How to safely scale down
This page outlines the general steps to follow when scaling down Charmed OpenSearch in order to prevent data loss and ensure the deployment remains highly available
To see an example of scaling down a real deployment scenario, check the following page from the Charmed OpenSearch Tutorial: 6. Scale horizontally.
Warning:
- The following steps are for removing one single Juju unit (node). This may be repeated as many times as necessary, but never remove multiple units in the same command.
- In highly available deployments, it is not safe to scale below 3 nodes.
Summary
1. Check cluster health before scaling down
First of all, make sure that removing nodes is a safe operation to do. For that, check the health of the cluster. This can be done via Juju or via the OpenSearch API.
Via Juju
The charm will reflect the current health of the cluster on the application status. This will display an active status when the cluster is in good health, and a blocked status along with an informative message when the cluster is not in good health.
Below is a sample output of the command juju status --watch 1s when the cluster is not healthy enough for scaling down.
Model Controller Cloud/Region Version SLA Timestamp
tutorial opensearch-demo localhost/localhost 2.9.42 unsupported 15:46:15Z
App Version Status Scale Charm Channel Rev Exposed Message
data-integrator active 1 data-integrator edge 11 no
opensearch blocked 2 opensearch edge 22 no 1 or more 'replica' shards are not assigned, please scale your application up.
tls-certificates-operator active 1 tls-certificates-operator stable 22 no
In this case, the cluster is not in good health because the status is blocked, and the message says 1 or more 'replica' shards are not assigned, please scale your application up.
Via the OpenSearch health API
To monitor the health more precisely, you can use the OpenSearch health API.
In order to authenticate your requests to the REST API, you need to retrieve the admin user’s credentials.
To get the admin user credentials, run the following command:
juju run opensearch/leader get-password
> unit-opensearch-0:
results:
ca-chain: |-
<certificate>
username: admin
password: admin_password
Cluster health statuses
A cluster health may return green, yellow, or red.
green
Scaling down might be safe to do. This is roughly equivalent to an active juju status.
It is imperative to check whether the node targeted for removal does not hold a primary shard of an index with no replicas. You can see this by making the following request and seeing which primary shards are allocated to a given node:
curl --cacert cert.pem -k -XGET https://admin:admin_pasword@10.180.162.96:9200/_cat/shards
It is generally not recommended to disable replication for indices, but if that’s the case: re-route the said shard manually to another node.
yellow
Scaling down might not be safe to do. This is roughly equivalent to a blocked juju status.
This means that some replica shards are unassigned. You can visualize that by using the cat API as shown below.
curl --cacert cert.pem -k -XGET https://10.180.162.96:9200/_cat/shards -u admin:admin_password
A general good course of action here would be to scale up (add a unit) to have a green state where all primary and replica shards are well assigned.
To investigate why is your cluster in a yellow state. You can make the following call to have an explanation:
curl --cacert cert.pem -k -XGET "https://10.180.162.96:9200/_cluster/allocation/explain?filter_path=index,shard,primary,**.node_name,**.node_decision,**.decider,**.decision,**.*explanation,**.unassigned_info,**.*delay" -u admin:admin_password
Depending on the output, there may be a different course of action. For example: scaling up, adding more storage to the existing nodes, or perhaps manually re-route the relevant shard manually to another node.
To scale up by one unit, run the following command:
juju add-unit -n 1
red
Scaling down is definitely not safe to do, as some primary shards are not assigned. This is roughly equivalent to a blocked juju status.
The course of action to follow here is to add units to the cluster. To scale up by one unit, run the following command:
juju add-unit -n 1
Note: If the health color is red after removing a unit, the charm will attempt to block the removal of the node, giving the administrator the opportunity to scale up (add units).
2. Scale down one unit
Once you made sure that removing a unit is safe to do, you can proceed to removing a single unit. It is unsafe to remove more than one unit at a time.
Note: Although we implement a rolling units removal, the internal state of OpenSearch is only reflected reactively. This means the charm does not know in advance whether a certain removal will put the cluster in a red (some primary shards are unassigned) or yellow (some replica shards are unassigned).
Read more about cluster health in the official OpenSearch documentation.
To remove one unit of the opensearch application, run the following command:
juju remove-unit opensearch/<unit-id>
Make sure you monitor the status of the application using: juju status --watch 1s.
3. Repeat cluster health check
After removing one unit, depending on the roles of the said unit, the charm may reconfigure and restart a unit to balance the node roles. You can monitor this with juju status --watch 1s.
Make sure you wait for the whole application to stabilize before you consider removing further units.
Once the application is stable, check the health of the cluster as detailed in the section Understand the meaning of the cluster status and react accordingly.