ChatGPT-LevelDB-Usage.md

Prompt

Using the header wrapper you just provided, can you please give me an example of using it with full CRUD including all operations that a batch supports?

Answer

Sure! Below is a full example demonstrating how to use the LevelDB Pascal wrapper for CRUD operations (Create, Read, Update, Delete) and utilizing batch operations. The example assumes you have already linked the LevelDB library to your Pascal project.

Example of CRUD operations with batch support:

program LevelDBExample;

{$mode objfpc}{$H+}

uses
  leveldb, ctypes, sysutils;

var
  db: Pleveldb_t;
  options: Pleveldb_options_t;
  writeOptions: Pleveldb_writeoptions_t;
  readOptions: Pleveldb_readoptions_t;
  batch: Pleveldb_writebatch_t;
  err: PChar;
  key, value: PChar;
  keylen, valuelen: csize_t;
  val: PChar;

procedure CheckError;
begin
  if err <> nil then
  begin
    Writeln('Error: ', StrPas(err));
    leveldb_free(err); // Free the error string allocated by LevelDB
    Halt(1);
  end;
end;

begin
  err := nil;

  // Initialize options
  options := leveldb_options_create();
  leveldb_options_set_create_if_missing(options, 1);

  // Open or create the database
  db := leveldb_open(options, 'testdb', @err);
  CheckError;

  // Initialize write options
  writeOptions := leveldb_writeoptions_create();
  leveldb_writeoptions_set_sync(writeOptions, 1); // Use synchronous writes

  // Initialize read options
  readOptions := leveldb_readoptions_create();

  // *** CREATE (PUT) OPERATION ***
  key := 'name';
  value := 'Gus';
  keylen := Length(key);
  valuelen := Length(value);
  leveldb_put(db, writeOptions, key, keylen, value, valuelen, @err);
  CheckError;
  Writeln('PUT: ', key, ' -> ', value);

  // *** READ OPERATION ***
  key := 'name';
  keylen := Length(key);
  val := leveldb_get(db, readOptions, key, keylen, @valuelen, @err);
  CheckError;
  Writeln('GET: ', key, ' -> ', StrPas(val));
  leveldb_free(val); // Free the value returned by LevelDB

  // *** UPDATE OPERATION ***
  key := 'name';
  value := 'GusUpdated';
  keylen := Length(key);
  valuelen := Length(value);
  leveldb_put(db, writeOptions, key, keylen, value, valuelen, @err);
  CheckError;
  Writeln('UPDATE: ', key, ' -> ', value);

  // *** READ UPDATED VALUE ***
  val := leveldb_get(db, readOptions, key, keylen, @valuelen, @err);
  CheckError;
  Writeln('GET Updated: ', key, ' -> ', StrPas(val));
  leveldb_free(val);

  // *** DELETE OPERATION ***
  key := 'name';
  keylen := Length(key);
  leveldb_delete(db, writeOptions, key, keylen, @err);
  CheckError;
  Writeln('DELETE: ', key);

  // *** BATCH OPERATIONS ***
  Writeln('Performing batch operations...');
  batch := leveldb_writebatch_create();

  // Add key-value pairs in the batch
  key := 'key1';
  value := 'value1';
  keylen := Length(key);
  valuelen := Length(value);
  leveldb_writebatch_put(batch, key, keylen, value, valuelen);

  key := 'key2';
  value := 'value2';
  keylen := Length(key);
  valuelen := Length(value);
  leveldb_writebatch_put(batch, key, keylen, value, valuelen);

  // Delete a key in the batch
  key := 'key1';
  keylen := Length(key);
  leveldb_writebatch_delete(batch, key, keylen);

  // Commit the batch
  leveldb_write(db, writeOptions, batch, @err);
  CheckError;
  Writeln('Batch operations completed.');

  // Read the values after batch commit
  key := 'key2';
  keylen := Length(key);
  val := leveldb_get(db, readOptions, key, keylen, @valuelen, @err);
  CheckError;
  Writeln('GET after batch: ', key, ' -> ', StrPas(val));
  leveldb_free(val);

  // Clean up resources
  leveldb_writebatch_destroy(batch);
  leveldb_readoptions_destroy(readOptions);
  leveldb_writeoptions_destroy(writeOptions);
  leveldb_options_destroy(options);
  leveldb_close(db);

  Writeln('Database operations completed successfully.');
end.

Explanation of the example:

1. Creating and Opening the Database:

   • We create the LevelDB options using leveldb_options_create() and set the create_if_missing option to create the database if it does not already exist.
   • The database is then opened using leveldb_open().

2. PUT Operation (Create):

   • We use leveldb_put() to insert a key-value pair into the database. In this case, the key is 'name', and the value is 'Gus'.

3. GET Operation (Read):

   • The value for the key 'name' is read using leveldb_get().

4. Update Operation:

   • We update the value for the key 'name' to 'GusUpdated'.

5. DELETE Operation:

   • We delete the key 'name' using leveldb_delete().

6. Batch Operations:

   • We create a write batch using leveldb_writebatch_create().
   • We add two key-value pairs to the batch and delete one key.
   • We use leveldb_write() to commit the batch to the database.

7. Resource Management:

   • We destroy all the created options, write batches, and free memory returned by leveldb_get() and error strings.

Compilation:

Ensure that you have the LevelDB C library properly linked in your Free Pascal environment.

Let me know if you need further assistance!