Berkeleydb.pm download

For example, say you are writing a web application and you want to keep a record of the number of times your site is accessed in a Berkeley DB database. That may look innocent enough, but there is a race condition lurking in there. If I rewrite the line of code using the low-level Berkeley DB API, which is what will actually be executed, the race condition may be more apparent:.

Consider what happens behind the scenes when you execute the commands above. A read lock will be used for this part of the update. This time a write lock will be used. Here's the problem - there is nothing to stop two or more processes executing the read part at the same time.

Remember multiple processes can hold a read lock on the database at the same time. So both will fetch the same value, let's say 7, from the database.

Both increment the value to 8 and attempt to write it to the database. Berkeley DB will ensure that only one of the processes gets a write lock, while the other will be blocked. So the process that happened to get the write lock will store the value 8 to the database and release the write lock. Now the other process will be unblocked, and it too will write the value 8 to the database. The result, in this example, is we have missed a hit in the counter.

To deal with this kind of scenario, you need to make the update atomic. This has the useful side-effect of holding a write-lock on the database until the cursor is deleted. This is how you create a write-cursor.

Remember the complete database will be locked to other process whilst the write lock is in place. This issue is easier to demonstrate with an example, so consider the code below. The intention of the code is to increment the values of all the elements in a database by one.

All Berkeley DB cursors will hold either a read lock or a write lock on the database for the existence of the cursor. In order to prevent blocking of other processes you need to make sure that they are not long lived. There are a number of instances where the Perl interface to Berkeley DB will create a cursor behind the scenes without you being aware of it.

Most of these are very short-lived and will not affect the running of your script, but there are a few notable exceptions. While that cursor is still active, a read lock will obviously be held against the database. If your application has any other writing processes, these will be blocked until the read cursor is closed.

That won't happen until the loop terminates. The locking behaviour of the values or keys functions, shown below, is subtly different. Just as in the each function, a read cursor will be created to iterate over the database in both of these cases. Where keys and values differ is the place where the cursor carries out the iteration through the database. Whilst each carried out a single iteration every time it was invoked, the keys and values functions will iterate through the entire database in one go -- the complete database will be read into memory before the first iteration of the loop.

Apart from the fact that a read lock will be held for the amount of time required to iterate through the database, the use of keys and values is not recommended because it will result in the complete database being read into memory.

If your CDS application uses multiple database files, and you need to write to more than one of them, you need to be careful you don't create a deadlock. For example, say you have two databases, D1 and D2, and two processes, P1 and P2.

Assume you want to write a record to each database. If P1 writes the records to the databases in the order D1, D2 while process P2 writes the records in the order D2, D1, there is the potential for a deadlock to occur.

This flag will make a write-lock apply to all the databases in the environment. An example is when you need to encode your data in UTF-8 before writing to the database and then decode the UTF-8 when reading from the database file. This module hides the complexity of the API defined below and comes with a number of "canned" filters that cover some of the common use-cases.

There are four methods associated with DBM Filters. All work identically, and each is used to install or uninstall a single DBM Filter. Each expects a single parameter, namely a reference to a sub. The only difference between them is the place that the filter is installed. If a filter has been installed with this method, it will be invoked every time you write a key to a DBM database. If a filter has been installed with this method, it will be invoked every time you write a value to a DBM database.

If a filter has been installed with this method, it will be invoked every time you read a key from a DBM database.

If a filter has been installed with this method, it will be invoked every time you read a value from a DBM database. The return code from the filter is ignored. Consider the following scenario. You have a DBM database that you need to share with a third-party C application.

When you write to the database you will have to use something like this:. It would be much better if you could ignore the NULL terminations issue in the main application code and have a mechanism that automatically added the terminating NULL to all keys and values whenever you write to the database and have them removed when you read from the database.

Hopefully the contents of each of the filters should be self-explanatory. Here is another real-life example. By default, whenever Perl writes to a DBM database it always writes the key and value as strings.

So when you use this:. If you actually want the key to be stored in the DBM database as a C int, you will have to use pack when writing, and unpack when reading. This time only two filters have been used -- we only need to manipulate the contents of the key, so it wasn't necessary to install any value filters.

See the MLDBM documentation for information on how to use the module and for details of its limitations. There is no technical reason why a Berkeley DB database cannot be shared by both a Perl and a C application. The vast majority of problems that are reported in this area boil down to the fact that C strings are NULL terminated, whilst Perl strings are not. Before Berkeley DB 2.

That means that it doesn't support transactions, locking or any of the other new features available in DB 2. Copyright c Paul Marquess. All rights reserved. Berkeley DB has its own copyright and its own license.

Please take the time to read it. If you are in any doubt about the license situation, contact either the Berkeley DB authors or the author of BerkeleyDB.

For more information on module installation, please visit the detailed CPAN module installation guide. Should I use it? Another Example -- Key is a C int. Expect it to be incomplete in places. The interface to Berkeley DB is implemented with a number of Perl classes. This option requires Berkeley DB 4.

This option requires Berkeley DB 3. This option requires Berkeley DB 6. If not specified blob files are stores in the environment directoy. Only valid when Berkeley DB 3.

Examples TODO. BerkeleyDB::Unknown This isn't a database format at all. See "" for an example. Methods BerkeleyDB::Hash only supports the standard database methods. Another Simple Hash Example Do the same as the previous example but not using tie. Methods BerkeleyDB::Btree supports the following database methods. All the methods below return 0 to indicate success. The proportion is returned as a double in the range 0. A Simple Btree Example The code below is a simple example of using a btree database.

Note it is not necessary to sort the keys as the btree will have kept them in order automatically. Smith Wall mouse Changing the sort order It is possible to supply your own sorting algorithm if the one that Berkeley DB used isn't suitable. Defaults to The destruction of the CDS lock object automatically calls this method. I do not know what your time frame is for your project but I can probably post a simple program that utilizes the Berkeley DB C API sometime later this week or early next week.

If it cannot wait that long you might consider looking at the Berkeley Docs and see if you can work one out yourself. Hi Eric, Thanks for your reply. Taht will be nice if you can post something simple to access the DB. Your email address will not be published. Notify me of follow-up comments by email.

Notify me of new posts by email. Skip to content Berkeley DB is a general purpose embedded database engine that provides data management features found in enterprise class databases. This is what have been provided to make it work such create.

Hello Zack, From looking at the code it appears that it is just one piece of a larger set code files. Are you sure that this code if for Berkeley Databases? Leave a Reply Cancel reply Your email address will not be published.

The solution is to use a local drive. Berkeley DB doesn't support network drives. Solution: You will have to build a private copy of the Berkeley DB library and use it when building this Perl module. Linux Notes Some versions of Linux e. RedHat 6, SuSe 6 ship with a C library that has version 2. This makes it difficult to build this module with anything other than the version of Berkeley DB that shipped with your Linux release.

If you do try to use a different version of Berkeley DB you will most likely get the error described in the "Incompatible versions of db. To make matters worse, prior to Perl 5. If you want to use a newer version of Berkeley DB with this module, the easiest solution is to use Perl 5. There are two approaches you can use to get older versions of Perl to work with specific versions of Berkeley DB.

Both have their advantages and disadvantages. The first approach will only work when you want to build a version of Perl older than 5. If you want to use Berkeley DB 2. This approach involves rebuilding your existing version of Perl after applying an unofficial patch. The "patches" directory in the this module's source distribution contains a number of patch files. There is one patch file for every stable version of Perl since 5. Apply the appropriate patch to your Perl source tree before re-building and installing Perl from scratch.

For example, assuming you are in the top-level source directory for Perl 5. Remember to replace the path shown below with one that points to this module's patches directory. You should now have a perl binary that can be used to build this module. The second approach will work with Berkeley DB 2.

Start by building Berkeley DB as a shared library. This is from the Berkeley DB build instructions: Building Shared Libraries for the GNU GCC compiler If you're using gcc and there's no better shared library example for your architecture, the following shared library build procedure will probably work. Rebuild all of your. This will create a Berkeley DB library that contains.

The file libdb. This may be a problem if you run any commands that access a database created by the version of Berkeley DB that shipped with your Linux distribution. Solaris 2. It has been reported that Sun patch or later revisions fixes this problem.

To find out if you have the patch installed, the command "showrev -p" will display the patches that are currently installed on your system. The linked output may not run on a PA 1. The build will fail with an error similar to: BerkeleyDB. How to report a problem with BerkeleyDB. The version of Perl and the operating system name and version you are running. The complete output from running "perl -V" will tell me all I need to know.

If your perl does not understand the "-V" option is too old.