72 lines
3.0 KiB
Markdown
72 lines
3.0 KiB
Markdown
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# Background update controller callbacks
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Background update controller callbacks allow module developers to control (e.g. rate-limit)
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how database background updates are run. A database background update is an operation
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Synapse runs on its database in the background after it starts. It's usually used to run
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database operations that would take too long if they were run at the same time as schema
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updates (which are run on startup) and delay Synapse's startup too much: populating a
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table with a big amount of data, adding an index on a big table, deleting superfluous data,
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etc.
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Background update controller callbacks can be registered using the module API's
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`register_background_update_controller_callbacks` method. Only the first module (in order
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of appearance in Synapse's configuration file) calling this method can register background
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update controller callbacks, subsequent calls are ignored.
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The available background update controller callbacks are:
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### `on_update`
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_First introduced in Synapse v1.49.0_
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```python
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def on_update(update_name: str, database_name: str, one_shot: bool) -> AsyncContextManager[int]
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```
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Called when about to do an iteration of a background update. The module is given the name
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of the update, the name of the database, and a flag to indicate whether the background
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update will happen in one go and may take a long time (e.g. creating indices). If this last
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argument is set to `False`, the update will be run in batches.
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The module must return an async context manager. It will be entered before Synapse runs a
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background update; this should return the desired duration of the iteration, in
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milliseconds.
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The context manager will be exited when the iteration completes. Note that the duration
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returned by the context manager is a target, and an iteration may take substantially longer
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or shorter. If the `one_shot` flag is set to `True`, the duration returned is ignored.
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__Note__: Unlike most module callbacks in Synapse, this one is _synchronous_. This is
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because asynchronous operations are expected to be run by the async context manager.
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This callback is required when registering any other background update controller callback.
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### `default_batch_size`
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_First introduced in Synapse v1.49.0_
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```python
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async def default_batch_size(update_name: str, database_name: str) -> int
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```
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Called before the first iteration of a background update, with the name of the update and
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of the database. The module must return the number of elements to process in this first
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iteration.
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If this callback is not defined, Synapse will use a default value of 100.
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### `min_batch_size`
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_First introduced in Synapse v1.49.0_
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```python
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async def min_batch_size(update_name: str, database_name: str) -> int
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```
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Called before running a new batch for a background update, with the name of the update and
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of the database. The module must return an integer representing the minimum number of
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elements to process in this iteration. This number must be at least 1, and is used to
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ensure that progress is always made.
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If this callback is not defined, Synapse will use a default value of 100.
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