What is strong cache and negotiated cache for nginx

1. Strong Cache

1. definition

• Strong cache tells the browser directly:No need to communicate with the server before the cache expires, directly use local cache.
• Pass the response header by the serverCache-ControlandExpirescontrol.

2. Response header

• Cache-Control: max-age=3600
Indicates that the resource is3600 seconds (1 hour)Valid within (priority higher thanExpires）。
• Expires: Thu, 31 Dec 2030 23:59:59 GMT
Specifies an absolute expiration time (dependent on client local time, errors may occur).

3. Nginx configuration example

location /static/ {
     # Set strong cache: valid within 1 year
     add_header Cache-Control "public, max-age=31536000";
     expires 1y;
 }

4. Behavior

• When the browser first requests a resource, the server returns the resource and comes with a cache header.
• During subsequent requests, the browser directly reads the local cache (status code)200 (from disk cache)），Do not send requests to the server。

5. Applicable scenarios

• Static resources (such as CSS, JS, pictures, font files) that remain unchanged for a long time.

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2. Negotiation cache (Weak Cache)

1. definition

• Negotiate cache requirements browserVerify with the server every time that the cache is expired, if it has not expired, it will return304 Not Modified, continue to use local cache.
• Pass the response header by the serverLast-ModifiedandETagcontrol.

2. Response header

• Last-Modified: Wed, 21 Oct 2023 07:28:00 GMT
Indicates the resource's last modification time (precision is seconds, which may be invalid due to time synchronization problems).
• ETag: "5d8c72a5-264"
A unique identifier (hash value or version number) of the resource, with higher precision.

3. Nginx configuration example

location /dynamic/ {
     # Enable negotiation cache (supported by default, no explicit configuration is required)
     add_header Last-Modified "";
     etag on;
 }

4. Behavior

1. When the browser first requests a resource, the server returns the resource and comes with itLast-ModifiedorETag。
2. On subsequent requests, the browser verifies the cache through the following request header:
   • If-Modified-Since: [Last-Modified value]
   Ask the server if the resource has been modified after the specified time.
   • If-None-Match: [ETag value]
   Verify the resource to the serverETagWhether it changes.
3. If the resource is not modified, the server returns304 Not Modified, the browser continues to use the cache; if modified, return a new resource (status code200）。

5. Applicable scenarios

• Resources that are frequently updated (such as HTML pages, dynamic API responses).

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3. Key differences

characteristic	Strong cache	Negotiate cache
Communication costs	No network requests (direct read cache)	Request verification cache is required
Response status code	200 (from disk cache)	304 Not Modified
Priority	Priority over negotiated cache	Triggered after the strong cache expires
Applicable resources	Static resources that remain unchanged for a long time	Frequently updated dynamic resources

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IV. Nginx best practices

1. Mix two caches

   location / {
        # Strong cache for 1 hour, and enable negotiation cache after expiration
        add_header Cache-Control "public, max-age=3600";
        etag on;
    }

2. Distinguish policies by file type

   # Strong cache of pictures, fonts, etc.
    location ~* \.(jpg|png|gif|woff2)$ {
        expires 1y;
        add_header Cache-Control "public, max-age=31536000";
    }
   
    # HTML files are disabled for strong cache (always negotiated)
    location ~* \.html$ {
        add_header Cache-Control "no-cache, must-revalidate";
    }

3. Solve cache update issues
   • Strong cache resources are recommended to passFile name hashControl version (e.g.main.）。
   • Negotiation cache can be modifiedETagorLast-ModifiedTrigger update.

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5. Debugging tools

1. Browser Developer Tools (Network Tag):
   • Check200 (from disk cache)(Strong cache) or304 Not Modified(Negotiation cache).
   • Check the request headerCache-Control、If-Modified-Since、If-None-Match。
2. Command line tools:

   curl -I /
   

By rationally configuring strong caching and negotiated caching, website performance can be significantly improved and server load can be reduced.