Inconsistent handling of node IDs in preference and _cat/nodes

[mtritschler]

Elasticsearch version (bin/elasticsearch --version): 5.4.0

Plugins installed: [x-pack]

JVM version (java -version):
java version "1.8.0_102"
Java(TM) SE Runtime Environment (build 1.8.0_102-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.102-b14, mixed mode)

OS version (uname -a if on a Unix-like system):
Linux searchlog001 4.4.0-96-generic #119-Ubuntu SMP

Description of the problem including expected versus actual behavior:
GET /_cat/nodes?h=id returns just the first four characters of the node id, so it seems as if this would suffice to identify a node. However, when using just the first four characters as value for preference=_prefer_nodes:..., the preference does not get honored and search results from replicas on other nodes may get returned. The preference does only apply when one uses the full-length ID of a node.

Steps to reproduce:

1. Setup a cluster with at least two nodes. Create an index with a single shard and one replica
2. Index a single document
3. Do curl $ELASTIC_HOST:9200/_cat/nodes?h=name,id to obtain a short node ID and the host name
4. Query for the document with "explain": true to see the node in the response and set preference=_prefer_nodes:$NODE_ID, repeat to see eventually varying node IDs in the response
5. Do curl $ELASTIC_HOST:9200/_nodes to get the full-length node IDs (keys in /nodes)
6. Query for the document as above but with the preference using the full, repeat to always see the specified node ID in the response

[jasontedor]

The short ID is for display purposes. There is a documented parameter for the cat nodes API (like full_id) that shows the full ID needed for preferences. I’m closing this as expected behavior and not a bug.

[mtritschler]

I see. But as specified in the report, I am using 5.4.0. The docs for 5.4 don't mention `full_id` at all ( https://www.elastic.co/guide/en/elasticsearch/reference/5.4/cat-nodes.html). Is this just undocumented in this version or has it been added later? 2017-12-15 18:55 GMT+01:00 Jason Tedor <notifications@github.com>:

…

The short ID is for display purposes. There is a documented parameter for the cat nodes API (like full_id) that shows the full ID needed for preferences. I’m closing this as expected behavior and not a bug. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#27843 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAZHd8yvgUsMNet-Ik9SxMb2nmbZ-8AGks5tArKpgaJpZM4RDwPU> .

[jasontedor]

Yes, it was undocumented previously although works but it is documented starting in 5.6. I hope that helps.