This feature is used to signal that a data file cannot be modified after writing. Once a record is added, this record cannot be modified or deleted.

• avoids programming errors by preventing modification or deletion (via HModify/HDelete, or via a query).
• adds a hash of the record content to each record. This hash is an HMAC(SHA256) whose key is derived from the data file's encryption password.

• To reinforce the security, we advise you to implement a password on the unalterable data file. In this case, the data file cannot be created without knowing the password. It will not be possible to delete and recreate the data file without knowing this password.
• Use of unalterable data files with PERC servers:
  In this case, writing to the data file may be slower. This slowness is amplified when reindexing an unalterable data file with deleted records (very rare case).

You have the ability to define an unalterable HFSQL data file:

• in the data model editor:
  1. Open the data file description window (for example, "Description of data file" option in the context menu).
  2. On the "Options" tab, check "Unalterable data file (automatically enables enhanced security)".
     
  3. Validate the description of data file. In the data model editor, the unalterable data file is represented with a lock.
     
• through programming, by defining a data file dynamically:
  1. Create a variable of type File description.
  2. Use the Unalterable property to define the file unalterability.
  3. Use HDescribeFile to create the file description.

• If the data file is defined as being unalterable with password, the reinforced security is automatically implemented.
  Reminder: the "Enable enhanced security " option is used to encrypt data without storing the encryption key in the data file. With this option and without the file password, the file content cannot be retrieved.
• If the "Unalterable data file" option is set on an existing data file, the data files will be updated automatically. The existing data file will be compressed (deletion of crossed and deleted records) during this update.
  Warning: As soon as a data file is unalterable, no automatic data modification can be performed on it..
• This option is available from version 23. For the HFSQL Client/Server data files, an HFSQL C/S engine in version 23 (or later) must be used.

The only operations that can be performed on an unalterable data file are the creation and the addition of records.

• HCreation and HCreationIfNotFound.
• HAdd (outside transaction).
• HWrite with a record number only if the record number is the one that follows the last record (in this case, an addition is performed).
• HModify with a record number only if the record number is the one that follows the last record (in this case, an addition is performed).
• HSave preceded by HReset (in this case, an addition is performed).
• INSERT queries.

• HModify, HCross, HWrite, HSave, HDelete, HDeleteAll.
  Indeed, these functions modify the content of data files.
• UPDATE and DELETE queries.
  Indeed, these queries modify the content of data files.
• Any modification of the contents of the data file (by whatever means).
• The reindex operation with compression or deletion of damaged records.
  Indeed, this type of reindex operation deletes all inactive or damaged records. Therefore, the data file can be modified.
• The addition in a transaction.
  Indeed, canceling a transaction triggers the modification of data file.
• The automatic modification of data files.
  Indeed, modifying the structure of a data file (item addition or deletion for example) can modify the data found in the data file.
  For example, adding an item with a default value will modify all records by adding this new item to them. Similarly, deleting an existing item deletes information from the saved data. The data integrity is guaranteed anymore.
  

A runtime error occurs if one of these functions or features is used.

• manage the archiving of data file in its earlier format,
• create a new data file with the new structure.

Reminder: An application can consult the data in an HFSQL data file of an old structure using the HCheckStructure function in mode hCompatible. If the data file is incompatible due to the change in structure, it will be possible to use the archived data file in its old format with the new application after calling function HDeclareExternal.