Release Synapse 1.35.0 (#1001)

* Release Synapse 1.35.0

Signed-off-by: Dan Callahan <danc@element.io>

gatsby/content/blog/2021/06/2021-06-01-synapse-1.35.0-released.mdx

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+---
+  date: '2021-06-01'
+  title: Synapse 1.35.0 released
+  categories:
+    - Releases
+  author: Dan Callahan
+---
+
+Synapse 1.35.0 is out! This release focused on improving internals as we drive toward better memory performance during room joins, but more on that below.
+
+## Spaces: On by Default
+
+Following the successful release of [Synapse 1.34](/blog/2021/05/17/synapse-1-34-0-released), the experimental Spaces flag is now enabled by default. If you had manually enabled the `experimental_features: { spaces_enabled: true }` flag in your homeserver configuration, you may now remove it.
+
+## Bug Squashing
+
+This release of Synapse fixes an issue which could cause federated room joins to fail when the join response exceeded a size limit which was too low ([#10082](/~https://github.com/matrix-org/synapse/pull/10082)). We've also improved what Synapse logs when it drops a connection in similar circumstances ([#10091](/~https://github.com/matrix-org/synapse/pull/10091)), which should aid diagnosis if a similar issue were to arise in the future.
+
+GitHub user thermaq contributed a fix ([#10014](/~https://github.com/matrix-org/synapse/pull/10014)) for a bug  which could cause user presence state to become stale.
+
+Lastly our OpenTracing support now allows for profiling end-to-end performance on a per-user basis ([#9978](/~https://github.com/matrix-org/synapse/pull/9978)).
+
+## An Update on Room Joins
+
+We've been hammering away at shrinking Synapse's memory footprint when joining large / complex rooms, and while we're not there yet, the end is in sight! In particular, this release includes many internal refactorings, including using [ijson](https://pypi.org/project/ijson/) to parse the JSON response to `/send_join` ([#9958](/~https://github.com/matrix-org/synapse/pull/9958)), clearing the way for substantial improvements.
+
+Memory usage still spikes because we're effectively doing the same work with a different library, but ijson's design allows for iterative parsing. This will pay dividends once we modify the code downstream of `/send_join` to take advantage of it.
+
+Concretely, Erik Johnston has an experimental branch of Synapse which completely eliminates the memory spike:
+
+![Memory usage graph for Synapse 1.33, 1.35, and an experimental branch](/blog/img/2021-06-01-synapse-1.35-join-memory.png)
+
+The remaining work is centered on splitting that branch into self-contained, reviewable pull requests, like a rewrite of the Synapse Keyring class ([#10035](/~https://github.com/matrix-org/synapse/pull/10035)). After that's merged, we'll need to make one further change to properly batch up work, at which point we should attain the efficiency gains from Erik's experiment.
+
+## Everything Else
+
+GitHub user savyajha contributed a security hardened systemd unit file which effectively sandboxes Synapse ([#9803](/~https://github.com/matrix-org/synapse/pull/9803)). While not enabled by default, we'd encourage security conscious users to review the [example file](/~https://github.com/matrix-org/synapse/blob/v1.35.0/contrib/systemd/override-hardened.conf) and associated [documentation](/~https://github.com/matrix-org/synapse/blob/v1.35.0/docs/systemd-with-workers/README.md#hardening).
+
+Please see the [Release Notes](/~https://github.com/matrix-org/synapse/blob/v1.35.0/CHANGES.md) for a complete list of changes in this release.
+
+Synapse is a Free and Open Source Software project, and we'd like to extend our thanks to everyone who contributed to this release, including [dklimpel](/~https://github.com/dklimpel), [jerinjtitus](/~https://github.com/jerinjtitus), [junquera](/~https://github.com/junquera), [lonyeon](/~https://github.com/lonyeon), [savyajha](/~https://github.com/savyajha), and [thermaq](/~https://github.com/thermaq).