1 .. This work is licensed under a Creative Commons Attribution 4.0
2 .. International License.
3 .. http://creativecommons.org/licenses/by/4.0
4 .. Copyright 2018-2020 Amdocs, Bell Canada, Orange, Samsung
8 .. _Curated applications for Kubernetes: https://github.com/kubernetes/charts
9 .. _Services: https://kubernetes.io/docs/concepts/services-networking/service/
10 .. _ReplicaSet: https://kubernetes.io/docs/concepts/workloads/controllers/replicaset/
11 .. _StatefulSet: https://kubernetes.io/docs/concepts/workloads/controllers/statefulset/
12 .. _Helm Documentation: https://docs.helm.sh/helm/
13 .. _Helm: https://docs.helm.sh/
14 .. _Kubernetes: https://Kubernetes.io/
15 .. _Kubernetes LoadBalancer: https://kubernetes.io/docs/concepts/services-networking/service/#loadbalancer
16 .. _oom_user_guide_helm3:
18 OOM User Guide helm3 (experimental)
19 ###################################
21 The ONAP Operations Manager (OOM) provide the ability to manage the entire
22 life-cycle of an ONAP installation, from the initial deployment to final
23 decommissioning. This guide provides instructions for users of ONAP to
24 use the Kubernetes_/Helm_ system as a complete ONAP management system.
26 This guide provides many examples of Helm command line operations. For a
27 complete description of these commands please refer to the `Helm
30 .. figure:: oomLogoV2-medium.png
33 The following sections describe the life-cycle operations:
35 - Deploy_ - with built-in component dependency management
36 - Configure_ - unified configuration across all ONAP components
37 - Monitor_ - real-time health monitoring feeding to a Consul UI and Kubernetes
38 - Heal_- failed ONAP containers are recreated automatically
39 - Scale_ - cluster ONAP services to enable seamless scaling
40 - Upgrade_ - change-out containers or configuration with little or no service
42 - Delete_ - cleanup individual containers or entire deployments
44 .. figure:: oomLogoV2-Deploy.png
50 The OOM team with assistance from the ONAP project teams, have built a
51 comprehensive set of Helm charts, yaml files very similar to TOSCA files, that
52 describe the composition of each of the ONAP components and the relationship
53 within and between components. Using this model Helm is able to deploy all of
54 ONAP with a few simple commands.
58 Your environment must have both the Kubernetes `kubectl` and Helm setup as a
63 Enter the following to install kubectl (on Ubuntu, there are slight differences
64 on other O/Ss), the Kubernetes command line interface used to manage a
67 > curl -LO https://storage.googleapis.com/kubernetes-release/release/v1.8.10/bin/linux/amd64/kubectl
69 > sudo mv ./kubectl /usr/local/bin/kubectl
72 Paste kubectl config from Rancher (see the :ref:`cloud-setup-guide-label` for
73 alternative Kubernetes environment setups) into the `~/.kube/config` file.
75 Verify that the Kubernetes config is correct::
77 > kubectl get pods --all-namespaces
79 At this point you should see six Kubernetes pods running.
83 Helm is used by OOM for package and configuration management. To install Helm,
86 > wget https://get.helm.sh/helm-v3.3.4-linux-amd64.tar.gz
87 > tar -zxvf helm-v3.3.4-linux-amd64.tar.gz
88 > sudo mv linux-amd64/helm /usr/local/bin/helm
90 Verify the Helm version with::
96 Once kubectl and Helm are setup, one needs to setup a local Helm server to
97 server up the ONAP charts::
99 > helm install osn/onap
102 The osn repo is not currently available so creation of a local repository is
105 Helm is able to use charts served up from a repository and comes setup with a
106 default CNCF provided `Curated applications for Kubernetes`_ repository called
107 stable which should be removed to avoid confusion::
109 > helm repo remove stable
111 .. To setup the Open Source Networking Nexus repository for helm enter::
112 .. > helm repo add osn 'https://nexus3.onap.org:10001/helm/helm-repo-in-nexus/master/'
114 To prepare your system for an installation of ONAP, you'll need to::
116 > git clone -b guilin --recurse-submodules -j2 http://gerrit.onap.org/r/oom
120 To install a local Helm server::
122 > curl -LO https://s3.amazonaws.com/chartmuseum/release/latest/bin/linux/amd64/chartmuseum
123 > chmod +x ./chartmuseum
124 > mv ./chartmuseum /usr/local/bin
126 To setup a local Helm server to server up the ONAP charts::
128 > mkdir -p ~/helm3-storage
129 > chartmuseum --storage local --storage-local-rootdir ~/helm3-storage -port 8879 &
131 Note the port number that is listed and use it in the Helm repo add as
134 > helm repo add local http://127.0.0.1:8879
136 To get a list of all of the available Helm chart repositories::
140 local http://127.0.0.1:8879
142 Then build your local Helm repository::
144 > make SKIP_LINT=TRUE [HELM_BIN=<HELM_PATH>] all
147 Sets the helm binary to be used. The default value use helm from PATH
149 The Helm search command reads through all of the repositories configured on the
150 system, and looks for matches::
152 > helm search repo local
153 NAME VERSION DESCRIPTION
154 local/appc 2.0.0 Application Controller
155 local/clamp 2.0.0 ONAP Clamp
156 local/common 2.0.0 Common templates for inclusion in other charts
157 local/onap 2.0.0 Open Network Automation Platform (ONAP)
158 local/robot 2.0.0 A helm Chart for kubernetes-ONAP Robot
159 local/so 2.0.0 ONAP Service Orchestrator
161 In any case, setup of the Helm repository is a one time activity.
163 Next, install Helm Plugins required to deploy the ONAP Casablanca release::
165 > cp -R ~/oom/kubernetes/helm/plugins/ ~/.local/share/helm/plugins
167 Once the repo is setup, installation of ONAP can be done with a single
170 > helm deploy development local/onap --namespace onap
172 This will install ONAP from a local repository in a 'development' Helm release.
173 As described below, to override the default configuration values provided by
174 OOM, an environment file can be provided on the command line as follows::
176 > helm deploy development local/onap --namespace onap -f overrides.yaml
178 To get a summary of the status of all of the pods (containers) running in your
181 > kubectl get pods --all-namespaces -o=wide
184 The Kubernetes namespace concept allows for multiple instances of a component
185 (such as all of ONAP) to co-exist with other components in the same
186 Kubernetes cluster by isolating them entirely. Namespaces share only the
187 hosts that form the cluster thus providing isolation between production and
188 development systems as an example. The OOM deployment of ONAP in Beijing is
189 now done within a single Kubernetes namespace where in Amsterdam a namespace
190 was created for each of the ONAP components.
193 The Helm `--name` option refers to a release name and not a Kubernetes namespace.
196 To install a specific version of a single ONAP component (`so` in this example)
197 with the given release name enter::
199 > helm deploy so onap/so --version 3.0.1
201 To display details of a specific resource or group of resources type::
203 > kubectl describe pod so-1071802958-6twbl
205 where the pod identifier refers to the auto-generated pod identifier.
207 .. figure:: oomLogoV2-Configure.png
213 Each project within ONAP has its own configuration data generally consisting
214 of: environment variables, configuration files, and database initial values.
215 Many technologies are used across the projects resulting in significant
216 operational complexity and an inability to apply global parameters across the
217 entire ONAP deployment. OOM solves this problem by introducing a common
218 configuration technology, Helm charts, that provide a hierarchical
219 configuration with the ability to override values with higher
220 level charts or command line options.
222 The structure of the configuration of ONAP is shown in the following diagram.
223 Note that key/value pairs of a parent will always take precedence over those
224 of a child. Also note that values set on the command line have the highest
232 oValues [label="values.yaml"]
233 demo [label="onap-demo.yaml"]
234 prod [label="onap-production.yaml"]
235 oReq [label="requirements.yaml"]
236 soValues [label="values.yaml"]
237 soReq [label="requirements.yaml"]
238 mdValues [label="values.yaml"]
241 oResources [label="resources"]
245 oResources -> environments
258 The top level onap/values.yaml file contains the values required to be set
259 before deploying ONAP. Here is the contents of this file:
261 .. include:: ../kubernetes/onap/values.yaml
264 One may wish to create a value file that is specific to a given deployment such
265 that it can be differentiated from other deployments. For example, a
266 onap-development.yaml file may create a minimal environment for development
267 while onap-production.yaml might describe a production deployment that operates
268 independently of the developer version.
270 For example, if the production OpenStack instance was different from a
271 developer's instance, the onap-production.yaml file may contain a different
272 value for the vnfDeployment/openstack/oam_network_cidr key as shown below.
278 apps: consul msb mso message-router sdnc vid robot portal policy appc aai
279 sdc dcaegen2 log cli multicloud clamp vnfsdk aaf kube2msb
280 dataRootDir: /dockerdata-nfs
282 # docker repositories
284 onap: nexus3.onap.org:10001
287 filebeat: docker.elastic.co
292 # vnf deployment environment
295 ubuntu_14_image: "Ubuntu_14.04.5_LTS"
296 public_net_id: "e8f51956-00dd-4425-af36-045716781ffc"
297 oam_network_id: "d4769dfb-c9e4-4f72-b3d6-1d18f4ac4ee6"
298 oam_subnet_id: "191f7580-acf6-4c2b-8ec0-ba7d99b3bc4e"
299 oam_network_cidr: "192.168.30.0/24"
303 To deploy ONAP with this environment file, enter::
305 > helm deploy local/onap -n onap -f environments/onap-production.yaml
307 .. include:: environments_onap_demo.yaml
310 When deploying all of ONAP a requirements.yaml file control which and what
311 version of the ONAP components are included. Here is an excerpt of this
316 # Referencing a named repo called 'local'.
317 # Can add this repo by running commands like:
319 # > helm repo add local http://127.0.0.1:8879
325 condition: so.enabled
328 The ~ operator in the `so` version value indicates that the latest "2.X.X"
329 version of `so` shall be used thus allowing the chart to allow for minor
330 upgrades that don't impact the so API; hence, version 2.0.1 will be installed
333 The onap/resources/environment/onap-dev.yaml (see the excerpt below) enables
334 for fine grained control on what components are included as part of this
335 deployment. By changing this `so` line to `enabled: false` the `so` component
336 will not be deployed. If this change is part of an upgrade the existing `so`
337 component will be shut down. Other `so` parameters and even `so` child values
338 can be modified, for example the `so`'s `liveness` probe could be disabled
339 (which is not recommended as this change would disable auto-healing of `so`).
343 #################################################################
344 # Global configuration overrides.
346 # These overrides will affect all helm charts (ie. applications)
347 # that are listed below and are 'enabled'.
348 #################################################################
352 #################################################################
353 # Enable/disable and configure helm charts (ie. applications)
354 # to customize the ONAP deployment.
355 #################################################################
359 so: # Service Orchestrator
365 # necessary to disable liveness probe when setting breakpoints
366 # in debugger so K8s doesn't restart unresponsive container
371 Accessing the ONAP Portal using OOM and a Kubernetes Cluster
372 ------------------------------------------------------------
374 The ONAP deployment created by OOM operates in a private IP network that isn't
375 publicly accessible (i.e. OpenStack VMs with private internal network) which
376 blocks access to the ONAP Portal. To enable direct access to this Portal from a
377 user's own environment (a laptop etc.) the portal application's port 8989 is
378 exposed through a `Kubernetes LoadBalancer`_ object.
380 Typically, to be able to access the Kubernetes nodes publicly a public address
381 is assigned. In OpenStack this is a floating IP address.
383 When the `portal-app` chart is deployed a Kubernetes service is created that
384 instantiates a load balancer. The LB chooses the private interface of one of
385 the nodes as in the example below (10.0.0.4 is private to the K8s cluster only).
386 Then to be able to access the portal on port 8989 from outside the K8s &
387 OpenStack environment, the user needs to assign/get the floating IP address that
388 corresponds to the private IP as follows::
390 > kubectl -n onap get services|grep "portal-app"
391 portal-app LoadBalancer 10.43.142.201 10.0.0.4 8989:30215/TCP,8006:30213/TCP,8010:30214/TCP 1d app=portal-app,release=dev
394 In this example, use the 10.0.0.4 private address as a key find the
395 corresponding public address which in this example is 10.12.6.155. If you're
396 using OpenStack you'll do the lookup with the horizon GUI or the OpenStack CLI
397 for your tenant (openstack server list). That IP is then used in your
398 `/etc/hosts` to map the fixed DNS aliases required by the ONAP Portal as shown
401 10.12.6.155 portal.api.simpledemo.onap.org
402 10.12.6.155 vid.api.simpledemo.onap.org
403 10.12.6.155 sdc.api.fe.simpledemo.onap.org
404 10.12.6.155 sdc.workflow.plugin.simpledemo.onap.org
405 10.12.6.155 sdc.dcae.plugin.simpledemo.onap.org
406 10.12.6.155 portal-sdk.simpledemo.onap.org
407 10.12.6.155 policy.api.simpledemo.onap.org
408 10.12.6.155 aai.api.sparky.simpledemo.onap.org
409 10.12.6.155 cli.api.simpledemo.onap.org
410 10.12.6.155 msb.api.discovery.simpledemo.onap.org
411 10.12.6.155 msb.api.simpledemo.onap.org
412 10.12.6.155 clamp.api.simpledemo.onap.org
413 10.12.6.155 so.api.simpledemo.onap.org
414 10.12.6.155 sdc.workflow.plugin.simpledemo.onap.org
416 Ensure you've disabled any proxy settings the browser you are using to access
417 the portal and then simply access now the new ssl-encrypted URL:
418 ``https://portal.api.simpledemo.onap.org:30225/ONAPPORTAL/login.htm``
421 Using the HTTPS based Portal URL the Browser needs to be configured to accept
422 unsecure credentials.
423 Additionally when opening an Application inside the Portal, the Browser
424 might block the content, which requires to disable the blocking and reloading
428 Besides the ONAP Portal the Components can deliver additional user interfaces,
429 please check the Component specific documentation.
433 | Alternatives Considered:
435 - Kubernetes port forwarding was considered but discarded as it would require
436 the end user to run a script that opens up port forwarding tunnels to each of
437 the pods that provides a portal application widget.
439 - Reverting to a VNC server similar to what was deployed in the Amsterdam
440 release was also considered but there were many issues with resolution, lack
441 of volume mount, /etc/hosts dynamic update, file upload that were a tall order
442 to solve in time for the Beijing release.
446 - If you are not using floating IPs in your Kubernetes deployment and directly attaching
447 a public IP address (i.e. by using your public provider network) to your K8S Node
448 VMs' network interface, then the output of 'kubectl -n onap get services | grep "portal-app"'
449 will show your public IP instead of the private network's IP. Therefore,
450 you can grab this public IP directly (as compared to trying to find the floating
451 IP first) and map this IP in /etc/hosts.
453 .. figure:: oomLogoV2-Monitor.png
459 All highly available systems include at least one facility to monitor the
460 health of components within the system. Such health monitors are often used as
461 inputs to distributed coordination systems (such as etcd, Zookeeper, or Consul)
462 and monitoring systems (such as Nagios or Zabbix). OOM provides two mechanisms
463 to monitor the real-time health of an ONAP deployment:
465 - a Consul GUI for a human operator or downstream monitoring systems and
466 Kubernetes liveness probes that enable automatic healing of failed
468 - a set of liveness probes which feed into the Kubernetes manager which
469 are described in the Heal section.
471 Within ONAP, Consul is the monitoring system of choice and deployed by OOM in
474 - a three-way, centralized Consul server cluster is deployed as a highly
475 available monitor of all of the ONAP components, and
476 - a number of Consul agents.
478 The Consul server provides a user interface that allows a user to graphically
479 view the current health status of all of the ONAP components for which agents
480 have been created - a sample from the ONAP Integration labs follows:
482 .. figure:: consulHealth.png
485 To see the real-time health of a deployment go to: ``http://<kubernetes IP>:30270/ui/``
486 where a GUI much like the following will be found:
489 .. figure:: oomLogoV2-Heal.png
495 The ONAP deployment is defined by Helm charts as mentioned earlier. These Helm
496 charts are also used to implement automatic recoverability of ONAP components
497 when individual components fail. Once ONAP is deployed, a "liveness" probe
498 starts checking the health of the components after a specified startup time.
500 Should a liveness probe indicate a failed container it will be terminated and a
501 replacement will be started in its place - containers are ephemeral. Should the
502 deployment specification indicate that there are one or more dependencies to
503 this container or component (for example a dependency on a database) the
504 dependency will be satisfied before the replacement container/component is
505 started. This mechanism ensures that, after a failure, all of the ONAP
506 components restart successfully.
508 To test healing, the following command can be used to delete a pod::
510 > kubectl delete pod [pod name] -n [pod namespace]
512 One could then use the following command to monitor the pods and observe the
513 pod being terminated and the service being automatically healed with the
514 creation of a replacement pod::
516 > kubectl get pods --all-namespaces -o=wide
518 .. figure:: oomLogoV2-Scale.png
524 Many of the ONAP components are horizontally scalable which allows them to
525 adapt to expected offered load. During the Beijing release scaling is static,
526 that is during deployment or upgrade a cluster size is defined and this cluster
527 will be maintained even in the presence of faults. The parameter that controls
528 the cluster size of a given component is found in the values.yaml file for that
529 component. Here is an excerpt that shows this parameter:
533 # default number of instances
536 In order to change the size of a cluster, an operator could use a helm upgrade
537 (described in detail in the next section) as follows::
539 > helm upgrade --set replicaCount=3 onap/so/mariadb
541 The ONAP components use Kubernetes provided facilities to build clustered,
542 highly available systems including: Services_ with load-balancers, ReplicaSet_,
543 and StatefulSet_. Some of the open-source projects used by the ONAP components
544 directly support clustered configurations, for example ODL and MariaDB Galera.
546 The Kubernetes Services_ abstraction to provide a consistent access point for
547 each of the ONAP components, independent of the pod or container architecture
548 of that component. For example, SDN-C uses OpenDaylight clustering with a
549 default cluster size of three but uses a Kubernetes service to and change the
550 number of pods in this abstract this cluster from the other ONAP components
551 such that the cluster could change size and this change is isolated from the
552 other ONAP components by the load-balancer implemented in the ODL service
555 A ReplicaSet_ is a construct that is used to describe the desired state of the
556 cluster. For example 'replicas: 3' indicates to Kubernetes that a cluster of 3
557 instances is the desired state. Should one of the members of the cluster fail,
558 a new member will be automatically started to replace it.
560 Some of the ONAP components many need a more deterministic deployment; for
561 example to enable intra-cluster communication. For these applications the
562 component can be deployed as a Kubernetes StatefulSet_ which will maintain a
563 persistent identifier for the pods and thus a stable network id for the pods.
564 For example: the pod names might be web-0, web-1, web-{N-1} for N 'web' pods
565 with corresponding DNS entries such that intra service communication is simple
566 even if the pods are physically distributed across multiple nodes. An example
567 of how these capabilities can be used is described in the Running Consul on
570 .. figure:: oomLogoV2-Upgrade.png
576 Helm has built-in capabilities to enable the upgrade of pods without causing a
577 loss of the service being provided by that pod or pods (if configured as a
578 cluster). As described in the OOM Developer's Guide, ONAP components provide
579 an abstracted 'service' end point with the pods or containers providing this
580 service hidden from other ONAP components by a load balancer. This capability
581 is used during upgrades to allow a pod with a new image to be added to the
582 service before removing the pod with the old image. This 'make before break'
583 capability ensures minimal downtime.
585 Prior to doing an upgrade, determine of the status of the deployed charts::
588 NAME REVISION UPDATED STATUS CHART NAMESPACE
589 so 1 Mon Feb 5 10:05:22 2018 DEPLOYED so-2.0.1 default
591 When upgrading a cluster a parameter controls the minimum size of the cluster
592 during the upgrade while another parameter controls the maximum number of nodes
593 in the cluster. For example, SNDC configured as a 3-way ODL cluster might
594 require that during the upgrade no fewer than 2 pods are available at all times
595 to provide service while no more than 5 pods are ever deployed across the two
596 versions at any one time to avoid depleting the cluster of resources. In this
597 scenario, the SDNC cluster would start with 3 old pods then Kubernetes may add
598 a new pod (3 old, 1 new), delete one old (2 old, 1 new), add two new pods (2
599 old, 3 new) and finally delete the 2 old pods (3 new). During this sequence
600 the constraints of the minimum of two pods and maximum of five would be
601 maintained while providing service the whole time.
603 Initiation of an upgrade is triggered by changes in the Helm charts. For
604 example, if the image specified for one of the pods in the SDNC deployment
605 specification were to change (i.e. point to a new Docker image in the nexus3
606 repository - commonly through the change of a deployment variable), the
607 sequence of events described in the previous paragraph would be initiated.
609 For example, to upgrade a container by changing configuration, specifically an
612 > helm deploy onap onap/so --version 2.0.1 --set enableDebug=true
614 Issuing this command will result in the appropriate container being stopped by
615 Kubernetes and replaced with a new container with the new environment value.
617 To upgrade a component to a new version with a new configuration file enter::
619 > helm deploy onap onap/so --version 2.0.2 -f environments/demo.yaml
621 To fetch release history enter::
624 REVISION UPDATED STATUS CHART DESCRIPTION
625 1 Mon Feb 5 10:05:22 2018 SUPERSEDED so-2.0.1 Install complete
626 2 Mon Feb 5 10:10:55 2018 DEPLOYED so-2.0.2 Upgrade complete
628 Unfortunately, not all upgrades are successful. In recognition of this the
629 lineup of pods within an ONAP deployment is tagged such that an administrator
630 may force the ONAP deployment back to the previously tagged configuration or to
631 a specific configuration, say to jump back two steps if an incompatibility
632 between two ONAP components is discovered after the two individual upgrades
635 This rollback functionality gives the administrator confidence that in the
636 unfortunate circumstance of a failed upgrade the system can be rapidly brought
637 back to a known good state. This process of rolling upgrades while under
638 service is illustrated in this short YouTube video showing a Zero Downtime
639 Upgrade of a web application while under a 10 million transaction per second
642 For example, to roll-back back to previous system revision enter::
647 REVISION UPDATED STATUS CHART DESCRIPTION
648 1 Mon Feb 5 10:05:22 2018 SUPERSEDED so-2.0.1 Install complete
649 2 Mon Feb 5 10:10:55 2018 SUPERSEDED so-2.0.2 Upgrade complete
650 3 Mon Feb 5 10:14:32 2018 DEPLOYED so-2.0.1 Rollback to 1
654 The description field can be overridden to document actions taken or include
657 Many of the ONAP components contain their own databases which are used to
658 record configuration or state information. The schemas of these databases may
659 change from version to version in such a way that data stored within the
660 database needs to be migrated between versions. If such a migration script is
661 available it can be invoked during the upgrade (or rollback) by Container
662 Lifecycle Hooks. Two such hooks are available, PostStart and PreStop, which
663 containers can access by registering a handler against one or both. Note that
664 it is the responsibility of the ONAP component owners to implement the hook
665 handlers - which could be a shell script or a call to a specific container HTTP
666 endpoint - following the guidelines listed on the Kubernetes site. Lifecycle
667 hooks are not restricted to database migration or even upgrades but can be used
668 anywhere specific operations need to be taken during lifecycle operations.
670 OOM uses Helm K8S package manager to deploy ONAP components. Each component is
671 arranged in a packaging format called a chart - a collection of files that
672 describe a set of k8s resources. Helm allows for rolling upgrades of the ONAP
673 component deployed. To upgrade a component Helm release you will need an
674 updated Helm chart. The chart might have modified, deleted or added values,
675 deployment yamls, and more. To get the release name use::
679 To easily upgrade the release use::
681 > helm upgrade [RELEASE] [CHART]
683 To roll back to a previous release version use::
685 > helm rollback [flags] [RELEASE] [REVISION]
687 For example, to upgrade the onap-so helm release to the latest SO container
690 - Edit so values.yaml which is part of the chart
691 - Change "so: nexus3.onap.org:10001/openecomp/so:v1.1.1" to
692 "so: nexus3.onap.org:10001/openecomp/so:v1.1.2"
693 - From the chart location run::
695 > helm upgrade onap-so
697 The previous so pod will be terminated and a new so pod with an updated so
698 container will be created.
700 .. figure:: oomLogoV2-Delete.png
706 Existing deployments can be partially or fully removed once they are no longer
707 needed. To minimize errors it is recommended that before deleting components
708 from a running deployment the operator perform a 'dry-run' to display exactly
709 what will happen with a given command prior to actually deleting anything. For
712 > helm undeploy onap --dry-run
714 will display the outcome of deleting the 'onap' release from the
716 To completely delete a release and remove it from the internal store enter::
720 One can also remove individual components from a deployment by changing the
721 ONAP configuration values. For example, to remove `so` from a running
724 > helm undeploy onap-so
726 will remove `so` as the configuration indicates it's no longer part of the
727 deployment. This might be useful if a one wanted to replace just `so` by
728 installing a custom version.