SolrCloud Leader Election Failing

At the time we speak ( Solr 7.3.0 ) SolrCloud is a reliable and stable distributed architecture for Apache Solr.
But it is not perfect and failures happen.
This lightening blog post will present some practical tips to follow when a specific shard of a collection is down with no leader and the situation is stuck.
The following problem has been experienced with the following Solr versions :

  • 4.10.2
  • 5.4.0

Steps to solve the problem may involve manual interaction with the Zookeeper Ensemble[1].
The following steps are extracted from an interesting thread of the Solr User mailing list[2] and practical experience on the field.
In particular, thanks to Jeff Wartes for the suggestions, that proved useful for me in a couple of occasions.

Problem

  • All nodes for a Shard in a Collection are up and running
  • There is no leader for the shard
  • All the nodes are in a “Recovering” / “Recovery Failed” state
  • Search is down and the situation persist after many minutes (> 5)

Solution

A possible explanation for this problem to occur is when the node-local version of the Zookeeper clusterstate has diverged from the centralized Zookeeper cluster state.
One possible cause for the leader election to break is a Zookeeper failure : for example you lose >=50% of the ensemble nodes or the connectivity among the ensemble nodes for a certain period of time ( this is the scenario I experimented directly)
This failure, even if resolved later, can bring a corruption to the Zookeeper file system.
Some of the SolrCloud collections may remain in a not consistent status.

It may be necessary to manually delete corrupted files from Zookeeper :
Let’s start from :

collections/<collection>/leader_elect/shard<x>/election
An healthy SolrCloud cluster presents as many core_nodeX as the total replicas for the shard.
You don’t want duplicates or missing nodes here.
If you’re having trouble getting a sane election, you can try deleting the lowest-numbered entries (as well as any lower-numbered duplicates) and try to foce the election again. Possibly followed by restarting the node with that lowest-numbered entry.

collections/<collection>/leader/shard<x>
Make sure that this folder exists and has the expected replica as a leader.

collections/<collection>/leader_initiated_recovery
This folder can be informative too, this represents replicas that the *leader* thinks are out of sync, usually due to a failed update request.

After having completed the verification above, there a couple of Collection API endpoints that may be useful :

Force Leader Election
/admin/collections?action=FORCELEADER&collection=<collectionName>&shard=<shardName>

Force Leader Rebalance
/admin/collections?action=REBALANCELEADERS&collection=collectionName

N.B. rebalancing all the leader will affect all the shards

 

[1] Apache Zookeeper Solr Cli

[2] Solr Mailing List Thread

[3] Solr Collection API

 

Distributed Search Tips for Apache Solr

Distributed search is the foundation for Apache Solr Scalability :

It’s possible to distributed search across different Apache Solr nodes of the same collection ( both in a  legacy[1] or SolrCloud[2] architecture), but it is also possible to distribute search across different collections in a SolrCloud cluster.
Aggregating results from different collections may be useful when you put in place different systems ( that were meant to be separate ) and you later realize that aggregating the results may be an additional useful use case.
This blog will focus on some tricky situations that can happen when running distributed search ( for configuration or details you can refer to the Solr wiki ).

IDF

Inverse Document Frequency affects the score.
This means that a document coming from a big collection can obtain a boost from IDF, in comparison to a similar document from a smaller collection.
This is because the maxDoc count is taken into account as corpus size, so even if a term has the same document frequency, IDF will be strongly affected by the collection size.
Distributed IDF[3] partially solved the problem :

When distributing the search across different shards of the same collection, it works quite well.
But using the ExactStatCache and alternating single collection distribution and multi collection distribution in the same SolrCloud cluster will create some caching conflict.

Specifically if we first execute the inter collection query, the global stats cached will be the inter collection global stats,  so if we then execute a single collection distributed search, the preview global stats will remain cached ( viceversa applies).

Debug Scoring

Real score and debug score is not aligned with the distributed IDF, this means that the debug query will not show the correct distributed IDF and correct scoring calculus for distributed searches[4].

Relevancy tuning

Lucene/Solr score is not probabilistic or normalised.
For the same collection we can have completely different score scales just with different queries.
The situation becomes more complicated when we tune our relevancy adding multiplicative or additive boosts.
Different collections may imply completely different boosting logic that could cause the score of a collection to be on a completely different scale in comparison to another.
We need to be extra careful when tuning relevancy for searches across different collections and try to configure the distributed request handler in the most compatible way as possible.

Request handler

It is important to carefully specify the request handler to be used when using distributed search.
The request will hit one collection in one node and then when distributing the same request handler will be called on the other collections across the other nodes.
If necessary it is possible to configure the aggregator request handler and local request handlers ( this may be useful if we want to use a different scoring formulas per collection, using local parameters) :

Aggregator Request Handler

It is executed on the first node receiving the request.
It will distribute the request and then aggregate the results.
It must describe parameters that are in common across all the collections involved in the search.
It is the one specified in the main request.
e.g.
http://localhost:8983/solr/collection1/select?

Local Request Handler

It is specified passing the parameter : shards.qt=
It is execute on each node that receive the distributed query AFTER the first one.
This can be used to use specific fields or parameter on a per collection basis.
A local request handler may use fields and search components that are local to the collection interested.

e.g.
http://localhost:8983/solr/collection1/select?q=*:*&collection=collection1,collection2&shards.qt=localSelect

N.B. the use of local request handler may be useful in case you want to define local query parser rules, such as local edismax configuration to affect the score.

Unique Key

The unique key field must be the same across the different collections.
Furthermore the value should be unique across the different collections to guarantee a proper behaviour.
If we don’t comply with this rule, Solr will fail in aggregating the results and raise an exception.

[1] https://lucene.apache.org/solr/guide/6_6/distributed-search-with-index-sharding.html
[2] https://lucene.apache.org/solr/guide/6_6/solrcloud.html
[3] https://lucene.apache.org/solr/guide/6_6/distributed-requests.html#DistributedRequests-ConfiguringstatsCache_DistributedIDF_
[4] https://issues.apache.org/jira/browse/SOLR-7759