Before we get into the actual topic, we need to understand what is an agent-less check and later delve into the answer to the question, what should we look for over the network? We define agent-less check as the Solr health check, which can be performed remotely.
The first and the foremost thing we should do is to have a host-level check, which can be expected to perform a network-level ping. Then, we may go for checking TCP connectivity to the servlet container port and then make an HTTP GET request to the Solr front page checking for a known string (for instance, Welcome to Solr Monitoring and Optimization).
Now we can start with performing Solr specific checks by monitoring items which may include:
Ping status
Number of documents
Number and rate (per second) of queries
Average response time
Number of modifications
Cache hit ratio
Replication status
Synthetic queries
As an example, we will walk-through one of the items from the list (for instance, Ping status).