Re: x-webobjects-request-id lacking uniqueness?
Re: x-webobjects-request-id lacking uniqueness?
- Subject: Re: x-webobjects-request-id lacking uniqueness?
- From: "email@hidden" <email@hidden>
- Date: Mon, 6 Dec 2010 13:52:04 +0000
Hi Patrick,
Couldn't you just use WOLongResponse so that it keeps the first connection alive until it responds?
Regards,
Rob.
On 6 Dec 2010, at 13:09, Patrick Middleton wrote:
> I have an app doing something superficially like web services which is sessionless and accessed via DirectActions. Because of other activity in the database sometimes, database i/o can block for several minutes. When this happens the Apache WebObjects adaptor's loadbalancing features come into play, and redirect the request it sent to an apparently unresponsive instance to a different instance, which may in turn block on database i/o. The caller's browser waits several minutes and then receives an "no instance available" page. When the other activity in the database clears an, if the request caused a database update, that update can be applied multiple times because of each instance attempting to process the request. If the first update changes the database in a way that prevents the other updates from succeeding, they might report "update failed", and if the database i/o hold up was only a few minutes, that page can be returned to the caller -- a page saying that the update failed, when in fact the update succeeded but the caller won't get to see that page.
>
> What I thought about doing, and have largely done, is announce and listen for activity via multicast datagrams. If an instance receives a request via a direct action and I don't want it to be redirected via the load balancer, enough information is broadcast such that other instances waiting for requests will be able to tell that another instance should already be processing it, and then generate a response (without blocking on database i/o) to the effect that database congestion may be in progress, that their request is being processed, any database updates may or may not be committed but will be committed at most once.
>
> But what constitutes "enough information"? The WebObjects adaptor adds a header "x-webobjects-request-id" (aka REQUEST_ID_HEADER, in config.h) to a received request before writing it to an instance (this header is not "leaked back" to the client) and this appears to be unique: 24 chars long corresponding to three hexstrings of 32-bit numbers, being the time at which some initialization code was called in the process (httpd), the process identifier (of httpd), and a unique sequence counter defended by a lock. The point at which this header is added means that a redirected request will have the same x-webobjects-request-id header.
>
> And so to the subject line of this message: "x-webobjects-request-id lacking uniqueness?" Of those 24 chars, the first 16 are effectively fixed whenever httpd starts, and I appear to be seeing values being reused for the last 8. I'd guess that either the shared memory or the locking isn't working as expected.
>
> Anybody else seeing this?
>
> ---
> Regards Patrick
> OneStep Solutions (Research) LLP
> www.onestep.co.uk
>
>
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