docs(upgrade) add 2.3.0 upgrade path

UPGRADE.md

@@ -36,6 +36,158 @@ starts new workers, which take over from old workers before those old workers
 are terminated. In this way, Kong will serve new requests via the new
 configuration, without dropping existing in-flight connections.
 
+## Upgrade to `2.3.0`
+
+Kong adheres to [semantic versioning](https://semver.org/), which makes a
+distinction between "major", "minor", and "patch" versions. The upgrade path
+will be different depending on which previous version from which you are migrating.
+
+If you are migrating from 2.0.0, 2.1.x, or 2.2.x upgrading into 2.3.x is a minor upgrade,
+but read below for important instructions on database migration, especially
+for Cassandra users.
+
+If you are migrating from 1.x, upgrading into 2.3.x is a major upgrade,
+so, in addition, be aware of any [breaking changes](#breaking-changes-2.0.0)
+between the 1.x and 2.x series below, further detailed in the
+[CHANGELOG.md](https://github.com/Kong/kong/blob/2.0.0/CHANGELOG.md) document.
+
+
+#### 1. Dependencies
+
+If you are using the provided binary packages, all necessary dependencies
+for the gateway are bundled and you can skip this section.
+
+If you are building your dependencies by hand, there are changes since the
+previous release, so you will need to rebuild them with the latest patches.
+
+The required OpenResty version for kong 2.3.x is
+[1.17.8.2](https://openresty.org/en/changelog-1017008.html). This is more recent
+than the version in Kong 2.1.0 (which used `1.15.8.3`). In addition to an upgraded
+OpenResty, you will need the correct [OpenResty
+patches](https://github.com/Kong/kong-build-tools/tree/master/openresty-build-tools/openresty-patches)
+for this new version, including the latest release of
+[lua-kong-nginx-module](https://github.com/Kong/lua-kong-nginx-module).
+The [kong-build-tools](https://github.com/Kong/kong-build-tools)
+repository contains [openresty-build-tools](https://github.com/Kong/kong-build-tools/tree/master/openresty-build-tools),
+which allows you to build OpenResty with the necessary patches
+and modules easily.
+
+There is a new way to deploy Go using Plugin Servers. You can read more about it the [Go section of the docs](https://docs.konghq.com/2.3.x/go).
+
+#### 2. Template Changes
+
+There are **Changes in the Nginx configuration file**, between kong 2.0.0,
+2.1.0, 2.2.0 and 2.3.0.
+
+To view the configuration changes between versions, clone the
+[Kong repository](https://github.com/kong/kong) and run `git diff`
+on the configuration templates, using `-w` for greater readability.
+
+Here's how to see the differences between 2.0.0 and 2.3.0:
+
+```
+git clone https://github.com/kong/kong
+cd kong
+git diff -w 2.0.0 2.3.0 kong/templates/nginx_kong*.lua
+```
+
+To produce a patch file, use the following command:
+
+```
+git diff 2.0.0 2.3.0 kong/templates/nginx_kong*.lua > kong_config_changes.diff
+```
+
+#### 3. Suggested Upgrade Path
+
+##### Upgrade from `0.x` to `2.3.0`
+
+The lowest version that Kong 2.3.0 supports migrating from is 1.0.0.
+If you are migrating from a version lower than 0.14.1, you need to
+migrate to 0.14.1 first. Then, once you are migrating from 0.14.1,
+please migrate to 1.5.0 first.
+
+The steps for upgrading from 0.14.1 to 1.5.0 are the same as upgrading
+from 0.14.1 to Kong 1.0. Please follow the steps described in the
+"Migration Steps from 0.14" in the [Suggested Upgrade Path for Kong
+1.0](#kong-1-0-upgrade-path), with the addition of the `kong
+migrations migrate-apis` command, which you can use to migrate legacy
+`apis` configurations.
+
+Once you migrated to 1.5.0, you can follow the instructions in the section
+below to migrate to 2.3.0.
+
+##### Upgrade from `1.0.0` - `2.2.0` to `2.3.0`
+
+**Postgres**
+
+Kong 2.3.0 supports a no-downtime migration model. This means that while the
+migration is ongoing, you will have two Kong clusters running, sharing the
+same database. (This is sometimes called the Blue/Green migration model.)
+
+The migrations are designed so that the new version of Kong is able to use
+the database as it is migrated while the old Kong cluster keeps working until
+it is time to decommission it. For this reason, the migration is split into
+two steps, performed via commands `kong migrations up` (which does
+only non-destructive operations) and `kong migrations finish` (which puts the
+database in the final expected state for Kong 2.3.0).
+
+1. Download 2.3.0, and configure it to point to the same datastore
+   as your old (1.0 to 2.0) cluster. Run `kong migrations up`.
+2. Once that finishes running, both the old (pre-2.1) and new (2.3.0)
+   clusters can now run simultaneously. Start provisioning 2.3.0 nodes,
+   but do not use their Admin API yet. If you need to perform Admin API
+   requests, these should be made to the old cluster's nodes. The reason
+   is to prevent the new cluster from generating data that is not understood
+   by the old cluster.
+3. Gradually divert traffic away from your old nodes, and into
+   your 2.3.0 cluster. Monitor your traffic to make sure everything
+   is going smoothly.
+4. When your traffic is fully migrated to the 2.3.0 cluster,
+   decommission your old nodes.
+5. From your 2.3.0 cluster, run: `kong migrations finish`.
+   From this point on, it will not be possible to start
+   nodes in the old cluster pointing to the same datastore anymore. Only run
+   this command when you are confident that your migration
+   was successful. From now on, you can safely make Admin API
+   requests to your 2.3.0 nodes.
+
+**Cassandra**
+
+Due to internal changes, the table schemas used by Kong 2.3.0 on Cassandra
+are incompatible with those used by Kong 2.0.0. Migrating using the usual commands
+`kong migrations up` and `kong migrations finish` will require a small
+window of downtime, since the old and new versions cannot use the
+database at the same time. Alternatively, to keep your previous version fully
+operational while the new one initializes, you will need to transfer the
+data to a new keyspace via a database dump, as described below:
+
+1. Download 2.3.0, and configure it to point to a new keyspace.
+   Run `kong migrations bootstrap`.
+2. Once that finishes running, both the old (pre-2.1) and new (2.3.0)
+   clusters can now run simultaneously, but the new cluster does not
+   have any data yet.
+3. On the old cluster, run `kong config db_export`. This will create
+   a file `kong.yml` with a database dump.
+4. Transfer the file to the new cluster and run
+   `kong config db_import kong.yml`. This will load the data into the new cluster.
+5. Gradually divert traffic away from your old nodes, and into
+   your 2.3.0 cluster. Monitor your traffic to make sure everything
+   is going smoothly.
+6. When your traffic is fully migrated to the 2.3.0 cluster,
+   decommission your old nodes.
+
+##### Installing 2.3.0 on a Fresh Datastore
+
+The following commands should be used to prepare a new 2.3.0 cluster from a
+fresh datastore. By default the `kong` CLI tool will load the configuration
+from `/etc/kong/kong.conf`, but you can optionally use the flag `-c` to
+indicate the path to your configuration file:
+
+```
+$ kong migrations bootstrap [-c /path/to/your/kong.conf]
+$ kong start [-c /path/to/your/kong.conf]
+```
+
 ## Upgrade to `2.2.0`
 
 Kong adheres to [semantic versioning](https://semver.org/), which makes a