federation: parallel sending per instance (#4623)

* federation: parallel sending

* federation: some comments

* lint and set force_write true when a request fails

* inbox_urls return vec

* split inbox functions into separate file

* cleanup

* extract sending task code to separate file

* move federation concurrent config to config file

* off by one issue

* improve msg

* fix both permanent stopping of federation queues and multiple creation of the same federation queues

* fix after merge

* lint fix

* Update crates/federate/src/send.rs

Co-authored-by: dullbananas <dull.bananas0@gmail.com>

* comment about reverse ordering

* remove crashable, comment

* comment

* move comment

* run federation tests twice

* fix test run

* prettier

* fix config default

* upgrade rust to 1.78 to fix diesel cli

* fix clippy

* delay

* add debug to make localhost urls not valid in ap crate, add some debug logs

* federation tests: ensure server stop after test and random activity id

* ci fix

* add test to federate 100 events

* fix send 100 test

* different data every time so activities are distinguishable

* allow out of order receives in test

* lint

* comment about https://github.com/LemmyNet/lemmy/pull/4623#discussion_r1565437391

* move sender for clarity, add comment

* move more things to members

* update test todo comment, use same env var as worker test but default to 1

* remove else below continue

* some more cleanup

* handle todo about smooth exit

* add federate inboxes collector tests

* lint

* actor max length

* don't reset fail count if activity skipped

* fix some comments

* reuse vars

* format

* Update .woodpecker.yml

* fix recheck time

* fix inboxes tests under fast mode

* format

* make i32 and ugly casts

* clippy

---------

Co-authored-by: dullbananas <dull.bananas0@gmail.com>

.woodpecker.yml

@@ -181,6 +181,7 @@ steps:
       LEMMY_DATABASE_URL: postgres://lemmy:password@database:5432/lemmy
       RUST_BACKTRACE: "1"
       CARGO_HOME: .cargo_home
+      LEMMY_TEST_FAST_FEDERATION: "1"
     commands:
       - export LEMMY_CONFIG_LOCATION=../../config/config.hjson
       - cargo test --workspace --no-fail-fast

Cargo.lock

@@ -1674,6 +1674,12 @@ dependencies = [
  "syn 1.0.109",
 ]
 
+[[package]]
+name = "downcast"
+version = "0.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1435fa1053d8b2fbbe9be7e97eca7f33d37b28409959813daefc1446a14247f1"
+
 [[package]]
 name = "downcast-rs"
 version = "1.2.1"
@@ -1915,6 +1921,12 @@ dependencies = [
  "percent-encoding",
 ]
 
+[[package]]
+name = "fragile"
+version = "2.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6c2141d6d6c8512188a7891b4b01590a45f6dac67afb4f255c4124dbb86d4eaa"
+
 [[package]]
 name = "fs2"
 version = "0.4.3"
@@ -2980,7 +2992,9 @@ name = "lemmy_federate"
 version = "0.19.5"
 dependencies = [
  "activitypub_federation",
+ "actix-web",
  "anyhow",
+ "async-trait",
  "chrono",
  "diesel",
  "diesel-async",
@@ -2990,14 +3004,19 @@ dependencies = [
  "lemmy_db_schema",
  "lemmy_db_views_actor",
  "lemmy_utils",
+ "mockall",
  "moka",
  "once_cell",
  "reqwest 0.11.27",
  "serde_json",
  "serial_test",
+ "test-context",
  "tokio",
  "tokio-util",
  "tracing",
+ "tracing-test",
+ "url",
+ "uuid",
 ]
 
 [[package]]
@@ -3475,6 +3494,33 @@ version = "1.12.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c9be0862c1b3f26a88803c4a49de6889c10e608b3ee9344e6ef5b45fb37ad3d1"
 
+[[package]]
+name = "mockall"
+version = "0.12.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "43766c2b5203b10de348ffe19f7e54564b64f3d6018ff7648d1e2d6d3a0f0a48"
+dependencies = [
+ "cfg-if",
+ "downcast",
+ "fragile",
+ "lazy_static",
+ "mockall_derive",
+ "predicates",
+ "predicates-tree",
+]
+
+[[package]]
+name = "mockall_derive"
+version = "0.12.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "af7cbce79ec385a1d4f54baa90a76401eb15d9cab93685f62e7e9f942aa00ae2"
+dependencies = [
+ "cfg-if",
+ "proc-macro2",
+ "quote",
+ "syn 2.0.66",
+]
+
 [[package]]
 name = "moka"
 version = "0.12.8"
@@ -4273,6 +4319,32 @@ version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "925383efa346730478fb4838dbe9137d2a47675ad789c546d150a6e1dd4ab31c"
 
+[[package]]
+name = "predicates"
+version = "3.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "68b87bfd4605926cdfefc1c3b5f8fe560e3feca9d5552cf68c466d3d8236c7e8"
+dependencies = [
+ "anstyle",
+ "predicates-core",
+]
+
+[[package]]
+name = "predicates-core"
+version = "1.0.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b794032607612e7abeb4db69adb4e33590fa6cf1149e95fd7cb00e634b92f174"
+
+[[package]]
+name = "predicates-tree"
+version = "1.0.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "368ba315fb8c5052ab692e68a0eefec6ec57b23a36959c14496f0b0df2c0cecf"
+dependencies = [
+ "predicates-core",
+ "termtree",
+]
+
 [[package]]
 name = "pretty_assertions"
 version = "1.4.0"
@@ -5676,6 +5748,33 @@ dependencies = [
  "winapi-util",
 ]
 
+[[package]]
+name = "termtree"
+version = "0.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3369f5ac52d5eb6ab48c6b4ffdc8efbcad6b89c765749064ba298f2c68a16a76"
+
+[[package]]
+name = "test-context"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6676ab8513edfd2601a108621103fdb45cac9098305ca25ec93f7023b06b05d9"
+dependencies = [
+ "futures",
+ "test-context-macros",
+]
+
+[[package]]
+name = "test-context-macros"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "78ea17a2dc368aeca6f554343ced1b1e31f76d63683fa8016e5844bd7a5144a1"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.66",
+]
+
 [[package]]
 name = "thiserror"
 version = "1.0.62"
@@ -6299,6 +6398,27 @@ dependencies = [
  "tracing-serde",
 ]
 
+[[package]]
+name = "tracing-test"
+version = "0.2.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "557b891436fe0d5e0e363427fc7f217abf9ccd510d5136549847bdcbcd011d68"
+dependencies = [
+ "tracing-core",
+ "tracing-subscriber",
+ "tracing-test-macro",
+]
+
+[[package]]
+name = "tracing-test-macro"
+version = "0.2.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "04659ddb06c87d233c566112c1c9c5b9e98256d9af50ec3bc9c8327f873a7568"
+dependencies = [
+ "quote",
+ "syn 2.0.66",
+]
+
 [[package]]
 name = "triomphe"
 version = "0.1.11"

api_tests/src/follow.spec.ts

@@ -11,6 +11,7 @@ import {
   betaUrl,
   registerUser,
   unfollows,
+  delay,
 } from "./shared";
 
 beforeAll(setupLogins);
@@ -21,39 +22,48 @@ test("Follow local community", async () => {
   let user = await registerUser(beta, betaUrl);
 
   let community = (await resolveBetaCommunity(user)).community!;
-  expect(community.counts.subscribers).toBe(1);
-  expect(community.counts.subscribers_local).toBe(1);
   let follow = await followCommunity(user, true, community.community.id);
 
   // Make sure the follow response went through
   expect(follow.community_view.community.local).toBe(true);
   expect(follow.community_view.subscribed).toBe("Subscribed");
-  expect(follow.community_view.counts.subscribers).toBe(2);
+  expect(follow.community_view.counts.subscribers).toBe(
-  expect(follow.community_view.counts.subscribers_local).toBe(2);
+    community.counts.subscribers + 1,
+  );
+  expect(follow.community_view.counts.subscribers_local).toBe(
+    community.counts.subscribers_local + 1,
+  );
 
   // Test an unfollow
   let unfollow = await followCommunity(user, false, community.community.id);
   expect(unfollow.community_view.subscribed).toBe("NotSubscribed");
-  expect(unfollow.community_view.counts.subscribers).toBe(1);
+  expect(unfollow.community_view.counts.subscribers).toBe(
-  expect(unfollow.community_view.counts.subscribers_local).toBe(1);
+    community.counts.subscribers,
+  );
+  expect(unfollow.community_view.counts.subscribers_local).toBe(
+    community.counts.subscribers_local,
+  );
 });
 
 test("Follow federated community", async () => {
   // It takes about 1 second for the community aggregates to federate
-  let betaCommunity = (
+  await delay(2000); // if this is the second test run, we don't have a way to wait for the correct number of subscribers
+  const betaCommunityInitial = (
     await waitUntil(
       () => resolveBetaCommunity(alpha),
-      c =>
+      c => !!c.community && c.community?.counts.subscribers >= 1,
-        c.community?.counts.subscribers === 1 &&
-        c.community.counts.subscribers_local === 0,
     )
   ).community;
-  if (!betaCommunity) {
+  if (!betaCommunityInitial) {
     throw "Missing beta community";
   }
-  let follow = await followCommunity(alpha, true, betaCommunity.community.id);
+  let follow = await followCommunity(
+    alpha,
+    true,
+    betaCommunityInitial.community.id,
+  );
   expect(follow.community_view.subscribed).toBe("Pending");
-  betaCommunity = (
+  const betaCommunity = (
     await waitUntil(
       () => resolveBetaCommunity(alpha),
       c => c.community?.subscribed === "Subscribed",
@@ -64,20 +74,24 @@ test("Follow federated community", async () => {
   expect(betaCommunity?.community.local).toBe(false);
   expect(betaCommunity?.community.name).toBe("main");
   expect(betaCommunity?.subscribed).toBe("Subscribed");
-  expect(betaCommunity?.counts.subscribers_local).toBe(1);
+  expect(betaCommunity?.counts.subscribers_local).toBe(
+    betaCommunityInitial.counts.subscribers_local + 1,
+  );
 
   // check that unfollow was federated
   let communityOnBeta1 = await resolveBetaCommunity(beta);
-  expect(communityOnBeta1.community?.counts.subscribers).toBe(2);
+  expect(communityOnBeta1.community?.counts.subscribers).toBe(
-  expect(communityOnBeta1.community?.counts.subscribers_local).toBe(1);
+    betaCommunityInitial.counts.subscribers + 1,
+  );
 
   // Check it from local
   let site = await getSite(alpha);
   let remoteCommunityId = site.my_user?.follows.find(
-    c => c.community.local == false,
+    c =>
+      c.community.local == false &&
+      c.community.id === betaCommunityInitial.community.id,
   )?.community.id;
   expect(remoteCommunityId).toBeDefined();
-  expect(site.my_user?.follows.length).toBe(2);
 
   if (!remoteCommunityId) {
     throw "Missing remote community id";
@@ -89,10 +103,21 @@ test("Follow federated community", async () => {
 
   // Make sure you are unsubbed locally
   let siteUnfollowCheck = await getSite(alpha);
-  expect(siteUnfollowCheck.my_user?.follows.length).toBe(1);
+  expect(
+    siteUnfollowCheck.my_user?.follows.find(
+      c => c.community.id === betaCommunityInitial.community.id,
+    ),
+  ).toBe(undefined);
 
   // check that unfollow was federated
-  let communityOnBeta2 = await resolveBetaCommunity(beta);
+  let communityOnBeta2 = await waitUntil(
-  expect(communityOnBeta2.community?.counts.subscribers).toBe(1);
+    () => resolveBetaCommunity(beta),
+    c =>
+      c.community?.counts.subscribers ===
+      betaCommunityInitial.counts.subscribers,
+  );
+  expect(communityOnBeta2.community?.counts.subscribers).toBe(
+    betaCommunityInitial.counts.subscribers,
+  );
   expect(communityOnBeta2.community?.counts.subscribers_local).toBe(1);
 });

api_tests/src/post.spec.ts

@@ -52,17 +52,23 @@ beforeAll(async () => {
 
 afterAll(unfollows);
 
-async function assertPostFederation(postOne: PostView, postTwo: PostView) {
+async function assertPostFederation(
+  postOne: PostView,
+  postTwo: PostView,
+  waitForMeta = true,
+) {
   // Link metadata is generated in background task and may not be ready yet at this time,
   // so wait for it explicitly. For removed posts we cant refetch anything.
-  postOne = await waitForPost(beta, postOne.post, res => {
+  if (waitForMeta) {
-    return res === null || res?.post.embed_title !== null;
+    postOne = await waitForPost(beta, postOne.post, res => {
-  });
+      return res === null || !!res?.post.embed_title;
-  postTwo = await waitForPost(
+    });
-    beta,
+    postTwo = await waitForPost(
-    postTwo.post,
+      beta,
-    res => res === null || res?.post.embed_title !== null,
+      postTwo.post,
-  );
+      res => res === null || !!res?.post.embed_title,
+    );
+  }
 
   expect(postOne?.post.ap_id).toBe(postTwo?.post.ap_id);
   expect(postOne?.post.name).toBe(postTwo?.post.name);
@@ -408,7 +414,11 @@ test("Remove a post from admin and community on same instance", async () => {
     p => p?.post_view.post.removed ?? false,
   );
   expect(alphaPost?.post_view.post.removed).toBe(true);
-  await assertPostFederation(alphaPost.post_view, removePostRes.post_view);
+  await assertPostFederation(
+    alphaPost.post_view,
+    removePostRes.post_view,
+    false,
+  );
 
   // Undelete
   let undeletedPost = await removePost(beta, false, betaPost.post);

api_tests/src/user.spec.ts

@@ -131,7 +131,11 @@ test("Requests with invalid auth should be treated as unauthenticated", async ()
 });
 
 test("Create user with Arabic name", async () => {
-  let user = await registerUser(alpha, alphaUrl, "تجريب");
+  let user = await registerUser(
+    alpha,
+    alphaUrl,
+    "تجريب" + Math.random().toString().slice(2, 10), // less than actor_name_max_length
+  );
 
   let site = await getSite(user);
   expect(site.my_user).toBeDefined();

config/defaults.hjson

@@ -108,10 +108,12 @@
   port: 8536
   # Whether the site is available over TLS. Needs to be true for federation to work.
   tls_enabled: true
-  # The number of activitypub federation workers that can be in-flight concurrently
+  federation: {
-  worker_count: 0
+    # Limit to the number of concurrent outgoing federation requests per target instance.
-  # The number of activitypub federation retry workers that can be in-flight concurrently
+    # Set this to a higher value than 1 (e.g. 6) only if you have a huge instance (>10 activities
-  retry_count: 0
+    # per second) and if a receiving instance is not keeping up.
+    concurrent_sends_per_instance: 1
+  }
   prometheus: {
     bind: "127.0.0.1"
     port: 10002

crates/api_common/src/context.rs

@@ -55,7 +55,7 @@ impl LemmyContext {
   /// Initialize a context for use in tests which blocks federation network calls.
   ///
   /// Do not use this in production code.
-  pub async fn init_test_context() -> Data<LemmyContext> {
+  pub async fn init_test_federation_config() -> FederationConfig<LemmyContext> {
     // call this to run migrations
     let pool = build_db_pool_for_tests().await;
 
@@ -70,14 +70,19 @@ impl LemmyContext {
     let rate_limit_cell = RateLimitCell::with_test_config();
 
     let context = LemmyContext::create(pool, client, secret, rate_limit_cell.clone());
-    let config = FederationConfig::builder()
+
+    FederationConfig::builder()
       .domain(context.settings().hostname.clone())
       .app_data(context)
+      .debug(true)
       // Dont allow any network fetches
       .http_fetch_limit(0)
       .build()
       .await
-      .expect("build federation config");
+      .expect("build federation config")
+  }
+  pub async fn init_test_context() -> Data<LemmyContext> {
+    let config = Self::init_test_federation_config().await;
     config.to_request_data()
   }
 }

crates/db_schema/src/newtypes.rs

@@ -107,7 +107,7 @@ pub struct PrivateMessageReportId(i32);
 #[cfg_attr(feature = "full", derive(DieselNewType, TS))]
 #[cfg_attr(feature = "full", ts(export))]
 /// The site id.
-pub struct SiteId(i32);
+pub struct SiteId(pub i32);
 
 #[derive(Debug, Copy, Clone, Hash, Eq, PartialEq, Serialize, Deserialize, Default)]
 #[cfg_attr(feature = "full", derive(DieselNewType, TS))]

crates/federate/Cargo.toml

@@ -34,6 +34,13 @@ tokio = { workspace = true, features = ["full"] }
 tracing.workspace = true
 moka.workspace = true
 tokio-util = "0.7.11"
+async-trait.workspace = true
 
 [dev-dependencies]
 serial_test = { workspace = true }
+url.workspace = true
+actix-web.workspace = true
+tracing-test = "0.2.5"
+uuid.workspace = true
+test-context = "0.3.0"
+mockall = "0.12.1"

crates/federate/src/inboxes.rs

@@ -0,0 +1,572 @@
+use crate::util::LEMMY_TEST_FAST_FEDERATION;
+use anyhow::Result;
+use async_trait::async_trait;
+use chrono::{DateTime, TimeZone, Utc};
+use lemmy_db_schema::{
+  newtypes::{CommunityId, DbUrl, InstanceId},
+  source::{activity::SentActivity, site::Site},
+  utils::{ActualDbPool, DbPool},
+};
+use lemmy_db_views_actor::structs::CommunityFollowerView;
+use once_cell::sync::Lazy;
+use reqwest::Url;
+use std::collections::{HashMap, HashSet};
+
+/// interval with which new additions to community_followers are queried.
+///
+/// The first time some user on an instance follows a specific remote community (or, more precisely:
+/// the first time a (followed_community_id, follower_inbox_url) tuple appears), this delay limits
+/// the maximum time until the follow actually results in activities from that community id being
+/// sent to that inbox url. This delay currently needs to not be too small because the DB load is
+/// currently fairly high because of the current structure of storing inboxes for every person, not
+/// having a separate list of shared_inboxes, and the architecture of having every instance queue be
+/// fully separate. (see https://github.com/LemmyNet/lemmy/issues/3958)
+static FOLLOW_ADDITIONS_RECHECK_DELAY: Lazy<chrono::TimeDelta> = Lazy::new(|| {
+  if *LEMMY_TEST_FAST_FEDERATION {
+    chrono::TimeDelta::try_seconds(1).expect("TimeDelta out of bounds")
+  } else {
+    chrono::TimeDelta::try_minutes(2).expect("TimeDelta out of bounds")
+  }
+});
+/// The same as FOLLOW_ADDITIONS_RECHECK_DELAY, but triggering when the last person on an instance
+/// unfollows a specific remote community. This is expected to happen pretty rarely and updating it
+/// in a timely manner is not too important.
+static FOLLOW_REMOVALS_RECHECK_DELAY: Lazy<chrono::TimeDelta> =
+  Lazy::new(|| chrono::TimeDelta::try_hours(1).expect("TimeDelta out of bounds"));
+
+#[async_trait]
+pub trait DataSource: Send + Sync {
+  async fn read_site_from_instance_id(
+    &self,
+    instance_id: InstanceId,
+  ) -> Result<Option<Site>, diesel::result::Error>;
+  async fn get_instance_followed_community_inboxes(
+    &self,
+    instance_id: InstanceId,
+    last_fetch: DateTime<Utc>,
+  ) -> Result<Vec<(CommunityId, DbUrl)>, diesel::result::Error>;
+}
+pub struct DbDataSource {
+  pool: ActualDbPool,
+}
+
+impl DbDataSource {
+  pub fn new(pool: ActualDbPool) -> Self {
+    Self { pool }
+  }
+}
+
+#[async_trait]
+impl DataSource for DbDataSource {
+  async fn read_site_from_instance_id(
+    &self,
+    instance_id: InstanceId,
+  ) -> Result<Option<Site>, diesel::result::Error> {
+    Site::read_from_instance_id(&mut DbPool::Pool(&self.pool), instance_id).await
+  }
+
+  async fn get_instance_followed_community_inboxes(
+    &self,
+    instance_id: InstanceId,
+    last_fetch: DateTime<Utc>,
+  ) -> Result<Vec<(CommunityId, DbUrl)>, diesel::result::Error> {
+    CommunityFollowerView::get_instance_followed_community_inboxes(
+      &mut DbPool::Pool(&self.pool),
+      instance_id,
+      last_fetch,
+    )
+    .await
+  }
+}
+
+pub(crate) struct CommunityInboxCollector<T: DataSource> {
+  // load site lazily because if an instance is first seen due to being on allowlist,
+  // the corresponding row in `site` may not exist yet since that is only added once
+  // `fetch_instance_actor_for_object` is called.
+  // (this should be unlikely to be relevant outside of the federation tests)
+  site_loaded: bool,
+  site: Option<Site>,
+  followed_communities: HashMap<CommunityId, HashSet<Url>>,
+  last_full_communities_fetch: DateTime<Utc>,
+  last_incremental_communities_fetch: DateTime<Utc>,
+  instance_id: InstanceId,
+  domain: String,
+  pub(crate) data_source: T,
+}
+
+pub type RealCommunityInboxCollector = CommunityInboxCollector<DbDataSource>;
+
+impl<T: DataSource> CommunityInboxCollector<T> {
+  pub fn new_real(
+    pool: ActualDbPool,
+    instance_id: InstanceId,
+    domain: String,
+  ) -> RealCommunityInboxCollector {
+    CommunityInboxCollector::new(DbDataSource::new(pool), instance_id, domain)
+  }
+  pub fn new(
+    data_source: T,
+    instance_id: InstanceId,
+    domain: String,
+  ) -> CommunityInboxCollector<T> {
+    CommunityInboxCollector {
+      data_source,
+      site_loaded: false,
+      site: None,
+      followed_communities: HashMap::new(),
+      last_full_communities_fetch: Utc.timestamp_nanos(0),
+      last_incremental_communities_fetch: Utc.timestamp_nanos(0),
+      instance_id,
+      domain,
+    }
+  }
+  /// get inbox urls of sending the given activity to the given instance
+  /// most often this will return 0 values (if instance doesn't care about the activity)
+  /// or 1 value (the shared inbox)
+  /// > 1 values only happens for non-lemmy software
+  pub async fn get_inbox_urls(&mut self, activity: &SentActivity) -> Result<Vec<Url>> {
+    let mut inbox_urls: HashSet<Url> = HashSet::new();
+
+    if activity.send_all_instances {
+      if !self.site_loaded {
+        self.site = self
+          .data_source
+          .read_site_from_instance_id(self.instance_id)
+          .await?;
+        self.site_loaded = true;
+      }
+      if let Some(site) = &self.site {
+        // Nutomic: Most non-lemmy software wont have a site row. That means it cant handle these
+        // activities. So handling it like this is fine.
+        inbox_urls.insert(site.inbox_url.inner().clone());
+      }
+    }
+    if let Some(t) = &activity.send_community_followers_of {
+      if let Some(urls) = self.followed_communities.get(t) {
+        inbox_urls.extend(urls.iter().cloned());
+      }
+    }
+    inbox_urls.extend(
+      activity
+        .send_inboxes
+        .iter()
+        .filter_map(std::option::Option::as_ref)
+        // a similar filter also happens within the activitypub-federation crate. but that filter
+        // happens much later - by doing it here, we can ensure that in the happy case, this
+        // function returns 0 urls which means the system doesn't have to create a tokio
+        // task for sending at all (since that task has a fair amount of overhead)
+        .filter(|&u| (u.domain() == Some(&self.domain)))
+        .map(|u| u.inner().clone()),
+    );
+    tracing::trace!(
+      "get_inbox_urls: {:?}, send_inboxes: {:?}",
+      inbox_urls,
+      activity.send_inboxes
+    );
+    Ok(inbox_urls.into_iter().collect())
+  }
+
+  pub async fn update_communities(&mut self) -> Result<()> {
+    if (Utc::now() - self.last_full_communities_fetch) > *FOLLOW_REMOVALS_RECHECK_DELAY {
+      tracing::debug!("{}: fetching full list of communities", self.domain);
+      // process removals every hour
+      (self.followed_communities, self.last_full_communities_fetch) = self
+        .get_communities(self.instance_id, Utc.timestamp_nanos(0))
+        .await?;
+      self.last_incremental_communities_fetch = self.last_full_communities_fetch;
+    }
+    if (Utc::now() - self.last_incremental_communities_fetch) > *FOLLOW_ADDITIONS_RECHECK_DELAY {
+      // process additions every minute
+      let (news, time) = self
+        .get_communities(self.instance_id, self.last_incremental_communities_fetch)
+        .await?;
+      if !news.is_empty() {
+        tracing::debug!(
+          "{}: fetched {} incremental new followed communities",
+          self.domain,
+          news.len()
+        );
+      }
+      self.followed_communities.extend(news);
+      self.last_incremental_communities_fetch = time;
+    }
+    Ok(())
+  }
+
+  /// get a list of local communities with the remote inboxes on the given instance that cares about
+  /// them
+  async fn get_communities(
+    &mut self,
+    instance_id: InstanceId,
+    last_fetch: DateTime<Utc>,
+  ) -> Result<(HashMap<CommunityId, HashSet<Url>>, DateTime<Utc>)> {
+    // update to time before fetch to ensure overlap. subtract some time to ensure overlap even if
+    // published date is not exact
+    let new_last_fetch = Utc::now() - *FOLLOW_ADDITIONS_RECHECK_DELAY / 2;
+
+    let inboxes = self
+      .data_source
+      .get_instance_followed_community_inboxes(instance_id, last_fetch)
+      .await?;
+
+    let map: HashMap<CommunityId, HashSet<Url>> =
+      inboxes.into_iter().fold(HashMap::new(), |mut map, (c, u)| {
+        map.entry(c).or_default().insert(u.into());
+        map
+      });
+
+    Ok((map, new_last_fetch))
+  }
+}
+
+#[cfg(test)]
+#[allow(clippy::unwrap_used)]
+#[allow(clippy::indexing_slicing)]
+mod tests {
+  use super::*;
+  use lemmy_db_schema::{
+    newtypes::{ActivityId, CommunityId, InstanceId, SiteId},
+    source::activity::{ActorType, SentActivity},
+  };
+  use mockall::{mock, predicate::*};
+  use serde_json::json;
+  mock! {
+      DataSource {}
+      #[async_trait]
+      impl DataSource for DataSource {
+          async fn read_site_from_instance_id(&self, instance_id: InstanceId) -> Result<Option<Site>, diesel::result::Error>;
+          async fn get_instance_followed_community_inboxes(
+              &self,
+              instance_id: InstanceId,
+              last_fetch: DateTime<Utc>,
+          ) -> Result<Vec<(CommunityId, DbUrl)>, diesel::result::Error>;
+      }
+  }
+
+  fn setup_collector() -> CommunityInboxCollector<MockDataSource> {
+    let mock_data_source = MockDataSource::new();
+    let instance_id = InstanceId(1);
+    let domain = "example.com".to_string();
+    CommunityInboxCollector::new(mock_data_source, instance_id, domain)
+  }
+
+  #[tokio::test]
+  async fn test_get_inbox_urls_empty() {
+    let mut collector = setup_collector();
+    let activity = SentActivity {
+      id: ActivityId(1),
+      ap_id: Url::parse("https://example.com/activities/1")
+        .unwrap()
+        .into(),
+      data: json!({}),
+      sensitive: false,
+      published: Utc::now(),
+      send_inboxes: vec![],
+      send_community_followers_of: None,
+      send_all_instances: false,
+      actor_type: ActorType::Person,
+      actor_apub_id: None,
+    };
+
+    let result = collector.get_inbox_urls(&activity).await.unwrap();
+    assert!(result.is_empty());
+  }
+
+  #[tokio::test]
+  async fn test_get_inbox_urls_send_all_instances() {
+    let mut collector = setup_collector();
+    let site_inbox = Url::parse("https://example.com/inbox").unwrap();
+    let site = Site {
+      id: SiteId(1),
+      name: "Test Site".to_string(),
+      sidebar: None,
+      published: Utc::now(),
+      updated: None,
+      icon: None,
+      banner: None,
+      description: None,
+      actor_id: Url::parse("https://example.com/site").unwrap().into(),
+      last_refreshed_at: Utc::now(),
+      inbox_url: site_inbox.clone().into(),
+      private_key: None,
+      public_key: "test_key".to_string(),
+      instance_id: InstanceId(1),
+      content_warning: None,
+    };
+
+    collector
+      .data_source
+      .expect_read_site_from_instance_id()
+      .return_once(move |_| Ok(Some(site)));
+
+    let activity = SentActivity {
+      id: ActivityId(1),
+      ap_id: Url::parse("https://example.com/activities/1")
+        .unwrap()
+        .into(),
+      data: json!({}),
+      sensitive: false,
+      published: Utc::now(),
+      send_inboxes: vec![],
+      send_community_followers_of: None,
+      send_all_instances: true,
+      actor_type: ActorType::Person,
+      actor_apub_id: None,
+    };
+
+    let result = collector.get_inbox_urls(&activity).await.unwrap();
+    assert_eq!(result.len(), 1);
+    assert_eq!(result[0], site_inbox);
+  }
+
+  #[tokio::test]
+  async fn test_get_inbox_urls_community_followers() {
+    let mut collector = setup_collector();
+    let community_id = CommunityId(1);
+    let url1 = "https://follower1.example.com/inbox";
+    let url2 = "https://follower2.example.com/inbox";
+
+    collector
+      .data_source
+      .expect_get_instance_followed_community_inboxes()
+      .return_once(move |_, _| {
+        Ok(vec![
+          (community_id, Url::parse(url1).unwrap().into()),
+          (community_id, Url::parse(url2).unwrap().into()),
+        ])
+      });
+
+    collector.update_communities().await.unwrap();
+
+    let activity = SentActivity {
+      id: ActivityId(1),
+      ap_id: Url::parse("https://example.com/activities/1")
+        .unwrap()
+        .into(),
+      data: json!({}),
+      sensitive: false,
+      published: Utc::now(),
+      send_inboxes: vec![],
+      send_community_followers_of: Some(community_id),
+      send_all_instances: false,
+      actor_type: ActorType::Person,
+      actor_apub_id: None,
+    };
+
+    let result = collector.get_inbox_urls(&activity).await.unwrap();
+    assert_eq!(result.len(), 2);
+    assert!(result.contains(&Url::parse(url1).unwrap()));
+    assert!(result.contains(&Url::parse(url2).unwrap()));
+  }
+
+  #[tokio::test]
+  async fn test_get_inbox_urls_send_inboxes() {
+    let mut collector = setup_collector();
+    collector.domain = "example.com".to_string();
+    let inbox_user_1 = Url::parse("https://example.com/user1/inbox").unwrap();
+    let inbox_user_2 = Url::parse("https://example.com/user2/inbox").unwrap();
+    let other_domain_inbox = Url::parse("https://other-domain.com/user3/inbox").unwrap();
+    let activity = SentActivity {
+      id: ActivityId(1),
+      ap_id: Url::parse("https://example.com/activities/1")
+        .unwrap()
+        .into(),
+      data: json!({}),
+      sensitive: false,
+      published: Utc::now(),
+      send_inboxes: vec![
+        Some(inbox_user_1.clone().into()),
+        Some(inbox_user_2.clone().into()),
+        Some(other_domain_inbox.clone().into()),
+      ],
+      send_community_followers_of: None,
+      send_all_instances: false,
+      actor_type: ActorType::Person,
+      actor_apub_id: None,
+    };
+
+    let result = collector.get_inbox_urls(&activity).await.unwrap();
+    assert_eq!(result.len(), 2);
+    assert!(result.contains(&inbox_user_1));
+    assert!(result.contains(&inbox_user_2));
+    assert!(!result.contains(&other_domain_inbox));
+  }
+
+  #[tokio::test]
+  async fn test_get_inbox_urls_combined() {
+    let mut collector = setup_collector();
+    collector.domain = "example.com".to_string();
+    let community_id = CommunityId(1);
+
+    let site_inbox = Url::parse("https://example.com/site_inbox").unwrap();
+    let site = Site {
+      id: SiteId(1),
+      name: "Test Site".to_string(),
+      sidebar: None,
+      published: Utc::now(),
+      updated: None,
+      icon: None,
+      banner: None,
+      description: None,
+      actor_id: Url::parse("https://example.com/site").unwrap().into(),
+      last_refreshed_at: Utc::now(),
+      inbox_url: site_inbox.clone().into(),
+      private_key: None,
+      public_key: "test_key".to_string(),
+      instance_id: InstanceId(1),
+      content_warning: None,
+    };
+
+    collector
+      .data_source
+      .expect_read_site_from_instance_id()
+      .return_once(move |_| Ok(Some(site)));
+
+    let subdomain_inbox = "https://follower.example.com/inbox";
+    collector
+      .data_source
+      .expect_get_instance_followed_community_inboxes()
+      .return_once(move |_, _| {
+        Ok(vec![(
+          community_id,
+          Url::parse(subdomain_inbox).unwrap().into(),
+        )])
+      });
+
+    collector.update_communities().await.unwrap();
+    let user1_inbox = Url::parse("https://example.com/user1/inbox").unwrap();
+    let user2_inbox = Url::parse("https://other-domain.com/user2/inbox").unwrap();
+    let activity = SentActivity {
+      id: ActivityId(1),
+      ap_id: Url::parse("https://example.com/activities/1")
+        .unwrap()
+        .into(),
+      data: json!({}),
+      sensitive: false,
+      published: Utc::now(),
+      send_inboxes: vec![
+        Some(user1_inbox.clone().into()),
+        Some(user2_inbox.clone().into()),
+      ],
+      send_community_followers_of: Some(community_id),
+      send_all_instances: true,
+      actor_type: ActorType::Person,
+      actor_apub_id: None,
+    };
+
+    let result = collector.get_inbox_urls(&activity).await.unwrap();
+    assert_eq!(result.len(), 3);
+    assert!(result.contains(&site_inbox));
+    assert!(result.contains(&Url::parse(subdomain_inbox).unwrap()));
+    assert!(result.contains(&user1_inbox));
+    assert!(!result.contains(&user2_inbox));
+  }
+
+  #[tokio::test]
+  async fn test_update_communities() {
+    let mut collector = setup_collector();
+    let community_id1 = CommunityId(1);
+    let community_id2 = CommunityId(2);
+    let community_id3 = CommunityId(3);
+
+    let user1_inbox_str = "https://follower1.example.com/inbox";
+    let user1_inbox = Url::parse(user1_inbox_str).unwrap();
+    let user2_inbox_str = "https://follower2.example.com/inbox";
+    let user2_inbox = Url::parse(user2_inbox_str).unwrap();
+    let user3_inbox_str = "https://follower3.example.com/inbox";
+    let user3_inbox = Url::parse(user3_inbox_str).unwrap();
+
+    collector
+      .data_source
+      .expect_get_instance_followed_community_inboxes()
+      .times(2)
+      .returning(move |_, last_fetch| {
+        if last_fetch == Utc.timestamp_nanos(0) {
+          Ok(vec![
+            (community_id1, Url::parse(user1_inbox_str).unwrap().into()),
+            (community_id2, Url::parse(user2_inbox_str).unwrap().into()),
+          ])
+        } else {
+          Ok(vec![(
+            community_id3,
+            Url::parse(user3_inbox_str).unwrap().into(),
+          )])
+        }
+      });
+
+    // First update
+    collector.update_communities().await.unwrap();
+    assert_eq!(collector.followed_communities.len(), 2);
+    assert!(collector.followed_communities[&community_id1].contains(&user1_inbox));
+    assert!(collector.followed_communities[&community_id2].contains(&user2_inbox));
+
+    // Simulate time passing
+    collector.last_full_communities_fetch = Utc::now() - chrono::TimeDelta::try_minutes(3).unwrap();
+    collector.last_incremental_communities_fetch =
+      Utc::now() - chrono::TimeDelta::try_minutes(3).unwrap();
+
+    // Second update (incremental)
+    collector.update_communities().await.unwrap();
+    assert_eq!(collector.followed_communities.len(), 3);
+    assert!(collector.followed_communities[&community_id1].contains(&user1_inbox));
+    assert!(collector.followed_communities[&community_id3].contains(&user3_inbox));
+    assert!(collector.followed_communities[&community_id2].contains(&user2_inbox));
+  }
+
+  #[tokio::test]
+  async fn test_get_inbox_urls_no_duplicates() {
+    let mut collector = setup_collector();
+    collector.domain = "example.com".to_string();
+    let community_id = CommunityId(1);
+    let site_inbox = Url::parse("https://example.com/site_inbox").unwrap();
+    let site_inbox_clone = site_inbox.clone();
+    let site = Site {
+      id: SiteId(1),
+      name: "Test Site".to_string(),
+      sidebar: None,
+      published: Utc::now(),
+      updated: None,
+      icon: None,
+      banner: None,
+      description: None,
+      actor_id: Url::parse("https://example.com/site").unwrap().into(),
+      last_refreshed_at: Utc::now(),
+      inbox_url: site_inbox.clone().into(),
+      private_key: None,
+      public_key: "test_key".to_string(),
+      instance_id: InstanceId(1),
+      content_warning: None,
+    };
+
+    collector
+      .data_source
+      .expect_read_site_from_instance_id()
+      .return_once(move |_| Ok(Some(site)));
+
+    collector
+      .data_source
+      .expect_get_instance_followed_community_inboxes()
+      .return_once(move |_, _| Ok(vec![(community_id, site_inbox_clone.into())]));
+
+    collector.update_communities().await.unwrap();
+
+    let activity = SentActivity {
+      id: ActivityId(1),
+      ap_id: Url::parse("https://example.com/activities/1")
+        .unwrap()
+        .into(),
+      data: json!({}),
+      sensitive: false,
+      published: Utc::now(),
+      send_inboxes: vec![Some(site_inbox.into())],
+      send_community_followers_of: Some(community_id),
+      send_all_instances: true,
+      actor_type: ActorType::Person,
+      actor_apub_id: None,
+    };
+
+    let result = collector.get_inbox_urls(&activity).await.unwrap();
+    assert_eq!(result.len(), 1);
+    assert!(result.contains(&Url::parse("https://example.com/site_inbox").unwrap()));
+  }
+}

crates/federate/src/lib.rs

@@ -1,6 +1,9 @@
 use crate::{util::CancellableTask, worker::InstanceWorker};
 use activitypub_federation::config::FederationConfig;
-use lemmy_api_common::context::LemmyContext;
+use lemmy_api_common::{
+  context::LemmyContext,
+  lemmy_utils::settings::structs::FederationWorkerConfig,
+};
 use lemmy_db_schema::{newtypes::InstanceId, source::instance::Instance};
 use lemmy_utils::error::LemmyResult;
 use stats::receive_print_stats;
@@ -14,6 +17,8 @@ use tokio_util::sync::CancellationToken;
 use tracing::info;
 use util::FederationQueueStateWithDomain;
 
+mod inboxes;
+mod send;
 mod stats;
 mod util;
 mod worker;
@@ -38,10 +43,15 @@ pub struct SendManager {
   context: FederationConfig<LemmyContext>,
   stats_sender: UnboundedSender<FederationQueueStateWithDomain>,
   exit_print: JoinHandle<()>,
+  federation_worker_config: FederationWorkerConfig,
 }
 
 impl SendManager {
-  fn new(opts: Opts, context: FederationConfig<LemmyContext>) -> Self {
+  fn new(
+    opts: Opts,
+    context: FederationConfig<LemmyContext>,
+    federation_worker_config: FederationWorkerConfig,
+  ) -> Self {
     assert!(opts.process_count > 0);
     assert!(opts.process_index > 0);
     assert!(opts.process_index <= opts.process_count);
@@ -56,14 +66,20 @@ impl SendManager {
         stats_receiver,
       )),
       context,
+      federation_worker_config,
     }
   }
 
-  pub fn run(opts: Opts, context: FederationConfig<LemmyContext>) -> CancellableTask {
+  pub fn run(
+    opts: Opts,
+    context: FederationConfig<LemmyContext>,
+    config: FederationWorkerConfig,
+  ) -> CancellableTask {
     CancellableTask::spawn(WORKER_EXIT_TIMEOUT, move |cancel| {
       let opts = opts.clone();
+      let config = config.clone();
       let context = context.clone();
-      let mut manager = Self::new(opts, context);
+      let mut manager = Self::new(opts, context, config);
       async move {
         let result = manager.do_loop(cancel).await;
         // the loop function will only return if there is (a) an internal error (e.g. db connection
@@ -120,22 +136,21 @@ impl SendManager {
           // create new worker
           let context = self.context.clone();
           let stats_sender = self.stats_sender.clone();
+          let federation_worker_config = self.federation_worker_config.clone();
+
           self.workers.insert(
             instance.id,
             CancellableTask::spawn(WORKER_EXIT_TIMEOUT, move |stop| {
-              // if the instance worker ends unexpectedly due to internal/db errors, this lambda is rerun by cancellabletask.
+              // if the instance worker ends unexpectedly due to internal/db errors, this lambda is
+              // rerun by cancellabletask.
               let instance = instance.clone();
-              let req_data = context.to_request_data();
+              InstanceWorker::init_and_loop(
-              let stats_sender = stats_sender.clone();
+                instance,
-              async move {
+                context.clone(),
-                InstanceWorker::init_and_loop(
+                federation_worker_config.clone(),
-                  instance,
+                stop,
-                  req_data,
+                stats_sender.clone(),
-                  stop,
+              )
-                  stats_sender,
-                )
-                .await
-              }
             }),
           );
         } else if !should_federate {
@@ -214,7 +229,14 @@ mod test {
         .app_data(context.clone())
         .build()
         .await?;
+      let concurrent_sends_per_instance = std::env::var("LEMMY_TEST_FEDERATION_CONCURRENT_SENDS")
+        .ok()
+        .and_then(|s| s.parse().ok())
+        .unwrap_or(1);
 
+      let federation_worker_config = FederationWorkerConfig {
+        concurrent_sends_per_instance,
+      };
       let pool = &mut context.pool();
       let instances = vec![
         Instance::read_or_create(pool, "alpha.com".to_string()).await?,
@@ -222,7 +244,7 @@ mod test {
         Instance::read_or_create(pool, "gamma.com".to_string()).await?,
       ];
 
-      let send_manager = SendManager::new(opts, federation_config);
+      let send_manager = SendManager::new(opts, federation_config, federation_worker_config);
       Ok(Self {
         send_manager,
         context,

crates/federate/src/send.rs

@@ -0,0 +1,148 @@
+use crate::util::get_actor_cached;
+use activitypub_federation::{
+  activity_sending::SendActivityTask,
+  config::Data,
+  protocol::context::WithContext,
+};
+use anyhow::{Context, Result};
+use chrono::{DateTime, Utc};
+use lemmy_api_common::{context::LemmyContext, federate_retry_sleep_duration};
+use lemmy_apub::{activity_lists::SharedInboxActivities, FEDERATION_CONTEXT};
+use lemmy_db_schema::{newtypes::ActivityId, source::activity::SentActivity};
+use reqwest::Url;
+use std::ops::Deref;
+use tokio::{sync::mpsc::UnboundedSender, time::sleep};
+use tokio_util::sync::CancellationToken;
+
+#[derive(Debug, Eq)]
+pub(crate) struct SendSuccessInfo {
+  pub activity_id: ActivityId,
+  pub published: Option<DateTime<Utc>>,
+  // true if the activity was skipped because the target instance is not interested in this
+  // activity
+  pub was_skipped: bool,
+}
+impl PartialEq for SendSuccessInfo {
+  fn eq(&self, other: &Self) -> bool {
+    self.activity_id == other.activity_id
+  }
+}
+/// order backwards because the binary heap is a max heap, and we need the smallest element to be on
+/// top
+impl PartialOrd for SendSuccessInfo {
+  fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+    Some(self.cmp(other))
+  }
+}
+impl Ord for SendSuccessInfo {
+  fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+    other.activity_id.cmp(&self.activity_id)
+  }
+}
+
+/// Represents the result of sending an activity.
+///
+/// This enum is used to communicate the outcome of a send operation from a send task
+/// to the main instance worker. It's designed to maintain a clean separation between
+/// the send task and the main thread, allowing the send.rs file to be self-contained
+/// and easier to understand.
+///
+/// The use of a channel for communication (rather than shared atomic variables) was chosen
+/// because:
+/// 1. It keeps the send task cleanly separated with no direct interaction with the main thread.
+/// 2. The failure event needs to be transferred to the main task for database updates anyway.
+/// 3. The main fail_count should only be updated under certain conditions, which are best handled
+///    in the main task.
+/// 4. It maintains consistency in how data is communicated (all via channels rather than a mix of
+///    channels and atomics).
+/// 5. It simplifies concurrency management and makes the flow of data more predictable.
+pub(crate) enum SendActivityResult {
+  Success(SendSuccessInfo),
+  Failure { fail_count: i32 },
+}
+/// Represents a task for retrying to send an activity.
+///
+/// This struct encapsulates all the necessary information and resources for attempting
+/// to send an activity to multiple inbox URLs, with built-in retry logic.
+pub(crate) struct SendRetryTask<'a> {
+  pub activity: &'a SentActivity,
+  pub object: &'a SharedInboxActivities,
+  /// Must not be empty at this point
+  pub inbox_urls: Vec<Url>,
+  /// Channel to report results back to the main instance worker
+  pub report: &'a mut UnboundedSender<SendActivityResult>,
+  /// The first request will be sent immediately, but subsequent requests will be delayed
+  /// according to the number of previous fails + 1
+  ///
+  /// This is a read-only immutable variable that is passed only one way, from the main
+  /// thread to each send task. It allows the task to determine how long to sleep initially
+  /// if the request fails.
+  pub initial_fail_count: i32,
+  /// For logging purposes
+  pub domain: String,
+  pub context: Data<LemmyContext>,
+  pub stop: CancellationToken,
+}
+
+impl<'a> SendRetryTask<'a> {
+  // this function will return successfully when (a) send succeeded or (b) worker cancelled
+  // and will return an error if an internal error occurred (send errors cause an infinite loop)
+  pub async fn send_retry_loop(self) -> Result<()> {
+    let SendRetryTask {
+      activity,
+      object,
+      inbox_urls,
+      report,
+      initial_fail_count,
+      domain,
+      context,
+      stop,
+    } = self;
+    debug_assert!(!inbox_urls.is_empty());
+
+    let pool = &mut context.pool();
+    let Some(actor_apub_id) = &activity.actor_apub_id else {
+      return Err(anyhow::anyhow!("activity is from before lemmy 0.19"));
+    };
+    let actor = get_actor_cached(pool, activity.actor_type, actor_apub_id)
+      .await
+      .context("failed getting actor instance (was it marked deleted / removed?)")?;
+
+    let object = WithContext::new(object.clone(), FEDERATION_CONTEXT.deref().clone());
+    let requests = SendActivityTask::prepare(&object, actor.as_ref(), inbox_urls, &context).await?;
+    for task in requests {
+      // usually only one due to shared inbox
+      tracing::debug!("sending out {}", task);
+      let mut fail_count = initial_fail_count;
+      while let Err(e) = task.sign_and_send(&context).await {
+        fail_count += 1;
+        report.send(SendActivityResult::Failure {
+          fail_count,
+          // activity_id: activity.id,
+        })?;
+        let retry_delay = federate_retry_sleep_duration(fail_count);
+        tracing::info!(
+          "{}: retrying {:?} attempt {} with delay {retry_delay:.2?}. ({e})",
+          domain,
+          activity.id,
+          fail_count
+        );
+        tokio::select! {
+          () = sleep(retry_delay) => {},
+          () = stop.cancelled() => {
+            // cancel sending without reporting any result.
+            // the InstanceWorker needs to be careful to not hang on receive of that
+            // channel when cancelled (see handle_send_results)
+            return Ok(());
+          }
+        }
+      }
+    }
+    report.send(SendActivityResult::Success(SendSuccessInfo {
+      activity_id: activity.id,
+      published: Some(activity.published),
+      was_skipped: false,
+    }))?;
+    Ok(())
+  }
+}

crates/federate/src/util.rs

@@ -1,7 +1,6 @@
 use anyhow::{anyhow, Context, Result};
 use diesel::prelude::*;
 use diesel_async::RunQueryDsl;
-use lemmy_api_common::lemmy_utils::CACHE_DURATION_FEDERATION;
 use lemmy_apub::{
   activity_lists::SharedInboxActivities,
   fetcher::{site_or_community_or_user::SiteOrCommunityOrUser, user_or_community::UserOrCommunity},
@@ -28,19 +27,28 @@ use tokio_util::sync::CancellationToken;
 
 /// Decrease the delays of the federation queue.
 /// Should only be used for federation tests since it significantly increases CPU and DB load of the
-/// federation queue.
+/// federation queue. This is intentionally a separate flag from other flags like debug_assertions,
+/// since this is a invasive change we only need rarely.
 pub(crate) static LEMMY_TEST_FAST_FEDERATION: Lazy<bool> = Lazy::new(|| {
   std::env::var("LEMMY_TEST_FAST_FEDERATION")
     .map(|s| !s.is_empty())
     .unwrap_or(false)
 });
 
-/// Recheck for new federation work every n seconds.
+/// Recheck for new federation work every n seconds within each InstanceWorker.
 ///
 /// When the queue is processed faster than new activities are added and it reaches the current time
 /// with an empty batch, this is the delay the queue waits before it checks if new activities have
 /// been added to the sent_activities table. This delay is only applied if no federated activity
-/// happens during sending activities of the last batch.
+/// happens during sending activities of the last batch, which means on high-activity instances it
+/// may never be used. This means that it does not affect the maximum throughput of the queue.
+///
+///
+/// This is thus the interval with which tokio wakes up each of the
+/// InstanceWorkers to check for new work, if the queue previously was empty.
+/// If the delay is too short, the workers (one per federated instance) will wake up too
+/// often and consume a lot of CPU. If the delay is long, then activities on low-traffic instances
+/// will on average take delay/2 seconds to federate.
 pub(crate) static WORK_FINISHED_RECHECK_DELAY: Lazy<Duration> = Lazy::new(|| {
   if *LEMMY_TEST_FAST_FEDERATION {
     Duration::from_millis(100)
@@ -49,6 +57,21 @@ pub(crate) static WORK_FINISHED_RECHECK_DELAY: Lazy<Duration> = Lazy::new(|| {
   }
 });
 
+/// Cache the latest activity id for a certain duration.
+///
+/// This cache is common to all the instance workers and prevents there from being more than one
+/// call per N seconds between each DB query to find max(activity_id).
+pub(crate) static CACHE_DURATION_LATEST_ID: Lazy<Duration> = Lazy::new(|| {
+  if *LEMMY_TEST_FAST_FEDERATION {
+    // in test mode, we use the same cache duration as the recheck delay so when recheck happens
+    // data is fresh, accelerating the time the tests take.
+    *WORK_FINISHED_RECHECK_DELAY
+  } else {
+    // in normal mode, we limit the query to one per second
+    Duration::from_secs(1)
+  }
+});
+
 /// A task that will be run in an infinite loop, unless it is cancelled.
 /// If the task exits without being cancelled, an error will be logged and the task will be
 /// restarted.
@@ -174,7 +197,7 @@ pub(crate) async fn get_activity_cached(
 pub(crate) async fn get_latest_activity_id(pool: &mut DbPool<'_>) -> Result<ActivityId> {
   static CACHE: Lazy<Cache<(), ActivityId>> = Lazy::new(|| {
     Cache::builder()
-      .time_to_live(CACHE_DURATION_FEDERATION)
+      .time_to_live(*CACHE_DURATION_LATEST_ID)
       .build()
   });
   CACHE

crates/utils/src/settings/structs.rs

@@ -43,12 +43,8 @@ pub struct Settings {
   #[default(None)]
   #[doku(skip)]
   pub opentelemetry_url: Option<Url>,
-  /// The number of activitypub federation workers that can be in-flight concurrently
+  #[default(Default::default())]
-  #[default(0)]
+  pub federation: FederationWorkerConfig,
-  pub worker_count: usize,
-  /// The number of activitypub federation retry workers that can be in-flight concurrently
-  #[default(0)]
-  pub retry_count: usize,
   // Prometheus configuration.
   #[default(None)]
   #[doku(example = "Some(Default::default())")]
@@ -237,3 +233,14 @@ pub struct PrometheusConfig {
   #[doku(example = "10002")]
   pub port: i32,
 }
+
+#[derive(Debug, Deserialize, Serialize, Clone, SmartDefault, Document)]
+#[serde(default)]
+// named federation"worker"config to disambiguate from the activitypub library configuration
+pub struct FederationWorkerConfig {
+  /// Limit to the number of concurrent outgoing federation requests per target instance.
+  /// Set this to a higher value than 1 (e.g. 6) only if you have a huge instance (>10 activities
+  /// per second) and if a receiving instance is not keeping up.
+  #[default(1)]
+  pub concurrent_sends_per_instance: i64,
+}

scripts/test.sh

@@ -14,6 +14,7 @@ source scripts/start_dev_db.sh
 # so to load the config we need to traverse to the repo root
 export LEMMY_CONFIG_LOCATION=../../config/config.hjson
 export RUST_BACKTRACE=1
+export LEMMY_TEST_FAST_FEDERATION=1 # by default, the persistent federation queue has delays in the scale of 30s-5min
 
 if [ -n "$PACKAGE" ];
 then

src/lib.rs

@@ -233,6 +233,7 @@ pub async fn start_lemmy_server(args: CmdArgs) -> LemmyResult<()> {
         process_count: args.federate_process_count,
       },
       cfg,
+      SETTINGS.federation.clone(),
     )
   });
   let mut interrupt = tokio::signal::unix::signal(SignalKind::interrupt())?;