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docs/advanced/debugging.md
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       ---

     

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       title: Debugging

     

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       order: 6

     

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       ---

     

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       # Debugging

     

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       We've tried to make debugging in `urql` as seamless as possible by creating tools for users of `urql` and those creating their own exchanges.

     

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       ## Debug events

     

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       Debug events are a means for seeing what's going on inside of `urql` exchanges. Before getting started, here are a few things to be aware of.

     

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       ### Anyone with client access can listen

     

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       This includes the creator of an `urql` client and any of it's exchanges (although the latter is not advised).

     

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       ### Debug events are disabled in production

     

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       Debug events are a fire-and-forget mechanism and should not be used as a means for messaging in your app.

     

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       ## Consuming debug events

     

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       Debugging events can be inspected either graphically using our [devtools](https://github.com/FormidableLabs/urql-devtools) or manually by subscribing to events on the client.

     

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       ### Devtools

     

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       The quickest way is going to be using our [devtools extension](https://github.com/FormidableLabs/urql-devtools/) which visualizes events on a timeline and provides tools to filter events, inspect the cache, and trigger custom queries via the running client.

     

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       ![Urql Devtools Timeline](../assets/devtools-timeline.png)

     

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       Visit [the repo](https://github.com/FormidableLabs/urql-devtools/) for instructions on getting started.

     

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       > Note: Devtools is unfortunately not currently supported for React Native but we're looking into it!

     

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       ### Manual consumption of events

     

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       For those not looking to use a GUI to view events, the client has a `subscribeToDebugTarget` function.

     

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       As demonstrated below, the `subscribeToDebugTarget` function takes a callback which is called with every debug event that is dispatched.

     

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       ```ts

     

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       const { unsubscribe } = client.subscribeToDebugTarget(event => {

     

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         if (event.source === 'dedupExchange') {

     

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           return;

     

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         }

     

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         console.log(event);

     

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       });

     

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       // ...

     

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       // Unsubscribe from events

     

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       unsubscribe();

     

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       ```

     

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       ## Dispatching debug events

     

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       Debug events are a means of sharing implementation details to consumers of an exchange.

     

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       #### Identify key events

     

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       The first step is to identify key events of the exchange in question.

     

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       For example, a [_fetchExchange_](https://github.com/FormidableLabs/urql/blob/master/packages/core/src/exchanges/fetch.ts) which triggers fetch requests may have an event of type `fetchRequest`.

     

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       #### Create an event type (optional)

     

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       For type safe events, and to prevent conflicts with other exchanges, [declaration merging](https://www.typescriptlang.org/docs/handbook/declaration-merging.html) can be used.

     

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       ```ts

     

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       // urql.d.ts

     

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       import '@urql/core';

     

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       declare module '@urql/core' {

     

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         interface DebugEventTypes {

     

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           fetchRequest: { targetUrl: string };

     

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         }

     

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       }

     

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       ```

     

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       #### Dispatch the event

     

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       A `dispatchDebug` function is now passed to every exchange and is used to dispatch debug events.

     

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       It is called with an object containing the following properties:

     

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       - `type` - a unique identifier for the event type.

     

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       - `message` - a human readable description of the event.

     

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       - `operation` - the operation in scope when the event occured.

     

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       - `data` _(optional)_ - any additional data useful for debugging

     

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       Here, we call `dispatchDebug` with our `fetchRequest` event we declared earlier.

     

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       ```ts

     

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       export const fetchExchange: Exchange = ({ forward, dispatchDebug }) => {

     

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         // ...

     

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         dispatchDebug({

     

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           type: 'fetchRequest',

     

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           message: 'A network request has been triggered',

     

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           operation,

     

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           data: { targetUrl },

     

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         });

     

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       };

     

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       ```

     

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       > Note: for a real world example, see the [_fetchExchange_](https://github.com/FormidableLabs/urql/blob/master/packages/core/src/exchanges/fetch.ts).

     

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       ### Tips

     

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       In summary, here are a few do's and don'ts.

     

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       #### ✅ Do share internal details

     

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       Frequent debug messages on key events inside your exchange is very useful for consumers.

     

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       #### ✅ Do create unique event types

     

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       Key events should be easy to identify and differentiate, so have a unique name and data format for each unique event and use that format consistently.

     

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       #### ❌ Don't listen to debug events inside your exchange

     

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       While it is possible, there isn't any value in doing this. Use the exchange pipeline to communicate with other exchanges.

     

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       #### ❌ Don't send warnings in debug events

     

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       Debug **events** are intended to document **events** inside an exchange, not as a way to send messages to the user. Use `console.warn` to send alerts to the user.
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