Using OSGi for script deployment in Apache Sling

Using OSGi for script deployment in Apache Sling
Radu Cotescu, Karl Pauls - Adobe

3
‣ Computer Scientist @ Adobe, Basel,
Switzerland
‣ Member of the Apache Software
Foundation
‣ Apache Sling PMC member
‣ Maintainer of HTL for Apache Sling

4
‣ Computer Scientist @ Adobe, Basel,
Switzerland
‣ Member of the Apache Software
Foundation
‣ Apache Sling and Apache Felix PMC (VP)
member
‣ Co-Author of OSGi in Action

Do you have a minute to talk about Apache Sling[0]?
6
‣ REST-centric web framework,
based on an extensible
content tree
‣ JCR for persistence (Apache
Jackrabbit Oak)
‣ Collection of OSGi modules,
deployed in Apache Felix
‣ Powers Adobe Experience
Manager

From URLs to Scripts - a simplified view
7

Scripts and Servlets are equal
8
@Component(service = Servlet.class,
name="org.apache.sling.servlets.get.DefaultGetServlet",
property = {
"service.description=Default GET Servlet",
"service.vendor=The Apache Software Foundation",
// Use this as a default servlet for Sling
"sling.servlet.resourceTypes=sling/servlet/default",
"sling.servlet.prefix:Integer=-1",
// Generic handler for all get requests
"sling.servlet.methods=GET",
"sling.servlet.methods=HEAD"
})
@Designate(ocd=DefaultGetServlet.Config.class)
public class DefaultGetServlet extends SlingSafeMethodsServlet {
}

Versioning and dependencies
9
‣ There is no standard way of defining either.
‣ An option would be to use resource type versioning through
path conventions.
‣ Dependencies can only be checked at runtime (but not
enforced).
‣ What happens if your evil colleagues delete a script you were
delegating to? Or worse, if they change the whole markup?

Performance
10
‣ Each script requires two trips to the
persistence layer when first compiled,
only to read the script.
‣ Sling needs to maintain some caches to
keep things snappy.

Performance
11
“There are only two hard things in Computer Science: cache
invalidation and naming things.” -- Phil Karlton

Reality check
12
1. What are scripts actually: content or
code?
2. Are scripts authored or developed?
3. Can scripts be used freely or do they
have constraints?
4. If scripts are code, then why do we treat
them diﬀerently?

Reality check
Code:
1. provides or implements an API (HTTP in our case)
2. evolves semantically
3. is bundled into a cohesive unit, managed by one or more
developers
13

But what if we…
14
1. pack scripts into OSGi bundles
2. define the resource types as versioned capabilities, with
versioned requirements (Java APIs, other resource types to
which scripts delegate or which scripts extend)
3. allow the platform to do what it’s made to: wire things

Let’s quickly consult the OSGi specification
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Capability - Describing a feature or function of the Resource when
installed in the Environment. A capability has attributes and directives.
Requirement - An assertion on the availability of a capability in the
Environment. A requirement has attributes and directives.
The filter directive contains the filter to assert the attributes of the
capability in the same Namespace.
https://osgi.org/specification/osgi.core/7.0.0/framework.module.html#framework.module.dependencies

How? Use the Apache Sling Scripting Bundle Tracker[1]
16
What:
1. add-on module to which bundles that provide scripts have to
be wired explicitly
2. reuses the already established mechanisms for registering
servlets in Apache Sling
3. allows building light-weight instances that can be thrown into
production with very little warm-up, when using precompiled
scripts

How? Use the Apache Sling Scripting Bundle Tracker[1]
17
4. provides the mechanism for deploying truly versionable
scripts, with explicit dependencies, by relying on the OSGi
framework
5. removes the need of a separate ScriptCache
6. removes additional pressure on the persistence layer
7. simplifies instance and application upgrades
8. there’s also a Maven plugin for generating requirements and
capabilities

So what’s diﬀerent?
18
Option 1: scripts packed as bundle entries

So what’s diﬀerent?
19
Option 2: precompiled scripts

How does it work in practice?
20

Sure, but how?
21
1 Provide-Capability / Script -> 1 Servlet / Script
Provide-Capability
sling.resourceType=“latte-art-maker”;
sling.servlet.methods:List<String>=“GET”;
version:Version=“1.0.0”

Demo* * or how we can embarrass ourselves if things don’t work
22

Where does all this lead?
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OSGi RFP 196[2]
‣ Provides a way to use an OSGi framework with custom
classloaders (a.k.a. OSGi Connect/PojoSR)
Graal/Substrate VM
‣ Ahead-of-Time (AOT) Java code compilation
Together with the precompiled bundled scripts it should be
possible to perform an AOT compilation of a Sling application as a
native image[3].

Resources
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[0] - https://sling.apache.org
[1] - https://github.com/apache/sling-org-apache-sling-scripting-bundle-tracker
[2] - https://github.com/osgi/design/blob/master/rfps/rfp-0196-OSGiConnect.pdf
[3] - https://adapt.to/2019/en/schedule/from-0-to-hero-in-under-10-seconds.html
Assets licensed from https://stock.adobe.com
Our diagrams were designed with https://whimsical.co/flowcharts
Demo available at https://github.com/raducotescu/eclipsecon-demo

Using OSGi for script deployment in Apache Sling

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