DATALINK is an SQL data type that allows to store a reference to a file that is external to the database system. The purpose of Datalink is to provide a mechanism to implement the referential integrity, recovery and access control of the external files and the SQL-data associated with them. Files can be modified using file system commands or via SQL.
This extension aims to add this feature to PostgreSQL.
This extension is a proof of concept for implementing DATALINK in PostgreSQL, it must not be used in production until an official release will be published. Expect changes in the implementation and behaviors before the official release.
The documentation is at its early stage, for more detailled information look at the presentation at PgConf Asia 2019 conference
The Datalink extension requires the uuid-ossp and the uri extensions.
In order to install the Datalink extension download latest development sources
from GitHub and use the following command
to build and install the extension. pg_config must be in found from your PATH
environment variable.
make
sudo make install
To test the extension run, it is required that the user running the script be PostgreSQL superuser and able to execute commands using sudo:
cd test/
sh dl_test.sh
To use the extension in your database execute:
CREATE EXTENSION uri;
The default values for GUC related to the DATALINK extension are forced to:
datalink.dl_naptime = 10
datalink.dl_base_directory = '/tmp/test_datalink'
datalink.dl_token_path = '/tmp/test_datalink/pg_dltoken'
datalink.dl_token_expiry = 60
datalink.dl_keep_max_copies = 5
This is the one I use for the proof of concept, feel free to adjust them in datalink.h before compiling. This is not possible to change them from the configuration file for the moment but it will be possible as soon as the extension move to beta stage.
SQL/MED allows a variety of attributes to be specified for DATALINK columns. They are used to defined how the database system controls the file. This can be from no control at all (the file does not even have to exist) to full control, where removal of the datalink value from the database leads to a deletion of the physical file.
Example of use:
CREATE TABLE persons
(
id integer,
fullname text,
picture DATALINK
);
INSERT INTO persons VALUES (1, 'Jon Doe',
DLVALUE('img1.jpg', 'MyBaseDir', 'A comment'));
A DATALINK is formally composed by:
- a base directory representing mount points where external files can be read or written
- an URI corresponding to the path of the file relative to its base directory
- a comment
The base directories must exists on file system and be registered into table
pg_datalink_bases, only PostgreSQL superuser can do that.
INSERT INTO pg_datalink_bases (dirname, base)
VALUES (‘MyBaseDir’, ‘file:///var/www/mysite1/’);
No verification if the path exists is done.
The postgres user must have read/write permission on the directory if files
will be managed by SQL.
See test/ directory for more example of use.
A background worker is started with PostgreSQL and the process is named "Datalink background worker". This background worker scan periodically a directory where access control token are stored. Default is each 10 seconds (controlled by GUC datalink.dl_naptime) for testing.
This background worker is responsible of removing token that have expired
from directory $PGDATA/pg_token/ (GUC datalink.dl_base_directory). The
default expiry time is 60 seconds, the value can be changed using GUC
datalink.dl_token_expiry. For read token it also remove all link to
external files when the token expires. For write access token the backround
worker remove all obsolete copies that correspond to a rollbacked transaction.
See file SQL-MED-DATALINK-PgConfAsia2019.pdf for detailed information about the DATALINK implementation.
DATALINKs supports two permission modes to read and write datalinks, namely
- READ/WRITE PERMISSION FS (Controlled by the File System)
- READ/WRITE PERMISSION DB (Controlled by the Database system)
There is a third mode WRITE PERMISSION BLOCKED that provides data recovery functionality for an SQL-mediated file (WRITE PERMISSION DB), as long as the RECOVERY option is specified as YES.
This third option mean that a user cannot update the file while the file is referenced in a DATALINK column. Updating the content of a file that in this mode requires three distinct steps:
1. Unlinking the file
2. Modifying the file
3. Re-linking the file
The extension use a Copy-On-Write mechanism so this mode is always used in term that it never modify a file in place.
When READ or WRITE PERMISSION FS is specified, the system that control the DATALINK functionality allows user to update a file while the file remains linked to the database. This mode does not provide consistency and file data recovery. Which means that in case of crashes or a user needs to restore the database, there is no backup data to recover. In cases of transaction failure, this may cause inconsistency between the file data and the database data.
In this mode, read/write access privileges are determined by the file system permissions assigned to the file. PostgreSQL only allow access to external file through the user running the database system, mostly the postgres user.
For all these reasons it is not recommended to used this mode. Although the DATALINK extension provide this mode, this is only through the postgres user.
READ/WRITE PERMISSION FS does not support a token-based access model like the one provided with READ/WRITE PERMISSION DB/ADMIN.
When READ or WRITE PERMISSION DB is specified, the system that control the DATALINK functionality allows user to update a file while the file remains
When READ PERMISSION DB is specified, this is the database server that control which users are authorized to access the file. Writing is controlled by the Database system if WRITE PERMISSION ADMIN option is set.
These options can be followed by either the keywords REQUIRING TOKEN FOR UPDATE or the keywords NOT REQUIRING TOKEN FOR UPDATE.
- "REQUIRING TOKEN FOR UPDATE" indicates that the token, which was included in the file reference returned to the user when he requested an access to the file, is needed to update the DATALINK value in question.
- "NOT REQUIRING TOKEN FOR UPDATE" indicates that this token is not needed to update the DATALINK value.
"NOT TOKEN FOR UPDATE" does not allow concurrency level at Database side, it must be handled by the application. Although it is implemented in the extension it is recommended to always used "REQUIRING TOKEN FOR UPDATE" by this way file data recovery can be supported in an implementation-dependent way, as long as the RECOVERY option is set to YES.
The DLVALUE function returns a DATALINK value. When the function is in a VALUES clause in an INSERT statement, it creates a link to an external file.
If only a comment is specified, the data-location is a zero-length string and the DATALINK value is created with no file link.
data-location
Uri data type containing an URL value or a path.
directory-base
An optional text expression that specifies the link base directory of the DATALINK value. The valid value are 'URL', 'FILE' and any other value representing a directory that is registered in pg_catalog.pg_datalink_bases table.
comment-string
An optional text value that provides a comment or additional information.
The result of the function is a DATALINK value. If the data-location is null, the result is the null value.
Examples
Insert a DATALINK URL using the default base directory:
INSERT INTO dl_example VALUES( 1, DLVALUE('http://www.darold.net/', NULL, 'Web site');
Insert a local path DATALINK:
INSERT INTO dl_example VALUES( 1, DLVALUE('/var/lib/pgsql/files/info.dat', 'FILE', 'Local path');
Insert a NULL DATALINK:
INSERT INTO dl_example VALUES( 1, DLVALUE(NULL, 'FILE', 'Return null');
Insert an empty URL:
INSERT INTO dl_example VALUES( 1, DLVALUE('', 'URL', 'Empty uri value') );
Insert just a comment with empty uri:
INSERT INTO dl_example VALUES( 1, DLVALUE('Empty uri value') );
The DLVALUE function returns a DATALINK value. In this form the function is to be used on the right side of a SET clause in an UPDATE statement, it replace the link to an external file.
Same description as DLVALUE for INSERT statement except first parameter that is the DATALINK column that has to be updated. In future implementation this parameter will be removed in respect to the standard SQL as the value must be taken from the left operand of the UPDATE statement. This mean that in a short term there will be just one version of the DLVALUE() function, the same as the one use with INSERT statement.
The result of the function is a DATALINK value. If the data-location is null, the result is the null value.
Examples
Update a DATALINK URL:
UPDATE dl_example SET efile = DLVALUE('http://www.darold.net/', MyWebSite', 'Web site') WHERE id = 1;
The DLCOMMENT function returns the comment value of a DATALINK record. The argument must be an expression that results in a DATALINK type value.
The result of the function is of type text and can be null if the value is null.
Examples
Given a DATALINK value that was inserted into column EFILE using function:
DLVALUE('http://pgcluu.darold.net/index.html','URL','Main page of pgCluu site')
then call to the function on that DATALINK will return:
SELECT DLCOMMENT(EFILE)
will return the value:
Main page of pgCluu site
The DLLINKTYPE function returns the link type value from a DATALINK. The argument must be a DATALINK data type.
The result of the function is of type text and can be null if the value is null.
The link type value is either FILE, URL or the base directory of the DATALINK
Examples
Given a DATALINK value that was inserted into column EFILE using function:
DLVALUE('http://pgcluu.darold.net/index.html','URL','Main page of pgCluu site')
or DLVALUE('http://pgcluu.darold.net/index.html', NULL,'Main page of pgCluu site')
then call to the function on that DATALINK will return:
SELECT DLLINKTYPE(EFILE)
will return the value:
URL
Given a DATALINK value that was inserted into column EFILE using function:
DLVALUE('/var/log/postgresql/postgresql-11.log','FILE','PostgreSQL log file')
or DLVALUE('/var/log/postgresql/postgresql-11.log',NULL,'PostgreSQL log file')
then call to the function on that DATALINK will return:
SELECT DLLINKTYPE(EFILE)
will return the value:
FILE
Given a DATALINK value that was inserted into column EFILE using function:
DLVALUE('/tmp/test_datalink/jatfttd.txt','test_directory','Just a temporary file to test directory')
then call to the function on that DATALINK will return:
SELECT DLLINKTYPE(EFILE)
will return the value:
test_directory
The DLURLSCHEME function returns the scheme from a DATALINK value. The value will always be in lower case. The argument must be a value with data type DATALINK.
The result of the function is text. If the argument is null, the result is the null value. If the DATALINK value only includes the comment the result returned is a zero length string.
Examples
Given a DATALINK value that was inserted into column EFILE using function:
DLVALUE('http://pgbadger.darold.net/images/logo.png','URL','a comment')
then the following function operating on that value:
DLURLSCHEME(EFILE)
will return the value:
http
Given a DATALINK value that was inserted into column EFILE using function:
DLVALUE('/var/log/postgresql/postgresql-11.log','FILE','PostgreSQL log file')
or DLVALUE('/var/log/postgresql/postgresql-11.log',NULL,'PostgreSQL log file')
the return value is:
file
The DLURLSERVER function returns the file server from a DATALINK value when the uri scheme is http. The value returned is always in lower case.
The argument must be an expression that results in a value with data type DATALINK.
The result of the function is text. If the argument is null, the result is the null value. If the DATALINK value only includes the comment the result returned is a zero length string.
Examples
Given a DATALINK value that was inserted into column EFILE using function:
DLVALUE('http://pgbadger.darold.net/index.html','URL','a comment')
then the following function operating on that value:
DLURLSERVER(EFILE)
will return the value:
pgbadger.darold.net
The DLURLCOMPLETE function returns the data location attribute from a DATALINK value in a form of an URL. When datalink is a DATALINK column defined with the attribute READ PERMISSION DB, the value includes a file access token.
The argument must be a value with data type DATALINK.
The result of the function of type text. If the argument is null, the result is the null value. If the DATALINK value only includes the comment the result returned is a zero length string.
Examples
Given a DATALINK value that was inserted into column EFILE using function:
DLVALUE('http://www.darold.net/logo.png','URL','a comment')
the following function operating on that value:
DLURLCOMPLETE(EFILE)
returns:
http://www.darold.net/00f59e3-60a5-4bfa-a45b-214ccb08e425;logo.png
where 00f59e3-60a5-4bfa-a45b-214ccb08e425 represents the access token.
The DLURLCOMPLETEONLY function returns the data location attribute from a DATALINK value in form of an URL. The value returned never includes a file access token.
The argument must be a DATALINK data type value.
The result of the function is text. If the argument is null, the result is the null value.
If the DATALINK value only includes a comment, the result is a zero length string.
Examples
Given a DATALINK value that was inserted into a DATALINK column EFILE (defined with READ PERMISSION DB) using function:
DLVALUE('http://pgbadger.darold.net/logo.png','URL','a comment')
the following function operating on that value:
DLURLCOMPLETEONLY(EFILE)
returns:
http://pgbadger.darold.net/logo.png
### DLURLPATH ( datalink )
The DLURLPATH function returns the full path to the external file linked from a DATALINK value. When datalink is a DATALINK column defined with the attribute READ PERMISSION DB, the value includes a file access token.
The argument must be an expression that results in a value with data type DATALINK.
The result of the function is text. If the argument is null, the result is the null value. If the DATALINK value only includes the comment the result returned is a zero length string.
Examples
Given a DATALINK value that was inserted into column EFILE using the function:
DLVALUE('http://pgbadgr.darold.net/images/logo.png','URL','a comment')
then the following function operating on that value:
DLURLPATH(EFILE)
returns the value:
/images/00f59e3-60a5-4bfa-a45b-214ccb08e425;logo.png
(where 00f59e3-60a5-4bfa-a45b-214ccb08e425 represents the access token)
The DLURLPATHONLY function returns the full path to an external file linked from a DATALINK value. The value returned never includes a file access token.
The argument must be an expression that results in a value with data type DATALINK. The result of the function is text. If the argument is null, the result is the null value. If the DATALINK value only includes the comment the result returned is a zero length string.
Examples
Given a DATALINK value that was inserted into column EFILE using function:
DLVALUE('http://pgbadger.darold.net/images/logo.png','URL','a comment')
then the following function operating on that value:
DLURLPATHONLY(EFILE)
returns the value:
/images/logo.png
### DLURLCOMPLETEWRITE ( datalink )
The DLURLCOMPLETEWRITE function returns the complete URL value from a DATALINK value in the form of an URL. If the DATALINK value comes from a DATALINK column defined with WRITE PERMISSION ADMIN, a write token is included in the return value.
The returned value can be used to locate and update the linked file. If the DATALINK column is defined with another WRITE PERMISSION option (not ADMIN) or NO LINK CONTROL, DLURLCOMPLETEWRITE returns just the URL value without a write token. If the file reference is derived from a DATALINK column defined with WRITE PERMISSION FS, a token is not required to write to the file, because write permission is controlled by the file system.
The argument must be a value with data type DATALINK.
The result of the function is text. If the argument is null, the result is the null value. If the DATALINK value only includes a comment, the result is a zero length string.
Examples
Given a DATALINK value that was inserted into a DATALINK column EFILE (defined with WRITE PERMISSION ADMIN) using function:
DLVALUE('http://pgbadger.darold.net/logo.png','URL','a comment')
the following function operating on that value:
DLURLCOMPLETEWRITE(EFILE)
returns:
http://pgbadger.darold.net/00f59e3-60a5-4bfa-a45b-214ccb08e425;logo.png
where 00f59e3-60a5-4bfa-a45b-214ccb08e425 represents the write token. If EFILE is not defined with WRITE PERMISSION ADMIN, the write token will not be present.
The DLURLPATHWRITE function returns the full path necessary to access an external file given from a DATALINK value in a form of an absolute file path. The value returned includes a write token if the DATALINK value specified as parameter comes from a DATALINK column defined with WRITE PERMISSION ADMIN.
If the DATALINK column is defined with other WRITE PERMISSION options (not ADMIN) or NO LINK CONTROL, DLURLPATHWRITE returns the full path to the external file without a write token. If the file reference is derived from a DATALINK column defined with WRITE PERMISSION FS, a token is not required to write to the file, because write permission is controlled by the file system.
The argument must be an expression that results in a value with data type DATALINK.
The result of the function is text. If the argument is null, the result is the null value. If the DATALINK value only includes a comment, the result is a zero length string.
Examples
Given a DATALINK value that was inserted into a DATALINK column EFILE (defined with WRITE PERMISSION ADMIN) using function:
DLVALUE('http://pgbadger.darold.net/images/logo.png','URL','a comment')
the following function operating on that value:
DLURLPATHWRITE(EFILE)
returns:
/images/00f59e3-60a5-4bfa-a45b-214ccb08e425;logo.png
where 00f59e3-60a5-4bfa-a45b-214ccb08e425 represents the write token. If EFILE is not defined with WRITE PERMISSION ADMIN, the write token will not be present.
The DLNEWCOPY function returns a DATALINK value indicating that the referenced file has changed with the new URI in the DATALINK value.
The value is assigned to a DATALINK column as a result of an UPDATE statement. If the DATALINK column/directory is defined with option RECOVERY YES, the new version of the linked file is archived asynchronously.
If DLNEWCOPY is not called in an UPDATE statement an error is returned.
datalink
The DATALINK to be modified. In future implementation this parameter will be removed in respect to the standard SQL as the value must be taken from the left operand of the UPDATE statement.
data-location
A text expression that specifies a complete URL value. The value may have been obtained earlier by a SELECT statement through the DLURLCOMPLETEWRITE function.
has-token
A boolean value that indicates whether the URL contains a write token. An error occurs if the token embedded in the data location is not valid.
The result of the function is a DATALINK value.
Parameters data-location and has-token can not be null.
For a DATALINK column defined with WRITE PERMISSION ADMIN REQUIRING TOKEN FOR UPDATE, the write token must be in the data location to complete the SQL UPDATE statement.
On the other hand, for WRITE PERMISSION ADMIN NOT REQUIRING TOKEN FOR UPDATE, the write token is not required, but is allowed in the data location.
For a DATALINK column defined with WRITE PERMISSION ADMIN and REQUIRING TOKEN FOR UPDATE, the write token must be the same as the one used to open the specified file, if it was opened.
For any WRITE PERMISSION ADMIN column, even if the write token has expired, the token is still considered valid as long as the transaction is still open.
In a case where no file update has taken place, or the DATALINK file is linked with other options, such as WRITE PERMISSION BLOCKED/FS or NO LINK CONTROL, this function will behave like DLVALUE.
Examples
Given a DATALINK value that was inserted into column EFILE (defined with WRITE PERMISSION ADMIN REQUIRING TOKEN FOR UPDATE) using function:
DLVALUE('file:///home/postgres/datafiles/file1.txt','FILE','A local file')
Use the scalar function DLURLCOMPLETEWRITE to fetch the value:
SELECT DLURLCOMPLETEWRITE(EFILE) FROM DL_EXAMPLE WHERE ...
It returns:
/home/postgres/datafiles/00f59e3-60a5-4bfa-a45b-214ccb08e425;file1.txt
where 00f59e3-60a5-4bfa-a45b-214ccb08e425 represents the write token.
Use the above value to locate and update the content of the file. Issue the following SQL UPDATE statement to indicate that the file has been successfully changed:
UPDATE t1 SET EFILE = DLNEWCOPY(EFILE, 'file:///home/postgres/datafiles/00f59e3-60a5-4bfa-a45b-214ccb08e425;file1.txt', 1)
WHERE ...
where 00f59e3-60a5-4bfa-a45b-214ccb08e425 represents the same write token used to modify the file referenced by the URL value. Note that if EFILE is defined with WRITE PERMISSION ADMIN NOT REQUIRING TOKEN FOR UPDATE, the write token is not required in the above example.
The DLPREVIOUSCOPY function returns a DATALINK value indicating that the previous version of the file should be restored.
The value is assigned to a DATALINK column as a result of an UPDATE statement. This function restore the linked file from the previously committed version.
If DLPREVIOUSCOPY is not called in a result of an UPDATE statement an error is returned.
datalink
The DATALINK to be modified. In future implementation this parameter will be removed in respect to the standard SQL as the value must be taken from the left operand of the UPDATE statement.
data-location
A text expression that specifies the complete URL value. The value may have been obtained earlier by a SELECT statement through the DLURLCOMPLETEWRITE function.
has-token
An boolean value that indicates whether the URL contains a write token. An error occurs if the token embedded in the data location is not valid.
The result of the function is a DATALINK value.
Parameters data-location and has-token can not be null.
For a DATALINK column defined with WRITE PERMISSION ADMIN REQUIRING TOKEN FOR UPDATE, the write token must be in the data location to complete the SQL UPDATE statement. On the other hand, for WRITE PERMISSION ADMIN NOT REQUIRING TOKEN FOR UPDATE, the write token is not required, but is allowed in the data location.
For a DATALINK column defined with WRITE PERMISSION ADMIN REQUIRING TOKEN FOR UPDATE, the write token must be the same as the one used to open the specified file, if it was opened.
For any WRITE PERMISSION ADMIN column, even if the write token has expired, the token is still considered valid as long as the transaction is open.
Examples
Given a DATALINK value that was inserted into column EFILE (defined with WRITE PERMISSION ADMIN REQUIRING TOKEN FOR UPDATE and RECOVERY YES) using function:
DLVALUE('http://pgcluu.darold.net/index.html','URL','Main page of pgCluu site')
Use the scalar function DLURLCOMPLETEWRITE to fetch the value:
SELECT DLURLCOMPLETEWRITE(EFILE) FROM DL_EXAMPLE WHERE ...
It returns:
http://pgcluu.darold.net/00f59e3-60a5-4bfa-a45b-214ccb08e425;index.html
where 00f59e3-60a5-4bfa-a45b-214ccb08e425 represents the write token.
Use the above value to locate and update the content of the file. Issue the following SQL UPDATE statement to back out the file changes and restore to the previous committed version:
UPDATE DL_EXAMPLE SET EFILE = DLPREVIOUSCOPY('http://pgcluu.darold.net/00f59e3-60a5-4bfa-a45b-214ccb08e425;index.html', 1) WHERE ...
where 00f59e3-60a5-4bfa-a45b-214ccb08e425 represents the same write token used to modify the file referenced by the URL value. Note that if EFILE is defined with WRITE PERMISSION ADMIN NOT REQUIRING TOKEN FOR UPDATE, the write token is not required in the above example.
The DLREPLACECONTENT function returns a DATALINK value. When the function is on the right hand side of a SET clause in an UPDATE statement, or is in a VALUES clause in an INSERT statement, the assignment of the returned value results in replacing the content of a file by another file and then creating a link to it.
The actual file replacement process is done during commit processing of the current transaction.
datalink-expression
The DATALINK to be modified. In future implementation this parameter will be removed in respect to the standard SQL as the value must be taken from the left operand of the UPDATE statement.
data-location-target
A text expression that specifies a complete URL value.
As long as a DATALINK parameter is specified it must be the same URI than in the DATALINK.
data-location-source
A text expression that specifies the data location of a file in URL format. As a result of an assignment in an UPDATE or an INSERT statement, this file is renamed to the name of the file that is pointed to by data-location-target; the ownership and permission attributes of the target file are retained.
There is a restriction that data-location-source can only be one of the following:
- A zero-length value
- A NULL value
- The value of data-location-target plus a suffix string. The suffix string can be up to 20 characters in length. The characters of the suffix string must belong to the URL character set. Moreover, the string cannot contain a “\” character under the UNC scheme, or the “/” character under other valid schemes.
comment-string
An optional text value that contains a comment or additional location information.
The result of the function is a DATALINK value. If data-location-target is null, the result is the null value. If data-location-source is null, a zero-length string, or exactly the same as data-location-target, the effect of DLREPLACECONTENT is the same as DLVALUE.
Examples
Replace the content of a linked file by another file. Given a DATALINK value that was inserted into column EFILE using the following INSERT statement:
INSERT INTO DL_EXAMPLE (ID, EFILE) VALUES (1, DLVALUE('http://www.darold.net/logo.png'));
Replace the content of this file with another file by issuing the following SQL UPDATE statement:
UPDATE DL_EXAMPLE SET EFILE = DLREPLACECONTENT('http://www.darold.net/logo.png', 'http://www.darold.net/logo.png.new') WHERE ID = 1;
Gilles Darold < gilles@darold.net >
This extension is free software distributed under the PostgreSQL Licence.
Copyright (c) 2015-2018, Gilles Darold