Adding a new REST endpoint to elasticsearch

2016-07-30
2016-10-19
7 min read
Tutorial
Plugin for Elasticsearch V5

This blog post is part of a series which will teach you:

Imagine that you wish to add a new REST endpoint so you can send requests like:

curl -XGET "http://localhost:9200/_hello?name=David&pretty"

# Or

curl -XGET "http://localhost:9200/_hello/David&pretty"

# Or

curl -XGET "http://localhost:9200/_hello?pretty" -d '{
    "name": "David"
}'

# Or

curl -XPOST "http://localhost:9200/_hello?pretty" -d '{
    "name": "David"
}'

And you want to get back something like:

{
    "message": "Hello David!"
}

Without any parameter:

curl -XGET "http://localhost:9200/_hello?pretty"

It should return:

{
    "message": "Hello World!"
}

Or get back a list of existing indices and the number of documents for a given type of indices with:

curl -XGET http://localhost:9200/_bano

Let’s see how to implement that!

With what we saw in the previous article, we now have a Plugin skeleton ready to host our REST endpoint code. We also have seen how to create real integration tests.

We are now all set to implement the REST endpoint and be able to test it with a REST Client.

Rest Handler

Rest Handler skeleton

Basically, we will extend the BaseRestHandler class. Let’s create a HelloRestAction in src/main/java/org/elasticsearch/ingest/bano/:

public class HelloRestAction extends BaseRestHandler {

    @Inject
    public HelloRestAction(Settings settings, RestController controller) {
        super(settings);
        // Register your handlers here
    }

    @Override
    protected RestChannelConsumer prepareRequest(RestRequest restRequest, NodeClient client) throws IOException {
        // Implement the REST logic here
    }
}

Register endpoints

In the constructor, we can define when this class will be called:

@Inject
public HelloRestAction(Settings settings, RestController controller) {
    super(settings);
    controller.registerHandler(GET, "/_hello", this);
}

Here we are expecting a GET request on /_hello endpoint.

Register the REST handler

We need to define the REST handler in the plugin:

@Override
public List<Class<? extends RestHandler>> getRestHandlers() {
    return Collections.singletonList(HelloRestAction.class);
}

Test the endpoint

We have to add some “hacks” to the default integration tests. Let’s create a BanoRestIT in src/test/java/org/elasticsearch/ingest/bano/. Note that it extends AbstractITCase, which we have have seen previously:

public class BanoRestIT extends AbstractITCase {
}

Let’s now implement our first test:

public void testHelloWorld() throws Exception {
    Response response = client.performRequest("GET", "/_hello");
    assertThat(entityAsMap(response), hasEntry("message", "Hello World!"));
}

Implement handleRequest

As we did not implemented the REST logic yet, this test will obviously fail. Let’s fix that in HelloRestAction.

First, we will send back a JSON object to the user. Elasticsearch provides XContent classes to deal with this. We can create a class which will represent our response:

class Message implements ToXContent {

    private final String name;

    public Message(String name) {
        if (name == null) {
            this.name = "World";
        } else {
            this.name = name;
        }
    }

    @Override
    public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
        return builder.field("message", "Hello " + name + "!");
    }
}

This class implements ToXContent so it provides a toXContent method where we build the actual JSON response.

Then, we implement prepareRequest method:

@Override
protected RestChannelConsumer prepareRequest(RestRequest restRequest, NodeClient client) throws IOException {
    String name = restRequest.param("name");
    return channel -> {
        Message message = new Message(name);
        XContentBuilder builder = channel.newBuilder();
        builder.startObject();
        message.toXContent(builder, restRequest);
        builder.endObject();
        channel.sendResponse(new BytesRestResponse(RestStatus.OK, builder));
    };
}

This code is usig a lambda which builds a new Message object. Then we create our JSON document. Basically builder.startObject() and builder.endObject() create a JSON object {}. message.toXContent(builder, restRequest) will fill this object with a JSON representation of the Message object: "message": "Hello name!". At the end, this code will produce:

{
    "message": "Hello name!"
}

Then we send the JSON response over the wire.

Dealing with parameters

Query Parameters

We want to support:

curl -XGET "http://localhost:9200/_hello?name=David&pretty"

Let’s first implement a test for it (yeah! I love test driven development):

public void testHelloDaviddWithQueryParameters() throws Exception {
    Response response = client.performRequest("GET", "/_hello?name=David");
    assertThat(entityAsMap(response), hasEntry("message", "Hello David!"));
}

And fix the test in handleRequest method…

protected void handleRequest(RestRequest request, RestChannel channel, NodeClient client) throws Exception {
    RestToXContentListener<ToXContent> listener = new RestToXContentListener<>(channel);
    String name = request.param("name");
    listener.onResponse(new Message(name));
}

Parameters in URL

We want to support:

curl -XGET "http://localhost:9200/_hello/David&pretty"

Let’s again implement a test for it:

public void testHelloDavidWithURLParameters() throws Exception {
    Response response = client.performRequest("GET", "/_hello/David");
    assertThat(entityAsMap(response), hasEntry("message", "Hello David!"));
}

It will send back an error 400 (Bad Request) as this URL pattern does not exist. Let’s add it in the HelloRestAction constructor by registering now:

controller.registerHandler(GET, "/_hello/{name}", this);

name will be the placeholder for this parameter. Well. That’s cool. We don’t need to change the implementation!

Parameters as JSON documents

We want to support:

curl -XGET "http://localhost:9200/_hello?pretty" -d '{
    "name": "David"
}'

# Or

curl -XPOST "http://localhost:9200/_hello?pretty" -d '{
    "name": "David"
}'

That’s a good practice to support both as some clients don’t support GET with body.

public void testHelloDavidWithGetBody() throws Exception {
    Response response = client.performRequest("GET", "/_hello",
            Collections.emptyMap(), new StringEntity("{\"name\":\"David\"}"));
    assertThat(entityAsMap(response), hasEntry("message", "Hello David!"));
}

public void testHelloDavidWithPostBody() throws Exception {
    Response response = client.performRequest("POST", "/_hello",
            Collections.emptyMap(), new StringEntity("{\"name\":\"David\"}"));
    assertThat(entityAsMap(response), hasEntry("message", "Hello David!"));
}

We need in prepareRequest to extract the name from the body:

@Override
protected RestChannelConsumer prepareRequest(RestRequest restRequest, NodeClient client) throws IOException {
    String name = restRequest.param("name");
    if (name == null && restRequest.content().length() > 0) {
        // Let's try to find the name from the body
        Map<String, Object> map = XContentHelper.convertToMap(restRequest.content(), false).v2();
        if (map.containsKey("name")) {
            name = (String) map.get("name");
        }
    }

    String finalName = name;
    return channel -> {
        Message message = new Message(finalName);
        XContentBuilder builder = channel.newBuilder();
        builder.startObject();
        message.toXContent(builder, restRequest);
        builder.endObject();
        channel.sendResponse(new BytesRestResponse(RestStatus.OK, builder));
    };
}

And we also need to register the POST request. Let’s add it in the HelloRestAction constructor by registering now:

controller.registerHandler(POST, "/_hello", this);

Et voilà!

Calling elasticsearch client

You can also call elasticsearch client to perform some tasks and then send the result back to the caller.

Here for example, we want to call:

curl -XGET http://localhost:9200/_bano

And get back an array containing the list of all indices starting with .bano in the cluster:

{
    "bano": [
        ".bano-17",
        ".bano-95",
        ".bano-29"
    ]
}

Add a test

As usual, we start adding a test:

public void testBano() throws Exception {
    // We first create some indices
    int numIndices = randomIntBetween(1, 10);
    for (int i = 0; i < numIndices; i++) {
        client.performRequest("PUT", "/.bano-" + i);
    }
    // We create some other indices
    int numOtherIndices = randomIntBetween(1, 10);
    for (int i = 0; i < numOtherIndices; i++) {
        client.performRequest("PUT", "/" + randomAsciiOfLengthBetween(6, 10).toLowerCase(Locale.getDefault()));
    }

    client.performRequest("GET", "/_cluster/health", Collections.singletonMap("wait_for_status", "yellow"));

    Response response = client.performRequest("GET", "/_bano");
    Map<String, Object> responseMap = entityAsMap(response);
    assertThat(responseMap, hasKey("bano"));
    List<String> bano = (List<String>) responseMap.get("bano");
    for (int i = 0; i < numIndices; i++) {
        assertThat(bano, hasItem(".bano-" + i));
    }
}

The test generates a random number of indices starting with .bano plus some other unrelated indices.

Let’s now create a new BanoRestAction as we did for HelloRestAction:

public class BanoRestAction extends BaseRestHandler {

    @Inject
    public BanoRestAction(Settings settings, RestController controller) {
        super(settings);
        controller.registerHandler(GET, "/_bano", this);
    }

    @Override
    protected RestChannelConsumer prepareRequest(RestRequest restRequest, NodeClient client) throws IOException {
    }
}

We register it in the plugin:

public List<Class<? extends RestHandler>> getRestHandlers() {
return Arrays.asList(
        HelloRestAction.class,
        BanoRestAction.class);
}

Let’s implement the logic in handleRequest:

@Override
protected RestChannelConsumer prepareRequest(RestRequest restRequest, NodeClient client) throws IOException {
    return channel -> client.admin().indices().prepareGetIndex()
        .setIndices(".bano*")
        .execute(new RestBuilderListener<GetIndexResponse>(channel) {
            @Override
            public RestResponse buildResponse(GetIndexResponse getIndexResponse, XContentBuilder builder) throws Exception {
                Indices indices = new Indices();
                for (String index : getIndexResponse.getIndices()) {
                    indices.addIndex(index);
                }
                builder.startObject();
                indices.toXContent(builder, restRequest);
                builder.endObject();
                return new BytesRestResponse(RestStatus.OK, builder);
            }
        });
}

As you can see, we don’t block the thread here as we are using a Listener which is called once the request in the cluster is over. This is extremly important in the context of elasticsearch as it frees the network socket which can hold other requests and is not blocked by the current one.

If everything goes well, buildResponse method will be called. Otherwise any exception will be thrown and elasticsearch REST layer mechanism will send that back to the caller.

Indices is a new class as we have seen before which will help us to serialize to JSON the response:

class Indices implements ToXContent {

    private final List<String> indices;

    public Indices() {
        indices = new ArrayList<>();
    }

    public void addIndex(String index) {
        indices.add(index);
    }

    @Override
    public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
        builder.startArray("bano");
        for (String index : indices) {
            builder.value(index);
        }
        return builder.endArray();
    }
}

What’s next?

In a coming blog post, I’ll explain how to create an ingest plugin which will helps you to transform a french postal address to geo coordinates or the other way around. We will use what we have just seen to add some administrative tasks like automatically create our datasources in elasticsearch (download CSV files from bano website, extract, transform to JSON and index in elasticsearch).

Stay tuned!

#Java #Elasticsearch #Plugin
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David Pilato

20+ years of experience, mostly in Java. Living in Cergy, France.
Next Elasticsearch real integration tests

Series of Posts

Elasticsearch real integration tests with security enabled
Creating Elasticsearch Transport Action
Adding a new REST endpoint to elasticsearch
Elasticsearch real integration tests
Creating an Ingest plugin for elasticsearch
Creating a plugin for elasticsearch 5.0 using Maven

See Also

Elasticsearch real integration tests
Creating an Ingest plugin for elasticsearch
Creating a plugin for elasticsearch 5.0 using Maven
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