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Persistent and Persistence, What's the Difference?

Persistent connections are links that do not close when the execution of your script ends. When a persistent connection is requested, PHP checks if there's already an identical persistent connection that remained open from earlier - and if it exists, it uses it. If it does not exist, it creates the link. An 'identical' connection is a connection that was opened to the same host, with the same username and the same password where applicable. People who aren't thoroughly familiar with the way web servers work and distribute the load may mistake persistent connects for what they're not.

In particular, they do not give you an ability to open 'user sessions' on the same link, they do not give you an ability to build up a transaction efficiently, and they don't do a whole lot of other things.

In fact, to be extremely clear about the subject, persistent connections don't give you any functionality that wasn't possible with their non-persistent brothers.

This has to do with the way web servers work. There are three ways in which your web server can utilize PHP to generate web pages. When run this way, an instance of the PHP interpreter is created and destroyed for every page request for a PHP page to your web server.

persistent database connections

Because it is destroyed after every request, any resources that it acquires such as a link to an SQL database server are closed when it is destroyed. In this case, you do not gain anything from trying to use persistent connections -- they simply don't persist.

The second, and most popular, method is to run PHP as a module in a multiprocess web server, which currently only includes Apache. A multiprocess server typically has one process the parent which coordinates a set of processes its children who actually do the work of serving up web pages. When a request comes in from a client, it is handed off to one of the children that is not already serving another client.

This means that when the same client makes a second request to the server, it may be served by a different child process than the first time. When opening a persistent connection, every following page requesting SQL services can reuse the same established connection to the SQL server.

The last method is to use PHP as a plug-in for a multithreaded web server. The behavior is essentially the same as for the multiprocess model described before. If persistent connections don't have any added functionality, what are they good for? The answer here is extremely simple -- efficiency. Persistent connections are good if the overhead to create a link to your SQL server is high. Whether or not this overhead is really high depends on many factors.

Like, what kind of database it is, whether or not it sits on the same computer on which your web server sits, how loaded the machine the SQL server sits on is and so forth. The bottom line is that if that connection overhead is high, persistent connections help you considerably. They cause the child process to simply connect only once for its entire lifespan, instead of every time it processes a page that requires connecting to the SQL server.

This means that for every child that opened a persistent connection will have its own open persistent connection to the server.

For example, if you had 20 different child processes that ran a script that made a persistent connection to your SQL server, you'd have 20 different connections to the SQL server, one from each child. Note, however, that this can have some drawbacks if you are using a database with connection limits that are exceeded by persistent child connections. If your database has a limit of 16 simultaneous connections, and in the course of a busy server session, 17 child threads attempt to connect, one will not be able to.

If there are bugs in your scripts which do not allow the connections to shut down such as infinite loopsthe database with only 16 connections may be rapidly swamped. Check your database documentation for information on handling abandoned or idle connections. There are a couple of additional caveats to keep in mind when using persistent connections. One is that when using table locking on a persistent connection, if the script for whatever reason cannot release the lock, then subsequent scripts using the same connection will block indefinitely and may require that you either restart the httpd server or the database server.

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.

Better yet, avoid the problem entirely by not using persistent connections in scripts which use table locks or transactions you can still use them elsewhere. An important summary. Persistent connections were designed to have one-to-one mapping to regular connections.

That means that you should always be able to replace persistent connections with non-persistent connections, and it won't change the way your script behaves.The English language is one of the most expressive, and confusing, in existence. Words can have different meaning based not only on context, but on placement within a given sentence.

Add in the twists that come from technical jargon and suddenly you've got words meaning completely different things.

This is evident in the use of persistent and persistence. While the conceptual basis of persistence and persistent are essentially the same, in reality they refer to two different technical concepts. Both persistent and persistence relate to the handling of connections. The former is often used as a general description of the behavior of HTTP and, necessarily, TCP connections, though it is also used in the context of database connections. Persistent connections are connections that are kept open and reused.

The most commonly implemented form of persistent connections are HTTP, with database connections a close second. Before HTTP 1. As the number of objects in a page grew, this became increasingly inefficient and significantly reduced the capacity of web servers while causing browsers to appear slow to retrieve data.

HTTP 1. Similarly, this notion was implemented by proxy-based load-balancers as a way to improve performance of web applications and increase capacity on web servers.

Persistent connections between a load-balancer and web servers is usually referred to as TCP multiplexing. Persistent connections, both in browsers and load-balancers, have several advantages:. Persistence, on the other hand, is related to the ability of a load-balancer or other traffic management solution to maintain a virtual connection between a client and a specific server.

Persistence is often referred to in the application delivery networking world as "stickiness" while in the web and application server demesne it is called "server affinity". Persistence ensures that once a client has made a connection to a specific server that subsequent requests are sent to the same server.

This is very important to maintain state and session-specific information in some application architectures and for handling of SSL-enabled applications. Load-balancers that have evolved into application delivery controllers are capable of implementing persistence based on any piece of data in the application message payloadheaders, or at in the transport protocol TCP and network protocol IP layers.Here in wordpress tutorial, we explained, how to do persistent database connection in wordpress.

Persistent connections are links that do not close when the execution of your script ends. If it does not exist, it creates the link. This line you will find two times in that file. You need to change the line both the times. Then upload this file to your wordpress installation. Persistent database connection will open only one connection and for every query that will check for connection is present or not. If connection is already present then your query will execute using that persistent database connection.

There are couple of issues with persistent database connection, When you are using the persistent connection you should keep following things in mind. Imp: There are a couple of additional caveats to keep in mind when using persistent connections. One is that when using table locking on a persistent connection, if the script for whatever reason cannot release the lock, then subsequent scripts using the same connection will block indefinitely and may require that you either restart the httpd server or the database server.

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. Better yet, avoid the problem entirely by not using persistent connections in scripts which use table locks or transactions you can still use them elsewhere. Your email address will not be published. Notify me of follow-up comments by email. Notify me of new posts by email.

Related Posts How to install wordpress multisite on subdomain 5 Comments Jun 24, No Comments Feb 1, Katie July 6, Reply. Thank you so much for writing this good info! Looking forward to seeintg more posts.

Ary curve November 3, Reply. Add a Comment Cancel reply Your email address will not be published.Connections are established by creating instances of the PDO base class.

It doesn't matter which driver you want to use; you always use the PDO class name. The constructor accepts parameters for specifying the database source known as the DSN and optionally for the username and password if any.

If there are any connection errors, a PDOException object will be thrown. Example 2 Handling connection errors. If your application does not catch the exception thrown from the PDO constructor, the default action taken by the zend engine is to terminate the script and display a back trace.

This back trace will likely reveal the full database connection details, including the username and password. Upon successful connection to the database, an instance of the PDO class is returned to your script. The connection remains active for the lifetime of that PDO object. To close the connection, you need to destroy the object by ensuring that all remaining references to it are deleted—you do this by assigning NULL to the variable that holds the object.

If you don't do this explicitly, PHP will automatically close the connection when your script ends. Example 3 Closing a connection.

persistent database connections

Many web applications will benefit from making persistent connections to database servers. Persistent connections are not closed at the end of the script, but are cached and re-used when another script requests a connection using the same credentials. The persistent connection cache allows you to avoid the overhead of establishing a new connection every time a script needs to talk to a database, resulting in a faster web application.

Example 4 Persistent connections. If setting this attribute with PDO::setAttribute after instantiation of the object, the driver will not use persistent connections. The ODBC Connection Pool is shared with other modules in the process; if PDO is told to cache the connection, then that connection would never be returned to the ODBC connection pool, resulting in additional connections being created to service those other modules.

Edit Report a Bug. Connections and Connection management Connections are established by creating instances of the PDO base class. Warning If your application does not catch the exception thrown from the PDO constructor, the default action taken by the zend engine is to terminate the script and display a back trace.As of PHP 7.

By default, every database connection opened by a script is either explicitly closed by the user during runtime or released automatically at the end of the script. A persistent connection is not. Instead it does not close when the execution of the script ends. When a persistent connection is requested, PHP checks if there's already an identical persistent connection that remained open from earlier - and if it exists, it uses it.

If it does not exist, it creates a new connection. An 'identical' connection is a connection that was opened to the same host, with the same username and the same password where applicable.

Reuse saves connection overhead. Depending on the web server deployment model, a PHP process may serve one or multiple requests. Therefore, a pooled connection may be used by one or more scripts subsequently. The web server may spawn many PHP processes. If that connection overhead is high, persistent connections help us considerably. They cause the child process to simply connect only once for its entire lifespan, instead of every time it processes a page that requires connecting to the MySQL server.

This means that for every child that opened a persistent connection will have its own open persistent connection to the server one from each child.

MS Access – Persistent Connection in a Split Database

For example, if you had 20 different child processes that ran a script that made a persistent connection to your MySQL server, you'd have 20 different connections to the MySQL server. This can have some drawbacks if we are using a database with connection limits that are exceeded by persistent child connections. If there are bugs in the scripts which do not allow the connections to shut down such as infinite loopsthe database with only N connections may be rapidly swamped.

There are a couple of additional caveats to keep in mind when using persistent connections. One is that when using table locking on a persistent connection, if the script for whatever reason cannot release the lock, then subsequent scripts using the same connection will block indefinitely and may require that you either restart the web server or the database server.

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. In either case, their state is not reset before reuse. The mysqli extension supports both interpretations of a persistent connection: state persisted, and state reset before reuse.

The default is reset. The persistent connection appears to the user as if it was just opened. Better yet, avoid the problem entirely by not using persistent connections in scripts which use table locks or transactions you can still use them elsewhere.

It is left to the user to choose between safe behavior and best performance. Both are valid optimization goals. For ease of use, the safe behavior has been made the default at the expense of maximum performance. MySQL supports a large variety of data types, and choosing the correct type Sometimes, it's possible to define multiple location blocks with different One way to improve the concurrency of a shared resource is to be moreAnyone that knows anything about developing a database knows that best practice dictates that you split your database into 2 components: a Back-End containing the tables your data and a Front-End containing everything else queries, forms, reports, macros, modules, ….

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What many developers seem to overlook however is the importance of always creating a persistent connection between the Front-End and the Back-end. Then you open another object, it must, yet again, establish a new connection, … You get the idea.

Each time, establishing the connection takes time. Think of it this way, imagine you have a multi-question survey to call people with. Is it better to dial a number, wait for someone to pick up at the other end, explain who you are, ask your first question and then hang up. Then repeat the same steps for your second question. The proper answer to this question is pretty clear cut if you ask me. Well, it is the same for your database! To minimize this impact on your database and improve performance it is critical, IMHO, to always establish a persistent connection as soon as your front-end load.

Connection Pool with an Oracle Database - DBArch Video 19

This way the database need not create a new connection each time you choose to do something, it already exists, so it can simply use it. So what have we done? For complicated, multi-Back-End databases, a separate persistent connection should be established with each Back-End file.

This works with a split. I am using the jstreet table lniking code with the macro. So whether or not you are using an mdb, or an mde, the setup is the same.

With the above in mind, then user should never need to see such a form and this is why we use the DoCmd. OpenForm to open the form at the startup of our database in Hidden mode so it is there, just not visible and it will close when the database is closed. Thank you for your article. Just for clarification, if our split db automatically opens a form that is sourced from a query of a few tables in our backend, this would also work establish a persistent correction — correct?

As in, the form need not be sourced from a table, a query would also work? Thanks for this Daniel, I hope its not too late to comment.A persistent connection HTTP persistent connection is a network communication channel that remains open for further HTTP requests and responses rather than closing after a single exchange. HTTP has a persistent connection function that allows the channel to remain open rather than be closed after a requested exchange of data.

TCP is a connection-oriented protocol: It starts a connection after confirmation from both ends that they are available and open to a data exchange.

In a non-persistent connection, the channel closes when one host signals that it wants to end communications or when a certain amount of time has elapsed with no exchange of data. To maintain a persistent connection, TCP keep-alive packets are sent to prevent the connection from timing out. An open connection is faster for frequent data exchanges. Communications overhead is saved by leaving a connection open rather than opening and closing sessions for each request.

Persistent connections can also be used with APIs to enable servers to push data to clients. Other benefits of persistent connections include reduced network congestion, latency and CPU and memory usage due to the lower number of connections; errors can also be reported without closing the connection.

Persistent connections added for HTTP 1. HTTP 2.

persistent database connections

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