Chapter 15. Cron
Only consumer is supported
The Cron component is a generic interface component that allows triggering events at specific time interval specified using the Unix cron syntax (e.g. 0/2 * * * * ? to trigger an event every two seconds).
Being an interface component, the Cron component does not contain a default implementation, instead it requires that the users plug the implementation of their choice.
The following standard Camel components support the Cron endpoints:
- Camel-quartz
- Camel-spring
The Cron component is also supported in Camel K, which can use the Kubernetes scheduler to trigger the routes when required by the cron expression. Camel K does not require additional libraries to be plugged when using cron expressions compatible with Kubernetes cron syntax.
Maven users will need to add the following dependency to their pom.xml for this component:
<dependency>
<groupId>org.apache.camel</groupId>
<artifactId>camel-cron</artifactId>
<version>3.14.5.redhat-00018</version>
<!-- use the same version as your Camel core version -->
</dependency>Additional libraries may be needed in order to plug a specific implementation.
15.1. Configuring Options
Camel components are configured on two separate levels:
- component level
- endpoint level
15.1.1. Configuring Component Options
The component level is the highest level which holds general and common configurations that are inherited by the endpoints. For example a component may have security settings, credentials for authentication, urls for network connection and so forth.
Some components only have a few options, and others may have many. Because components typically have pre configured defaults that are commonly used, then you may often only need to configure a few options on a component; or none at all.
Configuring components can be done with the Component DSL, in a configuration file (application.properties|yaml), or directly with Java code.
15.1.2. Configuring Endpoint Options
Where you find yourself configuring the most is on endpoints, as endpoints often have many options, which allows you to configure what you need the endpoint to do. The options are also categorized into whether the endpoint is used as consumer (from) or as a producer (to), or used for both.
Configuring endpoints is most often done directly in the endpoint URI as path and query parameters. You can also use the Endpoint DSL as a type safe way of configuring endpoints.
A good practice when configuring options is to use Property Placeholders, which allows to not hardcode urls, port numbers, sensitive information, and other settings. In other words placeholders allows to externalize the configuration from your code, and gives more flexibility and reuse.
The following two sections lists all the options, firstly for the component followed by the endpoint.
15.2. Component Options
The Cron component supports 3 options, which are listed below.
| Name | Description | Default | Type |
|---|---|---|---|
| bridgeErrorHandler (consumer) | Allows for bridging the consumer to the Camel routing Error Handler, which mean any exceptions occurred while the consumer is trying to pickup incoming messages, or the likes, will now be processed as a message and handled by the routing Error Handler. By default the consumer will use the org.apache.camel.spi.ExceptionHandler to deal with exceptions, that will be logged at WARN or ERROR level and ignored. | false | boolean |
| autowiredEnabled (advanced) | Whether autowiring is enabled. This is used for automatic autowiring options (the option must be marked as autowired) by looking up in the registry to find if there is a single instance of matching type, which then gets configured on the component. This can be used for automatic configuring JDBC data sources, JMS connection factories, AWS Clients, etc. | true | boolean |
| cronService (advanced) | The id of the CamelCronService to use when multiple implementations are provided. | String |
15.3. Endpoint Options
The Cron endpoint is configured using URI syntax:
cron:name
with the following path and query parameters:
15.3.1. Path Parameters (1 parameters)
| Name | Description | Default | Type |
|---|---|---|---|
| name (consumer) | Required The name of the cron trigger. | String |
15.3.2. Query Parameters (4 parameters)
| Name | Description | Default | Type |
|---|---|---|---|
| bridgeErrorHandler (consumer) | Allows for bridging the consumer to the Camel routing Error Handler, which mean any exceptions occurred while the consumer is trying to pickup incoming messages, or the likes, will now be processed as a message and handled by the routing Error Handler. By default the consumer will use the org.apache.camel.spi.ExceptionHandler to deal with exceptions, that will be logged at WARN or ERROR level and ignored. | false | boolean |
| schedule (consumer) | Required A cron expression that will be used to generate events. | String | |
| exceptionHandler (consumer (advanced)) | To let the consumer use a custom ExceptionHandler. Notice if the option bridgeErrorHandler is enabled then this option is not in use. By default the consumer will deal with exceptions, that will be logged at WARN or ERROR level and ignored. | ExceptionHandler | |
| exchangePattern (consumer (advanced)) | Sets the exchange pattern when the consumer creates an exchange. Enum values:
| ExchangePattern |
15.4. Usage
The component can be used to trigger events at specified times, as in the following example:
from("cron:tab?schedule=0/1+*+*+*+*+?")
.setBody().constant("event")
.log("${body}");
The schedule expression 0/3+10+*+? can be also written as 0/3 10 * * * ? and triggers an event every three seconds only in the tenth minute of each hour.
Parts in the schedule expression means (in order):
- Seconds (optional)
- Minutes
- Hours
- Day of month
- Month
- Day of week
- Year (optional)
Schedule expressions can be made of 5 to 7 parts. When expressions are composed of 6 parts, the first items is the "seconds" part (and year is considered missing).
Other valid examples of schedule expressions are:
-
0/2 * * * ?(5 parts, an event every two minutes) -
0 0/2 * * * MON-FRI 2030(7 parts, an event every two minutes only in year 2030)
Routes can also be written using the XML DSL.
<route>
<from uri="cron:tab?schedule=0/1+*+*+*+*+?"/>
<setBody>
<constant>event</constant>
</setBody>
<to uri="log:info"/>
</route>15.5. Spring Boot Auto-Configuration
When using cron with Spring Boot make sure to use the following Maven dependency to have support for auto configuration:
<dependency> <groupId>org.apache.camel.springboot</groupId> <artifactId>camel-cron-starter</artifactId> <version>3.14.5.redhat-00032</version> <!-- Use your Camel Spring Boot version --> </dependency>
The component supports 4 options, which are listed below.
| Name | Description | Default | Type |
|---|---|---|---|
| camel.component.cron.autowired-enabled | Whether autowiring is enabled. This is used for automatic autowiring options (the option must be marked as autowired) by looking up in the registry to find if there is a single instance of matching type, which then gets configured on the component. This can be used for automatic configuring JDBC data sources, JMS connection factories, AWS Clients, etc. | true | Boolean |
| camel.component.cron.bridge-error-handler | Allows for bridging the consumer to the Camel routing Error Handler, which mean any exceptions occurred while the consumer is trying to pickup incoming messages, or the likes, will now be processed as a message and handled by the routing Error Handler. By default the consumer will use the org.apache.camel.spi.ExceptionHandler to deal with exceptions, that will be logged at WARN or ERROR level and ignored. | false | Boolean |
| camel.component.cron.cron-service | The id of the CamelCronService to use when multiple implementations are provided. | String | |
| camel.component.cron.enabled | Whether to enable auto configuration of the cron component. This is enabled by default. | Boolean |
