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- <h1 id="background-update-controller-callbacks"><a class="header" href="#background-update-controller-callbacks">Background update controller callbacks</a></h1>
- <p>Background update controller callbacks allow module developers to control (e.g. rate-limit)
- how database background updates are run. A database background update is an operation
- Synapse runs on its database in the background after it starts. It's usually used to run
- database operations that would take too long if they were run at the same time as schema
- updates (which are run on startup) and delay Synapse's startup too much: populating a
- table with a big amount of data, adding an index on a big table, deleting superfluous data,
- etc.</p>
- <p>Background update controller callbacks can be registered using the module API's
- <code>register_background_update_controller_callbacks</code> method. Only the first module (in order
- of appearance in Synapse's configuration file) calling this method can register background
- update controller callbacks, subsequent calls are ignored.</p>
- <p>The available background update controller callbacks are:</p>
- <h3 id="on_update"><a class="header" href="#on_update"><code>on_update</code></a></h3>
- <p><em>First introduced in Synapse v1.49.0</em></p>
- <pre><code class="language-python">def on_update(update_name: str, database_name: str, one_shot: bool) -> AsyncContextManager[int]
- </code></pre>
- <p>Called when about to do an iteration of a background update. The module is given the name
- of the update, the name of the database, and a flag to indicate whether the background
- update will happen in one go and may take a long time (e.g. creating indices). If this last
- argument is set to <code>False</code>, the update will be run in batches.</p>
- <p>The module must return an async context manager. It will be entered before Synapse runs a
- background update; this should return the desired duration of the iteration, in
- milliseconds.</p>
- <p>The context manager will be exited when the iteration completes. Note that the duration
- returned by the context manager is a target, and an iteration may take substantially longer
- or shorter. If the <code>one_shot</code> flag is set to <code>True</code>, the duration returned is ignored.</p>
- <p><strong>Note</strong>: Unlike most module callbacks in Synapse, this one is <em>synchronous</em>. This is
- because asynchronous operations are expected to be run by the async context manager.</p>
- <p>This callback is required when registering any other background update controller callback.</p>
- <h3 id="default_batch_size"><a class="header" href="#default_batch_size"><code>default_batch_size</code></a></h3>
- <p><em>First introduced in Synapse v1.49.0</em></p>
- <pre><code class="language-python">async def default_batch_size(update_name: str, database_name: str) -> int
- </code></pre>
- <p>Called before the first iteration of a background update, with the name of the update and
- of the database. The module must return the number of elements to process in this first
- iteration.</p>
- <p>If this callback is not defined, Synapse will use a default value of 100.</p>
- <h3 id="min_batch_size"><a class="header" href="#min_batch_size"><code>min_batch_size</code></a></h3>
- <p><em>First introduced in Synapse v1.49.0</em></p>
- <pre><code class="language-python">async def min_batch_size(update_name: str, database_name: str) -> int
- </code></pre>
- <p>Called before running a new batch for a background update, with the name of the update and
- of the database. The module must return an integer representing the minimum number of
- elements to process in this iteration. This number must be at least 1, and is used to
- ensure that progress is always made.</p>
- <p>If this callback is not defined, Synapse will use a default value of 100.</p>
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