Your First Service

This page takes about five minutes and needs nothing but the agent installed on one machine. No shared storage, no second node, no cluster. At the end you will have created a service, started it, proven it is serving, and stopped it.

We will wrap a plain HTTP server, because it is the shortest thing whose running you can prove rather than take on trust.

Create it

An object is created from the command line, resource by resource. Here is a single app resource, told how to start:

om hello create \
  --kw app#1.type=simple \
  --kw 'app#1.start=/usr/bin/python3 -m http.server 8099'

Nothing was started, and nothing outside /etc/opensvc was touched. The command only wrote a configuration, which you can read back:

om hello config show

[DEFAULT]
id = 0e745444-6724-48f9-8167-e865b4f9ec94

[app#1]
type = simple
start = /usr/bin/python3 -m http.server 8099

[app#1] is a resource. app is its driver group, 1 its index, and type = simple picks the driver that supervises a long-running process.

Start it

om hello instance start

INF hello: >>> do start [om hello instance start] (origin user, sid 7b9772c8)
INF hello: app#1: run: om exec --pg /opensvc.slice/... -- /usr/bin/python3 -m http.server 8099
INF hello: <<< done start in 98.079925ms, instance status is now up

The agent tells you what it ran and what the instance status became.

Prove it

curl -o /dev/null -w "%{http_code}\n" http://localhost:8099/

Look at it

om hello instance status

hello
└ instances
  └ node1                    up  no-monitor
    └ resources
      └ app#1      ...../..  up  simple

The instance is up because its only resource is up. Had the resource failed, both lines would say so. This is the command you will reach for most often.

Stop it

om hello instance stop

INF hello: app#1: send termination signal to process 2615892
INF hello: app#1: process 2615892 is now terminated
INF hello: <<< done stop in 86.862589ms, instance status is now down

The port is closed and the status agrees:

curl --max-time 3 http://localhost:8099/
om hello instance status

curl: (7) Failed to connect to localhost port 8099: Connection refused

hello
└ instances
  └ node1                    down  no-monitor
    └ resources
      └ app#1      ...../..  down  simple

Clean up

om hello delete

OBJECT  ORCHESTRATION_ID                      STATUS
hello   068b4fb0-ba3f-4d62-81e7-92ed88039239  accepted

The configuration is gone, and with it the object.

Two ways to start

You used om hello instance start, which starts the instance on this node, now, and prints each step as it happens.

Its cluster counterpart is om hello start, which does not act directly. It asks the cluster to reach the started state, and answers with an orchestration id:

OBJECT  ORCHESTRATION_ID                      STATUS
hello   45c9de82-ed12-48df-bdb2-21a251fb039d  accepted

The daemon then picks a node and starts the instance there, which is what you want once the object runs on more than one node. Watch it happen with om mon. Note that the matching om hello stop also freezes the instance, so the daemon does not immediately start it again.

What you just did

You described a process in a configuration file, and the agent started it, tracked its state, and stopped it. Everything else the agent does is this same loop with more interesting resources: an ip that moves between nodes, a filesystem mounted from a shared disk, a container, a database volume.

Where to go next:

Naming to place objects in namespaces before you have many of them.
Create, Deploy for the other ways to create an object, including from a manifest.
Cluster Configuration to add a second node, which is where failover starts to matter.

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