diff --git a/src/client.ts b/src/client.ts
index abb5fdce..e3361163 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -1,5 +1,6 @@
 import { Conf } from '@/config.ts';
 import { type Event, type Filter, matchFilters, RelayPool, TTLCache } from '@/deps.ts';
+import * as pipeline from '@/pipeline.ts';
 import { Time } from '@/utils.ts';
 
 import type { GetFiltersOpts } from '@/types.ts';
@@ -37,6 +38,7 @@ function getFilters<K extends number>(filters: Filter<K>[], opts: GetFiltersOpts
       Conf.poolRelays,
       (event: Event | null) => {
         if (event && matchFilters(filters, event)) {
+          pipeline.handleEvent(event).catch(() => {});
           results.push({
             id: event.id,
             kind: event.kind,
@@ -70,14 +72,4 @@ function getFilters<K extends number>(filters: Filter<K>[], opts: GetFiltersOpts
   });
 }
 
-/** Publish an event to the Nostr relay. */
-function publish(event: Event, relays = Conf.publishRelays): void {
-  console.log('Publishing event', event, relays);
-  try {
-    getPool().publish(event, relays);
-  } catch (e) {
-    console.error(e);
-  }
-}
-
-export { getFilters, publish };
+export { getFilters };
diff --git a/src/controllers/api/accounts.ts b/src/controllers/api/accounts.ts
index 544447b2..da464efe 100644
--- a/src/controllers/api/accounts.ts
+++ b/src/controllers/api/accounts.ts
@@ -1,7 +1,7 @@
 import { type AppController } from '@/app.ts';
 import { type Filter, findReplyTag, z } from '@/deps.ts';
-import { publish } from '@/client.ts';
 import * as mixer from '@/mixer.ts';
+import * as pipeline from '@/pipeline.ts';
 import { getAuthor, getFollows } from '@/queries.ts';
 import { jsonMetaContentSchema } from '@/schemas/nostr.ts';
 import { signEvent } from '@/sign.ts';
@@ -167,7 +167,13 @@ const updateCredentialsController: AppController = async (c) => {
     created_at: nostrNow(),
   }, c);
 
-  publish(event);
+  try {
+    await pipeline.handleEvent(event);
+  } catch (e) {
+    if (e instanceof pipeline.RelayError) {
+      return c.json({ error: e.message }, 422);
+    }
+  }
 
   const account = await toAccount(event);
   return c.json(account);
diff --git a/src/controllers/api/statuses.ts b/src/controllers/api/statuses.ts
index 2f594ac7..aae7f6cc 100644
--- a/src/controllers/api/statuses.ts
+++ b/src/controllers/api/statuses.ts
@@ -1,6 +1,6 @@
 import { type AppController } from '@/app.ts';
-import { publish } from '@/client.ts';
 import { ISO6391, Kind, z } from '@/deps.ts';
+import * as pipeline from '@/pipeline.ts';
 import { getAncestors, getDescendants, getEvent } from '@/queries.ts';
 import { signEvent } from '@/sign.ts';
 import { toStatus } from '@/transformers/nostr-to-mastoapi.ts';
@@ -77,7 +77,13 @@ const createStatusController: AppController = async (c) => {
       created_at: nostrNow(),
     }, c);
 
-    publish(event);
+    try {
+      await pipeline.handleEvent(event);
+    } catch (e) {
+      if (e instanceof pipeline.RelayError) {
+        return c.json({ error: e.message }, 422);
+      }
+    }
 
     return c.json(await toStatus(event));
   } else {
@@ -118,7 +124,13 @@ const favouriteController: AppController = async (c) => {
       created_at: nostrNow(),
     }, c);
 
-    publish(event);
+    try {
+      await pipeline.handleEvent(event);
+    } catch (e) {
+      if (e instanceof pipeline.RelayError) {
+        return c.json({ error: e.message }, 422);
+      }
+    }
 
     const status = await toStatus(target);
 
diff --git a/src/controllers/nostr/relay.ts b/src/controllers/nostr/relay.ts
index 24316e7c..8befa566 100644
--- a/src/controllers/nostr/relay.ts
+++ b/src/controllers/nostr/relay.ts
@@ -1,5 +1,5 @@
 import * as eventsDB from '@/db/events.ts';
-import { findUser } from '@/db/users.ts';
+import * as pipeline from '@/pipeline.ts';
 import { jsonSchema } from '@/schema.ts';
 import {
   type ClientCLOSE,
@@ -15,12 +15,14 @@ import type { Event, Filter } from '@/deps.ts';
 /** Limit of events returned per-filter. */
 const FILTER_LIMIT = 100;
 
+/** NIP-01 relay to client message. */
 type RelayMsg =
   | ['EVENT', string, Event]
   | ['NOTICE', string]
   | ['EOSE', string]
   | ['OK', string, boolean, string];
 
+/** Set up the Websocket connection. */
 function connectStream(socket: WebSocket) {
   socket.onmessage = (e) => {
     const result = jsonSchema.pipe(clientMsgSchema).safeParse(e.data);
@@ -31,6 +33,7 @@ function connectStream(socket: WebSocket) {
     }
   };
 
+  /** Handle client message. */
   function handleMsg(msg: ClientMsg) {
     switch (msg[0]) {
       case 'REQ':
@@ -45,6 +48,7 @@ function connectStream(socket: WebSocket) {
     }
   }
 
+  /** Handle REQ. Start a subscription. */
   async function handleReq([_, sub, ...filters]: ClientREQ) {
     for (const event of await eventsDB.getFilters(prepareFilters(filters))) {
       send(['EVENT', sub, event]);
@@ -52,20 +56,28 @@ function connectStream(socket: WebSocket) {
     send(['EOSE', sub]);
   }
 
+  /** Handle EVENT. Store the event. */
   async function handleEvent([_, event]: ClientEVENT) {
-    if (await findUser({ pubkey: event.pubkey })) {
-      eventsDB.insertEvent(event);
+    try {
+      // This will store it (if eligible) and run other side-effects.
+      await pipeline.handleEvent(event);
       send(['OK', event.id, true, '']);
-    } else {
-      send(['OK', event.id, false, 'blocked: only registered users can post']);
+    } catch (e) {
+      if (e instanceof pipeline.RelayError) {
+        send(['OK', event.id, false, e.message]);
+      } else {
+        send(['OK', event.id, false, 'error: something went wrong']);
+      }
     }
   }
 
+  /** Handle CLOSE. Close the subscription. */
   function handleClose([_, _sub]: ClientCLOSE) {
     // TODO: ???
     return;
   }
 
+  /** Send a message back to the client. */
   function send(msg: RelayMsg) {
     return socket.send(JSON.stringify(msg));
   }
diff --git a/src/db/events.ts b/src/db/events.ts
index cb04b235..7910a400 100644
--- a/src/db/events.ts
+++ b/src/db/events.ts
@@ -119,16 +119,4 @@ async function getFilters<K extends number>(
   ));
 }
 
-/** Returns whether the pubkey is followed by a local user. */
-async function isLocallyFollowed(pubkey: string): Promise<boolean> {
-  return Boolean(
-    await getFilterQuery({
-      kinds: [3],
-      '#p': [pubkey],
-      limit: 1,
-      local: true,
-    }).executeTakeFirst(),
-  );
-}
-
-export { getFilters, insertEvent, isLocallyFollowed };
+export { getFilters, insertEvent };
diff --git a/src/firehose.ts b/src/firehose.ts
index c717c075..57cc1022 100644
--- a/src/firehose.ts
+++ b/src/firehose.ts
@@ -1,9 +1,8 @@
-import { insertEvent, isLocallyFollowed } from '@/db/events.ts';
-import { addRelays, getActiveRelays } from '@/db/relays.ts';
-import { findUser } from '@/db/users.ts';
+import { getActiveRelays } from '@/db/relays.ts';
 import { type Event, RelayPool } from '@/deps.ts';
-import { trends } from '@/trends.ts';
-import { isRelay, nostrDate, nostrNow } from '@/utils.ts';
+import { nostrNow } from '@/utils.ts';
+
+import * as pipeline from './pipeline.ts';
 
 const relays = await getActiveRelays();
 const pool = new RelayPool(relays);
@@ -20,48 +19,10 @@ pool.subscribe(
 );
 
 /** Handle events through the firehose pipeline. */
-async function handleEvent(event: Event): Promise<void> {
+function handleEvent(event: Event): Promise<void> {
   console.info(`firehose: Event<${event.kind}> ${event.id}`);
 
-  trackHashtags(event);
-  trackRelays(event);
-
-  if (await findUser({ pubkey: event.pubkey }) || await isLocallyFollowed(event.pubkey)) {
-    insertEvent(event).catch(console.warn);
-  }
-}
-
-/** Track whenever a hashtag is used, for processing trending tags. */
-function trackHashtags(event: Event): void {
-  const date = nostrDate(event.created_at);
-
-  const tags = event.tags
-    .filter((tag) => tag[0] === 't')
-    .map((tag) => tag[1])
-    .slice(0, 5);
-
-  if (!tags.length) return;
-
-  try {
-    console.info('tracking tags:', tags);
-    trends.addTagUsages(event.pubkey, tags, date);
-  } catch (_e) {
-    // do nothing
-  }
-}
-
-/** Tracks known relays in the database. */
-function trackRelays(event: Event) {
-  const relays = new Set<`wss://${string}`>();
-
-  event.tags.forEach((tag) => {
-    if (['p', 'e', 'a'].includes(tag[0]) && isRelay(tag[2])) {
-      relays.add(tag[2]);
-    }
-    if (event.kind === 10002 && tag[0] === 'r' && isRelay(tag[1])) {
-      relays.add(tag[1]);
-    }
-  });
-
-  return addRelays([...relays]);
+  return pipeline
+    .handleEvent(event)
+    .catch(() => {});
 }
diff --git a/src/pipeline.ts b/src/pipeline.ts
new file mode 100644
index 00000000..af72b794
--- /dev/null
+++ b/src/pipeline.ts
@@ -0,0 +1,73 @@
+import * as eventsDB from '@/db/events.ts';
+import { addRelays } from '@/db/relays.ts';
+import { findUser } from '@/db/users.ts';
+import { type Event } from '@/deps.ts';
+import { isLocallyFollowed } from '@/queries.ts';
+import { trends } from '@/trends.ts';
+import { isRelay, nostrDate } from '@/utils.ts';
+
+/**
+ * Common pipeline function to process (and maybe store) events.
+ * It is idempotent, so it can be called multiple times for the same event.
+ */
+async function handleEvent(event: Event): Promise<void> {
+  await Promise.all([
+    storeEvent(event),
+    trackRelays(event),
+    trackHashtags(event),
+  ]);
+}
+
+/** Maybe store the event, if eligible. */
+async function storeEvent(event: Event): Promise<void> {
+  if (await findUser({ pubkey: event.pubkey }) || await isLocallyFollowed(event.pubkey)) {
+    await eventsDB.insertEvent(event).catch(console.warn);
+  } else {
+    return Promise.reject(new RelayError('blocked', 'only registered users can post'));
+  }
+}
+
+/** Track whenever a hashtag is used, for processing trending tags. */
+// deno-lint-ignore require-await
+async function trackHashtags(event: Event): Promise<void> {
+  const date = nostrDate(event.created_at);
+
+  const tags = event.tags
+    .filter((tag) => tag[0] === 't')
+    .map((tag) => tag[1])
+    .slice(0, 5);
+
+  if (!tags.length) return;
+
+  try {
+    console.info('tracking tags:', tags);
+    trends.addTagUsages(event.pubkey, tags, date);
+  } catch (_e) {
+    // do nothing
+  }
+}
+
+/** Tracks known relays in the database. */
+function trackRelays(event: Event) {
+  const relays = new Set<`wss://${string}`>();
+
+  event.tags.forEach((tag) => {
+    if (['p', 'e', 'a'].includes(tag[0]) && isRelay(tag[2])) {
+      relays.add(tag[2]);
+    }
+    if (event.kind === 10002 && tag[0] === 'r' && isRelay(tag[1])) {
+      relays.add(tag[1]);
+    }
+  });
+
+  return addRelays([...relays]);
+}
+
+/** NIP-20 command line result. */
+class RelayError extends Error {
+  constructor(prefix: 'duplicate' | 'pow' | 'blocked' | 'rate-limited' | 'invalid' | 'error', message: string) {
+    super(`${prefix}: ${message}`);
+  }
+}
+
+export { handleEvent, RelayError };
diff --git a/src/queries.ts b/src/queries.ts
index 7d03c308..0f4b3ec8 100644
--- a/src/queries.ts
+++ b/src/queries.ts
@@ -1,8 +1,8 @@
+import * as eventsDB from '@/db/events.ts';
 import { type Event, type Filter, findReplyTag } from '@/deps.ts';
+import * as mixer from '@/mixer.ts';
 import { type PaginationParams } from '@/utils.ts';
 
-import * as mixer from './mixer.ts';
-
 interface GetEventOpts<K extends number> {
   /** Timeout in milliseconds. */
   timeout?: number;
@@ -83,4 +83,10 @@ function getDescendants(eventId: string): Promise<Event<1>[]> {
   return mixer.getFilters([{ kinds: [1], '#e': [eventId] }], { limit: 200, timeout: 2000 });
 }
 
-export { getAncestors, getAuthor, getDescendants, getEvent, getFeed, getFollows, getPublicFeed };
+/** Returns whether the pubkey is followed by a local user. */
+async function isLocallyFollowed(pubkey: string): Promise<boolean> {
+  const [event] = await eventsDB.getFilters([{ kinds: [3], '#p': [pubkey], local: true }], { limit: 1 });
+  return Boolean(event);
+}
+
+export { getAncestors, getAuthor, getDescendants, getEvent, getFeed, getFollows, getPublicFeed, isLocallyFollowed };