# Developer Guide: Angular Services: Managing Service Dependencies

Work in Progress This page is currently being revised. It might be incomplete or contain inaccuracies.

Angular allows services to declare other services as dependencies needed for construction of their instances.

To declare dependencies, you specify them in the factory function signature and via the `$inject` property, as an array of string identifiers. Optionally the `$inject` property declaration can be dropped (see "Inferring `$inject`" but note that that is currently an experimental feature).

Here is an example of two services that depend on each other, as well as on other services that are provided by angular's web framework:

    /**
    * batchLog service allows for messages to be queued in memory and flushed
    * to the console.log every 50 seconds.
    *
    * @param {*} message Message to be logged.
    */
    angular.service('batchLog', function($defer, $log) {
      var messageQueue = [];

      function log() {
        if (messageQueue.length) {
          $log('batchLog messages: ', messageQueue);
          messageQueue = [];
        }
        $defer(log, 50000);
      }

      // start periodic checking
      log();

      return function(message) {
        messageQueue.push(message);
      }
    }, {$inject: ['$defer', '$log']});
    // note how we declared dependency on built-in $defer and $log services above

    /**
    * routeTemplateMonitor monitors each $route change and logs the current
    * template via the batchLog service.
    */
    angular.service('routeTemplateMonitor', function($route, batchLog) {
      $route.onChange(function() {
        batchLog($route.current ? $route.current.template : null);
      });
    }, {$inject: ['$route', 'batchLog'], $eager: true});

Things to notice in this example:

- The `batchLog` service depends on the built-in `$defer` and `$log` services, and allows messages to be logged into the `console.log` in batches.
- The `routeTemplateMonitor` service depends on the built-in `$route` service as well as our custom `batchLog` service.
- The `routeTemplateMonitor` service is declared to be eager, so that it is started as soon as the application starts.
- To underline the need for the eager instantiation of the `routeTemplateMonitor` service, nothing else in the application depends on this service, and in this particular case the factory function of this service doesn't return anything at all.
- Both of our services use the factory function signature as well as the `$inject` property to declare their dependencies. It is important that the order of the string identifiers in the array associated with the `$inject` property is the same as the order of argument names in the signature of the factory function. Unless the dependencies are inferred from the function signature, it is this array with IDs and their order that the injector uses to determine which services and in which order to inject.

### Related Topics

- Understanding Angular Services
- Creating Angular Services
- Registering Services
- Injecting Services Into Controllers
- Testing Angular Services

### Related API

- `Angular Service API`
- `Angular Injector API`