private IndexShardWriter open(final long indexShardId) {
        return openShards.compute(indexShardId, (k, v) -> {
            if (v != null) {
                closeWriter(v);
            }
            return openWriter(indexShardId);
        });
    }
@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