Streaming Versions of API functions.

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## Session Module C Interface

## Streaming Versions of API functions.

>     int sqlite3changeset_apply_strm(
>       sqlite3 *db,                    /* Apply change to "main" db of this handle */
>       int (*xInput)(void *pIn, void *pData, int *pnData), /* Input function */
>       void *pIn,                                          /* First arg for xInput */
>       int(*xFilter)(
>         void *pCtx,                   /* Copy of sixth arg to _apply() */
>         const char *zTab              /* Table name */
>       ),
>       int(*xConflict)(
>         void *pCtx,                   /* Copy of sixth arg to _apply() */
>         int eConflict,                /* DATA, MISSING, CONFLICT, CONSTRAINT */
>         sqlite3_changeset_iter *p     /* Handle describing change and conflict */
>       ),
>       void *pCtx                      /* First argument passed to xConflict */
>     );
>     int sqlite3changeset_concat_strm(
>       int (*xInputA)(void *pIn, void *pData, int *pnData),
>       void *pInA,
>       int (*xInputB)(void *pIn, void *pData, int *pnData),
>       void *pInB,
>       int (*xOutput)(void *pOut, const void *pData, int nData),
>       void *pOut
>     );
>     int sqlite3changeset_invert_strm(
>       int (*xInput)(void *pIn, void *pData, int *pnData),
>       void *pIn,
>       int (*xOutput)(void *pOut, const void *pData, int nData),
>       void *pOut
>     );
>     int sqlite3changeset_start_strm(
>       sqlite3_changeset_iter **pp,
>       int (*xInput)(void *pIn, void *pData, int *pnData),
>       void *pIn
>     );
>     int sqlite3session_changeset_strm(
>       sqlite3_session *pSession,
>       int (*xOutput)(void *pOut, const void *pData, int nData),
>       void *pOut
>     );
>     int sqlite3session_patchset_strm(
>       sqlite3_session *pSession,
>       int (*xOutput)(void *pOut, const void *pData, int nData),
>       void *pOut
>     );
>     int sqlite3changegroup_add_strm(sqlite3_changegroup*,
>         int (*xInput)(void *pIn, void *pData, int *pnData),
>         void *pIn
>     );
>     int sqlite3changegroup_output_strm(sqlite3_changegroup*,
>         int (*xOutput)(void *pOut, const void *pData, int nData),
>         void *pOut
>     );

The six streaming API xxx_strm() functions serve similar purposes to the corresponding non-streaming API functions:

Streaming function | Non-streaming equivalent
--- | ---
sqlite3changeset_apply_str | sqlite3changeset_apply
sqlite3changeset_concat_str | sqlite3changeset_concat
sqlite3changeset_invert_str | sqlite3changeset_invert
sqlite3changeset_start_str | sqlite3changeset_start
sqlite3session_changeset_str | sqlite3session_changeset
sqlite3session_patchset_str | sqlite3session_patchset

Non-streaming functions that accept changesets (or patchsets) as input require that the entire changeset be stored in a single buffer in memory. Similarly, those that return a changeset or patchset do so by returning a pointer to a single large buffer allocated using sqlite3_malloc(). Normally this is convenient. However, if an application running in a low-memory environment is required to handle very large changesets, the large contiguous memory allocations required can become onerous.

In order to avoid this problem, instead of a single large buffer, input is passed to a streaming API functions by way of a callback function that the sessions module invokes to incrementally request input data as it is required. In all cases, a pair of API function parameters such as

          int nChangeset,
          void *pChangeset,

Is replaced by:

          int (*xInput)(void *pIn, void *pData, int *pnData),
          void *pIn,

Each time the xInput callback is invoked by the sessions module, the first argument passed is a copy of the supplied pIn context pointer. The second argument, pData, points to a buffer (*pnData) bytes in size. Assuming no error occurs the xInput method should copy up to (*pnData) bytes of data into the buffer and set (*pnData) to the actual number of bytes copied before returning SQLITE_OK. If the input is completely exhausted, (*pnData) should be set to zero to indicate this. Or, if an error occurs, an SQLite error code should be returned. In all cases, if an xInput callback returns an error, all processing is abandoned and the streaming API function returns a copy of the error code to the caller.

In the case of sqlite3changeset_start_strm(), the xInput callback may be invoked by the sessions module at any point during the lifetime of the iterator. If such an xInput callback returns an error, the iterator enters an error state, whereby all subsequent calls to iterator functions immediately fail with the same error code as returned by xInput.

Similarly, streaming API functions that return changesets (or patchsets) return them in chunks by way of a callback function instead of via a pointer to a single large buffer. In this case, a pair of parameters such as:

          int *pnChangeset,
          void **ppChangeset,

Is replaced by:

          int (*xOutput)(void *pOut, const void *pData, int nData),
          void *pOut

The xOutput callback is invoked zero or more times to return data to the application. The first parameter passed to each call is a copy of the pOut pointer supplied by the application. The second parameter, pData, points to a buffer nData bytes in size containing the chunk of output data being returned. If the xOutput callback successfully processes the supplied data, it should return SQLITE_OK to indicate success. Otherwise, it should return some other SQLite error code. In this case processing is immediately abandoned and the streaming API function returns a copy of the xOutput error code to the application.

The sessions module never invokes an xOutput callback with the third parameter set to a value less than or equal to zero. Other than this, no guarantees are made as to the size of the chunks of data returned.

See also lists of Objects, Constants, and Functions.

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