diff --git a/c#/GlobalSuppressions.cs b/c#/GlobalSuppressions.cs
index 551a02f9..a7ae8735 100644
--- a/c#/GlobalSuppressions.cs
+++ b/c#/GlobalSuppressions.cs
@@ -1,6 +1,8 @@
 // ReSharper disable once RedundantUsingDirective
 using System.Diagnostics.CodeAnalysis;
 
+[assembly: SuppressMessage("ApiDesign", "SS036:An enum should explicitly specify its values", Justification = "<Pending>")]
+[assembly: SuppressMessage("ApiDesign", "SS039:An enum should specify a default value", Justification = "<Pending>")]
 [assembly: SuppressMessage("Class Design", "AV1008:Class should not be static")]
 [assembly: SuppressMessage("Class Design", "AV1010:Member hides inherited member")]
 [assembly: SuppressMessage("Correctness", "SS019:Switch should have default label.")]
diff --git a/c#/crawler/src/Db/Revision/AuthorRevision.cs b/c#/crawler/src/Db/Revision/AuthorRevision.cs
index 0407759b..28184cec 100644
--- a/c#/crawler/src/Db/Revision/AuthorRevision.cs
+++ b/c#/crawler/src/Db/Revision/AuthorRevision.cs
@@ -7,5 +7,5 @@ public abstract class AuthorRevision
     public uint DiscoveredAt { get; set; }
     public uint Fid { get; set; }
     public long Uid { get; set; }
-    public required string TriggeredBy { get; set; }
+    public required PostType TriggeredBy { get; set; }
 }
diff --git a/c#/crawler/src/Db/Revision/UserRevisions.cs b/c#/crawler/src/Db/Revision/UserRevisions.cs
index bb987425..278a0a89 100644
--- a/c#/crawler/src/Db/Revision/UserRevisions.cs
+++ b/c#/crawler/src/Db/Revision/UserRevisions.cs
@@ -6,7 +6,7 @@ namespace tbm.Crawler.Db.Revision;
 public abstract class BaseUserRevision : RevisionWithSplitting<BaseUserRevision>
 {
     public long Uid { get; set; }
-    public required string TriggeredBy { get; set; }
+    public required PostType TriggeredBy { get; set; }
 }
 
 public class UserRevision : BaseUserRevision
diff --git a/c#/crawler/src/EntryPoint.cs b/c#/crawler/src/EntryPoint.cs
index e0deeb0e..a618d16e 100644
--- a/c#/crawler/src/EntryPoint.cs
+++ b/c#/crawler/src/EntryPoint.cs
@@ -42,7 +42,7 @@ protected override void ConfigureContainer(HostBuilderContext context, Container
         builder.RegisterType<CrawlerDbContext>();
         builder.RegisterType<ClientRequester>();
         builder.RegisterType<ClientRequesterTcs>().SingleInstance();
-        CrawlerLocks.RegisteredLocks.ForEach(type =>
+        Enum.GetValues<CrawlerLocks.Type>().ForEach(type =>
             builder.RegisterType<CrawlerLocks>()
                 .Keyed<CrawlerLocks>(type)
                 .WithParameter("lockType", type)
diff --git a/c#/crawler/src/PostType.cs b/c#/crawler/src/PostType.cs
new file mode 100644
index 00000000..cfb6a65d
--- /dev/null
+++ b/c#/crawler/src/PostType.cs
@@ -0,0 +1,8 @@
+namespace tbm.Crawler;
+
+public enum PostType
+{
+    Thread,
+    Reply,
+    SubReply
+}
diff --git a/c#/crawler/src/Tieba/Crawl/CrawlRequestFlag.cs b/c#/crawler/src/Tieba/Crawl/CrawlRequestFlag.cs
index 69419035..e58248ec 100644
--- a/c#/crawler/src/Tieba/Crawl/CrawlRequestFlag.cs
+++ b/c#/crawler/src/Tieba/Crawl/CrawlRequestFlag.cs
@@ -3,5 +3,5 @@ namespace tbm.Crawler.Tieba.Crawl;
 public enum CrawlRequestFlag
 {
     None = 0,
-    ThreadClientVersion602 = 1
+    ThreadClientVersion602
 }
diff --git a/c#/crawler/src/Tieba/Crawl/Crawler/ThreadLateCrawler.cs b/c#/crawler/src/Tieba/Crawl/Crawler/ThreadLateCrawler.cs
index b0b04671..d663bfca 100644
--- a/c#/crawler/src/Tieba/Crawl/Crawler/ThreadLateCrawler.cs
+++ b/c#/crawler/src/Tieba/Crawl/Crawler/ThreadLateCrawler.cs
@@ -4,10 +4,10 @@ public class ThreadLateCrawler(
     ILogger<ThreadLateCrawler> logger,
     ClientRequester requester,
     ClientRequesterTcs requesterTcs,
-    IIndex<string, CrawlerLocks> locks,
+    IIndex<CrawlerLocks.Type, CrawlerLocks> locks,
     Fid fid)
 {
-    private readonly CrawlerLocks _locks = locks["threadLate"]; // singleton
+    private readonly CrawlerLocks _locks = locks[CrawlerLocks.Type.ThreadLate]; // singleton
 
     public delegate ThreadLateCrawler New(Fid fid);
 
@@ -58,7 +58,7 @@ public class ThreadLateCrawler(
                         throw new TiebaException("Error from tieba client.") {Data = {{"raw", json}}};
                 }
 
-                var thread = json.GetProperty("thread");
+                var thread = json.GetProperty(Enum.GetName(PostType.Thread)!.ToLower(CultureInfo.InvariantCulture));
 #pragma warning disable S3358 // Ternary operators should not be nested
                 return thread.TryGetProperty("thread_info", out var threadInfo)
                     ? threadInfo.TryGetProperty("phone_type", out var phoneType)
diff --git a/c#/crawler/src/Tieba/Crawl/CrawlerLocks.cs b/c#/crawler/src/Tieba/Crawl/CrawlerLocks.cs
index 92cba78e..97e9f053 100644
--- a/c#/crawler/src/Tieba/Crawl/CrawlerLocks.cs
+++ b/c#/crawler/src/Tieba/Crawl/CrawlerLocks.cs
@@ -1,6 +1,6 @@
 namespace tbm.Crawler.Tieba.Crawl;
 
-public class CrawlerLocks(ILogger<CrawlerLocks> logger, IConfiguration config, string lockType)
+public class CrawlerLocks(ILogger<CrawlerLocks> logger, IConfiguration config, CrawlerLocks.Type lockType)
     : WithLogTrace(config, $"CrawlerLocks:{lockType}")
 {
     private readonly IConfigurationSection _config = config.GetSection($"CrawlerLocks:{lockType}");
@@ -9,8 +9,14 @@ public class CrawlerLocks(ILogger<CrawlerLocks> logger, IConfiguration config, s
     // inner value of field _failed with type ushort refers to failed times on this page and lockId before retry
     private readonly ConcurrentDictionary<LockId, ConcurrentDictionary<Page, FailureCount>> _failed = new();
 
-    public static IEnumerable<string> RegisteredLocks { get; } = ["thread", "threadLate", "reply", "subReply"];
-    public string LockType { get; } = lockType;
+    public enum Type
+    {
+        Thread,
+        ThreadLate,
+        Reply,
+        SubReply
+    }
+    public Type LockType { get; } = lockType;
 
     public IReadOnlySet<Page> AcquireRange(LockId lockId, IEnumerable<Page> pages)
     {
diff --git a/c#/crawler/src/Tieba/Crawl/Facade/BaseCrawlFacade.cs b/c#/crawler/src/Tieba/Crawl/Facade/BaseCrawlFacade.cs
index ddd0c938..b72b10aa 100644
--- a/c#/crawler/src/Tieba/Crawl/Facade/BaseCrawlFacade.cs
+++ b/c#/crawler/src/Tieba/Crawl/Facade/BaseCrawlFacade.cs
@@ -50,7 +50,7 @@ public virtual void Dispose()
         var savedPosts = Posts.IsEmpty ? null : postSaver.Save(db);
 
         var userSaver = userSaverFactory(_users);
-        userSaver.Save(db, postSaver.PostType, postSaver.UserFieldChangeIgnorance);
+        userSaver.Save(db, postSaver.CurrentPostType, postSaver.UserFieldChangeIgnorance);
 
         BeforeCommitSaveHook(db, userSaver);
         try
diff --git a/c#/crawler/src/Tieba/Crawl/Facade/ReplyCrawlFacade.cs b/c#/crawler/src/Tieba/Crawl/Facade/ReplyCrawlFacade.cs
index ae2d274c..c19594ec 100644
--- a/c#/crawler/src/Tieba/Crawl/Facade/ReplyCrawlFacade.cs
+++ b/c#/crawler/src/Tieba/Crawl/Facade/ReplyCrawlFacade.cs
@@ -4,7 +4,7 @@ public class ReplyCrawlFacade(
         ReplyCrawler.New crawlerFactory,
         Fid fid,
         Tid tid,
-        IIndex<string, CrawlerLocks> locks,
+        IIndex<CrawlerLocks.Type, CrawlerLocks> locks,
         ReplyParser postParser,
         ReplySaver.New postSaverFactory,
         UserParser.New userParserFactory,
@@ -12,7 +12,7 @@ public class ReplyCrawlFacade(
         CrawlerDbContext.New dbContextFactory,
         SonicPusher sonicPusher)
     : BaseCrawlFacade<ReplyPost, BaseReplyRevision, ReplyResponse, Reply>(
-        crawlerFactory(fid, tid), fid, new(fid, tid), locks["reply"],
+        crawlerFactory(fid, tid), fid, new(fid, tid), locks[CrawlerLocks.Type.Reply],
         postParser, postSaverFactory.Invoke,
         userParserFactory.Invoke, userSaverFactory.Invoke)
 {
diff --git a/c#/crawler/src/Tieba/Crawl/Facade/SubReplyCrawlFacade.cs b/c#/crawler/src/Tieba/Crawl/Facade/SubReplyCrawlFacade.cs
index b5c00303..11c4a3be 100644
--- a/c#/crawler/src/Tieba/Crawl/Facade/SubReplyCrawlFacade.cs
+++ b/c#/crawler/src/Tieba/Crawl/Facade/SubReplyCrawlFacade.cs
@@ -5,14 +5,14 @@ public class SubReplyCrawlFacade(
         Fid fid,
         Tid tid,
         Pid pid,
-        IIndex<string, CrawlerLocks> locks,
+        IIndex<CrawlerLocks.Type, CrawlerLocks> locks,
         SubReplyParser postParser,
         SubReplySaver.New postSaverFactory,
         UserParser.New userParserFactory,
         UserSaver.New userSaverFactory,
         SonicPusher sonicPusher)
     : BaseCrawlFacade<SubReplyPost, BaseSubReplyRevision, SubReplyResponse, SubReply>(
-        crawlerFactory(tid, pid), fid, new(fid, tid, pid), locks["subReply"],
+        crawlerFactory(tid, pid), fid, new(fid, tid, pid), locks[CrawlerLocks.Type.SubReply],
         postParser, postSaverFactory.Invoke,
         userParserFactory.Invoke, userSaverFactory.Invoke)
 {
diff --git a/c#/crawler/src/Tieba/Crawl/Facade/ThreadArchiveCrawlFacade.cs b/c#/crawler/src/Tieba/Crawl/Facade/ThreadArchiveCrawlFacade.cs
index 489c0e41..ee61f408 100644
--- a/c#/crawler/src/Tieba/Crawl/Facade/ThreadArchiveCrawlFacade.cs
+++ b/c#/crawler/src/Tieba/Crawl/Facade/ThreadArchiveCrawlFacade.cs
@@ -4,7 +4,7 @@ public class ThreadArchiveCrawlFacade(
         ThreadArchiveCrawler.New crawlerFactory,
         string forumName,
         Fid fid,
-        IIndex<string, CrawlerLocks> locks,
+        IIndex<CrawlerLocks.Type, CrawlerLocks> locks,
         ThreadParser postParser,
         ThreadSaver.New postSaverFactory,
         UserParser.New userParserFactory,
diff --git a/c#/crawler/src/Tieba/Crawl/Facade/ThreadCrawlFacade.cs b/c#/crawler/src/Tieba/Crawl/Facade/ThreadCrawlFacade.cs
index 41b65213..76dad0c3 100644
--- a/c#/crawler/src/Tieba/Crawl/Facade/ThreadCrawlFacade.cs
+++ b/c#/crawler/src/Tieba/Crawl/Facade/ThreadCrawlFacade.cs
@@ -4,13 +4,13 @@ public class ThreadCrawlFacade(
         ThreadCrawler.New crawlerFactory,
         string forumName,
         Fid fid,
-        IIndex<string, CrawlerLocks> locks,
+        IIndex<CrawlerLocks.Type, CrawlerLocks> locks,
         ThreadParser postParser,
         ThreadSaver.New postSaverFactory,
         UserParser.New userParserFactory,
         UserSaver.New userSaverFactory)
     : BaseCrawlFacade<ThreadPost, BaseThreadRevision, ThreadResponse, Thread>(
-        crawlerFactory(forumName), fid, new(fid), locks["thread"],
+        crawlerFactory(forumName), fid, new(fid), locks[CrawlerLocks.Type.Thread],
         postParser, postSaverFactory.Invoke,
         userParserFactory.Invoke, userSaverFactory.Invoke)
 {
diff --git a/c#/crawler/src/Tieba/Crawl/Saver/AuthorRevisionSaver.cs b/c#/crawler/src/Tieba/Crawl/Saver/AuthorRevisionSaver.cs
index 3844dc8f..ab3b56b4 100644
--- a/c#/crawler/src/Tieba/Crawl/Saver/AuthorRevisionSaver.cs
+++ b/c#/crawler/src/Tieba/Crawl/Saver/AuthorRevisionSaver.cs
@@ -2,14 +2,14 @@
 
 namespace tbm.Crawler.Tieba.Crawl.Saver;
 
-public class AuthorRevisionSaver(string triggeredByPostType)
+public class AuthorRevisionSaver(PostType triggeredByPostType)
 {
     // locks only using fid and uid field values from AuthorRevision
     // this prevents inserting multiple entities with similar time and other fields with the same values
     private static readonly HashSet<(Fid Fid, Uid Uid)> AuthorExpGradeLocks = [];
     private readonly List<(Fid Fid, Uid Uid)> _savedRevisions = [];
 
-    public delegate AuthorRevisionSaver New(string triggeredByPostType);
+    public delegate AuthorRevisionSaver New(PostType triggeredByPostType);
 
     public Action SaveAuthorExpGradeRevisions<TPostWithAuthorExpGrade>
         (CrawlerDbContext db, IReadOnlyCollection<TPostWithAuthorExpGrade> posts)
diff --git a/c#/crawler/src/Tieba/Crawl/Saver/Post/BasePostSaver.cs b/c#/crawler/src/Tieba/Crawl/Saver/Post/BasePostSaver.cs
index 21d5ee62..42398f70 100644
--- a/c#/crawler/src/Tieba/Crawl/Saver/Post/BasePostSaver.cs
+++ b/c#/crawler/src/Tieba/Crawl/Saver/Post/BasePostSaver.cs
@@ -6,7 +6,7 @@ public abstract class BasePostSaver<TPost, TBaseRevision>(
         ILogger<BasePostSaver<TPost, TBaseRevision>> logger,
         ConcurrentDictionary<PostId, TPost> posts,
         AuthorRevisionSaver.New authorRevisionSaverFactory,
-        string postType)
+        PostType currentPostType)
     : BaseSaver<TBaseRevision>(logger)
     where TPost : class, IPost
     where TBaseRevision : class, IRevision
@@ -17,9 +17,9 @@ public abstract class BasePostSaver<TPost, TBaseRevision>(
 
     public virtual IFieldChangeIgnorance.FieldChangeIgnoranceDelegates
         UserFieldChangeIgnorance => throw new NotSupportedException();
-    public string PostType { get; } = postType;
+    public PostType CurrentPostType { get; } = currentPostType;
     protected ConcurrentDictionary<PostId, TPost> Posts { get; } = posts;
-    protected AuthorRevisionSaver AuthorRevisionSaver { get; } = authorRevisionSaverFactory(postType);
+    protected AuthorRevisionSaver AuthorRevisionSaver { get; } = authorRevisionSaverFactory(currentPostType);
 
     [SuppressMessage("Misc", "AV1225:Method that raises an event should be protected virtual and be named 'On' followed by event name")]
     public void OnPostSaveEvent() => PostSaveEvent();
diff --git a/c#/crawler/src/Tieba/Crawl/Saver/Post/ReplySaver.cs b/c#/crawler/src/Tieba/Crawl/Saver/Post/ReplySaver.cs
index a77fd202..f63415e1 100644
--- a/c#/crawler/src/Tieba/Crawl/Saver/Post/ReplySaver.cs
+++ b/c#/crawler/src/Tieba/Crawl/Saver/Post/ReplySaver.cs
@@ -8,7 +8,7 @@ public partial class ReplySaver(
         ConcurrentDictionary<PostId, ReplyPost> posts,
         AuthorRevisionSaver.New authorRevisionSaverFactory)
     : BasePostSaver<ReplyPost, BaseReplyRevision>(
-        logger, posts, authorRevisionSaverFactory, "reply")
+        logger, posts, authorRevisionSaverFactory, PostType.Reply)
 {
     public delegate ReplySaver New(ConcurrentDictionary<PostId, ReplyPost> posts);
 
diff --git a/c#/crawler/src/Tieba/Crawl/Saver/Post/SubReplySaver.cs b/c#/crawler/src/Tieba/Crawl/Saver/Post/SubReplySaver.cs
index 400212d8..c16e8b47 100644
--- a/c#/crawler/src/Tieba/Crawl/Saver/Post/SubReplySaver.cs
+++ b/c#/crawler/src/Tieba/Crawl/Saver/Post/SubReplySaver.cs
@@ -7,7 +7,7 @@ public class SubReplySaver(
         ConcurrentDictionary<PostId, SubReplyPost> posts,
         AuthorRevisionSaver.New authorRevisionSaverFactory)
     : BasePostSaver<SubReplyPost, BaseSubReplyRevision>(
-        logger, posts, authorRevisionSaverFactory, "subReply")
+        logger, posts, authorRevisionSaverFactory, PostType.SubReply)
 {
     public delegate SubReplySaver New(ConcurrentDictionary<PostId, SubReplyPost> posts);
 
diff --git a/c#/crawler/src/Tieba/Crawl/Saver/Post/ThreadSaver.cs b/c#/crawler/src/Tieba/Crawl/Saver/Post/ThreadSaver.cs
index 9d398932..a941918f 100644
--- a/c#/crawler/src/Tieba/Crawl/Saver/Post/ThreadSaver.cs
+++ b/c#/crawler/src/Tieba/Crawl/Saver/Post/ThreadSaver.cs
@@ -7,7 +7,7 @@ public class ThreadSaver(
         ConcurrentDictionary<Tid, ThreadPost> posts,
         AuthorRevisionSaver.New authorRevisionSaverFactory)
     : BasePostSaver<ThreadPost, BaseThreadRevision>(
-        logger, posts, authorRevisionSaverFactory, "thread")
+        logger, posts, authorRevisionSaverFactory, PostType.Thread)
 {
     public delegate ThreadSaver New(ConcurrentDictionary<Tid, ThreadPost> posts);
 
diff --git a/c#/crawler/src/Tieba/Crawl/Saver/UserSaver.cs b/c#/crawler/src/Tieba/Crawl/Saver/UserSaver.cs
index 85a81246..614baa76 100644
--- a/c#/crawler/src/Tieba/Crawl/Saver/UserSaver.cs
+++ b/c#/crawler/src/Tieba/Crawl/Saver/UserSaver.cs
@@ -41,7 +41,7 @@ public partial class UserSaver(ILogger<UserSaver> logger, ConcurrentDictionary<U
 
     public void Save(
         CrawlerDbContext db,
-        string postType,
+        PostType postType,
         IFieldChangeIgnorance.FieldChangeIgnoranceDelegates userFieldChangeIgnorance)
     {
         if (users.IsEmpty) return;
diff --git a/c#/crawler/src/Worker/RetryCrawlWorker.cs b/c#/crawler/src/Worker/RetryCrawlWorker.cs
index 31918d51..2a61f625 100644
--- a/c#/crawler/src/Worker/RetryCrawlWorker.cs
+++ b/c#/crawler/src/Worker/RetryCrawlWorker.cs
@@ -2,7 +2,7 @@ namespace tbm.Crawler.Worker;
 
 public class RetryCrawlWorker(
         ILogger<RetryCrawlWorker> logger,
-        IIndex<string, CrawlerLocks> registeredLocksLookup,
+        IIndex<CrawlerLocks.Type, CrawlerLocks> registeredLocksLookup,
         CrawlPost crawlPost,
         Func<Owned<CrawlerDbContext.NewDefault>> dbContextDefaultFactory,
         Func<Owned<ThreadLateCrawlFacade.New>> threadLateCrawlFacadeFactory,
@@ -13,12 +13,12 @@ public class RetryCrawlWorker(
 {
     protected override async Task DoWork(CancellationToken stoppingToken)
     {
-        foreach (var lockType in CrawlerLocks.RegisteredLocks)
+        foreach (var lockType in Enum.GetValues<CrawlerLocks.Type>())
         {
             if (stoppingToken.IsCancellationRequested) return;
             var failed = registeredLocksLookup[lockType].RetryAllFailed();
             if (failed.Count == 0) continue; // skip current lock type if there's nothing needs to retry
-            if (lockType == "threadLate")
+            if (lockType == CrawlerLocks.Type.ThreadLate)
             {
                 await RetryThreadLate(failed, stoppingToken);
 
@@ -30,7 +30,7 @@ protected override async Task DoWork(CancellationToken stoppingToken)
     }
 
     private Func<KeyValuePair<CrawlerLocks.LockId, IReadOnlyDictionary<Page, FailureCount>>, Task> RetryFailed
-        (string lockType, CancellationToken stoppingToken = default) => async failedPagesKeyByLockId =>
+        (CrawlerLocks.Type lockType, CancellationToken stoppingToken = default) => async failedPagesKeyByLockId =>
     {
         if (stoppingToken.IsCancellationRequested) return;
         var ((fid, tid, pid), failureCountsKeyByPage) = failedPagesKeyByLockId;
@@ -39,15 +39,15 @@ protected override async Task DoWork(CancellationToken stoppingToken)
 
         switch (lockType)
         {
-            case "thread":
+            case CrawlerLocks.Type.Thread:
                 await RetryThread(fid, pages,
                     failureCountsKeyByPage.Count, FailureCountSelector, stoppingToken);
                 break;
-            case "reply" when tid != null:
+            case CrawlerLocks.Type.Reply when tid != null:
                 await RetryReply(fid, tid.Value, pages,
                     failureCountsKeyByPage.Count, FailureCountSelector, stoppingToken);
                 break;
-            case "subReply" when tid != null && pid != null:
+            case CrawlerLocks.Type.SubReply when tid != null && pid != null:
                 await RetrySubReply(fid, tid.Value, pid.Value, pages,
                     failureCountsKeyByPage.Count, FailureCountSelector, stoppingToken);
                 break;