Add rolling upgrade doc

doc/operation-and-maintenance/Rolling-upgrade.md

@@ -0,0 +1,80 @@
+## Rolling upgrade
+For all MongooseIM production deployments we recommend running multiple server nodes connected in a cluster behind a load-balancer.
+Rolling upgrade is a process of upgrading MongooseIM cluster, one node at a time.
+During the process, at least one MongooseIM node is available to handle incoming requests to guarantee availability and minimise the downtime.
+Running different MongooseIM versions at the same time beyond the duration of the upgrade is not recommended and not supported.
+
+Rolling upgrade procedure is recommended over configuration reload which is not supported since version 4.1.
+
+Please note that more complex upgrades that involve schema updates, customisations or have functional changes might require more specific and specially crafted migration procedure.
+
+The usual MongooseIM cluster upgrade can be achieved with the following steps:
+
+### 1. Check the cluster status.
+
+Use the following command on the running nodes and examine the status of the cluster:
+
+```bash
+mongooseimctl mnesia info | grep "running db nodes"
+
+running db nodes = [mongooseim@node1, mongooseim@node2]
+```
+
+This command shows all running nodes.
+A healthy cluster should list all nodes that are part of the cluster.
+
+Should you have any issues related to node clustering, please refer to [Cluster configuration and node management](Cluster-configuration-and-node-management.md) section.
+
+### 2. Apply the changes from the migration guide.
+
+All modifications of the configuration file or updates of the database schema, that are required to perform version upgrade, can be found in the Migration Guide section.
+When upgrading more than one version, please make sure to go over all consecutive, migration guides.
+
+For example, when migrating from MongooseIM 3.7 to 4.1, please familiarize yourself and apply all necessary changes described in the following pages of the Migration Guide section.
+
+* 3.7.0 to 4.0.0
+* 4.0.0 to 4.0.1
+* 4.0.1 to 4.1.0
+
+### 3. Stop the running node.
+
+Use the following command to stop the MognooseIM node:
+
+```bash
+mongooseimctl stop
+```
+
+### 4. Install new MongooseIM version.
+
+You can get the new version of the MongooseIM by either [building MongooseIM from source code](../user-guide/How-to-build.md) or [downloading and upgrading from package](../../user-guide/Getting-started/#download-a-package).
+
+### 5. Start the node.
+
+Use the following command to start and check the status of the MognooseIM node and the cluster:
+
+```bash
+mongooseimctl start
+mongooseimctl status
+
+mongooseimctl mnesia info | grep "running db nodes"
+```
+
+### 6. Test the cluster.
+
+Please verify that the nodes are running and part of the same cluster.
+If the cluster is working as expected, the migration of the node is complete.
+
+### 7. Upgrade the remaining nodes.
+
+Once all the prior steps are completed successfully, repeat the process for all nodes that are part of the MongooseIM cluster.
+
+## Further cluster upgrade considerations
+
+Another way to perform a cluster upgrade with minimising possible downtime would be to setup a parallel MongooseIM cluster running newer version.
+You can redirect the incoming traffic to the new cluster with use of a load-balancer.
+
+Once no connections are handled by the old cluster it can by safely stopped and the migration is complete.
+
+We highly recommend testing new software release in staging environment before it is deployed on production.
+
+Should you need any help with the upgrade, deployments or load testing of your MongooseIM cluster, please reach out to us. MongooseIM consultancy and support is part of [our commercial offering](https://www.erlang-solutions.com/products/mongooseim.html).

mkdocs.yml

@@ -84,6 +84,7 @@ nav:
       - 'Logging configuration': 'operation-and-maintenance/Logging.md'
       - 'Logging with Humio': 'operation-and-maintenance/Humio.md'
       - 'Logging fields': 'operation-and-maintenance/Logging-fields.md'
+      - 'Rolling upgrade': 'operation-and-maintenance/Rolling-upgrade.md'
       - 'Cluster restart': 'operation-and-maintenance/Cluster-restart.md'
       - 'Metrics': 'operation-and-maintenance/MongooseIM-metrics.md'
       - 'System Metrics Privacy Policy': 'operation-and-maintenance/System-Metrics-Privacy-Policy.md'