diff --git a/lib/ontologies_linked_data/services/rank_solr_propagator.rb b/lib/ontologies_linked_data/services/rank_solr_propagator.rb
index 0afc5602..dfd6c09c 100644
--- a/lib/ontologies_linked_data/services/rank_solr_propagator.rb
+++ b/lib/ontologies_linked_data/services/rank_solr_propagator.rb
@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require 'redis'
+
 module LinkedData
   module Services
     # Propagates the current ontology rank values into the denormalized
@@ -14,21 +16,39 @@ module Services
     # bioportalScore/umlsScore using the configured weights.
     #
     # Updates use true Solr atomic updates ({ id, ontologyRank: { set: score } })
-    # so no per-class triplestore read is needed.
+    # so no per-class triplestore read is needed. Batches are sent without a
+    # commit; a single commit is issued *between* ontologies (when no updates are
+    # in flight) to keep the transaction log bounded without pausing replicas
+    # mid-stream — committing during the update stream backs up SolrCloud's
+    # leader->replica forwarding queue and causes "distributed update stalled"
+    # 500s. Each Solr call is retried with backoff so a transient stall is
+    # survived rather than aborting the whole run.
+    #
+    # Ontologies whose rank is unchanged since the last successful propagation
+    # are skipped (tracked per acronym in Redis). Pass force: true to ignore the
+    # skip cache and re-propagate everything (e.g. after a collection rebuild).
     #
     # See https://github.com/ncbo/ncbo_cron/issues/132
     class RankSolrPropagator
-      DEFAULT_BATCH_SIZE = 1000
+      DEFAULT_BATCH_SIZE = 2_500
+      PROGRESS_EVERY = 50_000   # log per-ontology progress every N docs
+      MAX_RETRIES = 5
+      RETRY_BASE_SLEEP = 5      # seconds; exponential backoff: 5, 10, 20, 40, 80
+      # Redis key holding the last successfully propagated rank per acronym.
+      LAST_PROPAGATED_REDIS_FIELD = 'ontology_rank_solr_propagated'
 
-      def initialize(logger: nil, batch_size: DEFAULT_BATCH_SIZE)
+      # batch_size precedence: explicit arg > RANK_SOLR_BATCH_SIZE env > default.
+      # The env override lets the batch size be tuned on staging without a deploy.
+      def initialize(logger: nil, batch_size: nil)
         @logger = logger || Logger.new($stdout)
-        @batch_size = batch_size
+        @batch_size = (batch_size || ENV['RANK_SOLR_BATCH_SIZE'] || DEFAULT_BATCH_SIZE).to_i
+        @retry_count = 0
       end
 
-      # Reads the rank map and writes each ontology's normalizedScore onto all
-      # of its term_search docs. Returns the count of ontologies whose docs were
-      # updated.
-      def propagate(rank_map = nil)
+      # Reads the rank map and writes each ontology's normalizedScore onto all of
+      # its term_search docs. Unchanged ontologies are skipped unless force: true.
+      # Returns the number of ontologies whose docs were updated.
+      def propagate(rank_map = nil, force: false)
         rank_map ||= LinkedData::Models::Ontology.rank
 
         if rank_map.nil? || rank_map.empty?
@@ -36,39 +56,73 @@ def propagate(rank_map = nil)
           return 0
         end
 
+        last_propagated = force ? {} : load_last_propagated
+        total_onts = rank_map.size
         updated = 0
-        rank_map.each do |acronym, rank_info|
+        skipped = 0
+        @retry_count = 0
+
+        rank_map.each_with_index do |(acronym, rank_info), i|
           score = rank_info[:normalizedScore].to_f
+
+          if !force && last_propagated[acronym] == score
+            skipped += 1
+            next
+          end
+
+          log("[#{i + 1}/#{total_onts}] #{acronym} rank=#{score}")
           count = update_ontology_rank(acronym, score)
-          @logger.info("RankSolrPropagator: #{acronym} rank=#{score} (#{count} docs)")
+
+          last_propagated[acronym] = score
+          save_last_propagated(last_propagated)
           updated += 1 if count.positive?
         end
 
-        LinkedData::Models::Class.indexCommit(nil)
-        @logger.info("RankSolrPropagator: committed; updated #{updated} ontologies")
+        summary = "done; updated #{updated}, skipped #{skipped} unchanged (of #{total_onts}); Solr retries: #{@retry_count}"
+        if @retry_count.zero?
+          log(summary)
+        else
+          @logger.warn("RankSolrPropagator: BACKPRESSURE — #{summary}. " \
+                       'Solr stalled and was retried; consider a smaller RANK_SOLR_BATCH_SIZE.')
+          @logger.flush if @logger.respond_to?(:flush)
+        end
         updated
       end
 
       private
 
-      # Cursor-scans all term_search docs for the acronym and issues batched
-      # atomic updates to ontologyRank. Returns the number of docs updated.
+      # Cursor-scans all term_search docs for the acronym, issues batched atomic
+      # updates to ontologyRank (no commit), then commits once. Returns the
+      # number of docs updated.
       def update_ontology_rank(acronym, score)
+        query = "submissionAcronym:\"#{acronym}\""
+        num_found = (solr_search(query, rows: 0).dig('response', 'numFound') || 0).to_i
+
+        if num_found.zero?
+          log("  #{acronym}: no term_search docs; skipping")
+          return 0
+        end
+        log("  #{acronym}: #{num_found} docs to update")
+
         cursor = '*'
         total = 0
-        query = "submissionAcronym:\"#{acronym}\""
+        last_logged = 0
 
         loop do
-          resp = LinkedData::Models::Class.search(
-            query,
-            { fl: 'id', rows: @batch_size, sort: 'id asc', cursorMark: cursor }
-          )
+          resp = solr_search(query, fl: 'id', rows: @batch_size, sort: 'id asc', cursorMark: cursor)
           docs = resp.dig('response', 'docs') || []
 
           unless docs.empty?
             batch = docs.map { |d| { id: d['id'], ontologyRank: { set: score } } }
-            LinkedData::Models::Class.search_client.index_document(batch, commit: false)
+            with_retry("update #{acronym}") do
+              LinkedData::Models::Class.search_client.index_document(batch, commit: false)
+            end
             total += batch.size
+
+            if total - last_logged >= PROGRESS_EVERY
+              log("  #{acronym}: #{total}/#{num_found} docs…")
+              last_logged = total
+            end
           end
 
           next_cursor = resp['nextCursorMark']
@@ -77,8 +131,71 @@ def update_ontology_rank(acronym, score)
           cursor = next_cursor
         end
 
+        # Commit between ontologies (no updates in flight) — keeps the tlog
+        # bounded without stalling replica forwarding mid-stream.
+        log("  #{acronym}: committing #{total} docs…")
+        with_retry("commit #{acronym}") { LinkedData::Models::Class.indexCommit(nil) }
+        log("  #{acronym}: done (#{total} docs)")
         total
       end
+
+      def solr_search(query, **params)
+        with_retry('search') { LinkedData::Models::Class.search(query, params) }
+      end
+
+      # Retries transient Solr errors (e.g. distributed-update stalls under load)
+      # with exponential backoff. Re-raises once retries are exhausted.
+      def with_retry(what)
+        attempts = 0
+        begin
+          yield
+        rescue RSolr::Error::Http, RSolr::Error::ConnectionRefused,
+               Errno::ECONNREFUSED, Errno::ECONNRESET, Errno::EPIPE, Errno::ETIMEDOUT,
+               Net::ReadTimeout, Net::OpenTimeout, Net::WriteTimeout,
+               SocketError => e
+          attempts += 1
+          @retry_count += 1
+          first_line = e.message.to_s.lines.first&.strip
+
+          if attempts > MAX_RETRIES
+            @logger.error("RankSolrPropagator: BACKPRESSURE — #{what} still failing after " \
+                          "#{MAX_RETRIES} retries; aborting — #{first_line}")
+            @logger.flush if @logger.respond_to?(:flush)
+            raise
+          end
+
+          wait = RETRY_BASE_SLEEP * (2**(attempts - 1))
+          @logger.warn("RankSolrPropagator: BACKPRESSURE — #{what} failed (attempt " \
+                       "#{attempts}/#{MAX_RETRIES}); retrying in #{wait}s — #{first_line}")
+          @logger.flush if @logger.respond_to?(:flush)
+          sleep(wait)
+          retry
+        end
+      end
+
+      def log(msg)
+        @logger.info("RankSolrPropagator: #{msg}")
+        @logger.flush if @logger.respond_to?(:flush)
+      end
+
+      def redis
+        @redis ||= Redis.new(host: LinkedData.settings.ontology_analytics_redis_host,
+                             port: LinkedData.settings.ontology_analytics_redis_port)
+      end
+
+      def load_last_propagated
+        raw = redis.get(LAST_PROPAGATED_REDIS_FIELD)
+        raw ? Marshal.load(raw) : {}
+      rescue StandardError => e
+        @logger.warn("RankSolrPropagator: could not load skip cache (#{e.class}: #{e.message}); propagating all")
+        {}
+      end
+
+      def save_last_propagated(map)
+        redis.set(LAST_PROPAGATED_REDIS_FIELD, Marshal.dump(map))
+      rescue StandardError => e
+        @logger.warn("RankSolrPropagator: could not save skip cache (#{e.class}: #{e.message})")
+      end
     end
   end
 end
diff --git a/test/services/test_rank_solr_propagator.rb b/test/services/test_rank_solr_propagator.rb
index 3b83cb80..1c13c0dd 100644
--- a/test/services/test_rank_solr_propagator.rb
+++ b/test/services/test_rank_solr_propagator.rb
@@ -1,5 +1,6 @@
 require_relative '../models/test_ontology_common'
 require 'mocha/minitest'
+require 'stringio'
 
 class TestRankSolrPropagator < LinkedData::TestOntologyCommon
   ACRONYM = 'BRO'
@@ -12,6 +13,7 @@ def self.after_suite
 
   def setup
     self.class.after_suite
+    clear_propagation_cache
     # Index a small ontology's terms into the real local term_search collection.
     submission_parse(ACRONYM, 'BRO Ontology',
                      './test/data/ontology_files/BRO_v3.5.owl', 1,
@@ -22,6 +24,19 @@ def setup
 
   def teardown
     self.class.after_suite
+    clear_propagation_cache
+  end
+
+  def clear_propagation_cache
+    Redis.new(host: LinkedData.settings.ontology_analytics_redis_host,
+              port: LinkedData.settings.ontology_analytics_redis_port)
+         .del(LinkedData::Services::RankSolrPropagator::LAST_PROPAGATED_REDIS_FIELD)
+  end
+
+  def stub_rank(score)
+    LinkedData::Models::Ontology.stubs(:rank).returns(
+      { ACRONYM => { bioportalScore: 0.5, umlsScore: 0.0, normalizedScore: score } }
+    )
   end
 
   def term_docs
@@ -35,9 +50,7 @@ def test_propagate_writes_normalized_score_to_all_term_docs
     docs_before = term_docs
     refute_empty docs_before, 'expected BRO terms to be indexed in term_search'
 
-    LinkedData::Models::Ontology.stubs(:rank).returns(
-      { ACRONYM => { bioportalScore: 0.5, umlsScore: 0.0, normalizedScore: 0.642 } }
-    )
+    stub_rank(0.642)
 
     updated = LinkedData::Services::RankSolrPropagator.new.propagate
     assert_equal 1, updated
@@ -57,9 +70,7 @@ def test_propagate_preserves_searchable_fields
     )['response']['numFound']
     refute_equal 0, before, 'expected a prefLabel match before propagation'
 
-    LinkedData::Models::Ontology.stubs(:rank).returns(
-      { ACRONYM => { bioportalScore: 0.5, umlsScore: 0.0, normalizedScore: 0.642 } }
-    )
+    stub_rank(0.642)
     LinkedData::Services::RankSolrPropagator.new.propagate
 
     # The same search still works after atomic updates (copyField targets and
@@ -75,4 +86,58 @@ def test_propagate_returns_zero_for_empty_rank_map
     LinkedData::Models::Ontology.stubs(:rank).returns({})
     assert_equal 0, LinkedData::Services::RankSolrPropagator.new.propagate
   end
+
+  def test_propagate_skips_unchanged_ontology_on_second_run
+    stub_rank(0.642)
+
+    first = LinkedData::Services::RankSolrPropagator.new.propagate
+    assert_equal 1, first, 'first run should propagate the ontology'
+
+    # Rank unchanged -> the second run should skip it entirely.
+    second = LinkedData::Services::RankSolrPropagator.new.propagate
+    assert_equal 0, second, 'unchanged ontology should be skipped on the second run'
+  end
+
+  def test_force_repropagates_even_when_unchanged
+    stub_rank(0.642)
+
+    LinkedData::Services::RankSolrPropagator.new.propagate
+    # force: true ignores the skip cache and re-propagates.
+    forced = LinkedData::Services::RankSolrPropagator.new.propagate(nil, force: true)
+    assert_equal 1, forced, 'force should re-propagate even when rank is unchanged'
+  end
+
+  # Manifests the Solr-stall condition: the first atomic-update POST raises a
+  # retryable error, and we assert the run recovers AND emits a visible signal
+  # (a BACKPRESSURE warning plus a non-zero retry count in the summary).
+  def test_backpressure_is_retried_and_logged
+    stub_rank(0.642)
+
+    log_io = StringIO.new
+    propagator = LinkedData::Services::RankSolrPropagator.new(logger: Logger.new(log_io))
+    propagator.stubs(:sleep) # skip the backoff wait in the test
+
+    # RSolr::Error::ConnectionRefused is the error that previously slipped past
+    # the retry list and aborted a live run; assert it is now retried.
+    client = LinkedData::Models::Class.search_client
+    conn_refused = RSolr::Error::ConnectionRefused.new(uri: URI('http://solr.test/term_search/select'))
+    client.stubs(:index_document).raises(conn_refused).then.returns(true)
+
+    updated = propagator.propagate
+    assert_equal 1, updated, 'run should recover from the transient error'
+
+    log = log_io.string
+    assert_match(/BACKPRESSURE/, log, 'a retry must emit a filterable BACKPRESSURE warning')
+    assert_match(/Solr retries: 1/, log, 'summary must report the retry count')
+  end
+
+  # A clean run reports zero retries, so "Solr retries: 0" is the all-clear.
+  def test_clean_run_reports_zero_retries
+    stub_rank(0.642)
+
+    log_io = StringIO.new
+    LinkedData::Services::RankSolrPropagator.new(logger: Logger.new(log_io)).propagate
+
+    assert_match(/Solr retries: 0/, log_io.string, 'clean run must report zero retries')
+  end
 end