1 files changed, 32 insertions, 6 deletions
diff --git a/guides/source/caching_with_rails.md b/guides/source/caching_with_rails.md
index ebd67a4adb..6c734c1d78 100644
--- a/guides/source/caching_with_rails.md
+++ b/guides/source/caching_with_rails.md
@@ -100,8 +100,8 @@ called key-based expiration.
 
 Cache fragments will also be expired when the view fragment changes (e.g., the
 HTML in the view changes). The string of characters at the end of the key is a
-template tree digest. It is an md5 hash computed based on the contents of the
-view fragment you are caching. If you change the view fragment, the md5 hash
+template tree digest. It is an MD5 hash computed based on the contents of the
+view fragment you are caching. If you change the view fragment, the MD5 hash
 will change, expiring the existing file.
 
 TIP: Cache stores like Memcached will automatically delete old cache files.
@@ -258,7 +258,7 @@ comment format anywhere in the template, like:
 
 If you use a helper method, for example, inside a cached block and you then update
 that helper, you'll have to bump the cache as well. It doesn't really matter how
-you do it, but the md5 of the template file must change. One recommendation is to
+you do it, but the MD5 of the template file must change. One recommendation is to
 simply be explicit in a comment, like:
 
 ```html+erb
@@ -512,12 +512,38 @@ class ProductsController < ApplicationController
 end
 ```
 
-### A note on weak ETags
+### Strong v/s Weak ETags
 
-Etags generated by Rails are weak by default. Weak etags allow symantically equivalent responses to have the same etags, even if their bodies do not match exactly. This is useful when we don't want the page to be regenerated for minor changes in response body. If you absolutely need to generate a strong etag, it can be assigned to the header directly.
+Rails generates weak ETags by default. Weak ETags allow semantically equivalent
+responses to have the same ETags, even if their bodies do not match exactly.
+This is useful when we don't want the page to be regenerated for minor changes in
+response body.
+
+Weak ETags have a leading `W/` to differentiate them from strong ETags.
+
+```
+  W/"618bbc92e2d35ea1945008b42799b0e7" → Weak ETag
+  "618bbc92e2d35ea1945008b42799b0e7" → Strong ETag
+```
+
+Unlike weak ETag, strong ETag implies that response should be exactly the same
+and byte by byte identical. Useful when doing Range requests within a
+large video or PDF file. Some CDNs support only strong ETags, like Akamai.
+If you absolutely need to generate a strong ETag, it can be done as follows.
+
+```ruby
+  class ProductsController < ApplicationController
+    def show
+      @product = Product.find(params[:id])
+      fresh_when last_modified: @product.published_at.utc, strong_etag: @product
+    end
+  end
+```
+
+You can also set the strong ETag directly on the response.
 
 ```ruby
-  response.add_header "ETag", Digest::MD5.hexdigest(response.body)
+  response.strong_etag = response.body # => "618bbc92e2d35ea1945008b42799b0e7"
 ```
 
 References