Jun 06, 2020 • ☕️ 4 min read
Sounds like an ideal technique, right? Let’s dig deeper to see how it works, how it performs compare to other rendering techniques, and in which situation it really excels.
Basic understanding of how SSR and CSR work is expected because this technique combines the best of both; a key takeaway is that SSR excels at SEO performance and CSR excels at interactivity.
Hydration refers to filling an existing object with data. It is not a synonym for serialization. It is not a synonym for deserialization either. Deserialization involves creating the object with the data it had at the time it was serialized. Hydration refers to how you fill an object with data. Hydration is also used when discussing plans for loading data from databases or other data sources to construct application.
You could say that an object is partially hydrated when you have only loaded some of the fields into it, but not all of them.
In React, if there is a mismatch, it will bail hydration, discard existing DOM and render from scratch. In production mode, this assertion is disabled for maximum performance.
Rehydration is very SEO friendly like SSR when complete HTML markup already generated on server, it also delivers very good perceived performance, and solid first contentful pain on a wide range of devices.
One catch is the confusion caused by perceived performance and delayed interactivity, your sites often look deceptively loaded and interactive, but can’t actually respond to input until the client-side JS is executed and event handlers have been attached. This can take seconds or even minutes on mobile.
Rehydration also poses some challenges for caching, navigation, special HTML structures altered by browsers.
To overcome the downside of delayed interactivity, many variations to fully rehydration was proposed to only rehydrate some parts of the page.
Progressive rehydration — individual pieces of a server-rendered application are booted up over time, rather than the current common approach of initializing the entire application at once.
Partial rehydration — an extension of the idea of progressive rehydration, where the individual pieces (components / views / trees) to be progressively rehydrated are analyzed and those with little interactivity or no reactivity are identified.
Lazy hydration — delay the hydration of pre-rendered HTML util it needed.
But above assumption happens in an ideal world, in reality rehydration solution seem more complex than pure SSR or CSR when some parts of your code only work on server and some parts only work on browser.
You also need to pick 3rd-party libraries carefully when some don’t work properly with rehydration, especially those developed purely for CSR.
Next.js is the best React framework that implements rehydration, it is matured, battle-tested, has plenty examples, simple to use and tons of other features; definitely check it out if you’re using React and also want SSR.
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