Frequently Asked Questions (FAQ)#
What exactly is ADBC?#
A set of abstract APIs in different languages (C/C++, Go, and Java, with more on the way) for working with databases and Arrow data.
For example, result sets of queries in ADBC are all returned as streams of Arrow data, not row-by-row.
A set of implementations of that API in different languages (C/C++, Go, Java, Python, and Ruby) that target different databases (e.g. PostgreSQL, SQLite, any database supporting Flight SQL).
Why not just use JDBC/ODBC?#
JDBC uses row-based interfaces like ResultSet. When working with columnar data, like Arrow data, this means that we have to convert the data at least once and possibly twice:
Once (possibly) in the driver or database, to take columnar data and convert it into a row-based format so it can be returned through the JDBC APIs.
Once (always) when a client application pulls data from the JDBC API, to convert the rows into columns.
In keeping with Arrow’s “zero-copy” or “minimal-copy” ethos, we would like to avoid these unnecessary conversions.
ODBC is in a similar situation. Although ODBC does support “column-wise binding”, not all ODBC drivers support it, and it is more complex to use. Additionally, ODBC uses caller-allocated buffers (which often means forcing a data copy), and ODBC specifies data layouts that are not quite Arrow-compatible (requiring a data conversion anyways).
How do ADBC and Arrow Flight SQL differ?#
ADBC is a client API specification. It doesn’t define what goes on between your client and the database, just the API calls that you make as an application developer. Under the hood, a driver must take those API calls and talk to the actual database. Another perspective is that ADBC is all about the client-side, and specifies nothing about the network protocol or server-side implementation.
Flight SQL is a wire protocol. It specifies the exact commands to send to a database to perform various actions like authenticating with the database, creating prepared statements, or executing queries. Flight SQL specifies the network protocol that the client and the server must implement.
One more way of looking at it: an ADBC driver can be written for a database purely as a client library. (That’s how the PostgreSQL driver in this repository is implemented, for instance—as a wrapper around libpq.) But adding Flight SQL support to a database means either modifying the database to run a Flight SQL service, or putting the database behind a proxy that translates between Flight SQL and the database.
Why not just use Arrow Flight SQL?#
Because ADBC is client-side, ADBC can support databases that either don’t support returning Arrow data, or support Arrow data through a protocol besides Flight SQL.
Then do we even need Arrow Flight SQL?#
Flight SQL is a concrete protocol that database vendors can implement, instead of designing their own protocol. And Flight SQL also has JDBC and ODBC drivers for maximal compatibility.
As an analogy: many databases implement the PostgreSQL wire protocol, so that they can gain access to all the PostgreSQL clients, including JDBC and ODBC drivers. (And JDBC/ODBC users can still use other drivers to work with other databases.)
For the Arrow ecosystem, we hope databases will implement the Flight SQL wire protocol, giving them access to all the Flight SQL clients, including ADBC, JDBC, and ODBC drivers. (And ADBC users can still use other drivers to work with other databases.)
So what is the “ADBC Flight SQL driver” then?#
The ADBC Flight SQL driver implements the ADBC API standard (which an application interacts with) using the Flight SQL wire protocol (which a database server exposes). So it’s a generic driver that can talk to many databases, as long as those implement Flight SQL.
This is a little unusual, in that most database drivers and database protocols you’ll find were meant for a specific database. But Flight SQL was designed to be agnostic to the database from the start, and so was ADBC.
It sounds like they overlap, but they complement each other because they operate at different levels of abstraction. Database developers can just provide a Flight SQL service, which will give them ADBC, JDBC, and ODBC drivers for free, without having to build and maintain those drivers on their own. Database users can just use ADBC as the one Arrow-native API to work with both Arrow-native and non-Arrow-native databases, whether those databases support Flight SQL, a custom Arrow-native protocol, or no Arrow-native protocol.
And then what is the “ADBC JDBC driver”?#
The ADBC JDBC driver, or a hypothetical ADBC ODBC driver, adapts the JDBC API to the ADBC API, so that an ADBC user can interact with databases that have JDBC APIs available. While this doesn’t give you the best possible performance (you’re paying for tranposing the data back and forth!), it does save you the hassle of writing those conversions yourself.