EBOOK – REDIS IN ACTION

This book covers the use of Redis, an in-memory database/data structure server.

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10.2.2 Creating a server-sharded connection decorator

Now that we have a method to easily fetch a sharded connection, let’s use it to build a
decorator to automatically pass a sharded connection to underlying functions.

We’ll perform the same three-level function decoration we used in chapter 5,
which will let us use the same kind of “component” passing we used there. In addition
to component information, we’ll also pass the number of Redis servers we’re going to
shard to. The following listing shows the details of our shard-aware connection decorator.

Listing 10.3A shard-aware connection decorator
def sharded_connection(component, shard_count, wait=1):

Our decorator will take a component name, as well as the number of shards desired.

   def wrapper(function):

We’ll then create a wrapper that will actually decorate the function.

      @functools.wraps(function)

Copy some useful metadata from the original function to the configuration handler.

      def call(key, *args, **kwargs):

Create the function that will calculate a shard ID for keys, and set up the connection manager.

         conn = get_sharded_connection(
            component, key, shard_count, wait)

Fetch the sharded connection.

         return function(conn, key, *args, **kwargs)

Actually call the function, passing the connection and existing arguments.

      return call

Return the fully wrapped function.

   return wrapper

Return a function that can wrap functions that need a sharded connection.

Because of the way we constructed our connection decorator, we can decorate our
count_visit() function from chapter 9 almost completely unchanged. We need to be
careful because we’re keeping aggregate count information, which is fetched and/or
updated by our get_expected() function. Because the information stored will be
used and reused on different days for different users, we need to use a nonsharded
connection for it. The updated and decorated count_visit() function as well as the
decorated and slightly updated get_expected() function are shown next.

Listing 10.4A machine and key-sharded count_visit() function
@sharded_connection('unique', 16)

We’ll shard this to 16 different machines, which will automatically shard to multiple keys on each machine.

def count_visit(conn, session_id):
   today = date.today()
   key = 'unique:%s'%today.isoformat()
   conn2, expected = get_expected(key, today)

Our changed call to get_expected().

   id = int(session_id.replace('-', '')[:15], 16)
   if shard_sadd(conn, key, id, expected, SHARD_SIZE):
      conn2.incr(key)

Use the returned nonsharded connection to increment our unique counts.

@redis_connection('unique')

Use a nonsharded connection to get_expected().

def get_expected(conn, key, today):
   'all of the same function body as before, except the last line'
   return conn, EXPECTED[key]

Also return the nonsharded connection so that count_visit() can increment our unique count as necessary.

In our example, we’re sharding our data out to 16 different machines for the unique
visit SETs, whose configurations are stored as JSON-encoded strings at keys named
config:redis:unique:0 to config:redis:unique:15. For our daily count information,
we’re storing them in a nonsharded Redis server, whose configuration information
is stored at key config:redis:unique.

MULTIPLE REDIS SERVERS ON A SINGLE MACHINEThis section discusses sharding
writes to multiple machines in order to increase total memory available
and total write capacity. But if you’re feeling limited by Redis’s singlethreaded
processing limit (maybe because you’re performing expensive
searches, sorts, or other queries), and you have more cores available for processing,
more network available for communication, and more available disk
I/O for snapshots/AOF, you can run multiple Redis servers on a single
machine. You only need to configure them to listen on different ports and
ensure that they have different snapshot/AOF configurations.

ALTERNATE METHODS OF HANDLING UNIQUE VISIT COUNTS OVER TIMEWith the
use of SETBIT, BITCOUNT, and BITOP, you can actually scale unique visitor
counts without sharding by using an indexed lookup of bits, similar to what
we did with locations in chapter 9. A library that implements this in Python
can be found at https://github.com/Doist/bitmapist.

Now that we have functions to get regular and sharded connections, as well as decorators
to automatically pass regular and sharded connections, using Redis connections
of multiple types is significantly easier. Unfortunately, not all operations that we need
to perform on sharded datasets are as easy as a unique visitor count. In the next section,
we’ll talk about scaling search in two different ways, as well as how to scale our
social network example.