Information useful to developers

1. If you unfamiliar with servlet containers; here’s a small overview that a few collaborators found useful

   • Reading up on a few basics will help; there are several good sources of information on the net; but don’t get bogged down by the details.

   • Please do use Eclipse/Netbeans/Intelij to navigate the code. This makes life so much easier.

   • To get a quick sense of what a class/interface does, you can use the javadoc. Some attempts have been made to have some Javadoc in most classes and all interfaces

   • We use Tomcat purely as a servlet container; that is, a quick way of servicing HTTP requests using Java code.

   • A WAR file is basically a ZIP file (you can use unzip) with some conventions. For example, all the libraries (.jar’s) that the application depends on will be located in the WEB-INF/lib folder.

   • The starting point for servlet’s in a WAR file is the file WEB-INF/web.xml. For example, in the mgmt.war’s WEB-INF/web.xml, you can see that all HTTP requests matching the pattern /bpl/* are satisfied using the Java class org.epics.archiverappliance.mgmt.BPLServlet.

   • If you navigate to this class in Eclipse, you’ll see that it is basically a registry of BPLActions.

   • For example, the HTTP request, /mgmt/bpl/getAllPVs is satisfied using the GetAllPVs class. Breaking this down,

     1. /mgmt gets you into the mgmt.war file.

     2. /bpl gets you into the BPLServlet class.

     3. /getAllPVs gets you into the GetAllPVs class.

   • From a very high level

     • The engine.war establishes Channel Access monitors and then writes the data into the short term store (STS).

     • The etl.war file moves data between stores - that is from the STS to the MTS and from the MTS to the LTS and so on.

     • The retrieval.war gathers data from all the stores, stitches them together to satisfy data retrieval requests.

     • The mgmt.war manages all the other three and holds configuration state.

   • In terms of configuration, the most important is the PVTypeInfo; you can see what one looks like by looking at http://machine:17665/mgmt/bpl/getPVTypeInfo?pv=MYPV:111:BDES

   • The main interfaces are the ones in the org.epics.archiverappliance package.

   • The ConfigService class does all configuration management.

   • The customization guide is also a good guide to way in which this product can be customized.

ConfigService

All of the configuration in the archiver appliance is handled thru implementations of the ConfigService interface. Each webapp has one instance of this interface and this instance is dependency injected into the classes that need it. If all else fails, you can create your implementation of the ConfigService and register it in the servlet context listener.