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Trying to understand Etag

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Intro

I found out about Etag while reading thru Youtube Data API documentation.

One of the properties returned as part of JSON response was etag with description "The Etag of this resource."

Fascinating how I wasn't aware of this nice web feature.

This post is the summary of what I understood from reading the Etag documentation on MDN (Mozilla Developer Network) documentation.

What is an ETag?

It's a response header returned from a server to identify the resource version.
It's a simple string value (or prefixed with W/, more on this later).

e.g.) 
etag: "123dklj"
etag: W/"diekjfk"

Note: Youtube Data API returns it as part of the resource "body" instead of as a response header

Before diving into how it's useful, let's check out two types of etags (weak & strong).

Types of Etags

There are two types of Etags, "Weak" and "Strong".

A Weak Etag

This is a "fuzzy" resource match against what a client has against what the server has.
If a blog post content hasn't changed only the dates shown on the post, then the weak Etag would validate to be true.

You need to prefix the Etag value with W/ (a capital "W") to identify the value as weak.

e.g.)
etag: W/"3121lasjd"

A Strong ETag

This matches etag value for a "byte-to-byte" equivalence of resource on the server.
This is a digital finger print and can be generated using MD5 hash.

There is no prefix required unlike the weak version, just a string value.

e.g.)
etag: "12343hdhjkakiek"

What problems does it solve?

Etag solves two issues.

  1. Mid-air collision detection ("Optimistic concurrency control ")
  2. Unchanged Resource caching

1. Mid-air collision detection

Etag solves the issue of, is the resource I am updating the same or modified by someone else?

Suppose that there are two web clients, "Client #1" and "Client #2" and retrieved the same resource from a server (They both have the same etag).

When the client #1 updates the resource, the server will now generate a new etag as the content has changed.
If the client #2 tries to write to the same resource, the server will reject as it was modified by another client and client #2 needs to get a new resource, and try to write again.

In detail, clients will include If-Match: <EtagValue> request header to make sure to update only "if matching" etag exists (meaning, resource hasn't changed).

In database term, it's called Optimistic concurrency control.

You can see the flow in the drawing below.

if-match

2. Unchanged Resource caching

Etag also tries to solve the issue of loading resources from cache when the resource requested hasn't changed on the server.

Youtube Data API has a quota for API calls and you can

  1. Increase a resource load time if it hasn't changed.
  2. Save on a quota cost.

When a resource hasn't changed, the server implementing Etag returns the HTTP Status code, 304 (Not Modified), thus a client can load data from cache.
If it has changed, then the server would return an updated resource.

A client would include a request header, If-None-Match: <EtagValue> when making a request. The server checks if the specified etag value exists. The server would return a new resource "if none matching" etag is found.

if-none-match


Image by BRRT from Pixabay

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