apt install libpq5`. - For other pre-built packages, please consult the documentation from the relevant package. - If you installed synapse [in a virtualenv](setup/installation.md#installing-as-a-python-module-from-pypi), you can install the library with: ~/synapse/env/bin/pip install "matrix-synapse[postgres]" (substituting the path to your virtualenv for `~/synapse/env`, if you used a different path). You will require the postgres development files. These are in the `libpq-dev` package on Debian-derived distributions. ## Set up database Assuming your PostgreSQL database user is called `postgres`, first authenticate as the database user with: ```sh su - postgres # Or, if your system uses sudo to get administrative rights sudo -u postgres bash ``` Then, create a postgres user and a database with: ```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 ``` The above will create a user called `synapse_user`, and a database called `synapse`. Note that the PostgreSQL database *must* have the correct encoding set (as shown above), otherwise it will not be able to store UTF8 strings. You may need to enable password authentication so `synapse_user` can connect to the database. See . ## Synapse config When you are ready to start using PostgreSQL, edit the `database` section in your config file to match the following lines: ```yaml database: name: psycopg2 args: user: password: dbname: host: cp_min: 5 cp_max: 10 ``` All key, values in `args` are passed to the `psycopg2.connect(..)` function, except keys beginning with `cp_`, which are consumed by the twisted adbapi connection pool. See the [libpq documentation](https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-PARAMKEYWORDS) for a list of options which can be passed. You should consider tuning the `args.keepalives_*` 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: ```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 ``` ## Native Rust driver Synapse can talk to Postgres through its own driver, written in Rust on top of `tokio-postgres`, instead of psycopg2. It is experimental. Enable it by adding `use_rust_driver: true` to the `database` section; the engine `name` stays `psycopg2`: ```yaml database: name: psycopg2 use_rust_driver: true args: user: password: dbname: host: cp_max: 10 ``` The native driver prepares statements, pipelines batched writes, and keeps a pool of connections sized to `cp_max` (`cp_min` is ignored). A transaction waits at most `pool_checkout_timeout` milliseconds for a free connection before failing with an error that Synapse retries. The pool's occupancy is exported as the `synapse_rust_db_pool_*` metrics. ### Connection arguments The `args` are still libpq keywords, but the native driver accepts only the subset that `tokio-postgres` knows: `host`, `hostaddr`, `port`, `user`, `password`, `dbname` (or psycopg2's `database`), `application_name`, `connect_timeout`, `options`, `channel_binding`, `load_balance_hosts`, `target_session_attrs`, `sslnegotiation`, `tcp_user_timeout` and the `keepalives*` keys. `client_encoding` and `sslcompression` are ignored with a warning. Any other key, such as `service` or `passfile`, is an error at startup rather than being silently dropped, since dropping it could change which database Synapse connects to. Values missing from `args` fall back to the `PGHOST`, `PGPORT`, `PGUSER`, `PGPASSWORD` and `PGDATABASE` environment variables and to libpq's compiled-in default host, as they do with psycopg2. ### TLS The `sslmode`, `sslrootcert`, `sslcert`, `sslkey` and `sslpassword` keys are honoured, with the `PGSSL*` environment variables as fallbacks. `sslmode` defaults to `prefer`. `allow` is treated as `prefer`, and `verify-ca` and `verify-full` verify against `sslrootcert` or the system's CA store. Password-protected client keys are not supported. ### Differences from psycopg2 * Parameters are bound by the type Postgres infers for the placeholder rather than interpolated as text, so a value of the wrong Python type for a column is an error instead of being coerced. * `numeric` columns are returned as `Decimal`, as with psycopg2; `json` and `jsonb` columns are decoded to Python values. * Schema upgrades run as one transaction, so a failed upgrade rolls back cleanly rather than leaving earlier delta files applied. * `txn_limit` is enforced by the pool, which replaces a connection after that many transactions, rather than per worker thread. * Committing, rolling back or closing a connection discards any rows not yet fetched from it. To run Synapse's unit tests against the native driver, set `SYNAPSE_TEST_RUST_DRIVER=1` alongside `SYNAPSE_POSTGRES=1`; for Complement, pass `RUST_DRIVER=1` to `scripts-dev/complement.sh`. ### State resolution in Rust With the native driver, Synapse can also resolve state in Rust for rooms that use state resolution v2 or v2.1 (room versions 2 and later). This is an experimental feature, set with `rust_state_res` under `experimental_features`: ```yaml experimental_features: rust_state_res: shadow ``` * `off` (the default) resolves state in Python. * `shadow` resolves state in Python, then again in Rust, and logs a warning if the two disagree. The `synapse_state_res_rust_shadow_total` metric counts the comparisons by result. * `on` resolves state in Rust. Rooms without a chain cover index are always resolved in Python. To run the unit tests with it, set `SYNAPSE_TEST_RUST_STATE_RES` to `shadow` or `on` alongside `SYNAPSE_TEST_RUST_DRIVER=1`. ## Postgresql major version upgrades Postgres uses separate directories for database locations between major versions (typically `/var/lib/postgresql//main`). Therefore, it is recommended to stop Synapse and other services (MAS, etc) before upgrading Postgres major versions. It is also strongly recommended to [back up](./usage/administration/backups.md#database) your database beforehand to ensure no data loss arising from a failed upgrade. ## Backups Don't forget to [back up](./usage/administration/backups.md#database) your database! ## Tuning Postgres 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 . In particular, we've found tuning the following values helpful for performance: - `shared_buffers` - `effective_cache_size` - `work_mem` - `maintenance_work_mem` - `autovacuum_work_mem` Note that the appropriate values for those fields depend on the amount of free memory the database host has available. Additionally, admins of large deployments might want to consider using huge pages to help manage memory, especially when using large values of `shared_buffers`. You can read more about that [here](https://www.postgresql.org/docs/10/kernel-resources.html#LINUX-HUGE-PAGES). ## Porting from SQLite ### Overview The script `synapse_port_db` allows porting an existing synapse server backed by SQLite to using PostgreSQL. This is done as a two phase process: 1. Copy the existing SQLite database to a separate location and run the port script against that offline database. 2. 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. 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. It is safe to at any time kill the port script and restart it. However, under no circumstances should the SQLite database be `VACUUM`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 `auto_vacuum` mechanism is disabled, but the script is not able to protect against a manual `VACUUM` operation performed either by the administrator or by any automated task that the administrator may have configured. Note that the database may take up significantly more (25% - 100% more) space on disk after porting to Postgres. ### Using the port script Firstly, shut down the currently running synapse server and copy its database file (typically `homeserver.db`) to another location. Once the copy is complete, restart synapse. For instance: ```sh synctl stop cp homeserver.db homeserver.db.snapshot synctl start ``` Copy the old config file into a new config file: ```sh cp homeserver.yaml homeserver-postgres.yaml ``` Edit the database section as described in the section *Synapse config* above and with the SQLite snapshot located at `homeserver.db.snapshot` simply run: ```sh synapse_port_db --sqlite-database homeserver.db.snapshot \ --postgres-config homeserver-postgres.yaml ``` The flag `--curses` displays a coloured curses progress UI. (NOTE: if your terminal is too small the script will error out) 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. 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 `homeserver.db` run: ```sh synapse_port_db --sqlite-database homeserver.db \ --postgres-config homeserver-postgres.yaml ``` Once that has completed, change the synapse config to point at the PostgreSQL database configuration file `homeserver-postgres.yaml`: ```sh synctl stop mv homeserver.yaml homeserver-old-sqlite.yaml mv homeserver-postgres.yaml homeserver.yaml synctl start ``` Synapse should now be running against PostgreSQL. ## Troubleshooting ### Alternative auth methods If you get an error along the lines of `FATAL: Ident authentication failed for user "synapse_user"`, you may need to use an authentication method other than `ident`: * If the `synapse_user` user has a password, add the password to the `database:` section of `homeserver.yaml`. Then add the following to `pg_hba.conf`: ``` host synapse synapse_user ::1/128 md5 # or `scram-sha-256` instead of `md5` if you use that ``` * If the `synapse_user` user does not have a password, then a password doesn't have to be added to `homeserver.yaml`. But the following does need to be added to `pg_hba.conf`: ``` host synapse synapse_user ::1/128 trust ``` Note that line order matters in `pg_hba.conf`, so make sure that if you do add a new line, it is inserted before: ``` host all all ::1/128 ident ``` ### Fixing incorrect `COLLATE` or `CTYPE` Synapse will refuse to start when using a database with incorrect values of `COLLATE` and `CTYPE` unless the config flag `allow_unsafe_locale`, found in the `database` 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. 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 `REINDEX` on the entire database, however extreme care must be taken to avoid database corruption. 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.