ZooKeeper

Since Camel 2.9

Both producer and consumer are supported

The ZooKeeper component allows interaction with a ZooKeeper cluster and exposes the following features to Camel:

  1. Creation of nodes in any of the ZooKeeper create modes.

  2. Get and Set the data contents of arbitrary cluster nodes (data being set must be convertible to byte[]).

  3. Create and retrieve the list the child nodes attached to a particular node.

Maven users will need to add the following dependency to their pom.xml for this component:

<dependency>
    <groupId>org.apache.camel</groupId>
    <artifactId>camel-zookeeper</artifactId>
    <version>x.x.x</version>
    <!-- use the same version as your Camel core version -->
</dependency>

URI format

zookeeper://zookeeper-server[:port][/path][?options]

The path from the URI specifies the node in the ZooKeeper server (a.k.a. znode) that will be the target of the endpoint:

Configuring Options

Camel components are configured on two separate levels:

  • component level

  • endpoint level

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.

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 and DataFormat DSL as a type safe way of configuring endpoints and data formats in Java.

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.

Component Options

The ZooKeeper component supports 11 options, which are listed below.

Name Description Default Type

listChildren (common)

Whether the children of the node should be listed.

false

boolean

timeout (common)

The time interval to wait on connection before timing out.

5000

int

backoff (consumer)

The time interval to backoff for after an error before retrying.

5000

long

bridgeErrorHandler (consumer)

Allows for bridging the consumer to the Camel routing Error Handler, which mean any exceptions (if possible) occurred while the Camel consumer is trying to pickup incoming messages, or the likes, will now be processed as a message and handled by the routing Error Handler. Important: This is only possible if the 3rd party component allows Camel to be alerted if an exception was thrown. Some components handle this internally only, and therefore bridgeErrorHandler is not possible. In other situations we may improve the Camel component to hook into the 3rd party component and make this possible for future releases. 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

repeat (consumer)

Should changes to the znode be 'watched' and repeatedly processed.

false

boolean

sendEmptyMessageOnDelete (consumer)

Upon the delete of a znode, should an empty message be send to the consumer.

true

boolean

create (producer)

Should the endpoint create the node if it does not currently exist.

false

boolean

createMode (producer)

The create mode that should be used for the newly created node.

Enum values:

  • PERSISTENT

  • PERSISTENT_SEQUENTIAL

  • EPHEMERAL

  • EPHEMERAL_SEQUENTIAL

EPHEMERAL

String

lazyStartProducer (producer)

Whether the producer should be started lazy (on the first message). By starting lazy you can use this to allow CamelContext and routes to startup in situations where a producer may otherwise fail during starting and cause the route to fail being started. By deferring this startup to be lazy then the startup failure can be handled during routing messages via Camel’s routing error handlers. Beware that when the first message is processed then creating and starting the producer may take a little time and prolong the total processing time of the processing.

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

configuration (advanced)

To use a shared ZooKeeperConfiguration.

ZooKeeperConfiguration

Endpoint Options

The ZooKeeper endpoint is configured using URI syntax:

zookeeper:serverUrls/path

with the following path and query parameters:

Path Parameters (2 parameters)

Name Description Default Type

serverUrls (common)

Required The zookeeper server hosts (multiple servers can be separated by comma).

String

path (common)

Required The node in the ZooKeeper server (aka znode).

String

Query Parameters (11 parameters)

Name Description Default Type

listChildren (common)

Whether the children of the node should be listed.

false

boolean

timeout (common)

The time interval to wait on connection before timing out.

5000

int

backoff (consumer)

The time interval to backoff for after an error before retrying.

5000

long

repeat (consumer)

Should changes to the znode be 'watched' and repeatedly processed.

false

boolean

sendEmptyMessageOnDelete (consumer)

Upon the delete of a znode, should an empty message be send to the consumer.

true

boolean

bridgeErrorHandler (consumer (advanced))

Allows for bridging the consumer to the Camel routing Error Handler, which mean any exceptions (if possible) occurred while the Camel consumer is trying to pickup incoming messages, or the likes, will now be processed as a message and handled by the routing Error Handler. Important: This is only possible if the 3rd party component allows Camel to be alerted if an exception was thrown. Some components handle this internally only, and therefore bridgeErrorHandler is not possible. In other situations we may improve the Camel component to hook into the 3rd party component and make this possible for future releases. 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

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:

  • InOnly

  • InOut

ExchangePattern

create (producer)

Should the endpoint create the node if it does not currently exist.

false

boolean

createMode (producer)

The create mode that should be used for the newly created node.

Enum values:

  • PERSISTENT

  • PERSISTENT_SEQUENTIAL

  • EPHEMERAL

  • EPHEMERAL_SEQUENTIAL

EPHEMERAL

String

lazyStartProducer (producer (advanced))

Whether the producer should be started lazy (on the first message). By starting lazy you can use this to allow CamelContext and routes to startup in situations where a producer may otherwise fail during starting and cause the route to fail being started. By deferring this startup to be lazy then the startup failure can be handled during routing messages via Camel’s routing error handlers. Beware that when the first message is processed then creating and starting the producer may take a little time and prolong the total processing time of the processing.

false

boolean

Message Headers

The ZooKeeper component supports 7 message header(s), which is/are listed below:

Name Description Default Type

CamelZooKeeperNode (common)

Constant: ZOOKEEPER_NODE

The node.

String

CamelZooKeeperVersion (common)

Constant: ZOOKEEPER_NODE_VERSION

The node version.

-1

Integer

CamelZookeeperAcl (common)

Constant: ZOOKEEPER_ACL

The ACL.

Ids.OPEN_ACL_UNSAFE

List

CamelZookeeperCreateMode (common)

Constant: ZOOKEEPER_CREATE_MODE

The create mode.

CreateMode or String

CamelZookeeperStatistics (common)

Constant: ZOOKEEPER_STATISTICS

The statistics.

Stat

CamelZookeeperEventType (common)

Constant: ZOOKEEPER_EVENT_TYPE

The event type.

Enum values:

  • None

  • NodeCreated

  • NodeDeleted

  • NodeDataChanged

  • NodeChildrenChanged

  • DataWatchRemoved

  • ChildWatchRemoved

  • PersistentWatchRemoved

EventType

CamelZookeeperOperation (producer)

Constant: ZOOKEEPER_OPERATION

The operation to perform.

String

Use cases

Reading from a znode

The following snippet will read the data from the znode /somepath/somenode/ provided that it already exists. The data retrieved will be placed into an exchange and passed onto the rest of the route:

from("zookeeper://localhost:39913/somepath/somenode").to("mock:result");

If the node does not yet exist then a flag can be supplied to have the endpoint await its creation:

from("zookeeper://localhost:39913/somepath/somenode?awaitCreation=true").to("mock:result");

Reading from a znode

When data is read due to a WatchedEvent received from the ZooKeeper ensemble, the CamelZookeeperEventType header holds ZooKeeper’s EventType value from that WatchedEvent. If the data is read initially (not triggered by a WatchedEvent) the CamelZookeeperEventType header will not be set.

Writing to a znode

The following snippet will write the payload of the exchange into the znode at /somepath/somenode/ provided that it already exists:

from("direct:write-to-znode")
    .to("zookeeper://localhost:39913/somepath/somenode");

For flexibility, the endpoint allows the target znode to be specified dynamically as a message header. If a header keyed by the string CamelZooKeeperNode is present then the value of the header will be used as the path to the znode on the server. For instance using the same route definition above, the following code snippet will write the data not to /somepath/somenode but to the path from the header /somepath/someothernode.

the testPayload must be convertible to byte[] as the data stored in ZooKeeper is byte based. 
Object testPayload = ...
template.sendBodyAndHeader("direct:write-to-znode", testPayload, "CamelZooKeeperNode", "/somepath/someothernode");

To also create the node if it does not exist the create option should be used.

from("direct:create-and-write-to-znode")
    .to("zookeeper://localhost:39913/somepath/somenode?create=true");

It is also possible to delete a node using the header CamelZookeeperOperation by setting it to DELETE:

from("direct:delete-znode")
    .setHeader(ZooKeeperMessage.ZOOKEEPER_OPERATION, constant("DELETE"))
    .to("zookeeper://localhost:39913/somepath/somenode");

or equivalently:

<route>
  <from uri="direct:delete-znode" />
  <setHeader name="CamelZookeeperOperation">
     <constant>DELETE</constant>
  </setHeader>
  <to uri="zookeeper://localhost:39913/somepath/somenode" />
</route>

ZooKeeper nodes can have different types; they can be 'Ephemeral' or 'Persistent' and 'Sequenced' or 'Unsequenced'. For further information of each type you can check here. By default endpoints will create unsequenced, ephemeral nodes, but the type can be easily manipulated via a uri config parameter or via a special message header. The values expected for the create mode are simply the names from the CreateMode enumeration:

  • PERSISTENT

  • PERSISTENT_SEQUENTIAL

  • EPHEMERAL

  • EPHEMERAL_SEQUENTIAL

For example to create a persistent znode via the URI config:

from("direct:create-and-write-to-persistent-znode")
    .to("zookeeper://localhost:39913/somepath/somenode?create=true&createMode=PERSISTENT");

or using the header CamelZookeeperCreateMode.

the testPayload must be convertible to byte[] as the data stored in ZooKeeper is byte based. 
Object testPayload = ...
template.sendBodyAndHeader("direct:create-and-write-to-persistent-znode", testPayload, "CamelZooKeeperCreateMode", "PERSISTENT");