Why you should not store JSON in cookie

23 Jul 2014

Storing JSON in cookies will increase the size of cookie unnecessarly, which effects your web apps performance.

After a long time I fired up Firebug, which lead to a realization of mistake we done by storing JSON in cookies. I think Firebug is best to check cookie more than Chrome Devtools. Unfortunately Firefox devtools cookie panel is still not ready.

Firebug shows you two types of cookie size.

  • Raw Size
  • Size

Size is the actual content size before encoding the content.

Raw size is the size of cookie after encoding the content, the actual size when they send to server. When we store JSON, the {, :, ", } and , are encoded and converted to %7B, %3A, %22, %7D and %2C respectively.

Raw size

So if you use JSON, the characters like {}:", etc will be a unnecessary weight for the cookie. In the above image you can see that the difference of Raw size and size is around 48 Bytes which means this cookie’s ~40 Bytes are unnessary.

We can avoid this unnessary weight, by choosing plain text in the cookie and writing our own parser to extract the multiple values we stored in cookie.

For example, instead of storing

  "city": "kochi",
  "place": "kakkanad" 

we can store it as kochi|1|kakkanad and parse this ourselves which will reduce alot of weight.

Size of the cookie have effect on web performance.

As per the google best practice

the average size of cookies served off any domain be less than 400 bytes.

also Yahoo recommends to reduce the cookie’s size.

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