OLFS Extensions

1
OLFS Extensions

• Writing a Custom DispatchHandler

2
Extending the OLFS

• Extension modules written in Java
• Jar files added to a directory within Tomcat and
  called out in the olfs.xml file.
• The new modules have complete access to the
  request information
• Both HTTP GET and POST requests
• There is some significant processing done before
  the handler is called
• Conditional GET Requests (Handler is called on to
  provide last modified dates)
• Authorization Authentication

3
OLFS Dispatch Handlers

• The OLFS uses an ordered list of Dispatch
  Handlers to handle requests.
• Each handler on the list is asked if it can
  handle the incoming request.
• The first handler on the list that claims the
  request gets to handle it.
• List order is important, as some handlers (for
  example THREDDS) may claim to handle requests
  that could (should) be handled by a different
  handler.
• Handlers are free to do anything they need to
  handle a request Contact a remote
  system/process, read files, spawn threads, et
  cetera.

4
DispatchHandler Interface
Dispatch Handlers must implement the
DispatchHandler interface
public interface DispatchHandler public
void init(DispatchServlet servlet,
Element config) throws
Exception public boolean requestCanBeHandled
(HttpServletRequest request) throws
Exception public void handleRequest(HttpServ
letRequest request,
HttpServletResponse response) throws
Exception public long getLastModified(HttpSe
rvletRequest req) public void destroy()
5
Dispatch Handler Interfaceinit() method
Declaration public void init(DispatchServlet
servlet, Element config) throws
Exception
Parameters servlet This will be the
DispatchServlet that creates the instance of
DispatchHandler that is being initialized.
DispatchServlet is a child of HttpServlet.
config A JDOM Element object containing the XML
Element that announced which implementation of
DispatchHandler to use. It may (or may not)
contain additional confguration
information. Exceptions Throws an
Exception when the bad things happen.
6
Dispatch Handler InterfacerequestCanBeHandled()
method
Declaration public boolean requestCanBeHandled(H
ttpServletRequest request) throws
Exception
Parameters request The request to be
handled. Return True if the DispatchHandler
can service the request, false otherwise. Excepti
ons Throws an Exception when the bad things
happen.
7
Dispatch Handler InterfacehandleRequest() method
public void handleRequest(HttpServletRequest
request,
HttpServletResponse response)
throws Exception
Parameters request The request to be
handled. response The response object thru
which the response information will
placed/written. Exceptions Throws an
Exception when the bad things happen.
8
Dispatch Handler InterfacegetLastModified()
method
Declaration public long getLastModified(HttpSer
vletRequest request)
Parameters request The request for which
we need to get a last modified date. Return
The last modified date of the URI referenced in
the request. If the date cannot be determined
then return -1. Exceptions May not throw
an Exception.
9
Dispatch Handler Interfacedestroy() method
Declaration public void destroy()
Parameters None Return None. Exceptions
May not throw an Exception.
10
DispatchHandlerA Simple Example
11
requestCanBeHandled() method

• public boolean requestCanBeHandled(HttpServletReq
  uest request) throws Exception
• String sName ReqInfo.getFullSourceName(reque
  st)
• return sName.startsWith("/helloWorld")
• This simple test will cause our example
  DispatchHandler to claim all requests that begin
  with /helloWorld

12
handleRequest() method

• public void handleRequest(HttpServletRequest
  request,
• HttpServletResponse
  response)
• throws Exception
• response.setContentType("text/html")
• response.setStatus(HttpServletResponse.SC_
  OK)
• String msg "lthtmlgtltbodygtltbrgtltbrgtltbrgtltbrgt
  "
• msg "ltcentergtlth1gtHELLO
  WORLD!lt/h1gtlt/centergt"
• msg "lt/bodygtlt/htmlgt"
• response.getWriter().print(msg)
• Provides a simple Hello World response.

13
getLastModified() method

• public long getLastModified(HttpServletRequest
  request)
• return -1
• Returning a negative one will cause the page to
  always reload.

14
Installing your Handler

• Edit the olfs.xml file in CATALINA_HOME/content/o
  pendap
• Add your new DispatchHandler implementation to
  the list of handlers by inserting XML element
• ltHandler className"bom.tutorial.ExampleDispatc
  hHandler" /gtImmediately after the BESManager
  handler.
• Restart Tomcat.

15
Testing your Handler

• Point your browser at http//localhost8080/
  opendap/helloWorld

16
Experiment

• Edit the olfs.xml file in CATALINA_HOME/content/o
  pendap
• Move the element that defines your handler to the
  end of the handler list and restart Tomcat.
• Try other permutations of the Handler list (be
  sure to restart Tomcat after each change). This
  will probably break Hyrax )

17
DispatchHandlerA File Service
18
FileServiceinit() method

• public void init(DispatchServlet s, Element
  config) throws Exception
• if(initialized) return
• String dir
• Element dv config.getChild("TopDir")
• if(dv!null)
• dir dv.getTextTrim()
• if(dir!null)
• File f new File(dir)
• if (f.exists() f.isDirectory())
• fileRoot dir
• else
• fileRoot s.getServletContext().g
  etRealPath("docs")
• while(fileRoot.endsWith("/"))
• fileRoot fileRoot.substring(0,fileRoot.l
  ength()-1)

19
FileServiceinit() method

• public void init(DispatchServlet s, Element
  config) throws Exception
• if(initialized) return
• String dir
• Element dv config.getChild("TopDir")
• if(dv!null)
• dir dv.getTextTrim()
• if(dir!null)
• File f new File(dir)
• if (f.exists() f.isDirectory())
• fileRoot dir
• else
• fileRoot s.getServletContext().g
  etRealPath("docs")
• while(fileRoot.endsWith("/"))
• fileRoot fileRoot.substring(0,fileRoot.l
  ength()-1)

20
FileService requestCanBeHandled() and
handleRequest() methods

• public boolean requestCanBeHandled(HttpServletRequ
  est request)
• throws Exception
• return fileService(request,null,false)
• public void handleRequest(HttpServletRequest
  request,
• HttpServletResponse
  response)
• throws Exception
• fileService(request,response,true)
• Both of these methods call on the fileService()
  method to perform both the handling evaluation
  and the response evaluation.

21
FileService fileService() method

• private boolean fileService(HttpServletRequest
  request,
• HttpServletResponse
  response,
• boolean
  sendResponse)
• throws Exception
• String path getPath(request)
• boolean isFileRequest false
• if(path!null)
• isFileRequest true
• if(sendResponse)
• File file new File(path)
• if(file.exists())
• if(file.isDirectory())
• sendDirPage(file,request,
  response)
• else if(file.isFile())
• sendFile(file,response)
• else
• sendFileNotFound(response)
  
• else

22
FileService getPath() method

• private String getPath(HttpServletRequest req)
• String sName ReqInfo.getFullSourceName(req)
  
• String path null
• if(sName.startsWith(urlPrefix))
• path sName.substring(urlPrefix.length(),
  sName.length())
• if( ! path.startsWith("/"))
• path "/" path
• path fileRoot path
• return path

23
FileService getLastModifed() method

• public long getLastModified(HttpServletRequest
  req)
• String path getPath(req)
• if(path!null)
• File file new File(path)
• if(file.exists())
• return file.lastModified()
• return -1
• Utilizes the getPath() method. Ignores a bad
  return (that will be handled later with a 404
  page).
• Interacts directly with the file system to
  ascertain the last modified date of the
  file/directory.

24
Installing your Handler

• Edit the olfs.xml file in CATALINA_HOME/content/o
  pendap
• Add your new DispatchHandler implementation to
  the list of handlers by inserting XML element
• ltHandler className"bom.tutorial.FileServicegt
• ltTopDirgt/path/of/dir/to/accesslt/TopDirgt
• lt/HandlergtImmediately after the BESManager
  handler.
• Restart Tomcat.

25
FileDispatchHandlerA Hyrax DispatchHandler.

• Hyrax will serve files that it does not
  recognize as data.
• It is also possible to configure Hyrax to allow
  users to directly download the source data files.

26
FileDispatchHandlerinit() method

• public void init(DispatchServlet servlet,Element
  config)
• throwsException
• if(initialized) return
• Element dv config.getChild("AllowDirectDataS
  ourceAccess")
• if(dv!null)
• allowDirectDataSourceAccess true
• log.info("Intialized. Direct Data Source
  Access
• (allowDirectDataSourceAccess?"Enabled""Di
  sabled") )
• initialized true

27
FileDispatchHandlerrequestCanBeHandled() and
handleRequest() methods

• public boolean requestCanBeHandled(HttpServletRequ
  est request)
• throws Exception
• return fileDispatch(request, null, false)
• public void handleRequest(HttpServletRequest
  request,
• HttpServletResponse
  response)
• throws Exception
• if(!fileDispatch(request, response, true))
• log.debug("FileDispatch request failed
  inexplicably!")
• public boolean fileDispatch(HttpServletRequest
  request,
• 
  HttpServletResponse response,
• boolean
  sendResponse) throws Exception

28
FileDispatchHandlerfileDispatch() method

• public boolean fileDispatch(HttpServletRequest
  request,
• HttpServletResponse
  response,
• boolean sendResponse)
  throws Exception
• DataSourceInfo dsi new BESDataSource(ReqInfo
  .getFullSourceName(request))
• boolean isFileResponse false
• if (dsi.sourceExists())
• if (!dsi.isCollection())
• isFileResponse true
• if (sendResponse)
• if (!dsi.isDataset()
  allowDirectDataSourceAccess)
• sendFile(request, response)
• else
• sendDirectAccessDenied(request
  , response)

29
Summary

• DispatchHandler implementations are free to
  conduct business using any available tools
• Sockets
• JDBC
• File Systems
• Etc.
• Care should be taken in ordering them in the
  olfs.xml configuration file. Improper ordering
  can create unintended results, including
  disabling of desired functionality.