Scopes and Hubs

When an event is captured and sent to Sentry, SDKs will merge that event data with extra information from the current scope. SDKs will typically automatically manage the scopes for you in the framework integrations and you don't need to think about them. However, you should know what a scope is and how you can use it for your advantage.

What's a Scope, What's a Hub

You can think of the hub as the central point that our SDKs use to route an event to Sentry. When you call init() a hub is created and a client and a blank scope are created on it. That hub is then associated with the current thread and will internally hold a stack of scopes.

The scope will hold useful information that should be sent along with the event. For instance contexts or breadcrumbs are stored on the scope. When a scope is pushed, it inherits all data from the parent scope and when it pops all modifications are reverted.

The default SDK integrations will push and pop scopes intelligently. For instance web framework integrations will create and destroy scopes around your routes or controllers.

How the Scope and Hub Work

As you start using an SDK a scope and hub are automatically created for you out of the box. The hub you are unlikely to be interacting with directly unless you are writing an integration or you want to create or destroy scopes. Scopes on the other hand are more user facing. You can at any point in time call configure-scope to modify data stored on the scope. This is for instance used to modify the context.

When you call a global function such as capture_event internally Sentry discovers the current hub and asks it to capture an event. Internally the hub will then merge the event with the topmost scope's data.

Configuring the Scope

The most useful operation when working with scopes is the configure-scope function. It can be used to reconfigure the current scope.

You can, for instance, add custom tags or inform Sentry about the currently authenticated user.

import 'package:sentry/sentry.dart';

Sentry.configureScope((scope) => scope
  ..setTag('my-tag', 'my value')
  ..user = User(id: '42', email: ''));

You can also apply this configuration when unsetting a user at logout:

import 'package:sentry/sentry.dart';

Sentry.configureScope((scope) => scope.user = null);

To learn what useful information can be associated with scopes see the context documentation.

Local Scopes

We also have support for pushing and configuring a scope in one go. This is typically called with-scope or push-scope which is also very helpful if you only want to send data with one specific event. In the following example we are using that function to attach a level and a tag to only one specific error:

import 'package:sentry/sentry.dart';

    withScope: (scope) {
        // will be tagged with my-tag="my value"
        scope.setTag('my-tag', 'my value');
        scope.level = SentryLevel.warning;

// will not be tagged with my-tag

While this example looks similar to configure-scope, it's very different, in the sense that configure-scope actually changes the current active scope, all successive calls to configure-scope will keep the changes.

While on the other hand using with-scope creates a clone of the current scope and will stay isolated until the function call is completed. So you can either set context information in there that you don't want to be somewhere else or not attach any context information at all by calling clear on the scope, while the "global" scope remains unchanged.

Keep in mind that with-scope will not capture any exceptions that happen inside its callback function, and every error that happens there will be silently ignored and not reported.

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