Checking for a value within a stream in the condition of a routing rule

Yes — you’re correct to think about that edge case.

If you read the stream in fixed-size chunks (e.g., Read(32000)), a simple tChunk[pNeedle search can miss matches where the target string spans the boundary between two reads. For example, "HITL" could appear as "HI" at the end of one chunk and "TL" at the beginning of the next.

A common solution is to keep a small overlap buffer from the end of the previous chunk and prepend it to the next chunk before searching. The overlap only needs to be ($Length(pNeedle)-1) characters.

The general pattern looks like this:

1. Read the next chunk from the stream
2. Prepend the trailing characters from the previous iteration
3. Search the combined buffer
4. Preserve the last (needle length - 1) characters as the next overlap
5. Continue until a match is found or the stream ends
6. Rewind the stream before returning so downstream components can still read it

Here’s an example implementation

Class MyApp.Rule.Functions Extends Ens.Rule.FunctionSet
{
ClassMethod StreamContains(
   pMsg As EnsLib.HTTP.GenericMessage,
   pNeedle As %String = "HITL"
   ) As %Boolean
{
   Set tSC = $$$OK
   Set tFound = 0
   Set tTail = ""
   Set tChunkLen = 32000
   Set tNeedleLen = $Length(pNeedle)
   Set tStream = pMsg.Stream
   If '$IsObject(tStream) Quit 0
   If pNeedle="" Quit 0
   Do tStream.Rewind()
   While ('tStream.AtEnd) && ('tFound) {
       Set tChunk = tStream.Read(tChunkLen,.tSC)
       Quit:$$$ISERR(tSC)
       Set tData = tTail _ tChunk
       If tData[pNeedle {
           Set tFound = 1
           Quit
       }
       If tNeedleLen>1 {
           Set tTail = $Extract(tData,*-(tNeedleLen-2),*)
       } Else {
           Set tTail = ""
       }
   }
   Do tStream.Rewind()
   Quit tFound
}
}

Then your routing rule condition can simply call the function:

MyApp.Rule.Functions.StreamContains(Document,"HITL")

This keeps the routing rule readable while safely inspecting the stream without consuming it.