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Re: Question about asyncio and blocking operations

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  Re: Question about asyncio and blocking operations "Frank Millman" <frank@chagford.com> - 2016-01-26 16:15 +0200

#102121 — Re: Question about asyncio and blocking operations

"Frank Millman"  wrote in message news:n8038j$575$1@ger.gmane.org...
>
> I am developing a typical accounting/business application which involves a 
> front-end allowing clients to access the system, a back-end connecting to 
> a database, and a middle layer that glues it all together.
>
[...]
>
> There was one aspect that I deliberately ignored at that stage. I did not 
> change the database access to an asyncio approach, so all reading 
> from/writing to the database involved a blocking operation. I am now ready 
> to tackle that.

I am making some progress, but I have found a snag - possibly unavoidable, 
but worth a mention.

Usually when I retrieve rows from a database I iterate over the cursor -

    def get_rows(sql, params):
        cur.execute(sql, params)
        for row in cur:
            yield row

If I create a Future to run get_rows(), I have to 'return' the result so 
that the caller can access it by calling future.result().

If I return the cursor, I can iterate over it, but isn't this a blocking 
operation? As far as I know, the DB adaptor will only actually retrieve the 
row when requested.

If I am right, I should call fetchall() while inside get_rows(), and return 
all the rows as a list.

This seems to be swapping one bit of asynchronicity for another.

Does this sound right?

Frank

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