Startlevels

One of the primary authors of bnd has always objected to startlevels. His motivation was twofold:

• A dynamic OSGi system should be ablet to startup in any order. Startlevels are often abused to hide bundles that do not handle their dynamic dependencies correctly. Hiding these bugs by controlling the start ordering is dangerous since it removes the symptom but the cause can still bite at a later inconvenient time.
• Start levels are global data about a set of bundles. OSGi is quite elegant that it stores virtually all meta data inside the bundle, not requiring global information. However, for start levels you need data outside the bundles. This often becomes wrong over time.

With this disclaimer out of the way, there are actually cases where startlevels are quite important to improve the non-functional aspects.

• Jojo – Declarative services provides an elegant model to build components. However, they usually depend on configuration. If the component is started before configuration admin is started then it will start only to be immediately brought down when it receives its configuration. Using the OSGi Configurator, the component can be brought down a second time. This jojo effect is highly undesirable at startup. (Although the good part is that it exposes a lot of bugs.)
• Logging – A number of bundles, where the most prevalent is the logger, are more useful when they are running right from the beginning.

Startlevel Support in bnd

Run configurations in bnd are described in bndrun files. The list of bundles to run are listed in the -runbundles instruction. The runbundles instruction has a startlevel attribute that specifies the startlevel of each bundle.

Since the -runbundles instruction is frequently calculated by the resolver support in bnd it is not possible to manually assign the startlevel attributes to specific bundles. For this reason there is a decorator support in bnd for some selected instructions. A decorator is a header with the same name but ending with a plus sign (+). This instruction acts like a selector and can be used to assign startlevel attributes on the -runbundles.

Launching

The launcher is responsible for installing the bundles after starting or communicating with a framework. The default bnd launcher will assign each bundle a startlevel, where bundles that do not have a specified startlevel are assigned one level higher than the maximum specified levels.

The default launcher will then move the framework startlevel to 2 higher than the highest specified start level.

Resolving

The resolve support in bnd can automatically assign startlevel attributes to the -runbundles based on different ordering strategies. There are two strategies that use the dependency graph. This graph can be sorted in topological order. This means that a resource is always listed ahead of any of its dependencies when there are no cycles.

The -runstartlevel instruction controls the ordering and assigned start levels.

Strategies

At the time of this writing it is not yet clear what the best strategy is, and it may depend.

Ordering by the topological sort (a resource is followed by its dependencies) will start something like the log service last and any applications bundles first. Although at first sight this feels wrong (the log service should be started first to capture the events at startup) it does solve the jojo problem because one of the last bundles started will be the Service Component Runtime that will activate all components. At that time all initialization should have taken place.

Reversing the topplogical sort will start the something like SCR and log first because they have few or no dependencies. Application bundles are then started latest.

Numbering

Traditionally start levels are managed by an application bundle. Shell commands can increase and decrease start levels. For this reason, start levels often use nice numbers.

Using the resolver the need for nice numbers is diminished since bnd is now taking care. The basic default model is to assign bundles sequentially in selected order from an initial level stepping with 10. The -runstartlevel instruction can provide the initial level and the step if desired.

The launcher will by default move the framework then to a start level that includes any used start levels. This default behavior can be blocked by specifyig the org.osgi.framework.startlevel.beginning property. If this property is set bnd will assume that there is an agent that will handle the runtime start levels.