Datalink manual

pg_datalinker

pg_datalinker is a Datalink File Manager deamon, which normally runs in the background.

It is not meant to be run directly, but rather managed with dlfs command.

Datalinker uses entries in datalink.dl_linked_files table. Here it sees the results of transactions already commited in postgres.

It then attempts to modify linked files accordingly.

It also deletes (purges) temporary and aborted files created from postgres with datalink.write(), datalink.write_text() and dlreplacecontent() and such functions.

It needs to run as UNIX superuser root because it needs to change file ownership and permissions and whatnot.

Responsibilities

• Protect files from renaming or deletion (WRITE ACCESS BLOCKED or TOKEN or ADMIN)

  This is done by setting the immutable flag on the file with chattr +i command. This prevents file from being renamed or modified, even by root. Datalinker itself must run as root to change file attributes.

• Make backups of files (RECOVERY YES)

  Base file is designated by path, for example /dir1/dir2/file.ext.

  Backup file is base file with appended token, such as /dir1/dir2/file.ext#ae3cc23d-7a87-419a-b2f8-e6dc9d682d33.

  If backup file does not exist, it is created by creating a copy of base file.

• Restore files from backups (RECOVERY YES)

  Base file is designated by path, for example /dir1/dir2/file.ext.

  Backup file is base file with appended previous token, such as /dir1/dir2/file.ext#ae3cc23d-7a87-419a-b2f8-e6dc9d682d33.

  Base file is replaced by linking backup file as base file.

• Replace files with new versions (WRITE ACCESS TOKEN or ADMIN)

  Base file is designated by path, for example /dir1/dir2/file.ext.

  New file is base file with appended new token, such as /dir1/dir2/file.ext#ae3cc23d-7a87-419a-b2f8-e6dc9d682d33.

  Base file is replaced by linking new file as base file. New file will be deleted unless RECOVERY YES is also specified.

• Make files owned by database (READ ACCESS DB)

  File owner is set to the user postgres, making a file effectively owned by the postgres server process. File group is set to www-data, making file readable to apache web server. File mode is set to 0440, making file unreadable by normal users. Previous protection is stored in datalink.dl_linked_files table.

• Restore file permissions (ON UNLINK RESTORE)

  File owner, group and mode are restored from values previously stored in datalink.dl_linked_files table.

• Delete files no longer referenced (ON UNLINK DELETE)

• Purge files created from postgres

  This deletes files created by datalink.write() and such if the transaction was aborted or if the transaction was commited and file is temporary. File must also be owned by the user postgres.

Datalinker connects to postgres service "datalinker", so make sure it is configured in file /etc/postgresql-common/pg_service.conf.

Files in state LINK go into state LINKED when successfully linked otherwise they go into state ERROR.

Files in state UNLINK are unlinked and deleted from table datalink.dl_linked_files. If ON_UNLINK is DELETE then the files are also deleted from filesystem.

Options

$opt_O: if owner (of the table containing the datalink column) is equal to an existing OS user, it is set as the owner of the file thus making it readable to that user. This is meant to work with Postgres ident authetication on Debian linux and elsewhere, where database user is same as OS user.

Datalink manual