SolrCloud is highly available and fault tolerant in reads and writes.
Read Side Fault Tolerance
In a SolrCloud cluster each individual node load balances read requests across all the replicas in a collection. You still need a load balancer on the 'outside' that talks to the cluster, or you need a smart client which understands how to read and interact with Solr’s metadata in ZooKeeper and only requests the ZooKeeper ensemble’s address to start discovering to which nodes it should send requests. (Solr provides a smart Java SolrJ client called CloudSolrClient.)
Even if some nodes in the cluster are offline or unreachable, a Solr node will be able to correctly respond to a search request as long as it can communicate with at least one replica of every shard, or one replica of every relevant shard if the user limited the search via the shards or _route_ parameters. The more replicas there are of every shard, the more likely that the Solr cluster will be able to handle search results in the event of node failures.
Query Parameters for Query Routing
A Solr node will return the results of a search request as long as it can communicate with at least one replica of every shard that it knows about, even if it can not communicate with ZooKeeper at the time it receives the request. This is normally the preferred behavior from a fault tolerance standpoint, but may result in stale or incorrect results if there have been major changes to the collection structure that the node has not been informed of via ZooKeeper (i.e., shards may have been added or removed, or split into sub-shards).
A zkConnected header is included in every search response indicating if the node that processed the request was connected with ZooKeeper at the time:
To prevent stale or incorrect results in the event that the request-serving node can’t communicate with ZooKeeper, set the shards.tolerant parameter to requireZkConnected. This will cause requests to fail rather than setting a zkConnected header to false.
By default, SolrCloud will run searches on all shards and combine the results if the shards parameter is not specified. You can specify one or more shard names as the value of the shards parameter to limit the shards that you want to search against.
In the event that one or more shards queried are unavailable, then Solr’s default behavior is to fail the request. However, there are many use-cases where partial results are acceptable and so Solr provides a boolean shards.tolerant parameter (default false). In addition to true and false, shards.tolerant may also be set to requireZkConnected - see below.
If shards.tolerant=true then partial results may be returned. If the returned response does not contain results from all the appropriate shards then the response header contains a special flag called partialResults.
If shards.tolerant=requireZkConnected and the node serving the search request cannot communicate with ZooKeeper, the request will fail, rather than returning potentially stale or incorrect results. This will also cause requests to fail when one or more queried shards are completely unavailable, just like when shards.tolerant=false.
The client can specify 'shards.info' along with the shards.tolerant parameter to retrieve more fine-grained details.
Example response with partialResults flag set to true:
The collection parameter allows you to specify a collection or a number of collections on which the query should be executed. This allows you to query multiple collections at once and all the feature of Solr which work in a distributed manner can work across collections.
The _route_ parameter can be used to specify a route key which is used to figure out the corresponding shards. For example, if you have a document with a unique key "user1!123", then specifying the route key as "route=user1!" (notice the trailing '!' character) will route the request to the shard which hosts that user. You can specify multiple route keys separated by comma.
This parameter can be leveraged when we have shard data by users. See 'Document Routing' for more information
The debug parameter with a value of track can be used to trace the request as well as find timing information for each phase of a distributed request.
If set to true, the distrib.singlePass parameter changes the distributed search algorithm to fetch all requested stored fields from each shard in the first phase itself. This eliminates the need for making a second request to fetch the stored fields.
This can be faster when requesting a very small number of fields containing small values. However, if large fields are requested or if a lot of fields are requested then the overhead of fetching them over the network from all shards can make the request slower as compared to the normal distributed search path.
Note that this optimization only applies to distributed search. Certain features such as faceting may make additional network requests for refinements, etc.