Do all shards (within index) have the same content?

Question

Do all shards (within index) have the same content?

If yes, more shards = longer propagation (save) time? If no, when one of shards failed = data is incomplete when merging?

Answer 1

First, you need to understand what is sharding and why it's important in distributed systems like elasticsearch. You can read some good resources on shards here here and here.

Now Coming to your question,

Do all shards (within index) have the same content.

The answer, is no (assuming you are referring to primary shards here, of course, replica shard is just a copy of primary shard), let's take an example.

Your Index contains around 100 million docs and you have a 10 data nodes cluster, then you want to horizontally scale your index, so you started with the setting of 10 primary shards and 1 replica shards. In this case, elasticsearch will physically divide your data into 10 primary shards and each primary shard will be on a different node of a cluster as there are 10 data nodes and similarly every primary shards copy which is called replica of a shard which is on a different node of its primary shard.

Now coming to your follow-up question.

If yes, more shards = longer propagation (save) time? If no, when one of shards failed = data is incomplete when merging?

As elasticsearch doesn't store the same data in all the primary shards, so more shards mean longer propagation or save time is invalid and also when one of the shards is failed then elasticsearch recover its data from its replica shard as it's present physically on a different data node server.

Bonus tip:- Shards are used to split your data and to make your application horizontal scalable, while the replica is to make your application is highly available as it contains the duplicated data, so the application can recover easily from the scenario you just asked in your follow-up question.

Let me know if you need any clarification or more details.

Answer 2

short answer:

Q-1: no

if-no: if index has not a replica, it affects the whole index but not other shards of the index .

please read this document: https://www.elastic.co/guide/en/elasticsearch/reference/6.2/_basic_concepts.html