@Override
    public void readMessage(final HandlerContext ctx, final RegionPingEvent event) throws Exception {
        if (event.isReady()) {
            return;
        }
        final MetadataStore metadataStore = event.getMetadataStore();
        final RegionHeartbeatRequest request = event.getMessage();
        final long clusterId = request.getClusterId();
        final long storeId = request.getStoreId();
        final ClusterStatsManager clusterStatsManager = ClusterStatsManager.getInstance(clusterId);
        final List<Pair<Region, RegionStats>> regionStatsList = request.getRegionStatsList();
        for (final Pair<Region, RegionStats> stats : regionStatsList) {
            final Region region = stats.getKey();
            clusterStatsManager.addOrUpdateLeader(storeId, region.getId());
        }

        // check if the modelWorker
        final Pair<Set<Long>, Integer> modelWorkers = clusterStatsManager.findModelWorkerStores(1);
        final Set<Long> modelWorkerStoreIds = modelWorkers.getKey();
        final int modelWorkerLeaders = modelWorkers.getValue();
        if (!modelWorkerStoreIds.contains(storeId)) {
            return;
        }

        LOG.info("[Cluster: {}] model worker stores is: {}, it has {} leaders.", clusterId, modelWorkerStoreIds, modelWorkerLeaders);

        for (final Pair<Region, RegionStats> pair : regionStatsList) {
            final Region region = pair.getKey();
            final List<Peer> peers = region.getPeers();
            if (peers == null) {
                continue;
            }
            final List<Endpoint> endpoints = Lists.transform(peers, Peer::getEndpoint);
            final Map<Long, Endpoint> storeIds = metadataStore.unsafeGetStoreIdsByEndpoints(clusterId, endpoints);
            // find lazyWorkers
            final List<Pair<Long, Integer>> lazyWorkers = clusterStatsManager.findLazyWorkerStores(storeIds.keySet());
            if (lazyWorkers.isEmpty()) {
                return;
            }
            for (int i = lazyWorkers.size() - 1; i >= 0; i--) {
                final Pair<Long, Integer> worker = lazyWorkers.get(i);
                if (modelWorkerLeaders - worker.getValue() <= 1) { // no need to transfer
                    lazyWorkers.remove(i);
                }
            }
            if (lazyWorkers.isEmpty()) {
                continue;
            }
            final Pair<Long, Integer> laziestWorker = tryToFindLaziestWorker(clusterId, metadataStore, lazyWorkers);
            if (laziestWorker == null) {
                continue;
            }
            final Long lazyWorkerStoreId = laziestWorker.getKey();
            LOG.info("[Cluster: {}], lazy worker store is: {}, it has {} leaders.", clusterId, lazyWorkerStoreId,
                    laziestWorker.getValue());
            final Instruction.TransferLeader transferLeader = new Instruction.TransferLeader();
            transferLeader.setMoveToStoreId(lazyWorkerStoreId);
            transferLeader.setMoveToEndpoint(storeIds.get(lazyWorkerStoreId));
            final Instruction instruction = new Instruction();
            instruction.setRegion(region.copy());
            instruction.setTransferLeader(transferLeader);
            event.addInstruction(instruction);
            LOG.info("[Cluster: {}], send 'instruction.transferLeader': {} to region: {}.", clusterId, instruction, region);
            break; // Only do one thing at a time
        }
    }
Trying to balance the number of leaders in each store.

@author jiachun.fjc
jraft-rheakv/rheakv-pd/src/main/java/com/alipay/sofa/jraft/rhea/pipeline/handler/RegionLeaderBalanceHandler.java