# 24. Appendix B: Differences between python-oracledb Thin and Thick Modes

By default, python-oracledb runs in a ‘Thin’ mode which connects directly to Oracle Database. This mode does not need Oracle Client libraries. However, some additional functionality is available when python-oracledb uses them. Python-oracledb is said to be in ‘Thick’ mode when Oracle Client libraries are used. See Enabling python-oracledb Thick mode. Both modes have comprehensive functionality supporting the Python Database API v2.0 Specification.

This section details the differences between the python-oracledb Thin and Thick modes. Also see the summary feature comparison table in Appendix A: Oracle Database Features Supported by python-oracledb.

## 24.1. Connection Handling Differences between Thin and Thick Modes

Python-oracledb can create connections in either a Thin mode or a Thick mode. However, only one of these modes can be used in each Python process:

- By default, python-oracledb runs in a Thin mode which connects directly to Oracle Database.

- If `oracledb.init_oracle_client()` loads Oracle Client libraries before any standalone connection or pool is created, then the python-oracledb mode becomes Thick. The client libraries handle communication with Oracle Database. See Enabling python-oracledb Thick mode.

- If an application opens a connection or creates a pool and then calls `oracledb.init_oracle_client()`, an error will occur.

- Once a connection or pool has been opened, or `init_oracle_client()` has been called, you cannot change the mode.

Note

The parameters of connection and pool creation functions `oracledb.connect()` and `oracledb.create_pool()` are now keyword and not positional in both Thin and Thick modes. This change makes the python-oracledb driver compliant with the Python Database API specification PEP 249. The old usage will cause an error, see Common Connection Errors.

### 24.1.1. Connections to a Local Database

In Thin mode, there is no concept of a local database. Bequeath connections cannot be made since no Oracle Client libraries are used. The Thin mode does not de-reference environment variables such as `ORACLE_SID`, `TWO_TASK`, or `LOCAL` (the latter is specific to Windows). A connection string, or equivalent, must always be used.

### 24.1.2. Oracle Net Services and Client Configuration Files

In the python-oracledb Thin mode:

- The location of any `tnsnames.ora` files must explicitly be passed to the application.

- Any `sqlnet.ora` file will not be read. Instead, pass an equivalent setting when connecting.

- There is no support for `oraaccess.xml` since there are no Oracle Client libraries.

See Optional Oracle Net Configuration Files and Optional Oracle Client Configuration File for more information.

### 24.1.3. Connection Strings

The python-oracledb Thin mode accepts connection strings in the same formats as the Oracle Client libraries used by Thick mode does, but not all Oracle Net keywords will be supported.

The following table lists the parameters that are recognized in Thin mode either in Easy Connect Strings or in Full Connect Descriptor Strings that are either explicitly passed or referred to by a `tnsnames.ora` alias. All unrecognized parameters are ignored. The connection parameters shown can be used in `oracledb.connect()`, `oracledb.create_pool()`, `oracledb.ConnectParams()`, and `oracledb.PoolParams()`.

Table 24.1 Oracle Net Keywords Supported in the python-oracledb Thin Mode
Oracle Net Keyword | Equivalent Connection Parameter | Notes
--- | --- | ---
SSL_SERVER_CERT_DN | ssl_server_cert_dn | If specified, this value is used for any verification. Otherwise, the hostname will be used.
SSL_SERVER_DN_MATCH | ssl_server_dn_match | In Thin mode parsing the parameter supports case insensitive on/yes/true values similar to the Thick mode. Any other value is treated as disabling it.
WALLET_LOCATION | wallet_location | Used in Easy Connect Strings. It is same as `MY_WALLET_DIRECTORY` in a connect descriptor.
MY_WALLET_DIRECTORY | wallet_location |
EXPIRE_TIME | expire_time |
HTTPS_PROXY | https_proxy |
HTTPS_PROXY_PORT | https_proxy_port |
RETRY_COUNT | retry_count |
RETRY_DELAY | retry_delay |
TRANSPORT_CONNECT_TIMEOUT | tcp_connect_timeout |
POOL_CONNECTION_CLASS | cclass |
POOL_PURITY | purity |
SERVICE_NAME | service_name |
SID | sid |
PORT | port |
PROTOCOL | protocol |

In python-oracledb Thin mode, using the `POOL_CONNECTION_CLASS` or `POOL_PURITY` parameters in a connection string is similar to setting the equivalent attributes when creating a connection or connection pool.

In python-oracledb Thick mode, the `POOL_CONNECTION_CLASS` or `POOL_PURITY` values will only work when connected to Oracle Database 21c. Note if `POOL_PURITY=SELF` is used in a connect string, then python-oracledb Thick mode applications will ignore the action to drop the session when attempting to remove an unusable connections from a pool in some uncommon error cases. It is recommended to avoid using `POOL_PURITY=SELF` in a connect string with python-oracledb Thick mode. Instead, code the python-oracledb Thick mode application to explicitly specify the purity and connection class as attributes.

The `ENABLE=BROKEN` connect descriptor option is not supported in python-oracledb Thin mode. Use `expire_time` instead.

The `Session Data Unit (SDU)` connect descriptor option that is used to tune network transfers is not supported in python-oracledb Thin mode. The value is hard-coded as 8 KB. In python-oracledb Thick mode, the SDU connect descriptor option and equivalent `sqlnet.ora` setting are used.

If a name is given as a connect string, then the python-oracledb Thin mode will consider it as a Net Service Name and not as the minimal Easy Connect string of a hostname. The given connect string will be looked up in a `tnsnames.ora` file. This is different from the python-oracledb Thick mode. If supporting a bare name as a hostname is important to you in the python-oracledb Thin mode, then you can alter the connection string to include a port number such as `hostname:1521` or a protocol such as `tcp://hostname`.

### 24.1.4. Token Based Authentication

In the python-oracledb Thin mode:

- When connecting to Oracle Cloud Database with mutual TLS (mTLS) using OAuth2 tokens, you need to explicitly set the `config_dir`, `wallet_location`, and `wallet_password` parameters of `connect()` or `create_pool()`. See, Connecting to Oracle Cloud Autonomous Databases.

- Open Authorization (OAuth 2.0) token based authentication connection strings and Oracle Cloud Infrastructure (OCI) Identity and Access Management (IAM) token based authentication connection strings are not supported. Use `access_token` parameter of `oracledb.ConnectParams()` instead. See Token-Based Authentication.

### 24.1.5. Transport Layer Security (TLS) Support

When connecting with mutual TLS (mTLS) also known as two-way TLS, for example to Oracle Autonomous Database in Oracle Cloud using a wallet, the certificate must be in the correct format.

For the python-oracledb Thin mode, the certificate must be in a Privacy Enhanced Mail (PEM) `ewallet.pem` file. In python-oracledb Thick mode the certificate must be in a `cwallet.sso` file. See Connecting to Oracle Cloud Autonomous Databases for more information.

### 24.1.6. Native Network Encryption and Checksumming

The python-oracledb Thin mode does not support connections using Oracle Database native network encryption or checksumming. You can enable TLS instead of using native network encryption. If native network encryption or checksumming are required, then use python-oracledb in the Thick mode. See Enabling python-oracledb Thick mode.

For example, if you use python-oracledb Thin mode and try to connect to the Oracle Cloud Infrastructure (OCI) Oracle Base Database where by default native network encryption is set to REQUIRED in the `sqlnet.ora` file of the OCI Oracle Base Database server, the connection will fail with the error:

    DPY-6000: cannot connect to database. Listener refused connection.
    (Similar to ORA-12660)

## 24.2. Connection Pooling Differences between Thin and Thick Modes

Python-oracledb introduced the ConnectionPool Object class to replace `SessionPool`. A new `oracledb.create_pool()` method is now the recommended way to create a connection pool. The use of the equivalent `SessionPool()` constructor is deprecated.

The `create_pool()` method in the python-oracledb Thin mode differs from the python-oracledb Thick mode in the following ways:

- Not all the parameters of the `oracledb.create_pool()` method are applicable to both python-oracledb modes. Each mode ignores unrecognized parameters. The parameters that are ignored in Thin mode include `events`, `tag`, `matchanytag`, `appcontext`, `shardingkey`, `supershardingkey`, and `handle` parameters. The parameters that are ignored in the Thick mode include `wallet_password`, `disable_oob`, `config_dir`, and `debug_jdwp` parameters.

- The python-oracledb Thin mode only suppports homogeneous pools.

- The python-oracledb Thin mode creates connections in a daemon thread and so `oracledb.create_pool()` returns before any or all minimum number of connections are created. As soon as the pool is created, the `ConnectionPool.opened` attribute will not be equal to `ConnectionPool.min`. The `opened` attribute will increase to the minimum value over a short time as the connections are established. Note that this behavior may also be true of recent versions of the Oracle Call Interface (OCI) Session Pool used in the Thin mode.

  If the new `getmode` default value of `POOL_GETMODE_WAIT` is used, then this behavior will not be an issue. With this new default value, any immediate `ConnectionPool.acquire()` calls will wait for the connections to be created by the daemon thread. This improves the application start up time when compared to the python-oracledb Thick mode and cx_Oracle 8.3 driver, where `oracledb.create_pool()` will not return control to the application until all `pool.min` connections have been created.

  If the old default value `POOL_GETMODE_NOWAIT` is required, then the application could check if `ConnectionPool.opened` has reached `ConnectionPool.min` and then continue with application start up.

- In python-oracledb Thick mode, when you close a connection pool with the parameter `force=True`, the underlying Oracle Client libraries wait for the current SQL executions to complete before closing the connections. All of the connections are then dropped from the pool and the pool is closed. Closing the pool in python-oracledb Thick mode could stop responding indefinitely, depending on the network and Oracle Net Services timeout parameters. This is also applicable to the cx_Oracle 8.3 driver. In python-oracledb Thin mode, the parameter `force=True` disconnects each connection’s socket using a background thread, leaving the database to clean up its end of the connections.

- In python-oracledb Thin mode, the `cclass` parameter value is not used to tag connections in the application connection pool. It is only used for Database Resident Connection Pooling (DRCP).

- In python-oracledb Thin mode, the connection pool supports all the connection mode privileges.

  The python-oracledb Thick mode only supports the `AUTH_MODE_SYSDBA` privilege.

## 24.3. Supported Database Data Types in Thin and Thick Modes

The python-oracledb Thin and Thick modes support different Oracle database data types. See Supported Oracle Database Data Types.

## 24.4. Query Metadata in Thin and Thick Modes

In python-oracledb Thin mode, `Cursor.description` metadata can distinguish the ROWID and UROWID database types. The UROWID database type shows the new value `DB_TYPE_UROWID` and the database type ROWID uses the existing value `DB_TYPE_ROWID`.

In python-oracledb Thick mode, the value `DB_TYPE_ROWID` is shown for both ROWID and UROWID database types. In python-oracledb Thick and Thin modes, comparison with the type `oracledb.ROWID` (defined in the Python DB API) will match both ROWID and UROWID database types.

## 24.5. Statement Caching in Thin and Thick Modes

The statement cache implemented in the python-oracledb Thin mode is capable of determining when different database data types are used for the same bind variables when a statement is re-executed. This capability is not supported in the Oracle Client libraries that are used in python-oracledb Thick mode. Note changing the type of bind variables for the same SQL text is inappropriate and gives indeterminate results in both modes.

## 24.6. Fetching JSON in Thin and Thick Modes

The python-oracledb Thin mode does not natively handle the Oracle Database 21c JSON data type but a type handler can be used when fetching the type, see Using JSON Data.

## 24.7. Error Handling in Thin and Thick Modes

The python-oracledb Thin and Thick modes handle some errors differently. See Error Handling in Thin and Thick Modes.

## 24.8. Globalization in Thin and Thick Modes

All NLS environment variables, and the `ORA_SDTZ` and `ORA_TZFILE` environment variables, are ignored by the python-oracledb Thin mode. Use Python’s capabilities instead.

The python-oracledb Thin mode can only use NCHAR, NVARCHAR2, and NCLOB data when Oracle Database’s secondary character set is AL16UTF16.

See Character Sets and Globalization.

## 24.9. Tracing in Thin and Thick Modes

In the python-oracledb Thin mode, low level tracing is different because there are no Oracle Client libraries. See Tracing python-oracledb.