@Override
    public void checkInitialization(InitializableProblemHandler problemHandler) throws TikaConfigException {
        super.checkInitialization(problemHandler);
        mustNotBeEmpty("solrCollection", this.solrCollection);
        mustNotBeEmpty("urlFieldName", this.idField);
        mustNotBeEmpty("parsingIdField", this.parsingIdField);
        mustNotBeEmpty("failCountField", this.failCountField);
        mustNotBeEmpty("sizeFieldName", this.sizeFieldName);
        if ((this.solrUrls == null || this.solrUrls.isEmpty()) && (this.solrZkHosts == null || this.solrZkHosts.isEmpty())) {
            throw new IllegalArgumentException("expected either param solrUrls or param solrZkHosts, but neither was specified");
        }
        if (this.solrUrls != null && !this.solrUrls.isEmpty() && this.solrZkHosts != null && !this.solrZkHosts.isEmpty()) {
            throw new IllegalArgumentException("expected either param solrUrls or param solrZkHosts, but both were specified");
        }
    }
You can specify solrUrls, or you can specify solrZkHosts and use use zookeeper to determine the solr server urls.
tika-pipes/tika-pipes-iterators/tika-pipes-iterator-solr/src/main/java/org/apache/tika/pipes/pipesiterator/solr/SolrPipesIterator.java