I have a small scale PHP social network, running with a MySQL database. Users on the network can join various groups and receive updates.
I want to notify the user when a new update has been released by a group.
I don't want to do anything fancy with sockets, I'd just like a display of how many updates have been posted since the user was last active.
I'd thought of recording the current time against a user every time they refresh the page, this way I can compare the date of updates vs. the last time a user was active.
I'm not sure that writing to the database on every page load is a good idea though. Any other suitable suggestions?
I think the best way to do this is indeed writing to the database. However, in terms of performance there are a few ways to make it faster. Two ways I can think of are caching the updates for popular groups and creating a table which will only have users and timestamps, both indexed, so that should work very quickly.
Your solution is fine. Make sure you index the correct fields in your database.
Now if you ever have a question like this again.. go through the following steps:
Try it
Measure
If you're worried about scalability problems down the road.. do the same thing again, except you measure with how many records you expect (or want to support). It will be easy to just generate 10.000 records or even millions and try again.
Now if you actually run into unacceptable speeds, write down your problem and ask again here how you could potentially optimize.
This probably seems like a very simple question, and I would probably know if I had a more in depth knowledge of computer processes and the like, but anyway..
If two people request the same page from my server, is the PHP page processed once for the first person, and then a second time for the second person, or might these run along side each other at the same time?
Take this as an example. I have one stock Item left in my PHP driven online shop. A user adds this to their cart. Php script 1) checks to see if it is in stock, Yup, its in stock, so it 2)reserves it for him.
What If, in between checking if its in stock and reserving it, the same PHP page was loading for someone else, and just after user A checked if it was in stock, so did user B, before user A got a chance to reserve it, so they both end up reserving it!
Sorry if this seems silly, can't seem to find an answer on it, which is it?
Congratulations, you have identified a race condition! :-)
Whether PHP pages run in parallel or one after the other depends on the web server. Typically a web server allocates several threads to handle multiple incoming requests at once. So it may indeed happen that several instances of the same script are run in parallel if two or more users request the same page at the same time. Due to timing and scheduling differences it is unpredictable when each page will execute which action exactly.
Hence for such situations as you describe it is important to program actions in an atomic way, meaning that they either complete in their entirety or not at all. In your case you could use locks, transactions, cleverly formed UPDATE statements, UNIQUE indexes or a number of other techniques that avoid the possibility of two users reserving the same thing.
Yes, in general, without getting into too much detail: PHP scripts are executed simultanously for each request separately.
For making sure the problem you mentioned does not occur, you should probably implement feature of your database management system called "transactions". This way if you do something on the database layer and at the end you will find out the reservation can not happen, all the actions made within transaction will be rolled back.
In addition to transactions you should design your application keeping in mind that the problem you mentioned may occur. Thus you should design your database & application in a way allowing you to 1) shorten the time between "checking" and "reserving" as much as possible, 2) stopping the action if you cannot make reservation, and finally - in case of emergency - 3) identifying which reservation came first and which should be revoked.
Another idea, falling into category of "your application's design", may be something we could call "temporary reservation". That means you can temporarily (eg. for a couple of seconds) lock your reservation if you are about to make reservation. After that you can check if you really can make that reservation and either turn it into permanent reservation or just revoke it. I believe some systems also make longer temporary reservations right after the customer begins the process of reserving his/her places. Then, if the process is successful, the reservation is changed into permanent, but if some specific amount of time passes without success, the reservation can be simply revoked, allowing another customer to begin the process.
yes definately, they are parallel for php but when the database concerns you should learn transaction portion of database management system.
Yes and no. PHP may run in simultaneous processes depending on server setup, but on a small-scale, you'll only have one database. Database queries are handled sequentially, so you'll never have that kind of conflict. (As long as you check to see if an item's in stock immediately before you reserve it for someone.) More information
Of course, Users A + B might both see that it's in stock, and A might request it before B. But your code can realize that it's now out of stock and display an error to User B.
(You get into trouble with multiple database servers. If you have the same data stored across multiple servers, there's lag time before data can be fully replicated. But you won't have that issue. We're talking like top 1,000 sites here.)
i have a LAPP (linux, apache, postgresql and php) environment, but the question is pretty the same both on Postgres or Mysql.
I have an cms app i developed, that handle clients, documents (estimates, invoices, etc..) and other data, structured in 1 postgres DB with many schemas (one for each our customer using the app); let's assume around 200 schemas, each of them used concurrently by 15 people (avg).
EDIT: I do have an timestamp field named last_update on every table, and a trigger that update the timestamp every time the row is update.
The situation is:
People Foo and Bar are editing the document 0001, using a form with every document details.
Foo change the shipment details, for example.
Bar change the phone numbers, and some items in the document.
Foo press the 'Save' button, the app update the db.
Bar press the 'Save' button after bar, resending the form with the old shipment details.
In the database, the Foo changes have been lost.
The situation i want to have:
People Foo, Bar, John, Mary, Paoul are editing the document 0001, using a form with every document details.
Foo change the shipment details, for example.
Bar and the others change something else.
Foo press the 'Save' button, the app update the db.
Bar and the others get an alert 'Warning! this document has been changet by someone else. Click here to load the actuals data'.
I've wondered to use ajax to do this; simply using an hidden field with the id of the document and the last-updated timestamp, every 5 seconds check if the last-updated time is the same and do nothing, else, show the alert dialog box.
So, the page check-last-update.php should look something like:
<?php
//[connect to db, postgres or mysql]
$documentId = isset($_POST['document-id']) ? $_POST['document-id'] : 0;
$lastUpdateTime = isset($_POST['last-update-time']) ? $_POST['last-update-time'] : 0;
//in the real life i sanitize the data and use prepared statements;
$qr = pg_query("
SELECT
last_update_time
FROM
documents
WHERE
id = '$documentId'
");
$ray = pg_fetch_assoc($qr);
if($ray['last_update_time'] > $lastUpdateTime){
//someone else updated the document since i opened it!
echo 'reload';
}else{
echo 'ok';
}
?>
But i dont like to stress the db every 5 seconds for every user that have one (or more...) documents opened.
So, what can be another efficent solution without nuking the db?
I thought to use files, creating for example an empty txt file for each document, and everytime the document is updated, i 'touch' the file updating the 'last modified time' as well... but i guess that this would be slower than db and give problems when i have much users editing the same document.
If someone else have a better idea or any suggestion, please describe it in details!
* - - - - - UPDATE - - - - - *
I definitely choosen to NOT hit the db for check the 'last update timestamp', dont mind if the query will be pretty fast, the (main) database server has other tasks to fullfill, dont like the idea to increase his overload for that thing.
So, im taking this way:
Every time a document is updated by someone, i must do something to sign the new timestamp outside the db environment, e.g. without asking the db. My ideas are:
File-system: for each document i create an empry txt files named as the id of the document, everytime the document is update, i 'touch' the file. Im expecting to have thousands of those empty files.
APC, php cache: this will be probably a more flexible way than the first one, but im wondering if keeping thousands and thousands of data permanently in the apc wont slow down the php execution itself, or consume the server memory. Im little bit afraid to choose this way.
Another db, sqlite or mysql (that are faster and lighter with simple db structures) used to store just the documents ID and timestamps.
Whatever way i choose (files, apc, sub-db) im seriously thinking to use another web-server (lighttp?) on a sub-domain, to handle all those.. long-polling requests.
YET ANOTHER EDIT:
The file's way wouldnt work.
APC can be the solution.
Hitting the DB can be the solution too, creating a table just to handle the timestamps (with only two column, document_id and last_update_timestamp) that need to be as fast and light as possible.
Long polling: that's the way i'll choose, using lighttpd under apache to load static files (images, css, js, etc..), and just for this type of long-polling; This will lighten the apache2 load, specially for the polling.
Apache will proxy-up all those request to lighttpd.
Now, i only have to decide between db solution and APC solution..
p.s: thanks to all whom already answered me, you have been really usefull!
I agree that I probably wouldn't hit the database for this. I suppose I would use APC cache (or some other in-memory cache) to maintain this information. What you are describing is clearly optimistic locking at the detailed record level. The higher the level in the database structure the less you need to deal with. It sounds like you want to check with multiple tables within a structure.
I would maintain a cache (in APC) of the IDs and the timestamps of the last updated time keyed by the table name. So for example I might have an array of table names where each entry is keyed by ID and the actual value is the last updated timestamp. There are probably many ways to set this up with arrays or other structures but you get the idea. I would probably add a timeout to the cache so that entries in the cache are removed after a certain period of time - i.e., I wouldn't want the cache to grow and assume that 1 day old entries aren't useful anymore).
With this architecture you would need to do the following (in addition to setting up APC):
on any update to any (applicable) table, update the APC cache entry with the new timestamp.
within ajax just go as far "back" as php (to obtain the APC cache to check the entry) rather than all of the way "back" to the database.
I think you can use a condition in the UPDATE statement like WHERE ID=? AND LAST_UPDATE=?.
The idea is that you will only succeed in updating when you are the last one reading that row. If someone else has committed something, you will fail, and once you know you've failed, you can query the changes.
Hibernate uses a version field to do that. Give every table such a field and use a trigger to increment it on every update. When storing an update, compare the current version with the version when the data was read earlier. If those don't match, throw an exception. Use transactions to make the check-and-update atomic.
You will need some type of version stamp field for each record. What it is doesn't matter as long as you can guarantee that making any change to a record will result in that version stamp being different. Best practice is to then check and make sure the loaded record's version stamp is the same as the version stamp in the DB when the user clicks save, and if it's different handle it.
How you handle it is up to you. At the very least you'd want to offer to reload from the DB so the user can verify that they still want to save. One up from that would be to attempt to merge their changes into the new DB record and then ask them to verify that the merge worked correctly.
If you want to periodically poll any DB capable of handling your system should be able to take the poll load. 10 users polling once every 5 seconds is 2 transactions per second. This is a trivial load, and should be no problem at all. To keep the average load close to the actual load, just jitter the polling time slightly (instead of doing it exactly every 5 seconds, do it every 4-6 seconds, for example).
Donnie's answer (polling) is probably your best option - simple and works. It'll cover almost every case (its unlikely a simple PK lookup would hurt performance, even on a very popular site).
For completeness, and if you wanted to avoid polling, you can use a push-model. There's various ways described in the Wikipedia article. If you can maintain a write-through cache (everytime you update the record, you update the cache), then you can almost completely eliminate the database load.
Don't use a timestamp "last_updated" column, though. Edits within the same second aren't unheard of. You could get away with it if you add extra information (server that did the update, remote address, port, etc) to ensure that, if two requests came in at the same second, to the same server, you could detect the difference. If you need that precision, though, you might as well use a unique revision field (it doesn't necessarily have to be an incrementing integer, just unique within that record's lifespan).
Someone mentioned persistent connections - this would reduce the setup cost of the polling queries (every connection consumes resources on the database and host machine, naturally). You would keep a single connection (or as few as possible) open all the time (or as long as possible) and use that (in combination with caching and memoization, if desired).
Finally, there are SQL statements that allow you to add a condition on UPDATE or INSERT. My SQl is really rusting, but I think its something like UPDATE ... WHERE .... To match this level of protection, you would have to do your own row locking prior to sending the update (and all the error handling and cleanup that might entail). Its unlikely you'd need this; I'm just mentioning it for completness.
Edit:
Your solution sounds fine (cache timestamps, proxy polling requests to a another server). The only change I'd make is to update the cached timestamps on every save. This will keep the cache fresher. I'd also check the timestamp directly from the db when saving to prevent a save sneaking in due to stale cache data.
If you use APC for caching, then a second HTTP server doesn't make sense - you'd have to run it on the same machine (APC uses shared memory). The same physical machine would be doing the work, but with the additional overhead of a second HTTP server. If you want to off load the polling requests to a second server (lighttpd, in your case), then it would be better to setup lightttpd in front of Apache on a second physical machine and use a shared caching server (memcache) so that the lighttpd server can read the cached timestamps, and Apache can update the cached timestamps. The rationale for putting lighttpd in front of Apache is, if most requests are polling requests, to avoid the heavier-weight Apache process usage.
You probably don't need a second server at all, really. Apache should be able to handle the additional requests. If it can't, then I'd revisit your configuration (specifically the directives that control how many worker processes you run and how many requests they are allowed to handle before being killed).
Your approach of querying the database is the best one. If you do it every 5 seconds and you have 15 concurrent users then you're looking at ~3 queries a second. It should be a very small query too, returning only one row of data. If your database can't handle 3 transactions a second then you might have to look at a better database because 3 queries/second is nothing.
Timestamp the records in the table so you can quickly see if anything has changed without having to diff each field.
This is slightly off topic, but you can use the PEAR package (or PECL package, I forget which) xdiff to send back good user guidance when you do get a collision.
First off only update the fields that have changed on when writing to the database, this will decrease database load.
Second, query the timestamp of the last update, if you have a older timestamp then the current version in the database then throw the warning to the client.
Third is to somehow push this information to the client, though some kind of persistent connection with the server, enabling a concurrent two way connection.
Polling is rarely a nice solution.
You could do the timstamp check only when the user (with the open document) is doing something active with the document like scrolling, moving the mouse over it or starts to edit. Then the user gets an alert if the document has been changed.
.....
I know it was not what you asked for but ... why not a edit-singleton?
The singleton could be a userID column in the document-table.
If a user wants to edit the document, the document is locked for edit by other users.
Or have edit-singletons on the individual fields/groups of information.
Only one user can edit the document at a time. If another user has the document open and want to edit a single timestamp check reveal that the document has been altered and is reloaded.
With a singleton there is no polling and only one timestamp check when the user "touches" and/or wants to edit the document.
But perhaps a singleton mechanism doesn't fit your system.
Regards
Sigersted
Ahhh, i though it was easyer.
So, lets make the point: i have a generic database (pgsql or mysql doesn't matter), that contains many generic objects.
I have $x (actually $x = 200, but is growing, hoping will reach 1000 soon) of exact copy of this database, and for each of them up to 20 (avg 10) users for 9 hours at day.
If one of those users is viewing a record, any record, i must advice him if someone edit the same record.
Let's say Foo is watching the document 0001, sit up for a coffee, Bar open and edit the same document, when Foo come back he must see an 'Warning, someone else edited this document! click here to refresh tha page.'.
That'all i need atm, probably i'll extend this situation, adding a way to see the changes and rollback, but this is not the point.
Some of you suggested to check the 'last update' timestamp only when foo try to save the document; Can be a solution too, but i need something in real-time ( 10 sec deelay ).
Long polling, bad way, but seem to be the only one.
So, what i've done:
Installed Lighttp on my machine (and php5 as fastcgi);
Loaded apache2's proxy module (all, or 403 error will hit you);
Changed the lighttpd port from 80 (that is used by apache2) to 81;
Configured apache2 to proxying the request from mydomain.com/polling/* to polling.mydomain.com (served with Lighttp)
Now, i have another sub http-service that i'll use both for polling and load static content (images, etc..), in order to reduce the apache2's load.
Becose i dont want to nuke the database for the timestamp check, i've tryed some caches system (that can be called from php).
APC: quite simple to install and manage, very lightweight and faster, this would be my first choice.. if only the cache would be sharable between two cgi process (i need to store in cache a value from apache2's php process, and read it from lighttpd's php process)
Memcached: around 4-5 times slower than APC, but run as a single process that can be touched everywhere in my environment. I'll go with this one, atm. (even if is slower, the use i'll do of it is relatively simple).
Now, i just have to try this system loading some test datas to see ho will move 'under pressure' and optimize it.
I suppost this environment will work for other long-polling situations (chat?)
Thanks to everyone who gave me hear!
I suggest: when you first query the record that might be changed, hang onto a local copy. When "updating", compare the copy in the locked table/row against your copy, and if it's changed, kick it back to the user.
We have this PHP application which selects a row from the database, works on it (calls an external API which uses a webservice), and then inserts a new register based on the work done. There's an AJAX display which informs the user of how many registers have been processed.
The data is mostly text, so it's rather heavy data.
The process is made by thousands of registers a time. The user can choose how many registers to start working on. The data is obtained from one table, where they are marked as "done". No "WHERE" condition, except the optional "WHERE date BETWEEN date1 AND date2".
We had an argument over which approach is better:
Select one register, work on it, and insert the new data
Select all of the registers, work with them in memory and insert them in the database after all the work was done.
Which approach do you consider the most efficient one for a web environment with PHP and PostgreSQL? Why?
It really depends how much you care about your data (seriously):
Does reliability matter in this case? If the process dies, can you just re-process everything? Or can't you?
Typically when calling a remote web service, you don't want to be calling it twice for the same data item. Perhaps there are side effects (like credit card charges), or maybe it is not a free API...
Anyway, if you don't care about potential duplicate processing, then take the batch approach. It's easy, it's simple, and fast.
But if you do care about duplicate processing, then do this:
SELECT 1 record from the table FOR UPDATE (ie. lock it in a transaction)
UPDATE that record with a status of "Processing"
Commit that transaction
And then
Process the record
Update the record contents, AND
SET the status to "Complete", or "Error" in case of errors.
You can run this code concurrently without fear of it running over itself. You will be able to have confidence that the same record will not be processed twice.
You will also be able to see any records that "didn't make it", because their status will be "Processing", and any errors.
If the data is heavy and so is the load, considering the application is not real time dependant the best approach is most definately getting the needed data and working on all of it, then putting it back.
Efficiency speaking, regardless of language is that if you are opening single items, and working on them individually, you are probably closing the database connection. This means that if you have 1000's of items, you will open and close 1000's of connections. The overhead on this far outweighs the overhead of returning all of the items and working on them.
I would like to create an interface for manipulating invoices in a transaction-like manner.
The database consists of an invoices table, which holds billing information, and an invoice_lines table, which holds line items for the invoices. The website is a set of scripts which allow the addition, modification, and removal of invoices and their corresponding lines.
The problem I have is this, I would like the ACID properties of the database to be reflected in the web application.
Atomic: When the user hits save, either the entire invoice is modified or the entire invoice is not changed at all.
Consistent: The application code already ensures consistency, lines cannot be added to non-existent invoices. Invoice IDs cannot be duplicated.
Isolated: If a user is in the middle of a set of changes to an invoice, I would like to hide those changes from other users until the user clicks save.
Durable: If the web site dies, the data should be safe. This already works.
If I were writing a desktop application, it would maintain a connection to the MySQL database at all times, allowing me to simply use the BEGIN TRANSACTION and COMMIT at the beginning and end of the edit.
From what I understand you cannot BEGIN TRANSACTION on one PHP page and COMMIT on a different page because the connection is closed between pages.
Is there a way to make this possible without extensions? From what I have found, only SQL Relay does this (but it is an extension).
you don't want to have long running transactions, because that'll limit concurrency. http://en.wikipedia.org/wiki/Command_pattern
The translation on the web for this type of processing is the use of session data or data stored in the page itself. Typically what is done is that after each web page is completed the data is stored in the session (or in the page itself) and at the point in which all of the pages have been completed (via data entry) and a "Process" (or "Save") button is hit, the data is converted into the database form and saved - even with the relational aspect of data like you mentioned. There are many ways to do this but I would say that most developers have an architecture similar to what I mentioned (using session data or state within the page) to satisfy what you are talking about.
You'll get much advice here on different architectures but I can say that the Zend Framework (http://framework.zend.com) and the use of Doctrine (http://www.doctrine-project.org/) make this fairy easy since Zend provides much of the MVC architecture and session management and Doctrine provides the basic CRUD (create, retrieve, update, delete) you are looking for - plus all of the other aspects (uniqueness, commit, rollback, etc). Keeping the connection open to mysql may cause timeouts and lack of available connections.
Database transactions aren't really intended for this purpose - if you did use them, you'd probably run into other problems.
But also you can't use them as each page request uses its own connection (potentially) so cannot share a transaction with any others.
Keep the modifications to the invoice somewhere else while the user is editing them, then apply them when she hits save; you can do this final apply step in a transaction (albeit quite a short-lived one).
Long-lived transactions are usually bad.
The solution is not to open the transaction during the GET phase. Do all aspects of the transaction—BEGIN TRANSACTION, processing, and COMMIT—all during the POST triggered by the "save" button.
Persistent connections may help you:
http://php.net/manual/en/features.persistent-connections.php
Another is that when using
transactions, a transaction block will
also carry over to the next script
which uses that connection if script
execution ends before the transaction
block does.
But I recommend you to find another approach to the problem.
For example: create a cache table.
When you need to "commit", transfer the records from the cache table to the "real" tables.
Altough there are some good answers, I think that found some good responses to your question, that I was stuck with also. I think the best approach is using a framework like Doctrine (O/R mapping) that has this kind of approach somehow implemented. Here you have a link to what I'm talking about.