Tuesday, January 24, 2012

Error pages in Domino

First of all I'd like to briefly describe few another ways we can use to manage error pages in Domino.
Also notice, I will definitely update post few times more after all.

1. Lazy solution $$ReturnGeneralError and MessageString
It's the most fast and easy solutions, it require to create 1 design element (form) $$ReturnGeneralError, style it and add MessageString somewhere to explain what is wrong
- I see only 1 advantage compare to another approaches, its time to implement. Once you create $$ReturnGeneralError it starts to work. So few clicks and few minutes to manage output UI and you have solution.
- works inside of application only. So if you have 10 web applications, you need to manage error page in each of it. However you can setup inheritance and manage all error pages from 1 place etc.
- not very flexible on my opinion as we can operate with form-design elements only.

2. HTTP response headers
Simply create error page as design element or as document (does not matter) and create rule for your website in Domino Directory database. It does not require special knowledge and it is simple to use.

- easy to manage
- path to Error page (means we can use different database to keep error pages)
- we can use both: Design Elements and Documents as error page.
- require minor Administration of Domino skills.
it works in the same way as client side redirection. i.e. user will see “blink” of standard 404 page before loading custom 404 page.

3. "Whole server" solution
Use HTTPMultiErrorPage property in the Notes.ini file, for example HTTPMultiErrorPage=/error.html. Create rule on the Domino server for /error.html to be substituted corresponding page.

4. DSPAI as error handler for Domino
Most complicated approach but also most flexible from my point of view. There is quite low information about how to use it. It requires knowledge of Notes C Api as well. If you want to see how it looks, here is very good exampe (it helped me a lot) on loggin using DSAPI
Company I worked in use DSAPI for handling URL already, and now we are near to implement error handling using DSAPI as well (it already implemented it on development server, so soon I will show you real links etc).
DSAPI allows us to catch responses Domino generated for users and we can replace output in case if responce 400, 404 or any another.

I will post most important part of logic here, just to how overview how DSAPI works and what is gives to us. Notice I've changed code a bit, to show only most important part of it (I will copy comments from Paul's example just to make things faster) 

1. initiation of filter and register for response event
* FilterInit() - Required filter entry point. Called upon
* filter startup, which occurs when HTTP server task is started.
DLLEXPORT unsigned int FilterInit(FilterInitData* filterInitData) {
   filterInitData->eventFlags = kFilterResponse;
so what we did there, say to filter that we want to process response events
2. Link events we registered with functions
* HttpFilterProc() - Required filter entry point. Dispatches the event notifications to the appropriate functions for handling the events.
DLLEXPORT DWORD HttpFilterProc(FilterContext *pContext, DWORD dwEventType, void *pEventData)
 switch (dwEventType) {
  case kFilterResponse:
   return Response(pContext, pEventData);

 return kFilterNotHandled;
Now we can process Responses to users
3. This is how we process Response to users
int Response(FilterContext* context, FilterResponseHeaders* eventData) {
 int responce = eventData->responseCode;

 if (responce==404) {
  if (Send404(context) == TRUE) {
   return kFilterHandledRequest;
 return kFilterNotHandled;
4. Finally code for Send404() how we replace content to users
While you read code you need to know that when we load filter we also initiate a table with KEY-HTML arrays. Yes we keep in memory Error HTML pages with keys we need. Our Keys - domain, we handle error pages on domain level, means for domain1.com - is 1 error page, for domain2.com is another error page. But you can do it in another way its up to you.
int Send404(FilterContext* context) {
 FilterResponseHeaders         response;
 unsigned int   errID = 0;
 char     szBuffer[DEFAULT_BUFFER_SIZE]={0};
 char     pszErrorPage[ERRRO_PAGE_SIZE]={0};
 FilterParsedRequestLine pRequestLine;
 unsigned int   pErrID;
 int      i;

 // As we are returning the entire page to the browser, we can 
 // set the response code to 404 (so the domino web log will show the error)
 response.reasonText="Bad Request";
 // get domain name (its our key), could be done faster? right now its simple walk via 10-20 documents which is fine for now
 context->ServerSupport(context, kGetParsedRequest, &pRequestLine, NULL, NULL, &pErrID);
 for(i=0; i<errorPagesCount;i++) {
  if (strcmp(errorKey[i], pRequestLine.pHostName)==0) {
   strcpy(pszErrorPage, errorHTML[i]);

 if(strlen(pszErrorPage)<10) {
  sprintf(szBuffer, "Error page on %s is very small, something wrong", pRequestLine.pHostName);
  writeToLog(CRITICAL_MSG, szBuffer);
  return FALSE;

 sprintf(szBuffer, "Content-Type: text/html; charset=UTF-8\n\nContent-length: %i\n\n", strlen(pszErrorPage));
 response.headerText = szBuffer;

 if (context->ServerSupport(context, kWriteResponseHeaders, &response, 0, 0, &errID) != TRUE) {
  sprintf(szBuffer, "Error sending redirect, code: %d", errID);
  writeToLog(CRITICAL_MSG, szBuffer);
  return FALSE;
    if (context->WriteClient(context, pszErrorPage, (unsigned int) strlen(pszErrorPage), 0, &errID) != TRUE) {
  sprintf(szBuffer, "Error sending redirect, code: %d", errID);
  writeToLog(CRITICAL_MSG, szBuffer);
  return FALSE;
 return TRUE;
OK guys, I've shared most complicated part of this DSAPI for error handling, rest you have complete yourself (homework :)), but feel free to ask my help here if you need.
- most flexible approach (at least from those which I know)
- our logic control everything we want and we clearly see what is going on
- most complicated ways from all I described
- require knowledge of Notes C API
- I did not try yet this solution with Linux servers (but I know it is possible to do)
- it may crash your Domino server in case if you did you filter wrong (memory leak etc)

5. xPage error handling is on way (on pause actually) and would be nice if somebody help me with that. I've read few articles in past from Per Henrik: XPages custom 404 and error page and Controlling the HTTP response status code in XPages, I think they can be very useful for those who doing in xPages. We will try this approach as we have ongoing huge projects based on xPage. 

- ask to update/more details to article
- notify me about issues
- let me know if you know better way to handle errors

strongly recommended as it can help to many developers in future.

1 comment:

nick wall said...

It looks like the DSAPI filter would give you the most control, but you can also use domcfg.nsf and the 'Error & Response' Form Mapping, then set it to be used across all or a subset of your websites.