Updated Filer Change Data Capture (markdown)

Filer-Change-Data-Capture.md

@@ -2,6 +2,40 @@ Is it too much a dream to have something similar to [inotify](https://man7.org/l
 
 Actually SeaweedFS can give you more!
 
+# Experience it first
+
+You can watch the SeaweedFS meta data changes.
+
+```
+$ weed watch -h
+Example: weed watch [-filer=localhost:8888] [-target=/]
+Default Usage:
+  -filer string
+    	filer hostname:port (default "localhost:8888")
+  -pathPrefix string
+    	path to a folder or file, or common prefix for the folders or files on filer (default "/")
+  -pattern string
+    	full path or just filename pattern, ex: "/home/?opher", "*.pdf", see https://golang.org/pkg/path/filepath/#Match
+  -timeAgo duration
+    	start time before now. "300ms", "1.5h" or "2h45m". Valid time units are "ns", "us" (or "µs"), "ms", "s", "m", "h"
+Description:
+  See recent changes on a filer.
+
+$ weed watch
+dir:/home new_entry:{name:"3190.pdf" chunks:{size:1130805 mtime:1608763500740201000 e_tag:"5c745d0a109bde58e0baaa6515c3694a" fid:{volume_id:1 file_key:35 cookie:4265699614}} attributes:{file_size:1130805 mtime:1608763500 file_mode:432 uid:502 gid:20 crtime:1608763500 mime:"application/pdf" replication:"000" md5:"\\t]\n\x10\x9b\xdeXສe\x15\xc3iJ"}} delete_chunks:true new_parent_path:"/home" signatures:1622972984
+```
+
+# How it works?
+
+The `weed watch` code is nothing fancy. It is calls a gRPC stream API to subscribe to all meta data changes and simply print out the meta data.
+
+The gRPC API has several important use cases within SeaweedFS:
+* Replicate data to other SeaweedFS clusters in `weed filer.sync`.
+* Replicate meta data to other filers if not sharing the same filer meta store.
+* Replicate meta data to `weed mount` asynchronously.
+
+The gRPC API is also open to public and can support all other languages. 
+
 # Example
 
 Here is an example, in Java: