Apollo provides two techniques to allow your applications to load quickly, avoiding unnecessary delays to users:
- Store rehydration, which allows your initial set of queries to return data immediately without a server roundtrip.
- Server side rendering, which renders the initial HTML view on the server before sending it to the client.
You can use one or both of these techniques to provide a better user experience.
Before we dive more into SSR, let's create an example to work on.
You can render your entire Angular-based Apollo application on a Node server the same way as you would normally do with an Angular app.
No changes are required to client queries to support this, so your Apollo-based Angular UI should support SSR out of the box.
SSR works out of the box when using
apollo-angular-link-httpbecause it uses Angular's
HttpClientinternally. This would't be that easy with
apollo-link-http. That non-angular Link uses Fetch API which would have to schedule a macroTask (Zone.js) so Angular could wait for the request to finish.
For applications that can perform some queries on the server prior to rendering the UI on the client, Apollo allows for setting the initial state of data. This is sometimes called rehydration, since the data is "dehydrated" when it is serialized and included in the initial HTML payload.
For example, a typical approach is to use
BrowserTransferStateModule so you can then rehydrate the client using the initial state passed from the server:
Adding store rehydration to your app is straightforward since Angular itself has a mechanism to transfer data from server to client.
Let's explain step by step how we implemented store rehydration in the example.
First, we imported
Then we created a
STATE_KEY which allows to attach data to it:
Now we can implement some logic by using
this.transferState.hasKey<any>(STATE_KEY) will return a value only if it runs within a browser, on the client side.
Here is an interesting part, the actual store rehydration.
onServer we extract data from cache when TransferState says it is ready to save and serialize data.
OnBrowser we do opposite thing, we receive transfered data to restore cache.
Super easy and clean!
With all this when the client runs the first set of queries, the data will be returned instantly because it is already in the store!
If you are using
forceFetch on some of the initial queries, you can pass the
ssrForceFetchDelay option to skip force fetching during initialization, so that even those queries run using the cache:
You saw how to use Server-Side Rendering and Store Rehydration in your application, but you will need to be a little careful in how you create Apollo on the server to ensure everything works there as well:
When creating Apollo (
Apollo.create) on the server, you'll need to set up your HttpLink to connect to the API server correctly. This might look different to how you do it on the client, since you'll probably have to use an absolute URL to the server if you were using a relative URL on the client.
Since you only want to fetch each query result once, pass the
ssrMode: trueoption to the
Apollo.createto avoid repeated force-fetching.
You need to ensure that you create a new client or store instance for each request, rather than re-using the same client for multiple requests. Otherwise the UI will be getting stale data and you'll have problems with authentication.
You can take a look on a simple example with the implementation we talked about.