diff --git a/categories/apps/elasticsearch.md b/categories/apps/elasticsearch.md
index 546f20c..1e5318a 100644
--- a/categories/apps/elasticsearch.md
+++ b/categories/apps/elasticsearch.md
@@ -1,3 +1,17 @@
 # Elasticsearch
 
 Elasticsearch is a search engine based on the Lucene library. It provides a distributed, multitenant-capable full-text search engine with an HTTP web interface and schema-free JSON documents.
+
+
+## Tips
+
+## Calculate number of replicas
+
+> N >= R + 1
+
+N : number of nodes
+R: Number of replicas
+
+### How to resolve unassigned shards
+
+https://www.datadoghq.com/blog/elasticsearch-unassigned-shards/#reason-3-you-need-to-reenable-shard-allocation