mirror of
https://mau.dev/maunium/synapse.git
synced 2024-10-01 01:36:05 -04:00
877bdfa889
Summarized from @richvdh's reply at https://github.com/matrix-org/synapse/pull/13589#discussion_r961116999
218 lines
8.7 KiB
Markdown
218 lines
8.7 KiB
Markdown
# Synapse database schema files
|
||
|
||
Synapse's database schema is stored in the `synapse.storage.schema` module.
|
||
|
||
## Logical databases
|
||
|
||
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.
|
||
|
||
At the time of writing, the following "logical" databases are supported:
|
||
|
||
* `state` - used to store Matrix room state (more specifically, `state_groups`,
|
||
their relationships and contents).
|
||
* `main` - stores everything else.
|
||
|
||
Additionally, the `common` directory contains schema files for tables which must be
|
||
present on *all* physical databases.
|
||
|
||
## Synapse schema versions
|
||
|
||
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:
|
||
|
||
* The Synapse codebase defines a constant `synapse.storage.schema.SCHEMA_VERSION`
|
||
which represents the expectations made about the database by that version. For
|
||
example, as of Synapse v1.36, this is `59`.
|
||
|
||
* The database stores a "compatibility version" in
|
||
`schema_compat_version.compat_version` which defines the `SCHEMA_VERSION` of the
|
||
oldest version of Synapse which will work with the database. On startup, if
|
||
`compat_version` is found to be newer than `SCHEMA_VERSION`, Synapse will refuse to
|
||
start.
|
||
|
||
Synapse automatically updates this field from
|
||
`synapse.storage.schema.SCHEMA_COMPAT_VERSION`.
|
||
|
||
* Whenever a backwards-incompatible change is made to the database format (normally
|
||
via a `delta` file), `synapse.storage.schema.SCHEMA_COMPAT_VERSION` is also updated
|
||
so that administrators can not accidentally roll back to a too-old version of Synapse.
|
||
|
||
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.
|
||
|
||
As a worked example: we want to remove the `room_stats_historical` table. Here is how it
|
||
might pan out.
|
||
|
||
1. Replace any code that *reads* from `room_stats_historical` with alternative
|
||
implementations, but keep writing to it in case of rollback to an earlier version.
|
||
Also, increase `synapse.storage.schema.SCHEMA_VERSION`. In this
|
||
instance, there is no existing code which reads from `room_stats_historical`, so
|
||
our starting point is:
|
||
|
||
v1.36.0: `SCHEMA_VERSION=59`, `SCHEMA_COMPAT_VERSION=59`
|
||
|
||
2. Next (say in Synapse v1.37.0): remove the code that *writes* to
|
||
`room_stats_historical`, but don’t yet remove the table in case of rollback to
|
||
v1.36.0. Again, we increase `synapse.storage.schema.SCHEMA_VERSION`, but
|
||
because we have not broken compatibility with v1.36, we do not yet update
|
||
`SCHEMA_COMPAT_VERSION`. We now have:
|
||
|
||
v1.37.0: `SCHEMA_VERSION=60`, `SCHEMA_COMPAT_VERSION=59`.
|
||
|
||
3. 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 `SCHEMA_COMPAT_VERSION` accordingly.
|
||
There is no need to update `synapse.storage.schema.SCHEMA_VERSION`, since there is no
|
||
change to the Synapse codebase here. So we end up with:
|
||
|
||
v1.38.0: `SCHEMA_VERSION=60`, `SCHEMA_COMPAT_VERSION=60`.
|
||
|
||
If in doubt about whether to update `SCHEMA_VERSION` or not, it is generally best to
|
||
lean towards doing so.
|
||
|
||
## Full schema dumps
|
||
|
||
In the `full_schemas` directories, only the most recently-numbered snapshot is used
|
||
(`54` at the time of writing). Older snapshots (eg, `16`) are present for historical
|
||
reference only.
|
||
|
||
### Building full schema dumps
|
||
|
||
If you want to recreate these schemas, they need to be made from a database that
|
||
has had all background updates run.
|
||
|
||
To do so, use `scripts-dev/make_full_schema.sh`. This will produce new
|
||
`full.sql.postgres` and `full.sql.sqlite` files.
|
||
|
||
Ensure postgres is installed, then run:
|
||
|
||
```sh
|
||
./scripts-dev/make_full_schema.sh -p postgres_username -o output_dir/
|
||
```
|
||
|
||
NB at the time of writing, this script predates the split into separate `state`/`main`
|
||
databases so will require updates to handle that correctly.
|
||
|
||
## Delta files
|
||
|
||
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.
|
||
|
||
Synapse remembers which delta files it has applied to a database (they are stored in the
|
||
`applied_schema_deltas` table) and will not re-apply them (even if a given file is
|
||
subsequently updated).
|
||
|
||
Delta files should be placed in a directory named `synapse/storage/schema/<database>/delta/<version>/`.
|
||
They are applied in alphanumeric order, so by convention the first two characters
|
||
of the filename should be an integer such as `01`, to put the file in the right order.
|
||
|
||
### SQL delta files
|
||
|
||
These should be named `*.sql`, or — for changes which should only be applied for a
|
||
given database engine — `*.sql.posgres` or `*.sql.sqlite`. For example, a delta which
|
||
adds a new column to the `foo` table might be called `01add_bar_to_foo.sql`.
|
||
|
||
Note that our SQL parser is a bit simple - it understands comments (`--` and `/*...*/`),
|
||
but complex statements which require a `;` in the middle of them (such as `CREATE
|
||
TRIGGER`) are beyond it and you'll have to use a Python delta file.
|
||
|
||
### Python delta files
|
||
|
||
For more flexibility, a delta file can take the form of a python module. These should
|
||
be named `*.py`. Note that database-engine-specific modules are not supported here –
|
||
instead you can write `if isinstance(database_engine, PostgresEngine)` or similar.
|
||
|
||
A Python delta module should define either or both of the following functions:
|
||
|
||
```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."""
|
||
...
|
||
```
|
||
|
||
## Boolean columns
|
||
|
||
Boolean columns require special treatment, since SQLite treats booleans the
|
||
same as integers.
|
||
|
||
There are three separate aspects to this:
|
||
|
||
* Any new boolean column must be added to the `BOOLEAN_COLUMNS` list in
|
||
`synapse/_scripts/synapse_port_db.py`. This tells the port script to cast
|
||
the integer value from SQLite to a boolean before writing the value to the
|
||
postgres database.
|
||
|
||
* Before SQLite 3.23, `TRUE` and `FALSE` were not recognised as constants by
|
||
SQLite, and the `IS [NOT] TRUE`/`IS [NOT] FALSE` operators were not
|
||
supported. This makes it necessary to avoid using `TRUE` and `FALSE`
|
||
constants in SQL commands.
|
||
|
||
For example, to insert a `TRUE` value into the database, write:
|
||
|
||
```python
|
||
txn.execute("INSERT INTO tbl(col) VALUES (?)", (True, ))
|
||
```
|
||
|
||
* Default values for new boolean columns present a particular
|
||
difficulty. Generally it is best to create separate schema files for
|
||
Postgres and SQLite. For example:
|
||
|
||
```sql
|
||
# in 00delta.sql.postgres:
|
||
ALTER TABLE tbl ADD COLUMN col BOOLEAN DEFAULT FALSE;
|
||
```
|
||
|
||
```sql
|
||
# in 00delta.sql.sqlite:
|
||
ALTER TABLE tbl ADD COLUMN col BOOLEAN DEFAULT 0;
|
||
```
|
||
|
||
Note that there is a particularly insidious failure mode here: the Postgres
|
||
flavour will be accepted by SQLite 3.22, but will give a column whose
|
||
default value is the **string** `"FALSE"` - which, when cast back to a boolean
|
||
in Python, evaluates to `True`.
|
||
|
||
|
||
## `event_id` global uniqueness
|
||
|
||
In room versions `1` and `2` it's possible to end up with two events with the
|
||
same `event_id` (in the same or different rooms). After room version `3`, that
|
||
can only happen with a hash collision, which we basically hope will never
|
||
happen.
|
||
|
||
There are several places in Synapse and even Matrix APIs like [`GET
|
||
/_matrix/federation/v1/event/{eventId}`](https://spec.matrix.org/v1.1/server-server-api/#get_matrixfederationv1eventeventid)
|
||
where we assume that event IDs are globally unique.
|
||
|
||
But hash collisions are still possible, and by treating event IDs as room
|
||
scoped, we can reduce the possibility of a hash collision. When scoping
|
||
`event_id` in the database schema, it should be also accompanied by `room_id`
|
||
(`PRIMARY KEY (room_id, event_id)`) and lookups should be done through the pair
|
||
`(room_id, event_id)`.
|
||
|
||
There has been a lot of debate on this in places like
|
||
https://github.com/matrix-org/matrix-spec-proposals/issues/2779 and
|
||
[MSC2848](https://github.com/matrix-org/matrix-spec-proposals/pull/2848) which
|
||
has no resolution yet (as of 2022-09-01).
|
||
|