RequestDispatcher in SolrConfig | Apache Solr Reference Guide 7.1

The requestDispatcher element of solrconfig.xml controls the way the Solr HTTP RequestDispatcher implementation responds to requests.

Included are parameters for defining if it should handle /select urls (for Solr 1.1 compatibility), if it will support remote streaming, the maximum size of file uploads and how it will respond to HTTP cache headers in requests.

handleSelect Element

handleSelect is for legacy back-compatibility; those new to Solr do not need to change anything about the way this is configured by default.

The first configurable item is the handleSelect attribute on the <requestDispatcher> element itself. This attribute can be set to one of two values, either "true" or "false". It governs how Solr responds to requests such as /select?qt=XXX. The default value "false" will ignore requests to /select if a requestHandler is not explicitly registered with the name /select. A value of "true" will route query requests to the parser defined with the qt value.

In recent versions of Solr, a /select requestHandler is defined by default, so a value of "false" will work fine. See the section RequestHandlers and SearchComponents in SolrConfig for more information.

<requestDispatcher handleSelect="true" >
  ...
</requestDispatcher>

requestParsers Element

The <requestParsers> sub-element controls values related to parsing requests. This is an empty XML element that doesn’t have any content, only attributes.

enableRemoteStreaming: This attribute controls whether remote streaming of content is allowed. If omitted or set to false (the default), streaming will not be allowed. Setting it to true lets you specify the location of content to be streamed using stream.file or stream.url parameters.
enableStreamBody: This attribute controls whether streaming content from the HTTP parameter stream.body is allowed. If omitted or set to false (the default), streaming will not be allowed. Setting it to true lets you pass data in the stream.body parameter.

If you enable remote streaming, be sure that you have authentication enabled. Otherwise, someone could potentially gain access to your content by accessing arbitrary URLs. It’s also a good idea to place Solr behind a firewall to prevent it from being accessed from untrusted clients.
multipartUploadLimitInKB: This attribute sets an upper limit in kilobytes on the size of a document that may be submitted in a multi-part HTTP POST request. The value specified is multiplied by 1024 to determine the size in bytes. A value of -1 means MAX_INT, which is also the system default if omitted.
formdataUploadLimitInKB: This attribute sets a limit in kilobytes on the size of form data (application/x-www-form-urlencoded) submitted in a HTTP POST request, which can be used to pass request parameters that will not fit in a URL. A value of -1 means MAX_INT, which is also the system default if omitted.
addHttpRequestToContext: This attribute can be used to indicate that the original HttpServletRequest object should be included in the context map of the SolrQueryRequest using the key httpRequest. This HttpServletRequest is not used by any Solr component, but may be useful when developing custom plugins.

<requestParsers enableRemoteStreaming="false"
                enableStreamBody="false"
                multipartUploadLimitInKB="2048"
                formdataUploadLimitInKB="2048"
                addHttpRequestToContext="false" />

The below command is an example of how to enable RemoteStreaming and BodyStreaming through the Config API:

curl http://localhost:8983/solr/gettingstarted/config -H 'Content-type:application/json' -d'{
    "set-property" : {"requestDispatcher.requestParsers.enableRemoteStreaming":true},
    "set-property" : {"requestDispatcher.requestParsers.enableStreamBody":true}
}'

httpCaching Element

The <httpCaching> element controls HTTP cache control headers. Do not confuse these settings with Solr’s internal cache configuration. This element controls caching of HTTP responses as defined by the W3C HTTP specifications.

This element allows for three attributes and one sub-element. The attributes of the <httpCaching> element control whether a 304 response to a GET request is allowed, and if so, what sort of response it should be. When an HTTP client application issues a GET, it may optionally specify that a 304 response is acceptable if the resource has not been modified since the last time it was fetched.

never304: If present with the value true, then a GET request will never respond with a 304 code, even if the requested resource has not been modified. When this attribute is set to true, the next two attributes are ignored. Setting this to true is handy for development, as the 304 response can be confusing when tinkering with Solr responses through a web browser or other client that supports cache headers.
lastModFrom: This attribute may be set to either openTime (the default) or dirLastMod. The value openTime indicates that last modification times, as compared to the If-Modified-Since header sent by the client, should be calculated relative to the time the Searcher started. Use dirLastMod if you want times to exactly correspond to when the index was last updated on disk.
etagSeed: This value of this attribute is sent as the value of the ETag header. Changing this value can be helpful to force clients to re-fetch content even when the indexes have not changed---for example, when you’ve made some changes to the configuration.

<httpCaching never304="false"
             lastModFrom="openTime"
             etagSeed="Solr">
  <cacheControl>max-age=30, public</cacheControl>
</httpCaching>

cacheControl Element

In addition to these attributes, <httpCaching> accepts one child element: <cacheControl>. The content of this element will be sent as the value of the Cache-Control header on HTTP responses. This header is used to modify the default caching behavior of the requesting client. The possible values for the Cache-Control header are defined by the HTTP 1.1 specification in Section 14.9.

Setting the max-age field controls how long a client may re-use a cached response before requesting it again from the server. This time interval should be set according to how often you update your index and whether or not it is acceptable for your application to use content that is somewhat out of date. Setting must-revalidate will tell the client to validate with the server that its cached copy is still good before re-using it. This will ensure that the most timely result is used, while avoiding a second fetch of the content if it isn’t needed, at the cost of a request to the server to do the check.

Query Settings in SolrConfig Update Request Processors