@Override
    public void flush(final long indexShardId) {
        final Optional<IndexShardWriter> optional = cache.getIfPresent(indexShardId);
        optional.ifPresent(indexShardWriter -> {
            LOGGER.debug(() -> "Flush index shard " + indexShardId);
            indexShardWriter.flush();
        });
    }
@param indexShardId The id of the index shard.
@return A writer for the index shard.
stroom-index/stroom-index-impl/src/main/java/stroom/index/impl/IndexShardWriterCacheImpl.java