I am new to ES7 and trying to understand optimistic concurrency control.
I think I understand that when I get-request a document and send its _seq_no and _primary_term values in a later write-request to the same document, if the values differ, the write will be completely ignored.
But what happens to the document in the default case where I don't send the _seq_no and _primary_term values? Will the write go through even if it has older _seq_no and _primary_term values (therefore making the index inconsistent), or only be processed if the values are newer?
If the former, will the document eventually be consistent?
I'm trying to figure out if I need to send these values to get eventual consistency or if I get it for free without sending those values.
It's a great distributed system question. Let me break down the problem into sub-parts for readability and even before explain what is _seq_no and _primary_term as there isn't much explanation of those on the ES site.
_seq_no is the incremental counter which is assigned to ES document for each operation(update, delete, index), for example:- the first time you index a doc, it will have value 1, next update will have 2, next delete operation will have three and so on. Read operation doesn't update it._primary_term is the also an incremental counter, but change only when a replica shard is promoted as primary, due to network or any other failure, so if everything is excellent in your cluster it will not be changed, but in case of some failure and other replica promoted to primary then it would be increased.Coming to the first question,
Q:- What happens to the document in the default case where I don't send the _seq_no and _primary_term values?
Ans:- you can have lost update issue, suppose you have a counter which you are updating, simultaneously 2 requests read the counter value to 1 and trying to increment by 1. now when you don't specify these above terms explicitly, then it's calculated by ES. Now both the requests reach simultaneously to ES, then ES(primary shard) will process them one by one by increasing the sequence number, so at the end, your counter will have value 2, instead of 3. to make sure this doesn't happen, you pass these term values explicitly, and when ES tries to update them will see different sequence number and will reject your request. To prevent such lost updates, use-cases, its always recommended sending explicit version number.
Q:- I'm trying to figure out if I need to send these values to get eventual consistency or if I get it for free without sending those values..
Answer:- These are related to concurrency control and nothing to deal with eventual consistency. In ES, write always happens to primary shards, but read can happen to any replicas(may contain obsolete data), which makes ES eventual consistent.
Important read