RE: [PHP] Multiple Access Question
-Original Message- From: Robert Cummings [mailto:rob...@interjinn.com] Sent: Wednesday, July 07, 2010 10:28 PM To: Paul M Foster Cc: php-general@lists.php.net Subject: Re: [PHP] Multiple Access Question Paul M Foster wrote: On Wed, Jul 07, 2010 at 11:28:56PM -0400, Robert Cummings wrote: Paul M Foster wrote: snip @Paul, The OPs question is about concurrency on the record itself. How to avoid two users accessing the same record and potentially damaging each others changes My approach is the same as Rob's. Flag it locked and let the second user gets a read only copy I can't think of a way to do this using MySQL or PostgreSQL. And one of the biggest issues with the solution you suggest is the user who opens a record for writing and then goes out for coffee. Everyone's locked out of the record (for writes) until they come back and finish. Okay, to solve that, we start a timer. But when the locker's time is up, how do we let the locker know they're not allowed to store whatever edits they've made? And how do we fix it so that those locked out are now unlocked? Plus, they're probably in a queue, so we really only let one of them know that they can now make edits. Since this is a PHP list, I assume we're talking about a web interface. So how do we do all this back end jockeying? Javascript is about the only way. But every time you fire off one of these javascript dealies, it has to be on its own timer so that it can let the user know that the original locker is gone and now the golden ticket is yours. It essentially has to sleep and ping, sleep and ping. Actually, it's more like a spinlock. But a spinlock would eat CPU for every user, if it was running on the server. So it would have to be running on the client, and ping the server every once in a while. Then you'd have to figure out some kind of messaging infrastrucure for the DBMS, so that it would quickly answer pings without tying up a lot of CPU cycles. It would have to be something outside the normal query infrastructure. When you actually get into this, it's an incredibly complex solution. I vote instead for allowing edits to be queued, log changes to the database. If there is a true contention problem, you can look at the journal and see who made what edits in what order and resolve the situation. The best analogy I can think of is when using a DVCS like git, and trying to merge changes where two people have edited the same area of a file. Ultimately, git throws up its hands and asks a human to resolve the situation. Bottom line: I've heard about concurrency problems since I started using databases, and I've never heard of a foolproof solution for them that wasn't incredibly complex. And I don't think I've ever seen a solution in actual practice. If I'm wrong, someone show me where it's been viably solved and how. I think you're overthinking the issue. The timer handles the issue of holding onto a lock for too long. That's why I suggested it. As for a write queue... don't bother. If a user finds that another user has a lock then tell them when it expires. They can come back and try for the lock on their own. You can set up AJAX polling to see if the lock has been removed and indicate this to the user (if they've bothered to wait on the page) but this is optional. That's why I suggested it. Yes, we could just tell users come back later if they wanted to edit a locked page. I was just imagining a 100% complete wipe-your-butt-for-you solution. Queuing edits is not a good solution. And yet, it appears to adequate for the DBMSes I'm familiar with. Imagine document X: UserA requests X UserB requests X UserC requests X UserD requests X UserA modifies X and saves X.1 UserB modifies X and saves X.2 UserC modifies X and saves X.3 UserD modifies X and saves X.4 In this scenario all the work done by UserA, UserB, and UserC is clobbered by the submission by UserD. This can be resolved via merging such as used by versioning systems, ... if automatic merging can be done in a particular case. But there's a non-zero probability that a merge will require human intervention. Yes of course, without version/merging or some type of write-locks, there is potential contention. but this makes less sense in a high traffic collaborative content system such as a wiki. In the lock scenario we have the following: UserA requests X UserA modifies X and saves X.1 UserB requests X.1 UserB modifies X.1 and saves X.2 UserC requests X.2 UserC modifies X.2 and saves X.3 UserD requests X.3 UserD modifies X.3 and saves X.4 ... assuming UserB waits until UserA stores his edits, UserC waits until UserB stores his edits, etc. The above assumes locking, and probably versioning
Re: [PHP] Multiple Access Question
On 7 July 2010 17:59, tedd t...@sperling.com wrote: Hi gang: I have *my way* of handling this problem, but I would like to hear how you guys do it. Here's the problem -- let's say you have a database containing names and addresses and you want approved users to be able to access the data. As such, a user must login before accessing an editing script that would allow them to review and edit the data -- nothing complicated about that. However, let's say you have more than one user accessing the editing script at the same time and you want to make sure that any changes made to the database are done in the most efficient manner possible. For example, if two users access the database at the same time and are editing different records, then there's no real problem. When each user finishes editing they simply click submit and their changes are recorded in the database. However, if two (or more) users want to access the same record, then how do you handle that? Cheers, tedd I've developed many multi-user applications (specialising in Accounting and EPOS systems). Primarily for DOS and Windows and using SQL Server and D-ISAM data storage systems. In all instances, multi-user locking is handled through the use of a semaphore at the row level. We did look at semaphore locking at the field level. For DOS and Windows, the necessity for handling timeouts was reduced by the fact that if the user was logged in to the app, then the lock stays - and much to the annoyance of all other users. A simple process to allow locks for a user to be dropped was required to handle the instances were the user powered off/crashed/forgot password over lunch situations. In real terms, the number of collisions was very low. Few users would actually be entering data into the same record at the same time. In the vast majority, all the users are in the same room or, at least, the same building/office. When I started the web versions of these apps, the semaphore locking was still required, but was extended to include a timeout based upon the session timeout. We did realize that by using web based technology, some of our clients were looking to allow their users to work from home. So informing the user that a row was locked, how long it was locked for and who else wanted it was well worth while. So we introduced a new structure of lock requests. This was a simple table containing the id of the current semaphore (all our semaphores are in a single table), the user id of the person wanting the row and when they requested the lock. A user who is placed in the lock requests loop could reject their request (i.e. come back later sort of thing). The lock requests would be displayed to the lock holder so that they could essentially get the message to hurry up - in the office someone would call out Is anyone in so-and-so? and then someone would reply Oh yes! Sorry, just coming out now. sort of thing. This process worked fine. The clients knew that only 1 edit could take place at a time. With a distributed work force, the on screen visualisation worked well. A small little drop down in the corner of the app. Nice and easy. As the server is more or less constantly being told of the presence of a user (say once every 30 seconds), the server can easily detect a dropped user and therefore undo the lock. The user next in the lock requests table would automatically get the lock and be informed via the ping that they had control of the row (disabled inputs would become enabled, etc.). Their entry in the lock requests table would be deleted and anyone else would see that they have moved up the queue. I hope this is of some use to you. Semaphores worked really well for our apps. The transition from DOS/Windows to the web wasn't as easy as it could have been - primarily due to the statelessness of the requests. We did manage to do it without the need for any additional monitors running on the server. All based upon simply tracking the datetime of the lock requests, the table lock timeout and the session timeout. In all instances, if a lock has to be cleared due to a timeout, all the unwritten data (we used AJAX to send edits to the server for holding) would be held, but the lock removed and the user forced to login again. On the old system, a lock could persist and cause everyone to wait. On the web, a lock could NOT persist and the user creating the lock would suffer the punishment of having to relogin, retrieve their edits (which may now be stale) and to try again. Whatever technique you use, I would recommend getting it as close to the database as possible. The use of stored procedures, for us, was essential. Lock evaluation and enforcement was all done within the SQL server - all apps have to use the stored procedures and this makes the apps a lot simpler. And allows any app to use the same locking techniques irrespective of the language the developer wanted to use - our apps allowed for third party additions to the
Re: [PHP] Multiple Access Question
At 11:48 AM +0100 7/8/10, Richard Quadling wrote: On 7 July 2010 17:59, tedd t...@sperling.com wrote: Hi gang: I have *my way* of handling this problem, but I would like to hear how you guys do it. -snip- I hope this is of some sense to you. It certainly is an interesting topic. Richard. Richard: Yes, it was very informative and useful -- it gave me an idea to try out. Thanks very much for your most extensive answer. Cheers, tedd -- --- http://sperling.com http://ancientstones.com http://earthstones.com -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php
Re: [PHP] Multiple Access Question
tedd wrote: Hi gang: I have *my way* of handling this problem, but I would like to hear how you guys do it. Here's the problem -- let's say you have a database containing names and addresses and you want approved users to be able to access the data. As such, a user must login before accessing an editing script that would allow them to review and edit the data -- nothing complicated about that. However, let's say you have more than one user accessing the editing script at the same time and you want to make sure that any changes made to the database are done in the most efficient manner possible. For example, if two users access the database at the same time and are editing different records, then there's no real problem. When each user finishes editing they simply click submit and their changes are recorded in the database. However, if two (or more) users want to access the same record, then how do you handle that? I would provide read access whatever the case. For write access I would place a lock on the data for the first user to make the edit request. The lock would have an expiry time, and the user in possession of the lock could reset it if they need more time. Other users would be alerted that another user is currently editing the data and that the lock will expire in X minutes. If the user completes their edit before the timer expires, then the timer will be automatically cleared upon commit of the changes. Optionally, the user who doesn't get the lock could be alerted to what user currently has the lock. Additionally, if we get complicated we could allow a higher authority to steal the lock and alert the user making edits. Cheers, Rob. -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php
Re: [PHP] Multiple Access Question
On Wed, Jul 07, 2010 at 12:59:30PM -0400, tedd wrote: Hi gang: I have *my way* of handling this problem, but I would like to hear how you guys do it. Here's the problem -- let's say you have a database containing names and addresses and you want approved users to be able to access the data. As such, a user must login before accessing an editing script that would allow them to review and edit the data -- nothing complicated about that. However, let's say you have more than one user accessing the editing script at the same time and you want to make sure that any changes made to the database are done in the most efficient manner possible. For example, if two users access the database at the same time and are editing different records, then there's no real problem. When each user finishes editing they simply click submit and their changes are recorded in the database. However, if two (or more) users want to access the same record, then how do you handle that? Use a DBMS? I'm sorry if that seems flippant, but a DBMS handles this by queuing the requests, which is one of the advantages of a client-server DBMS. So maybe I don't understand your question. Paul -- Paul M. Foster -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php
Re: [PHP] Multiple Access Question
On Wed, Jul 7, 2010 at 8:47 PM, Paul M Foster pa...@quillandmouse.com wrote: On Wed, Jul 07, 2010 at 12:59:30PM -0400, tedd wrote: Hi gang: I have *my way* of handling this problem, but I would like to hear how you guys do it. Here's the problem -- let's say you have a database containing names and addresses and you want approved users to be able to access the data. As such, a user must login before accessing an editing script that would allow them to review and edit the data -- nothing complicated about that. However, let's say you have more than one user accessing the editing script at the same time and you want to make sure that any changes made to the database are done in the most efficient manner possible. For example, if two users access the database at the same time and are editing different records, then there's no real problem. When each user finishes editing they simply click submit and their changes are recorded in the database. However, if two (or more) users want to access the same record, then how do you handle that? Use a DBMS? I'm sorry if that seems flippant, but a DBMS handles this by queuing the requests, which is one of the advantages of a client-server DBMS. So maybe I don't understand your question. Paul -- Paul M. Foster -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php @Paul, The OPs question is about concurrency on the record itself. How to avoid two users accessing the same record and potentially damaging each others changes My approach is the same as Rob's. Flag it locked and let the second user gets a read only copy -- Bastien Cat, the other other white meat -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php
Re: [PHP] Multiple Access Question
On Wed, Jul 07, 2010 at 10:01:05PM -0400, Bastien Koert wrote: On Wed, Jul 7, 2010 at 8:47 PM, Paul M Foster pa...@quillandmouse.com wrote: On Wed, Jul 07, 2010 at 12:59:30PM -0400, tedd wrote: Hi gang: I have *my way* of handling this problem, but I would like to hear how you guys do it. Here's the problem -- let's say you have a database containing names and addresses and you want approved users to be able to access the data. As such, a user must login before accessing an editing script that would allow them to review and edit the data -- nothing complicated about that. However, let's say you have more than one user accessing the editing script at the same time and you want to make sure that any changes made to the database are done in the most efficient manner possible. For example, if two users access the database at the same time and are editing different records, then there's no real problem. When each user finishes editing they simply click submit and their changes are recorded in the database. However, if two (or more) users want to access the same record, then how do you handle that? Use a DBMS? I'm sorry if that seems flippant, but a DBMS handles this by queuing the requests, which is one of the advantages of a client-server DBMS. So maybe I don't understand your question. Paul -- Paul M. Foster -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php @Paul, The OPs question is about concurrency on the record itself. How to avoid two users accessing the same record and potentially damaging each others changes My approach is the same as Rob's. Flag it locked and let the second user gets a read only copy I can't think of a way to do this using MySQL or PostgreSQL. And one of the biggest issues with the solution you suggest is the user who opens a record for writing and then goes out for coffee. Everyone's locked out of the record (for writes) until they come back and finish. Okay, to solve that, we start a timer. But when the locker's time is up, how do we let the locker know they're not allowed to store whatever edits they've made? And how do we fix it so that those locked out are now unlocked? Plus, they're probably in a queue, so we really only let one of them know that they can now make edits. Since this is a PHP list, I assume we're talking about a web interface. So how do we do all this back end jockeying? Javascript is about the only way. But every time you fire off one of these javascript dealies, it has to be on its own timer so that it can let the user know that the original locker is gone and now the golden ticket is yours. It essentially has to sleep and ping, sleep and ping. Actually, it's more like a spinlock. But a spinlock would eat CPU for every user, if it was running on the server. So it would have to be running on the client, and ping the server every once in a while. Then you'd have to figure out some kind of messaging infrastrucure for the DBMS, so that it would quickly answer pings without tying up a lot of CPU cycles. It would have to be something outside the normal query infrastructure. When you actually get into this, it's an incredibly complex solution. I vote instead for allowing edits to be queued, log changes to the database. If there is a true contention problem, you can look at the journal and see who made what edits in what order and resolve the situation. The best analogy I can think of is when using a DVCS like git, and trying to merge changes where two people have edited the same area of a file. Ultimately, git throws up its hands and asks a human to resolve the situation. Bottom line: I've heard about concurrency problems since I started using databases, and I've never heard of a foolproof solution for them that wasn't incredibly complex. And I don't think I've ever seen a solution in actual practice. If I'm wrong, someone show me where it's been viably solved and how. Paul -- Paul M. Foster -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php
Re: [PHP] Multiple Access Question
Paul M Foster wrote: On Wed, Jul 07, 2010 at 10:01:05PM -0400, Bastien Koert wrote: On Wed, Jul 7, 2010 at 8:47 PM, Paul M Foster pa...@quillandmouse.com wrote: On Wed, Jul 07, 2010 at 12:59:30PM -0400, tedd wrote: Hi gang: I have *my way* of handling this problem, but I would like to hear how you guys do it. Here's the problem -- let's say you have a database containing names and addresses and you want approved users to be able to access the data. As such, a user must login before accessing an editing script that would allow them to review and edit the data -- nothing complicated about that. However, let's say you have more than one user accessing the editing script at the same time and you want to make sure that any changes made to the database are done in the most efficient manner possible. For example, if two users access the database at the same time and are editing different records, then there's no real problem. When each user finishes editing they simply click submit and their changes are recorded in the database. However, if two (or more) users want to access the same record, then how do you handle that? Use a DBMS? I'm sorry if that seems flippant, but a DBMS handles this by queuing the requests, which is one of the advantages of a client-server DBMS. So maybe I don't understand your question. Paul -- Paul M. Foster -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php @Paul, The OPs question is about concurrency on the record itself. How to avoid two users accessing the same record and potentially damaging each others changes My approach is the same as Rob's. Flag it locked and let the second user gets a read only copy I can't think of a way to do this using MySQL or PostgreSQL. And one of the biggest issues with the solution you suggest is the user who opens a record for writing and then goes out for coffee. Everyone's locked out of the record (for writes) until they come back and finish. Okay, to solve that, we start a timer. But when the locker's time is up, how do we let the locker know they're not allowed to store whatever edits they've made? And how do we fix it so that those locked out are now unlocked? Plus, they're probably in a queue, so we really only let one of them know that they can now make edits. Since this is a PHP list, I assume we're talking about a web interface. So how do we do all this back end jockeying? Javascript is about the only way. But every time you fire off one of these javascript dealies, it has to be on its own timer so that it can let the user know that the original locker is gone and now the golden ticket is yours. It essentially has to sleep and ping, sleep and ping. Actually, it's more like a spinlock. But a spinlock would eat CPU for every user, if it was running on the server. So it would have to be running on the client, and ping the server every once in a while. Then you'd have to figure out some kind of messaging infrastrucure for the DBMS, so that it would quickly answer pings without tying up a lot of CPU cycles. It would have to be something outside the normal query infrastructure. When you actually get into this, it's an incredibly complex solution. I vote instead for allowing edits to be queued, log changes to the database. If there is a true contention problem, you can look at the journal and see who made what edits in what order and resolve the situation. The best analogy I can think of is when using a DVCS like git, and trying to merge changes where two people have edited the same area of a file. Ultimately, git throws up its hands and asks a human to resolve the situation. Bottom line: I've heard about concurrency problems since I started using databases, and I've never heard of a foolproof solution for them that wasn't incredibly complex. And I don't think I've ever seen a solution in actual practice. If I'm wrong, someone show me where it's been viably solved and how. I think you're overthinking the issue. The timer handles the issue of holding onto a lock for too long. As for a write queue... don't bother. If a user finds that another user has a lock then tell them when it expires. They can come back and try for the lock on their own. You can set up AJAX polling to see if the lock has been removed and indicate this to the user (if they've bothered to wait on the page) but this is optional. Queuing edits is not a good solution. Imagine document X: UserA requests X UserB requests X UserC requests X UserD requests X UserA modifies X and saves X.1 UserB modifies X and saves X.2 UserC modifies X and saves X.3 UserD modifies X and saves X.4 In this scenario all the work done by UserA, UserB, and UserC is clobbered by the submission by UserD. This can be resolved via merging such as used by versioning systems, but this makes less sense in a high traffic collaborative content system such as a wiki. In the lock scenario we
Re: [PHP] Multiple Access Question
On Wed, Jul 07, 2010 at 11:28:56PM -0400, Robert Cummings wrote: Paul M Foster wrote: snip @Paul, The OPs question is about concurrency on the record itself. How to avoid two users accessing the same record and potentially damaging each others changes My approach is the same as Rob's. Flag it locked and let the second user gets a read only copy I can't think of a way to do this using MySQL or PostgreSQL. And one of the biggest issues with the solution you suggest is the user who opens a record for writing and then goes out for coffee. Everyone's locked out of the record (for writes) until they come back and finish. Okay, to solve that, we start a timer. But when the locker's time is up, how do we let the locker know they're not allowed to store whatever edits they've made? And how do we fix it so that those locked out are now unlocked? Plus, they're probably in a queue, so we really only let one of them know that they can now make edits. Since this is a PHP list, I assume we're talking about a web interface. So how do we do all this back end jockeying? Javascript is about the only way. But every time you fire off one of these javascript dealies, it has to be on its own timer so that it can let the user know that the original locker is gone and now the golden ticket is yours. It essentially has to sleep and ping, sleep and ping. Actually, it's more like a spinlock. But a spinlock would eat CPU for every user, if it was running on the server. So it would have to be running on the client, and ping the server every once in a while. Then you'd have to figure out some kind of messaging infrastrucure for the DBMS, so that it would quickly answer pings without tying up a lot of CPU cycles. It would have to be something outside the normal query infrastructure. When you actually get into this, it's an incredibly complex solution. I vote instead for allowing edits to be queued, log changes to the database. If there is a true contention problem, you can look at the journal and see who made what edits in what order and resolve the situation. The best analogy I can think of is when using a DVCS like git, and trying to merge changes where two people have edited the same area of a file. Ultimately, git throws up its hands and asks a human to resolve the situation. Bottom line: I've heard about concurrency problems since I started using databases, and I've never heard of a foolproof solution for them that wasn't incredibly complex. And I don't think I've ever seen a solution in actual practice. If I'm wrong, someone show me where it's been viably solved and how. I think you're overthinking the issue. The timer handles the issue of holding onto a lock for too long. That's why I suggested it. As for a write queue... don't bother. If a user finds that another user has a lock then tell them when it expires. They can come back and try for the lock on their own. You can set up AJAX polling to see if the lock has been removed and indicate this to the user (if they've bothered to wait on the page) but this is optional. That's why I suggested it. Yes, we could just tell users come back later if they wanted to edit a locked page. I was just imagining a 100% complete wipe-your-butt-for-you solution. Queuing edits is not a good solution. And yet, it appears to adequate for the DBMSes I'm familiar with. Imagine document X: UserA requests X UserB requests X UserC requests X UserD requests X UserA modifies X and saves X.1 UserB modifies X and saves X.2 UserC modifies X and saves X.3 UserD modifies X and saves X.4 In this scenario all the work done by UserA, UserB, and UserC is clobbered by the submission by UserD. This can be resolved via merging such as used by versioning systems, ... if automatic merging can be done in a particular case. But there's a non-zero probability that a merge will require human intervention. Yes of course, without version/merging or some type of write-locks, there is potential contention. but this makes less sense in a high traffic collaborative content system such as a wiki. In the lock scenario we have the following: UserA requests X UserA modifies X and saves X.1 UserB requests X.1 UserB modifies X.1 and saves X.2 UserC requests X.2 UserC modifies X.2 and saves X.3 UserD requests X.3 UserD modifies X.3 and saves X.4 ... assuming UserB waits until UserA stores his edits, UserC waits until UserB stores his edits, etc. The above assumes locking, and probably versioning and merging. But a wiki is not a DBMS. And perhaps the OP was talking about a wiki. In which case, all this may be moot. I just checked, and Wikipedia does not lock pages under edit. They do versioning, but their locking is on the honor system. For a discourteous user, this would allow contention. I don't know if other wikis perform locking. I doubt it, but I could be
Re: [PHP] Multiple Access Question
Paul M Foster wrote: On Wed, Jul 07, 2010 at 11:28:56PM -0400, Robert Cummings wrote: Paul M Foster wrote: snip @Paul, The OPs question is about concurrency on the record itself. How to avoid two users accessing the same record and potentially damaging each others changes My approach is the same as Rob's. Flag it locked and let the second user gets a read only copy I can't think of a way to do this using MySQL or PostgreSQL. And one of the biggest issues with the solution you suggest is the user who opens a record for writing and then goes out for coffee. Everyone's locked out of the record (for writes) until they come back and finish. Okay, to solve that, we start a timer. But when the locker's time is up, how do we let the locker know they're not allowed to store whatever edits they've made? And how do we fix it so that those locked out are now unlocked? Plus, they're probably in a queue, so we really only let one of them know that they can now make edits. Since this is a PHP list, I assume we're talking about a web interface. So how do we do all this back end jockeying? Javascript is about the only way. But every time you fire off one of these javascript dealies, it has to be on its own timer so that it can let the user know that the original locker is gone and now the golden ticket is yours. It essentially has to sleep and ping, sleep and ping. Actually, it's more like a spinlock. But a spinlock would eat CPU for every user, if it was running on the server. So it would have to be running on the client, and ping the server every once in a while. Then you'd have to figure out some kind of messaging infrastrucure for the DBMS, so that it would quickly answer pings without tying up a lot of CPU cycles. It would have to be something outside the normal query infrastructure. When you actually get into this, it's an incredibly complex solution. I vote instead for allowing edits to be queued, log changes to the database. If there is a true contention problem, you can look at the journal and see who made what edits in what order and resolve the situation. The best analogy I can think of is when using a DVCS like git, and trying to merge changes where two people have edited the same area of a file. Ultimately, git throws up its hands and asks a human to resolve the situation. Bottom line: I've heard about concurrency problems since I started using databases, and I've never heard of a foolproof solution for them that wasn't incredibly complex. And I don't think I've ever seen a solution in actual practice. If I'm wrong, someone show me where it's been viably solved and how. I think you're overthinking the issue. The timer handles the issue of holding onto a lock for too long. That's why I suggested it. As for a write queue... don't bother. If a user finds that another user has a lock then tell them when it expires. They can come back and try for the lock on their own. You can set up AJAX polling to see if the lock has been removed and indicate this to the user (if they've bothered to wait on the page) but this is optional. That's why I suggested it. Yes, we could just tell users come back later if they wanted to edit a locked page. I was just imagining a 100% complete wipe-your-butt-for-you solution. Queuing edits is not a good solution. And yet, it appears to adequate for the DBMSes I'm familiar with. Imagine document X: UserA requests X UserB requests X UserC requests X UserD requests X UserA modifies X and saves X.1 UserB modifies X and saves X.2 UserC modifies X and saves X.3 UserD modifies X and saves X.4 In this scenario all the work done by UserA, UserB, and UserC is clobbered by the submission by UserD. This can be resolved via merging such as used by versioning systems, ... if automatic merging can be done in a particular case. But there's a non-zero probability that a merge will require human intervention. Yes of course, without version/merging or some type of write-locks, there is potential contention. but this makes less sense in a high traffic collaborative content system such as a wiki. In the lock scenario we have the following: UserA requests X UserA modifies X and saves X.1 UserB requests X.1 UserB modifies X.1 and saves X.2 UserC requests X.2 UserC modifies X.2 and saves X.3 UserD requests X.3 UserD modifies X.3 and saves X.4 ... assuming UserB waits until UserA stores his edits, UserC waits until UserB stores his edits, etc. The above assumes locking, and probably versioning and merging. No, not at all. Each user can only edit the version last saved. It only assumes locking. But a wiki is not a DBMS. And perhaps the OP was talking about a wiki. In which case, all this may be moot. I just checked, and Wikipedia does not lock pages under edit. They do versioning, but their locking is on the honor system. For a discourteous user, this would allow contention. I don't know if other
