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Why session locking is necessary for Seaside?

GLASS mailing list
Hi,

I was reading this old post from Dale [1] which explains the topic very nice. After having watched the #seasideProcessRequestWithRetry:resultBlock:  and WASession >> handle:   and the post, I have some questions. 

On one paragraph you said:

"Object locking as a technique for avoiding commit conflicts shares the ‘retry on failure’ model of WATally, so it isn’t necessarily superior to ‘retry on commit failure’. It is a superior technique, if you need to protect logical updates where a physical conflict cannot be guaranteed (i.e., updates to different portions of an object graph).
"

And that's EXACTLY what I was wondering... why the commit failure (conflicts) / abort / retry would NOT be enough for sessions and avoid the lock?  Is that because of that sentence " if you need to protect logical updates where a physical conflict cannot be guaranteed"  
So that's the case for Seaside sessions? It's not safe to update (without conflict) different parts of the session subgraph ?
This is sad, because many many many requests would be read-only to what the session matters (unless there is something obvious I am not seeing?) and in that case, the requests WOULD be able to be parallelized on different Gems.  


I know Johan recommends session affinity [2] and I understand why. It's just that I would prefer to be able to have multiple requests to the same session in parallel in different gems. At least those that wouldn't fail due to a commit conflict on the session. 


Do you have more advice on this topic besides the one I already got on the mailing lists?

Thank you very much. 



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Re: Why session locking is necessary for Seaside?

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On 03/21/2017 12:25 PM, Mariano Martinez Peck via Glass wrote:
Hi,

I was reading this old post from Dale [1] which explains the topic very nice. After having watched the #seasideProcessRequestWithRetry:resultBlock:  and WASession >> handle:   and the post, I have some questions. 

On one paragraph you said:

"Object locking as a technique for avoiding commit conflicts shares the ‘retry on failure’ model of WATally, so it isn’t necessarily superior to ‘retry on commit failure’. It is a superior technique, if you need to protect logical updates where a physical conflict cannot be guaranteed (i.e., updates to different portions of an object graph).
"

And that's EXACTLY what I was wondering... why the commit failure (conflicts) / abort / retry would NOT be enough for sessions and avoid the lock?  Is that because of that sentence " if you need to protect logical updates where a physical conflict cannot be guaranteed"  
So that's the case for Seaside sessions? It's not safe to update (without conflict) different parts of the session subgraph ?
The primary reason for the Seaside session object lock is that Seaside itself had (and presumably still has) a mutex on the session object that only allows Pharo-based Seaside to handle one request at a time in a single vm.  The session object lock emulates the in-vm mutex semantics for multi-gem GemStone and guarantees logical consistency for the session state. Unfortunately it is not enough to rely conflicts to guarantee logical consistency.

This is sad, because many many many requests would be read-only to what the session matters (unless there is something obvious I am not seeing?) and in that case, the requests WOULD be able to be parallelized on different Gems.  

On the other hand, if you know that you have read-only requests, you could arrange for a pool of seaside gems to be serving read-only requests by putting the gems behind a different ip address, disabling the session lock altogether and unconditionally aborting after finishing request processing (this is roughly the equivalent of what Johan is doing with his pool of REST gems -- I think). I suppose you could embed something in the http request itself to tell the server to process a read only request as well... that way your entire pool of seaside gems could alternate between server session-based requests and session-less requests ...

I know Johan recommends session affinity [2] and I understand why. It's just that I would prefer to be able to have multiple requests to the same session in parallel in different gems. At least those that wouldn't fail due to a commit conflict on the session. 

This does bring up a question that I haven't thought about before:

  How does an Ajax request "bypass" the session mutex in a Pharo-based Seaside vm?

Dale

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Re: Why session locking is necessary for Seaside?

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On Tue, Mar 21, 2017 at 5:57 PM, Dale Henrichs via Glass <[hidden email]> wrote:



On 03/21/2017 12:25 PM, Mariano Martinez Peck via Glass wrote:
Hi,

I was reading this old post from Dale [1] which explains the topic very nice. After having watched the #seasideProcessRequestWithRetry:resultBlock:  and WASession >> handle:   and the post, I have some questions. 

On one paragraph you said:

"Object locking as a technique for avoiding commit conflicts shares the ‘retry on failure’ model of WATally, so it isn’t necessarily superior to ‘retry on commit failure’. It is a superior technique, if you need to protect logical updates where a physical conflict cannot be guaranteed (i.e., updates to different portions of an object graph).
"

And that's EXACTLY what I was wondering... why the commit failure (conflicts) / abort / retry would NOT be enough for sessions and avoid the lock?  Is that because of that sentence " if you need to protect logical updates where a physical conflict cannot be guaranteed"  
So that's the case for Seaside sessions? It's not safe to update (without conflict) different parts of the session subgraph ?
The primary reason for the Seaside session object lock is that Seaside itself had (and presumably still has) a mutex on the session object that only allows Pharo-based Seaside to handle one request at a time in a single vm. 


I wonder if that's still the case on latest Seaside. Do you have a clue where I can check myself?

 
The session object lock emulates the in-vm mutex semantics for multi-gem GemStone and guarantees logical consistency for the session state.

I am intrigued about the ORIGINAL needs of such a need. Because as Pharo and GemStone and too it makes me wonder if the lock would be needed on GemStone.

 
Unfortunately it is not enough to rely conflicts to guarantee logical consistency.

Do you have more details about this? An example maybe? 

 


This is sad, because many many many requests would be read-only to what the session matters (unless there is something obvious I am not seeing?) and in that case, the requests WOULD be able to be parallelized on different Gems.  

On the other hand, if you know that you have read-only requests, you could arrange for a pool of seaside gems to be serving read-only requests by putting the gems behind a different ip address, disabling the session lock altogether and unconditionally aborting after finishing request processing (this is roughly the equivalent of what Johan is doing with his pool of REST gems -- I think). I suppose you could embed something in the http request itself to tell the server to process a read only request as well... that way your entire pool of seaside gems could alternate between server session-based requests and session-less requests ...


I just analyzed this carefully, and I cannot guarantee my requests are fully read only ahahahahhaha. 
 
I know Johan recommends session affinity [2] and I understand why. It's just that I would prefer to be able to have multiple requests to the same session in parallel in different gems. At least those that wouldn't fail due to a commit conflict on the session. 

This does bring up a question that I haven't thought about before:

  How does an Ajax request "bypass" the session mutex in a Pharo-based Seaside vm?


Does it?


 
Dale

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Re: Why session locking is necessary for Seaside?

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What about the Rc... Classes?
Why are they not mentioned here?
The reducedConflict Classes?
I thought the reduced conflict Classes were
     the preferred way to reduce conflicts?
From the stone soup article i got the impression that 
     not using the Rc Classes
     was some kind of stop gap sub par way out
Is this not the case?
Why wouldn't you just use the Rc Classes and be done 
     with it?
Oh right there are some caviates
     some Rc rules that have to be followed
     They don't seem very restrictive 
          and if you violate them once in a blue moon
          then you would just get a retry?
My idea i got from what i read is
     just use the Rc Classes
     problem solved
Is this not the case?
It's because Seaside has no Rc Classes?
Well where are the Rc Classes then

So what i think I'm hearing is
     you can have gemstone gems
     and Seaside gems
     and the Seaside gems don't have the Reduced conflict Classes

or something

please correct me if I'm wrong



On Tue, Mar 21, 2017 at 13:57 Dale Henrichs via Glass <[hidden email]> wrote:



On 03/21/2017 12:25 PM, Mariano Martinez Peck via Glass wrote:
Hi,

I was reading this old post from Dale [1] which explains the topic very nice. After having watched the #seasideProcessRequestWithRetry:resultBlock:  and WASession >> handle:   and the post, I have some questions. 

On one paragraph you said:

"Object locking as a technique for avoiding commit conflicts shares the ‘retry on failure’ model of WATally, so it isn’t necessarily superior to ‘retry on commit failure’. It is a superior technique, if you need to protect logical updates where a physical conflict cannot be guaranteed (i.e., updates to different portions of an object graph).
"

And that's EXACTLY what I was wondering... why the commit failure (conflicts) / abort / retry would NOT be enough for sessions and avoid the lock?  Is that because of that sentence " if you need to protect logical updates where a physical conflict cannot be guaranteed"  
So that's the case for Seaside sessions? It's not safe to update (without conflict) different parts of the session subgraph ?
The primary reason for the Seaside session object lock is that Seaside itself had (and presumably still has) a mutex on the session object that only allows Pharo-based Seaside to handle one request at a time in a single vm.  The session object lock emulates the in-vm mutex semantics for multi-gem GemStone and guarantees logical consistency for the session state. Unfortunately it is not enough to rely conflicts to guarantee logical consistency.


This is sad, because many many many requests would be read-only to what the session matters (unless there is something obvious I am not seeing?) and in that case, the requests WOULD be able to be parallelized on different Gems.  

On the other hand, if you know that you have read-only requests, you could arrange for a pool of seaside gems to be serving read-only requests by putting the gems behind a different ip address, disabling the session lock altogether and unconditionally aborting after finishing request processing (this is roughly the equivalent of what Johan is doing with his pool of REST gems -- I think). I suppose you could embed something in the http request itself to tell the server to process a read only request as well... that way your entire pool of seaside gems could alternate between server session-based requests and session-less requests ...

I know Johan recommends session affinity [2] and I understand why. It's just that I would prefer to be able to have multiple requests to the same session in parallel in different gems. At least those that wouldn't fail due to a commit conflict on the session. 

This does bring up a question that I haven't thought about before:

  How does an Ajax request "bypass" the session mutex in a Pharo-based Seaside vm?


Dale
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Re: Why session locking is necessary for Seaside?

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On 03/21/2017 02:35 PM, Kjell Godo via Glass wrote:
> What about the Rc... Classes?
> Why are they not mentioned here?
> The reducedConflict Classes?
> I thought the reduced conflict Classes were
>      the preferred way to reduce conflicts?

A very short, but I hope helpful, answer:

The RC classes allow you to do things that are physical conflicts
(concurrent operations modify the same object) but are not logical
conflicts (in practice, both operations can complete in either order
with the same result).

This thread, If I'm recalling correctly from a few posts back, is more
concerned with detecting and preventing logical conflicts (things that
if done concurrently *would* mess things up) that are *not* physical
conflicts (no single object is modified by the conflicting operations).

Regards,

-Martin

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Re: Why session locking is necessary for Seaside?

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To expand upon Martin's answer, there _are_ rc collections used for
collections that can be updated by multiple sessions ... I'm not sure it
is practical to rewrite all of the Seaside session state code, to
convert it all to rc collections ... last time I looked there was quite
a bit of object copying going on so that the old session state can be
restored based on the which session is involved, so it may not even be
possible to rewrite Seaside using rc collections ... of course, this is
all of the detail of logical session state that the session mutex aims
to protect ...

Dale

On 03/21/2017 02:46 PM, Martin McClure via Glass wrote:

> On 03/21/2017 02:35 PM, Kjell Godo via Glass wrote:
>> What about the Rc... Classes?
>> Why are they not mentioned here?
>> The reducedConflict Classes?
>> I thought the reduced conflict Classes were
>>       the preferred way to reduce conflicts?
> A very short, but I hope helpful, answer:
>
> The RC classes allow you to do things that are physical conflicts
> (concurrent operations modify the same object) but are not logical
> conflicts (in practice, both operations can complete in either order
> with the same result).
>
> This thread, If I'm recalling correctly from a few posts back, is more
> concerned with detecting and preventing logical conflicts (things that
> if done concurrently *would* mess things up) that are *not* physical
> conflicts (no single object is modified by the conflicting operations).
>
> Regards,
>
> -Martin
>
> _______________________________________________
> Glass mailing list
> [hidden email]
> http://lists.gemtalksystems.com/mailman/listinfo/glass

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Re: Why session locking is necessary for Seaside?

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On 21 Mar 2017, at 22:35, Kjell Godo via Glass <[hidden email]> wrote:

So what i think I'm hearing is
     you can have gemstone gems
     and Seaside gems
     and the Seaside gems don't have the Reduced conflict Classes

No this is not what you should be hearing. 
This discussion is about serving requests for the same Seaside session concurrently.
A single Seaside session is about one user who is browsing your web app.

This has nothing to do with serving multiple sessions concurrently and processing requests for different sessions (and thus different users) in parallel.
Seaside can do that very well in GemStone, and using Rc classes inside your application will be very necessary for any shared state between the different sessions.

Hope this avoids confusion
Johan

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Re: Why session locking is necessary for Seaside?

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In reply to this post by GLASS mailing list



On 03/21/2017 02:13 PM, Mariano Martinez Peck wrote:

On Tue, Mar 21, 2017 at 5:57 PM, Dale Henrichs via Glass <[hidden email]> wrote:



On 03/21/2017 12:25 PM, Mariano Martinez Peck via Glass wrote:
Hi,

I was reading this old post from Dale [1] which explains the topic very nice. After having watched the #seasideProcessRequestWithRetry:resultBlock:  and WASession >> handle:   and the post, I have some questions. 

On one paragraph you said:

"Object locking as a technique for avoiding commit conflicts shares the ‘retry on failure’ model of WATally, so it isn’t necessarily superior to ‘retry on commit failure’. It is a superior technique, if you need to protect logical updates where a physical conflict cannot be guaranteed (i.e., updates to different portions of an object graph).
"

And that's EXACTLY what I was wondering... why the commit failure (conflicts) / abort / retry would NOT be enough for sessions and avoid the lock?  Is that because of that sentence " if you need to protect logical updates where a physical conflict cannot be guaranteed"  
So that's the case for Seaside sessions? It's not safe to update (without conflict) different parts of the session subgraph ?
The primary reason for the Seaside session object lock is that Seaside itself had (and presumably still has) a mutex on the session object that only allows Pharo-based Seaside to handle one request at a time in a single vm. 


I wonder if that's still the case on latest Seaside. Do you have a clue where I can check myself?

 
The session object lock emulates the in-vm mutex semantics for multi-gem GemStone and guarantees logical consistency for the session state.

I am intrigued about the ORIGINAL needs of such a need. Because as Pharo and GemStone and too it makes me wonder if the lock would be needed on GemStone.
I've read some of the newer Seaside code and while I can easily tell that there is still session state being copied and restored (see senders and implementers of snapShotCopy and restoreFromSnapShot:), it is not easy to tell how it is being protected from logical or physical corruption ... WACache has a mutex protecting the cache updates, but I still don't see how the component snapshots are being protected from concurrent updates ...

Unfortunately it is not enough to rely conflicts to guarantee logical consistency.

Do you have more details about this? An example maybe?

Logical inconsistency occurs in transactions when you use stale data to make a calculation or produce a report or update persistent state. For a simple example, let's say that you look at a persistent count and you use that count to generate a report. The report will be logically consistent if the persistent count and the data used to generate the report are guaranteed to be in the same transactional view. If you read the persistent count, abort and then generate the report, you will have a logical inconsistency if the count changed between the time that you read the count and generated your report and there will be no commit conflict to indicate that there is an inconsistency, because you are reading inconsistent data. 

A persistent logical inconsistency can occur if you update an object based upon the stale counter value and then commit ... There is no guaranteed commit conflict in this case and no indication that you've introduced corruption.

if you use an object lock to protect the counter and the data used to generate the report, then you can know that your data is consistent by acquiring the object lock, read the counter abort, generate your report and release the lock ...

 


This is sad, because many many many requests would be read-only to what the session matters (unless there is something obvious I am not seeing?) and in that case, the requests WOULD be able to be parallelized on different Gems.  

On the other hand, if you know that you have read-only requests, you could arrange for a pool of seaside gems to be serving read-only requests by putting the gems behind a different ip address, disabling the session lock altogether and unconditionally aborting after finishing request processing (this is roughly the equivalent of what Johan is doing with his pool of REST gems -- I think). I suppose you could embed something in the http request itself to tell the server to process a read only request as well... that way your entire pool of seaside gems could alternate between server session-based requests and session-less requests ...


I just analyzed this carefully, and I cannot guarantee my requests are fully read only ahahahahhaha.
It's not only the Seaside session state that is being protected from concurrent updates by the Seaside session lock. Your application data is also being protected and you don't have to carefully analyze your code to know whether it is safe or not ...

I would say that it is theoretically possible to move the transaction boundaries further down into the Seaside code, but I'm not so sure that it can be done without radically changing how Seaside works ...

It occurs to me that you could still use a pool of REST gems that do not lock session state as long as you put your own protections in place ... perhaps objects locks at a finer level of granularity?
 
I know Johan recommends session affinity [2] and I understand why. It's just that I would prefer to be able to have multiple requests to the same session in parallel in different gems. At least those that wouldn't fail due to a commit conflict on the session. 

This does bring up a question that I haven't thought about before:

  How does an Ajax request "bypass" the session mutex in a Pharo-based Seaside vm?


Does it?
If I knew, I wouldn't ask the question:)

Dale

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Re: [Seaside] Why session locking is necessary for Seaside?

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>> I am intrigued about the ORIGINAL needs of such a need. Because as Pharo and GemStone and too it makes me wonder if the lock would be needed on GemStone.
> I've read some of the newer Seaside code and while I can easily tell that there is still session state being copied and restored (see senders and implementers of snapShotCopy and restoreFromSnapShot:), it is not easy to tell how it is being protected from logical or physical corruption ... WACache has a mutex protecting the cache updates, but I still don't see how the component snapshots are being protected from concurrent updates ...

That should be the WAMutualExclusionFilter, no?
This one is never installed in the Gemstone code.

cheers
Johan
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