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Index writers in Nutch

An index writer is a component of the indexing job, which is used for sending documents from one or more segments to an external server. In Nutch, these components are found as plugins. Nutch includes these out-of-the-box indexers:

Indexer

Description

indexer-solr

Indexer for a Solr server

indexer-rabbit

Indexer for a RabbitMQ server

indexer-dummy

Indexer usually used for debugging, it writes in a plain text file

indexer-elastic

Indexer for an Elasticsearch server

indexer-elastic-rest

Indexer for Elasticsearch, but using Jest to connect with the REST API provided by Elasticsearch

indexer-cloudsearch

Indexer for Amazon <<GetText(CloudSearch)>>

indexer-csv

Indexer for writing documents to a CSV file

Structure of index-writers.xml

The configuration for the indexers is in the index-writers.xml file, included in the official Nutch distribution. The structure of this file is quite simple and consists mainly of a list of indexers (<writer> element):

{{{#!highlight xml
<writers>
<writer id="<writer_id>" class="<implementation_class>">
<mapping>
...
</mapping>
<parameters>
...
</parameters>
</writer>
...
</writers>
}}}

Each <writer> element has two mandatory attributes:

  1. <writer_id> is a unique identification for each configuration. This feature allows Nutch to distinguish each configuration, even when they are for the same index writer. In addition, it allows to have multiple instances for the same index writer, but with different configurations.
  2. <implementation_class> corresponds to the canonical name of the class that implements the IndexWriter extension point. For the indexers provided by Nutch out-of-the-box the possible values of <implementation_class> are:

Each <writer> element contains two child elements: <mapping> and <parameters>

Mapping section

The <mapping> element is independent for each configuration and is where you configure the modifications that will be applied to each document before it is sent to its final destination. The <mapping> element contains 3 child elements: <copy>, <rename> and <remove>

  • <copy> indicates which fields should be copied from the document and to which field they should be copied. Each child element of <copy> element, has this form: <field source="<source>" dest="<destination>"/>
    • <source> indicates the field's name to be copied.
    • <destination> indicates to which field or fields should be copied. The value of this attribute can be a comma separated list. In this case, the value of source attribute will be copied into each field in the list. For example: if the configuration is <field source="title" dest="description,search"/>, the value of the title field will be copied for the description and search fields.
  • <rename> indicates which fields of the document should be renamed. Each child element of <rename> element, has this form: <field source="<source>" dest="<destination>"/>
    • <source> indicates the field's name to be renamed.
    • <destination> indicates the new name of the field. For example: if the configuration is <field source="metatag.description" dest="description"/>, the field metatag.description will be renamed as description.
  • <remove> indicates which fields of the document should be removed. Each child element of <remove> element, has the form: <field source="<source>"/>
    • <source> indicates the field's name to be remove.

{{note}
Mapping section can't be empty

If you don't want to modify the document, just leave <copy>, <rename> and <remove> empty, like: <mapping> <copy /> <rename /> <remove /> </mapping>

{note}}

Use case

We have two servers previously configured (Solr and RabbitMQ). We want to send documents to each one, but with a different structure. Prior to the index step, each document has this hypothetical structure:

{{{#!highlight properties host: "www.example.org" domain: "example.org" title: "Example page" metatag.description: "Example page description" metatag.keywords: ["example", "page"] segment: 20180621163128
}}}
With this configuration we modify the structure of each document in different ways, depending the index writer:

{{{#!highlight xml
<writer id="indexer_solr_1" class="org.apache.nutch.indexwriter.solr.SolrIndexWriter">
<parameters>
<!-- Parameters here -->
</parameters>
<mapping>
<copy/>
<rename>
<field source="metatag.description" dest="description"/>
<field source="metatag.keywords" dest="keywords"/>
</rename>
<remove>
<field source="segment"/>
</remove>
</mapping>
</writer>
<writer id="indexer_rabbit_1" class="org.apache.nutch.indexwriter.rabbit.RabbitIndexWriter">
<parameters>
<!-- Parameters here -->
</parameters>
<mapping>
<copy>
<field source="title" dest="search"/>
</copy>
<rename>
<field source="metatag.description" dest="description"/>
<field source="metatag.keywords" dest="keywords"/>
<field source="domain" dest="domain_name"/>
</rename>
<remove />
</mapping>
</writer>
}}}

For indexer-solr we'll get documents like:

{{

Unknown macro: {#!highlight properties host}

}

For indexer-rabbit the document's structure is like:

{{

Unknown macro: {#!highlight properties host}

}

Parameters section

The <parameters> element is independent for each configuration and is where the parameters that the indexer needs are specified. Each parameter has the form <param name="<name>" value="<value>"/> and the values it can take depend on the indexer that you want to configure. Below is a description of the arguments of each indexer provided by Nutch out-of-the-box individually.

Solr indexer properties

Parameter Name

Description

Default value

type

Specifies the SolrClient implementation to use. This is a string value of one of the following cloud or http. The values represent CloudSolrServer or HttpSolrServer respectively.

http

url

Defines the fully qualified URL of Solr into which data should be indexed. Multiple URL can be provided using comma as a delimiter. When the value of type property is cloud, the URL should not include any collections or cores; just the root Solr path.

http://localhost:8983/solr/nutch

collection

The collection used in requests. Only used when the value of type property is cloud.

 

weight.field

Field's name where the weight of the documents will be written. If it is empty no field will be used.

 

commitSize

Defines the number of documents to send to Solr in a single update batch. Decrease when handling very large documents to prevent Nutch from running out of memory.
Note: It does not explicitly trigger a server side commit.

1000

auth

Whether to enable HTTP basic authentication for communicating with Solr. Use the username and password properties to configure your credentials.

false

username

The username of Solr server.

username

password

The password of Solr server.

password

Rabbit indexer properties

Parameter Name

Description

Default value

server.uri

URI with connection parameters in the form amqp://<username>:<password>@<hostname>:<port>/<virtualHost>
Where: <<Include(IndexWriters/RabbitURIParts)>>

amqp://guest:guest@localhost:5672/

binding

Whether the relationship between an exchange and a queue is created automatically.
NOTE: Binding between exchanges is not supported.

false

binding.arguments

Arguments used in binding. It must have the form key1=value1,key2=value2. This value is only used when the exchange's type is headers and the value of binding property is true. In other cases is ignored.

 

exchange.name

Name for the exchange where the messages will be sent.

 

exchange.options

Options used when the exchange is created. Only used when the value of binding property is true. It must have the form type=<type>,durable=<durable>
Where: <<Include(IndexWriters/RabbitExchangeOptions)>>

type=direct,durable=true

queue.name

Name of the queue used to create the binding. Only used when the value of binding property is true.

nutch.queue

queue.options

Options used when the queue is created. Only used when the value of binding property is true. It must have the form durable=<durable>,exclusive=<exclusive>,auto-delete=<auto-delete>,arguments=<arguments>
Where: <<Include(IndexWriters/RabbitQueueOptions)>>

durable=true,exclusive=false,auto-delete=false

routingkey

The routing key used to route messages in the exchange. It only makes sense when the exchange type is topic or direct.

Value of queue.name property

commit.mode

single if a message contains only one document. In this case, a header with the action (write, update or delete) will be added. multiple if a message contains all documents.

multiple

commit.size

Amount of documents to send into each message if the value of commit.mode property is multiple. In single mode this value represents the amount of messages to be sent.

250

headers.static

Headers to add to each message. It must have the form key1=value1,key2=value2.

 

headers.dynamic

Document's fields to add as headers to each message. It must have the form field1,field2. Only used when the value of commit.mode property is single

 

Dummy indexer properties

Parameter Name

Description

Default value

path

Path where the file will be created.

./dummy-index.txt

delete

If delete operations should be written to the file.

false

Elasticsearch indexer properties

Parameter Name

Description

Default value

host

Comma-separated list of hostnames to send documents to using TransportClient. Either host and port must be defined or cluster.

 

port

The port to connect to using TransportClient.

9300

cluster

The cluster name to discover. Either host and port must be defined or cluster.

 

index

Default index to send documents to.

nutch

max.bulk.docs

Maximum size of the bulk in number of documents.

250

max.bulk.size

Maximum size of the bulk in bytes.

2500500

exponential.backoff.millis

Initial delay for the BulkProcessor exponential backoff policy.

100

exponential.backoff.retries

Number of times the BulkProcessor exponential backoff policy should retry bulk operations.

10

bulk.close.timeout

Number of seconds allowed for the BulkProcessor to complete its last operation.

600

Elasticsearch rest indexer properties

Parameter Name

Description

Default value

host

The hostname or a list of comma separated hostnames to send documents to using Elasticsearch Jest. Both host and port must be defined.

 

port

The port to connect to using Elasticsearch Jest.

9200

index

Default index to send documents to.

nutch

max.bulk.docs

Maximum size of the bulk in number of documents.

250

max.bulk.size

Maximum size of the bulk in bytes.

2500500

user

Username for auth credentials (only used when https is enabled)

user

password

Password for auth credentials (only used when https is enabled)

password

type

Default type to send documents to.

doc

https

true to enable https, false to disable https. If you've disabled http access (by forcing https), be sure to set this to true, otherwise you might get "connection reset by peer".

false

trustallhostnames

true to trust elasticsearch server's certificate even if its listed domain name does not match the domain they are hosted or false to check if the elasticsearch server's certificate's listed domain is the same domain that it is hosted on, and if it doesn't, then fail to index (only used when https is enabled)

false

languages

A list of strings denoting the supported languages (e.g. en, de, fr, it). If this value is empty all documents will be sent to index property. If not empty the Rest client will distribute documents in different indices based on their languages property. Indices are named with the following schema: index separator language (e.g. nutch_de). Entries with an unsupported languages value will be added to index index separator sink (e.g. nutch_others).

 

separator

Is used only if languages property is defined to build the index name (i.e. index separator lang).

_

sink

Is used only if languages property is defined to build the index name where to store documents with unsupported languages (i.e. index separator sink).

others

CloudSearch indexer properties

Parameter Name

Description

Default value

endpoint

Endpoint where service requests should be submitted.

 

region

Region name.

 

batch.dump

true to send documents to a local file.

false

batch.maxSize

Maximum number of documents to send as a batch to <<GetText(CloudSearch)>>.

-1

CSV indexer properties

Parameter Name

Description

Default value

fields

Ordered list of fields (columns) in the CSV file

id,title,content

charset

Encoding of CSV file

UTF-8

separator

Separator between fields (columns)

,

valuesep

Separator between multiple values of one field

 

 

quotechar

Quote character used to quote fields containing separators or quotes

"

escapechar

Escape character used to escape a quote character

"

maxfieldlength

Max. length of a single field value in characters

4096

maxfieldvalues

Max. number of values of one field, useful for, e.g., the anchor texts field

12

header

Write CSV column headers

true

outpath

Output path / directory (local filesystem path, relative to current working directory)

csvindexwriter

The CSV indexer does not work in distributed mode, the output is written to the local filesystem, not to HDFS, see NUTCH-1541.

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