In this blog, we'll talk about simple yet very useful Solr handler which is PingRequestHandler. We can use Ping Request Handler as an endpoint for an HTTP Load-Balancer(Like HAProxy) to use when checking the “up status” of a Solr server.

We can configure PingRequestHandler with some defaults indicating a request that should be executed. If the configured request succeeds then the PingRequestHandler will respond back with a simple “OK” status. If the request fails then the PingRequestHandler will respond back with the corresponding HTTP Error code. Clients can be configured to poll the PingRequestHandler monitoring for these types of responses to know if there is a problem with the Solr server.

1. If the health check file exists, the handler will execute the query and returns status as described previously.

1. If the health check file does not exist, the handler will throw an HTTP error even though the server is working fine and the query would have succeeded.

• http://host:port/solr/admin/ping?action=enable – creates the health check file if it does not already exist

• http://host:port/solr/admin/ping?action=disable – deletes the health check file if it exists

• http://host:port/solr/admin/ping?action=status – returns a status code indicating if the healthcheck file exists (“enabled”) or not (“disabled”)

The Ping option will not open new page we can see the status of the request on the core overview page. The length of time the request has taken is displayed next to the Ping option, in milliseconds.

To configure this handler in Solr we need to add request handler in Solrconfig.xml file like below:

<requestHandler name="/admin/ping" class="solr.PingRequestHandler">

    <lst name="invariants">

      <str name="q">Management of Rubella in pregnancy</str>

    </lst>

    <lst name="defaults">

      <str name="echoParams">all</str>

      <str name="df">article</str>

    </lst>

    <str name="healthcheckFile">server-enabled.txt</str>

</requestHandler>

We can configure healthcheckFile that can be used to enable/disable the PingRequestHandler. We should always remember the below points while using healthcheck file:

This health check file feature can be used as a way to indicate to some Load Balancers that the server should be “removed from rotation” for maintenance, or upgrades, or whatever reason you may wish.

The health check file may be created/deleted by any external system, or the PingRequestHandler itself can be used to create/delete the file by specifying an “action” param in a request:

Below are screenshots of Solr Admin page after configuring PingRequestHandler

PingRequestHandler