Add links to the fundamentals briefs

docs/topics/distributed.md

@@ -40,7 +40,7 @@ Key distributed-systems building blocks and trade-offs, grouped by subtopic.
 ### Single-leader replication
 
 - All writes go to one leader; [sync](https://www.postgresql.org/docs/current/warm-standby.html#SYNCHRONOUS-REPLICATION) followers are durable but add latency, async followers are fast but lose recent writes on failover.
-- [Semi-sync](https://mariadb.com/docs/server/ha-and-performance/standard-replication/semisynchronous-replication) waits for at least one follower.
+- [Semi-sync](https://dev.mysql.com/doc/refman/8.4/en/replication-semisync.html) waits for at least one follower.
 - Replication lag breaks read-your-writes: a user can't see their own update right after writing.
 
 ### Multi-leader and leaderless
@@ -203,7 +203,7 @@ Key distributed-systems building blocks and trade-offs, grouped by subtopic.
 
 ### Change data capture
 
-- [Log-based CDC](https://debezium.io/documentation/reference/stable/features.html) (Debezium) tails the [WAL](https://www.postgresql.org/docs/current/wal-intro.html) or [binlog](https://debezium.io/documentation/reference/stable/connectors/mysql.html): every committed change in commit order, deletes included, with no dual writes in the application.
+- [Log-based CDC](https://debezium.io/documentation/reference/stable/features.html) (Debezium) tails the [WAL](https://www.postgresql.org/docs/current/wal-intro.html) or [binlog](https://dev.mysql.com/doc/refman/8.4/en/binary-log.html): every committed change in commit order, deletes included, with no dual writes in the application.
 - Polling an `updated_at` column misses hard deletes and rows that commit late with an older timestamp.
 - Bootstrap from a [consistent snapshot](https://debezium.io/documentation/reference/stable/connectors/postgresql.html#postgresql-snapshots), then stream from the log position it was taken at.
 - Events are ordered per key (partition by primary key); connectors deliver at least once, so consumers must be idempotent.

docs/topics/system.md

@@ -117,7 +117,7 @@ Key system design building blocks and trade-offs, grouped by subtopic. For CAP,
 ### Distributed rate limiting
 
 - Enforce at the gateway, keyed by user, API key, or IP; reject with [429](https://www.rfc-editor.org/rfc/rfc6585#section-4) and [`Retry-After`](https://www.rfc-editor.org/rfc/rfc9110#field.retry-after).
-- Share counters in Redis so every server sees one limit; wrap [`INCR`](https://redis.io/docs/latest/commands/incr/) + [`EXPIRE`](https://redis.io/docs/latest/commands/expire/) in [`MULTI`](https://redis.io/docs/latest/develop/interact/transactions/) or a [Lua script](https://redis.io/docs/latest/develop/interact/programmability/eval-intro/), or a crash between them leaves a counter with no TTL.
+- Share counters in Redis so every server sees one limit; wrap [`INCR`](https://redis.io/docs/latest/commands/incr/) + [`EXPIRE`](https://redis.io/docs/latest/commands/expire/) in [`MULTI`](https://redis.io/docs/latest/develop/using-commands/transactions/) or a [Lua script](https://redis.io/docs/latest/develop/programmability/eval-intro/), or a crash between them leaves a counter with no TTL.
 - A local per-instance limiter skips the round trip but is only approximate (limit ÷ instances).
 
 ## Reliability and operations

scripts/check_links.py

@@ -10,6 +10,7 @@
 import pathlib
 import re
 import sys
+import urllib.error
 import urllib.parse
 import urllib.request
 
@@ -18,18 +19,27 @@
 LINK = re.compile(r"\]\((https?://[^)\s]+)\)")
 # Minified HTML (the WHATWG spec, Hugo sites) leaves attribute values unquoted.
 ANCHOR = re.compile(r"""(?<![\w-])(?:id|name)=(?:"([^"]+)"|'([^']+)'|([^\s"'>]+))""")
-# Wikipedia rejects requests without a descriptive User-Agent.
-USER_AGENT = "interview-briefs-link-checker (+https://github.com/pinebit/interviews)"
+# Wikipedia rejects requests without a descriptive User-Agent; dev.mysql.com rejects
+# agents it doesn't recognize, so a 403 is retried with urllib's default agent.
+USER_AGENTS = ["interview-briefs-link-checker (+https://github.com/pinebit/interviews)", None]
 
 
 def fetch(url):
     """Return (error, set of anchor ids) for a page URL without a fragment."""
-    request = urllib.request.Request(url, headers={"User-Agent": USER_AGENT})
-    try:
-        with urllib.request.urlopen(request, timeout=30) as response:
-            body = response.read().decode("utf-8", "replace")
-    except (OSError, http.client.HTTPException) as e:  # URLError and timeouts are OSErrors
-        return str(e), set()
+    for user_agent in USER_AGENTS:
+        headers = {"User-Agent": user_agent} if user_agent else {}
+        try:
+            with urllib.request.urlopen(urllib.request.Request(url, headers=headers), timeout=30) as response:
+                body = response.read().decode("utf-8", "replace")
+            break
+        except urllib.error.HTTPError as e:
+            error = str(e)
+            if e.code != 403:
+                return error, set()
+        except (OSError, http.client.HTTPException) as e:  # URLError and timeouts are OSErrors
+            return str(e), set()
+    else:
+        return error, set()
     return None, {html.unescape("".join(groups)) for groups in ANCHOR.findall(body)}