docs(administration): add opensearch configuration documentation (#209)

docs/administration/configuration/configuring-opensearch/index.md

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+# Configuring OpenSearch
+
+OpenSearch with FluentBit is required for live job logs functionality. It is deployed using [OpenSearch Helm chart](https://github.com/opensearch-project/helm-charts) as REANA Helm chart dependency and should be enabled by changing `opensearch.enabled` value to `true`. Configuration can be passed to the chart by putting it under `opensearch` field in REANA Helm chart.
+
+Example Helm command to add live logs to an existing REANA deployment (check [Authentication/authorization section](index.md#authenticationauthorization) on how to create password hashes):
+
+```bash
+helm upgrade reana reanahub/reana --wait \
+  --set opensearch.customSecurityConfig.internalUsers.reana.hash='$So$mE$reAnAPaSswoRDhASh' \
+  --set opensearch.customSecurityConfig.internalUsers.fluentbit.hash='$So$fLUenTmE$PaSswoRDhASh' \
+  --set fluent-bit.outputConfig.httpPasswd='FluentBitUserPassword' \
+  --set opensearch.enabled=true \
+  --set fluent-bit.enabled=true
+```
+
+External OpenSearch service can be used instead (recommended).
+
+## Number of instances
+
+Only one OpenSearch instance is deployed by default. To add more instances, each of these instances needs to have its own `PersistentVolumeClaim` and `PersistentVolume` as each instance writes to a data directory with the same name but different contents - this will not work with `reana-shared-persistent-volume` or `reana-infrastructure-persistent-volume`. OpenSearch Helm chart uses `volumeClaimTemplates` to create a separate volume for each instance. It is possible to use `volumeClaimTemplates` with the `StorageClass` of choice (see [Multiple volumes with custom storage class](index.md#multiple-volumes-with-custom-storage-class)).
+
+Change Helm chart vlaues to configure 3 OpenSearch instances:
+
+```yaml
+opensearch:
+  <...>
+  singleNode: false
+  replicas: 3
+  <...>
+```
+
+OpenSearch allows deploying nodes of different types, e. g. `master` and `data`. Current Helm setup does not allow it, and each node has all possible roles, see [default Helm chart roles value](https://github.com/opensearch-project/helm-charts/blob/4253842c1e4d3ac6d4aee294e905c1f20469adc2/charts/opensearch/values.yaml#L16-L20).
+
+## Persistent storage
+
+There are multiple ways to configure persistent strage for OpenSearch.
+
+### Host path
+
+This is the simplest method, but not usable for production, see [Configuring storage volumes](../configuring-storage-volumes/index.md#hostpath).
+
+OpenSearch for REANA is configured with `hostPath` by default.
+
+### Shared/infrastructure volume
+
+To use shared or infrastructure volume, change `opensearch.extraVolumes` parameter in REANA `values.yaml` file:
+
+```yaml
+opensearch:
+  <...>
+  extraVolumes:
+    - name: reana-opensearch-volume
+      persistentVolumeClaim:
+        claimName: reana-shared-persistent-volume  # or reana-infrastructure-persistent-volume
+        readOnly: false
+  <...>
+```
+
+Bear in mind that this only works with one OpenSearch node. To deploy multiple nodes, see [Multiple volumes with custom storage class](index.md#multiple-volumes-with-custom-storage-class).
+
+### Multiple volumes with custom storage class
+
+To deploy [multiple OpenSearch nodes](./index.md#number-of-instances), persistence configuration provided by OpenSearch chart should be used. It employs [`volumeClaimTemplates`](https://github.com/opensearch-project/helm-charts/blob/4253842c1e4d3ac6d4aee294e905c1f20469adc2/charts/opensearch/templates/statefulset.yaml#L27-L53) to create a separate `PersistentVolume` and `PersistentVolumeClaim` for each OpenSearch instance. You need to provide the name of your `StorageClass`, which you should have created as described in [NFS](../configuring-storage-volumes/index.md#nfs) or [CephFS](../configuring-storage-volumes/index.md#cephfs) configuration docs. The name of the `StorageClass` should be `reana-shared-volume-storage-class` or `reana-infrastructure-volume-storage-class`:
+
+```yaml
+opensearch:
+  <...>
+  persistence:
+    enabled: true
+    storageClass: "reana-shared-volume-storage-class"  # or reana-infrastructure-volume-storage-class
+  <...>
+  extraVolumes: []
+  extraVolumeMounts: []
+```
+
+Additionally, you can configure persistence `size`, `annotations` and other attributes, see [docs](https://github.com/opensearch-project/helm-charts/tree/main/charts/opensearch).
+
+Alternatively, `PersistentVolume` and `PersistentVolumeClaim` can be created for each node manually. `PersistentVolumeClaim` should be named as `<clusterName|nameOverride|fullnameOverride>-<nodeGroup>-<clusterName|nameOverride|fullnameOverride>-<nodeGroup>-<N>`, where `N` is a number of an OpenSearch node. The name of each node is either `clusterName-nodeGroup-X`, or `nameOverride-nodeGroup-X` if a `nameOverride` is specified, or `fullnameOverride-X` if a `fullnameOverride` is specified. By default it is `opensearch-cluster-master-0`, `opensearch-cluster-master-1`, etc., hence `PersistentVolumeClaim` names should be `opensearch-cluster-master-opensearch-cluster-master-0`, `opensearch-cluster-master-opensearch-cluster-master-1`, etc. `volumeClaimTemplates` will pick the existing volumes by their names and will not create new ones.
+
+## TLS
+
+OpenSearch uses TLS certificates for communication between nodes (not possible to turn off if using security features, even when only one OpenSearch node is deployed) and for REST API HTTPS (can be disabled). By default both internode communication and REST API use TLS. REST API HTTPS has to be enabled in order to use Basic Authentication for users as in this case username and password in plain text are sent to the server, hence without TLS credentials risk being exposed. You can read more on TLS configuration in [Opensearch documentation](https://opensearch.org/docs/latest/security/configuration/tls).
+
+By default TLS certificates for OpenSearch nodes are generated automatically by a Helm function, similar to how it is done for [Ingress](../configuring-tls-certificates/index.md#default-self-signed-certificate). In production deployments, it is recommended to supply your own certificates by putting them in a secret and mounting to an OpenSearch pod (in `opensearch.secretMounts`).
+
+When certificates change, OpenSearch has to be manually restarted or have dynamic certificate reload as described in the [documentation](https://opensearch.org/docs/latest/security/access-control/api/#reload-transport-certificates). If root CA has changed, services that connect to OpenSearch should also be restarted.
+
+To use custom TLS certificates, change `opensearch.tls.generate` to `false` and manually create a secret with TLS certificates, which should have the following structure:
+
+```yaml
+apiVersion: v1
+kind: Secret
+metadata:
+  name: reana-opensearch-tls-secret
+  namespace: <reana opensearch namespace>
+type: kubernetes.io/tls
+stringData:
+  ca.crt: |
+    <root CA public key>
+  tls.crt: |
+    <internode public key>
+  tls.key: |
+    <internode private key>
+  admin.crt: |
+    <admin public key>
+  admin.key: |
+    <admin private key>
+
+```
+
+Common name (CN) of each certificate is used in `opensearch.yml` configuration and by default should be `reana.io` (CA), `opensearch-cluster-master.default.svc.cluster.local` (internode) and `opensearch-admin.reana.io` (admin). If you generate the certificates with different common names, you should change `opensearch.yml` values `plugins.security.nodes_dn` and `plugins.security.admin_dn` accordingly (via `opensearch.tls.cert.cn` and `opensearch.tls.admin.cn` Helm chart values).
+
+Admin TLS certificates provide superadmin permissions and are stored in the same secret - there is no `admin` user, hence Basic Authentication credentials are not needed when making requests. Helm generated admin certificates can be used while making REST API requests requiring superadmin permissions (e. g. TLS certificates reload), but not while executing `securityconfig.sh` script.
+
+### Certificate renewal
+
+Certificate validity in days can be configured via `opensearch.tls.ca.ttl`, `opensearch.tls.cert.ttl` and `opensearch.tls.admin.ttl`. At some point the certificates will have to be renewed whether they are Helm generated or custom.
+
+Certificates can be updated on OpenSearch pod restart or using hot reload without OpenSearch pod restart.
+
+OpenSearch restart takes up to several minutes, hence without hot reload there is a bigger chance of losing some job logs data. If you put REANA in maintenance mode and stopped all jobs, this method is sufficient.
+
+TLS certificate hot reload is enabled in the chart configuration and allows updating TLS certificates without restarting OpenSearch instance, see [OpenSearch docs](https://opensearch.org/docs/latest/security/configuration/tls/#hot-reloading-tls-certificates).
+
+Certificate renewal steps:
+
+1. Take note what certificates are currently used:
+
+    ```bash
+      kubectl exec statefulset/opensearch-cluster-master -- cat ./config/certs/tls.crt
+    ```
+
+1. Update the certificates. Either:
+
+    - Delete the secret and upgrade Helm to generate new one:
+
+      ```bash
+      kubectl delete secret reana-opensearch-tls-secret
+      helm upgrade <...>
+      ```
+
+    - Or edit it by adding your new custom certificates:
+
+      ```bash
+      kubectl edit secret reana-opensearch-tls-secret
+      ```
+
+1. Wait until new secret is mounted into the pods:
+
+    ```bash
+    sleep 60
+    ```
+
+1. Check that certificates have changed:
+
+    ```bash
+    kubectl exec statefulset/opensearch-cluster-master -- cat ./config/certs/tls.crt
+    ```
+
+1. Load new OpenSearch certificates. Either:
+
+    - Restart OpenSearch:
+
+    ```bash
+    kubectl rollout restart statefulset opensearch-cluster-master
+    ```
+
+    - Hot reload certificates:
+
+    ```bash
+    kubectl port-forward service/opensearch-cluster-master 9200:9200
+    curl --cacert ca.crt --cert admin.crt --key admin.key \
+      --insecure -XPUT https://localhost:9200/_plugins/_security/api/ssl/transport/reloadcerts
+    curl --cacert ca.crt --cert admin.crt --key admin.key \
+      --insecure -XPUT https://localhost:9200/_plugins/_security/api/ssl/http/reloadcerts
+    ```
+
+1. Restart related services if root CA has changed. There might be errors related to TLS until the services are restarted. You probably will lose some logs if there are running workflows or jobs as it can take up to several minutes to restart the services:
+
+    ```bash
+    kubectl rollout restart daemonset reana-fluent-bit
+    kubectl rollout restart deployment reana-workflow-controller
+    ```
+
+## Authentication/authorization
+
+REANA Helm chart configures system users using configuration files in OpenSearch `config/opensearch-security` folder. You can change security configuration by overriding `opensearch.customSecurityConfig` value which creates `internal_users.yaml`, `roles.yaml` and `roles_mapping.yaml` files. This configuration is loaded only once when OpenSearch is first deployed.
+
+!!! warning
+    Subsequent changes to OpenSearch security configuration files will not take effect unless you run `securityadmin.sh` script, which can also delete users that were created via REST API. Read the [documentation on Opensearch security configuration](https://opensearch.org/docs/latest/security/configuration/security-admin/) before attempting to make changes.
+    To run `securityadmin.sh` script, you will need [custom admin certificates in PKCS#8 format](https://opensearch.org/docs/latest/security/configuration/generate-certificates/#generate-an-admin-certificate) - it will not work with Helm generated certificates.
+
+By default two users are configured - `reana` and `fluentbit`. Their passwords need to be prepared by first spinning up OpenSearch instance in development environment, connecting to a pod and [running `plugins/opensearch-security/tools/hash.sh` script](https://opensearch.org/docs/latest/security/configuration/yaml/#internal_usersyml):
+
+```bash
+kubectl exec statefulset/opensearch-cluster-master -- ./plugins/opensearch-security/tools/hash.sh -p <somepassword>
+```
+
+The generated hashes for the passwords should be supplied to Helm with `--set opensearch.customSecurityConfig.internalUsers.reana.hash='$So$Me$pASsWOrD.HasH' --set opensearch.customSecurityConfig.internalUsers.fluentbit.hash='$So$Me$pASsWOrD.HasH'` options.