When implementing dynamic content inclusion in static pages at the web server level, two primary technologies emerge: Server Side Includes (SSI) and Edge Side Includes (ESI). While both serve similar purposes, their architectures and capabilities differ significantly.

SSI traces back to NCSA HTTPd in the mid-90s, becoming a standard feature in web servers like Apache and Nginx. ESI emerged later (2001) as a W3C specification primarily driven by Akamai, designed for content delivery networks and edge computing.

Here's a comparison table of their capabilities:

Feature	SSI	ESI
Processing Location	Web server	Edge cache/CDN
Conditional Logic	Basic	Advanced
Cache Control	Limited	Granular
Fragment Caching	No	Yes
Error Handling	Basic	Robust

SSI Example (Nginx):

# nginx.conf
server {
    ssi on;
    ...
}


<!--# include virtual="/header.html" -->

ESI Example (Varnish):

# VCL configuration
sub vcl_backend_response {
    if (beresp.http.Surrogate-Control ~ "ESI/1.0") {
        unset beresp.http.Surrogate-Control;
        set beresp.do_esi = true;
    }
}


<esi:include src="/userbar" />

Opt for SSI when:

• You're using Nginx or Apache without Varnish
• Your dynamic content requirements are simple
• You need minimal setup overhead

Choose ESI when:

• Using a CDN or edge caching solution
• Require fragment-level caching
• Need advanced conditional logic
• Implementing personalization at scale

ESI's fragment caching can significantly reduce server load by caching page components independently. For example:

<esi:include src="/weather?zip=90210" max-age="300" />
<esi:include src="/user-profile" max-age="3600" />

This allows the weather widget to refresh every 5 minutes while keeping the user profile cached for an hour.

If starting with SSI but anticipating future ESI needs:

# Use comments to indicate future ESI tags
<!-- SSI: <!--# include virtual="/header" --> -->
<!-- Future ESI: <esi:include src="/header" /> -->

ESI supports powerful features like:

<esi:choose>
  <esi:when test="$(HTTP_COOKIE{group})=='premium'">
    <esi:include src="/premium-content"/>
  </esi:when>
  <esi:otherwise>
    <esi:include src="/standard-content"/>
  </esi:otherwise>
</esi:choose>

---

When building modern web applications, developers often face the challenge of including dynamic content within static pages. Two primary solutions have emerged:

// SSI Example (nginx config)
server {
    ssi on;
    location / {
        # Enable SSI processing
    }
}

// ESI Example (Varnish config)
sub vcl_recv {
    set req.http.Surrogate-Capability = "ESI/1.0";
}

SSI operates at the web server level (Apache, nginx), while ESI works at the edge caching layer (Varnish, CDNs). This fundamental distinction impacts several aspects:

• Processing Location: SSI executes on origin servers, ESI on edge nodes
• Caching Strategy: ESI enables fragment-level caching
• Network Topology: ESI supports distributed processing

Feature	SSI	ESI
Conditional Logic	Basic (if/else)	Advanced (try/except)
Fragment Caching	No	Yes
CDN Support	Limited	Native
Error Handling	Basic	Robust

SSI is ideal when:

    Mobile version

ESI shines for:

<esi:include src="http://cdn.example.com/userbar" onerror="continue"/>

<esi:try>
    <esi:attempt>
        <esi:include src="http://api.example.com/recommendations"/>
    </esi:attempt>
    <esi:except>
        Fallback content
    </esi:except>
</esi:try>

ESI's fragment caching can dramatically improve performance for personalized content:

// Varnish VCL for ESI caching
sub vcl_backend_response {
    if (beresp.http.Surrogate-Control ~ "ESI/1.0") {
        set beresp.do_esi = true;
        set beresp.ttl = 24h;
    }
}

For projects starting with SSI but anticipating future needs:

1. Design modular includes with clear separation
2. Avoid SSI-specific features when possible
3. Use wrapper patterns for easy ESI conversion

Here's how to implement a user bar with both technologies:

// SSI Implementation (nginx)
<!--#set var="user_name" value="$cookie_user" -->
<div class="userbar">
    <!--#if expr="$user_name" -->
        Welcome <!--#echo var="user_name" -->
    <!--#else -->
        <a href="/login">Sign in</a>
    <!--#endif -->
</div>

// ESI Implementation
<esi:include src="http://auth-service/userbar?$(QUERY_STRING)"
             cache-control="public, max-age=300"
             ttl="5m"/>