evsql
- Author:
- Tero Marttila
Evsql is a C-language SQL library designed for use with
libevent, and primarily
PostgreSQL's
libpq.
Evsql was born as a result of wanting to use a SQL database from within a libevent application, and discovering libpq's asynchronous API. Since the libpq API is somewhat cumbersome to use, I wrote a separate interface that drives libpq using libevent and makes writing asynchronous SQL clients a lot easier - plus adding some conveniance for parametrized queries and such along the way.
The evsql.h API doesn't expose the underlying database library's API, although since the only currently existing implementation is libpq, this should really be thought of as a generically-named PostgreSQL library rather than a database-agnostic API...
Include the top-level
evsql.h header, making sure you also have the evpq and lib modules available.
Evsql sessions are represented using an opaque struct, called simply
evsql. Use the
evsql_new_* function corresponding to your database engine (PostgreSQL ->
evsql_new_pq()) to allocate this handle. It is valid for use immediately, although the initial connection may not yet be complete.
There is an evsql_close() function, but it is currently not implemented.
- See also:
- Session creation interface
Evsql supports both non-transactional queries and transactional queries. A
evsql_trans is allocated its own dedicated connection which it can use for its queries without being interfered by other queries/transactions. Evsql takes care of sending the initial "BEGIN TRANSACTION" query, and provides
evsql_trans_commit()/evsql_trans_abort() functions to send the "COMMIT TRANSACTION" and "ROLLBACK TRANSACTION" queries.
- See also:
- evsql_trans()
Transaction interface
There is a single
evsql_query() function used for both transactional and non-transactional queries; you can pass NULL as the
trans argument.
The given evsql_query_cb() callback function is invoked once the query has been processed and the evsql_result is available, or the query failed.
The important distinction between transactional and non-transactional queries is that transactions only support one outstanding query at a time, meaning that you must wait for your callback to be invoked before calling evsql_query() again for the transaction. Non-transactional queries are sent using an idle connection, and will be enqueued for later execution if no idle connections are available.
evsql_query() returns an evsql_query handle that can be passed to evsql_query_abort() to abort the query, ensuring that the evsql_query_cb() given to evsql_query() is not invoked, and any associated resources released.
- See also:
- evsql_query()
Query interface
Evsql also provides functions to send parametrized queries, the behaviour of
evsql_query explained above applies as well.
The first of these is evsql_query_params(), which takes the parametrized SQL command and a evsql_query_params containing the parameter types and values as arguments.
The second of these is evsql_query_exec(), which takes a evsql_query_info struct containing the parametrized SQL command and the parameter types, and the parameter values themselves as a list of variable arguments (of the correct type!).
- See also:
- Query interface
Parameter interface
Once a
evsql_query completes (sucess or failure, unless the query/transaction is aborted), the query's
evsql_query_cb() is called. It receives a
evsql_result handle, which can then be used with the
result interface to get information about the number of rows returned, access induvidual fields, iterate over the rows, etc. It is important to note that the query callback is responsible for releasing the
evsql_result using
evsql_result_free() (or equivalent) once it is done with it.
- See also:
- evsql_query_cb
Result interface
The entire API is defined in the top-level
evsql.h header, divided into various groups.
