在有状态的水平缩放节点上安装插件的效果

Question

I am interested on the effects of installing an add-on on a stateful horizontally scaled node. When a node is scaled and has 3 nodes (1 master and 2 regular), does the installed add-on only have effect on the Master node or is it installed on the regular nodes too? If not, can we explicitly install an add-on on a regular node? Could not find information about this situation on the documents.

Answer 1

This is somewhat defined by the add-on itself depending on what it has defined in its onBeforeScaleOut and onAfterScaleOut event handlers.

Ideally, every add-on should be written with those handlers in place (or appropriate behaviour without the necessity of those handlers), but in practice that's not guaranteed... depends entirely on the author.

If we assume that the add-on in question did not define what should happen, stateful scaling means that the entire filesystem of the master node will be copied - so any configuration files or system packages that were deployed on the master node by the add-on will also appear on the newly horizontally scaled node.

The documentation mentions a warning for the inverse scenario (applicable for stateless scaling):

• add-ons - any add-ons installed on the layer won't be available