CODEWITHSUNDEEP
elasticsearch
SGiux
SGiux

Reputation: 839

Why elasticsearch introduced the _primary_term to obtain optimistic concurrency control?

From the official Elasticsearch's documentation about optimistic concurrency control I read about the _seq_no and _primary_term as parameter needed to implement the optimistic locking.

I didn't understand the utility of the _seq_no parameter. Is not enough a sequence number to indentify uniquely a change in a document?

Maybe a possible scenario/example can explain to me why the primary term is needed.

Upvotes: 2

Views: 783

Answers (1)

SGiux
SGiux

Reputation: 839

I found an example that can be useful to understand the utility of the _primary_term:

Suppose we have 3 nodes A, B and C where A is the primary shard. Three operations are written on the primary shard:

  1. operation 1: _seq_no = 1 _primary_shard = 1
  2. operation 2: _seq_no = 2 _primary_shard = 1
  3. operation 3: _seq_no = 3 _primary_shard = 1

the primary shard starts to send these operations to be applied to the replicas shard. After a while suppose we have this situation:

  • Node A (operation 1, 2 and 3 completed)
  • Node B (operation 1 and 3 completed)
  • Node C (operation 2 completed)

of course node B and C are not aligned with the primary shard yet. Suppose that before the Node A sends the remain operations fails and suppose that Node B becames the primary shard. It sends so all its operations history (all operations after the global checkpoint. All the operations before the global checkpoint are completed in all the active nodes. For sake of simplicity we can assume that operation 1 is the first operation executed after the last global checkpoint) to the replicas shard. Node C sees that operations 1 and 3 arrived from Node B have the _primary_shard = 2. It understand that the operation 2 is associated with the old primary shard and because of that it executed the rollback on operation 2 (the _primary_term = 1) and executed the operation 1 and 3 to be aligned with the new primary node.

So the primary term is useful to distinguish between old and new primary shard.

Reference: https://www.elastic.co/blog/elasticsearch-sequence-ids-6-0.

P.S. I think the gif showed in the link above is not correct. When the primary shard sends the operation 2 and 3 to the replicas shard, the global checkpoint on the replicas is not updated.

Upvotes: 1

Related Questions