diff --git a/.circleci/config.yml b/.circleci/config.yml
index 702804c4a222f..482378015c09c 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -34,12 +34,12 @@ jobs:
             markdownlint $(git diff-tree --name-only --no-commit-id -r upstream/release-3.0..HEAD -- '*.md' ':(exclude).github/*')
 
       - run:
-          name: "Check links"
+          name: "Check internal links"
           command: |
             scripts/verify-links.sh
 
       - run:
-          name: "Check link anchors"
+          name: "Check internal link anchors"
           command: |
             scripts/verify-link-anchors.sh
 
diff --git a/.github/workflows/link-check.yaml b/.github/workflows/link-check.yaml
new file mode 100644
index 0000000000000..02697bdb03f04
--- /dev/null
+++ b/.github/workflows/link-check.yaml
@@ -0,0 +1,31 @@
+name: external-link-check
+
+on: [pull_request]
+
+jobs:
+  base:
+    runs-on: ubuntu-18.04
+    steps:
+      - name: Check out code
+        uses: actions/checkout@v2
+
+      # NOTE:
+      #  - the check process can be very fast,
+      #    but if `-c` is too large then `Too Many Requests (HTTP error 429)` may be reported from `https://github.com/*`.
+      #  - we hardcode `--document-root` to `/github/workspace` in the container now.
+      #  - we use `http://172.*` as sample addresses in some docs, so we need to exclude them.
+      - name: Check external links
+        id: lc
+        uses: peter-evans/link-checker@v1
+        with:
+          args: -c 32 -d /github/workspace -r -x "http://172.*|https://github.com/.*/pull/.*|https://github.com/.*/issues/.*|https://github.com/[^.\n]*$|http://127.0.0.1.*|http://localhost.*|http://.*:.*|https://static.googleusercontent.com/media/research.google.com/.*|https://www.crunchbase.com/organization/.*|http://www.brendangregg.com/.*|.*.md" .
+
+      - name: Fail if there were external link errors
+        if: ${{ steps.lc.outputs.exit_code }}
+        run: |
+          echo "Please check the broken links reported in the previous step \"Check external links\"."
+          exit ${{ steps.lc.outputs.exit_code }}
+      # Debug via SSH if previous steps failed
+      #- name: Set up tmate session
+      #  if: ${{ failure() }}
+      #  uses: mxschmitt/action-tmate@v2
diff --git a/adopters.md b/adopters.md
index bdffdacd2152f..93f8c507c6c82 100644
--- a/adopters.md
+++ b/adopters.md
@@ -24,7 +24,7 @@ This is a list of TiDB adopters in various industries.
 |[Xiaohongshu](https://en.wikipedia.org/wiki/Xiaohongshu)|E-commerce|[English](https://pingcap.com/case-studies/how-we-use-a-scale-out-htap-database-for-real-time-analytics-and-complex-queries); Chinese [#1](https://pingcap.com/cases-cn/user-case-xiaohongshu/), [#2](https://pingcap.com/cases-cn/user-case-xiaohongshu-2/)|
 |[Happigo.com](https://www.crunchbase.com/organization/happigo-com)|E-commerce||
 |[Yimutian](http://www.ymt.com/)|E-commerce||
-|[Youju Tech](https://www.ujuz.cn/)|E-commerce||
+|[Youju Tech](https://nn.yjyz.com/)|E-commerce||
 |[Maizuo](https://www.crunchbase.com/organization/maizhuo)|E-commerce||
 |[Mogujie](https://www.crunchbase.com/organization/mogujie)|E-commerce||
 |[Zhihu](https://en.wikipedia.org/wiki/Zhihu)|Knowledge Sharing|[English](https://pingcap.com/case-studies/lesson-learned-from-queries-over-1.3-trillion-rows-of-data-within-milliseconds-of-response-time-at-zhihu/); [Chinese](https://pingcap.com/cases-cn/user-case-zhihu/)|
@@ -43,9 +43,9 @@ This is a list of TiDB adopters in various industries.
 |[CAASDATA](https://www.caasdata.com/)|Big Data|[Chinese](https://pingcap.com/cases-cn/user-case-kasi/)|
 |[Mobikok](http://www.mobikok.com/en/)|AdTech|[Chinese](https://pingcap.com/cases-cn/user-case-mobikok/)|
 |[ZTO Express](https://www.crunchbase.com/organization/zto-express)| Logistics|[English](https://pingcap.com/case-studies/why-we-migrated-from-exadata-to-a-scale-out-htap-database-for-near-real-time-analytics); [Chinese](https://pingcap.com/cases-cn/user-case-zto-express/)|
-|[G7 Networks](https://www.english.g7.com.cn/)| Logistics|[Chinese](https://pingcap.com/cases-cn/user-case-g7/)|
+|[G7 Networks](https://www.crunchbase.com/organization/g7)| Logistics|[Chinese](https://pingcap.com/cases-cn/user-case-g7/)|
 |[Hive-Box](http://www.fcbox.com/en/pc/index.html#/)|Logistics|[Chinese](https://pingcap.com/cases-cn/user-case-fengchao/)|
-|[GAEA](http://www.gaea.com/en/)|Gaming|[English](https://pingcap.com/blog/2017-05-22-Comparison-between-MySQL-and-TiDB-with-tens-of-millions-of-data-per-day/); [Chinese](https://pingcap.com/cases-cn/user-case-gaea-ad/)|
+|[GAEA](http://www.gaea.com/en/)|Gaming|[English](https://pingcap.com/case-studies/2017-05-22-Comparison-between-MySQL-and-TiDB-with-tens-of-millions-of-data-per-day); [Chinese](https://pingcap.com/cases-cn/user-case-gaea-ad/)|
 |[YOOZOO Games](https://www.crunchbase.com/organization/yoozoo-games)|Gaming|[Chinese](https://pingcap.com/cases-cn/user-case-youzu/)|
 |[Seasun Games](https://www.crunchbase.com/organization/seasun)|Gaming|[Chinese](https://pingcap.com/cases-cn/user-case-xishanju/)|
 |[NetEase Games](https://game.163.com/en/)|Gaming||
@@ -72,15 +72,14 @@ This is a list of TiDB adopters in various industries.
 |[360 Finance](https://www.crunchbase.com/organization/360-finance)|FinTech|[Chinese](https://pingcap.com/cases-cn/user-case-360/)|
 |[Tongdun Technology](https://www.crunchbase.com/organization/tongdun-technology)|FinTech||
 |[Wacai](https://www.crunchbase.com/organization/wacai)|FinTech||
-|[Tree Finance](https://www.treefinance.com.cn/)|FinTech||
+|[Tree Finance](http://treefinancegroup.com.au/)|FinTech||
 |[Mashang Consumer Finance](https://www.crunchbase.com/organization/ms-finance)|FinTech||
 |[Snowball Finance](https://www.crunchbase.com/organization/snowball-finance)|FinTech||
-|[Shuangchuang Huipu](http://scphjt.com/)|FinTech||
 |[QuantGroup](https://www.crunchbase.com/organization/quantgroup)|FinTech||
 |[FINUP](https://www.crunchbase.com/organization/finup)|FinTech||
 |[Meili Finance](https://www.crunchbase.com/organization/meili-jinrong)|FinTech||
 |[Guolian Securities](https://www.crunchbase.com/organization/guolian-securities)|Financial Services||
-|[Founder Securities](https://www.linkedin.com/company/founder-securities-co-ltd-/)|Financial Services||
+|[Founder Securities](https://www.crunchbase.com/organization/keruyun-technology-beijing-co-ltd)|Financial Services||
 |[China Telecom Shanghai](http://www.189.cn/sh/)|Telecom||
 |[State Administration of Taxation](https://en.wikipedia.org/wiki/State_Administration_of_Taxation)|Finance||
 |[Hainan eKing Technology](https://www.crunchbase.com/organization/hainan-eking-technology)|Enterprise Technology|[Chinese](https://pingcap.com/cases-cn/user-case-ekingtech/)|
diff --git a/connectors-and-apis.md b/connectors-and-apis.md
index 5aa4317d6f001..31e8529a5a1a4 100644
--- a/connectors-and-apis.md
+++ b/connectors-and-apis.md
@@ -15,7 +15,7 @@ TiDB is compatible with all Connectors and APIs of MySQL (5.6, 5.7), including:
 - [MySQL Connector/Net](https://dev.mysql.com/doc/refman/5.7/en/connector-net-info.html)
 - [MySQL Connector/ODBC](https://dev.mysql.com/doc/refman/5.7/en/connector-odbc-info.html)
 - [MySQL Connector/Python](https://dev.mysql.com/doc/refman/5.7/en/connector-python-info.html)
-- [MySQL C API](https://dev.mysql.com/doc/refman/5.7/en/c-api.html)
+- [MySQL C API](https://dev.mysql.com/doc/refman/5.7/en/c-api-info.html)
 - [MySQL PHP API](https://dev.mysql.com/doc/refman/5.7/en/apis-php-info.html)
 - [MySQL Perl API](https://dev.mysql.com/doc/refman/5.7/en/apis-perl.html)
 - [MySQL Python API](https://dev.mysql.com/doc/refman/5.7/en/apis-python.html)
@@ -40,7 +40,7 @@ Oracle develops the following APIs and TiDB is compatible with all of them:
 
 ## Connect to TiDB using MySQL C API
 
-If you use C language programs to connect to TiDB, you can connect to `libmysqlclient` directly and use the MySQL [C API](https://dev.mysql.com/doc/refman/5.7/en/c-api.html). This is one of the major connection methods using C language, widely used by various clients and APIs, including Connector/C.
+If you use C language programs to connect to TiDB, you can connect to `libmysqlclient` directly and use the MySQL [C API](https://dev.mysql.com/doc/refman/5.7/en/c-api-info.html). This is one of the major connection methods using C language, widely used by various clients and APIs, including Connector/C.
 
 ## Connect to TiDB using third-party MySQL APIs
 
@@ -49,7 +49,7 @@ The third-party APIs are not developed by Oracle. The following table lists the
 | Environment    | API                                      | Type                             | Notes                                    |
 | -------------- | ---------------------------------------- | -------------------------------- | ---------------------------------------- |
 | Ada            | GNU Ada MySQL Bindings                   | `libmysqlclient`                 | See [MySQL Bindings for GNU Ada](http://gnade.sourceforge.net/) |
-| C              | C API                                    | `libmysqlclient`                 | See [Section 27.8, “MySQL C API”](https://dev.mysql.com/doc/refman/5.7/en/c-api.html) |
+| C              | C API                                    | `libmysqlclient`                 | See [MySQL C API](https://dev.mysql.com/doc/refman/5.7/en/c-api-info.html) |
 | C++            | Connector/C++                            | `libmysqlclient`                 | See [MySQL Connector/C++ Developer Guide](https://dev.mysql.com/doc/connector-cpp/en/) |
 |                | MySQL++                                  | `libmysqlclient`                 | See [MySQL++ Web site](http://tangentsoft.net/mysql++/doc/) |
 |                | MySQL wrapped                            | `libmysqlclient`                 | See [MySQL wrapped](http://www.alhem.net/project/mysql/) |
diff --git a/sql-statements/sql-statement-admin.md b/sql-statements/sql-statement-admin.md
index 18ae69defdc16..6e5cb5c59248a 100644
--- a/sql-statements/sql-statement-admin.md
+++ b/sql-statements/sql-statement-admin.md
@@ -93,7 +93,7 @@ mysql> admin show ddl jobs;
 * `JOB_TYPE`: the type of the DDL operations.
 * `SCHEMA_STATE`: the current state of the schema. If the `JOB_TYPE` is `add index`, it is the state of the index; if the `JOB_TYPE` is `add column`, it is the state of the column; if the `JOB_TYPE` is `create table`, it is the state of the table. The common states include:
     * `none`: it indicates not existing. When the `drop` or `create` operation fails and rolls back, it usually becomes the `none` state.
-    * `delete only`, `write only`, `delete reorganization`, `write reorganization`: these four states are intermediate states. For details, see the paper [Online, Asynchronous Schema Change in F1](http://static.googleusercontent.com/media/research.google.com/zh-CN//pubs/archive/41376.pdf). These states are not visible in common operations, because the conversion from the intermediate states is so quick. You can see the `write reorganization` state only in `add index` operations, which means that the index data is being added.
+    * `delete only`, `write only`, `delete reorganization`, `write reorganization`: these four states are intermediate states. For details, see the paper [Online, Asynchronous Schema Change in F1](https://static.googleusercontent.com/media/research.google.com/zh-CN//pubs/archive/41376.pdf). These states are not visible in common operations, because the conversion from the intermediate states is so quick. You can see the `write reorganization` state only in `add index` operations, which means that the index data is being added.
     * `public`: it indicates existing and usable. When operations like `create table` and `add index/column` are finished, it usually becomes the `public` state, which means that the created table/column/index can be normally read and written now.
 * `SCHEMA_ID`: the ID of the database on which the DDL operations are performed.
 * `TABLE_ID`: the ID of the table on which the DDL operations are performed.