Link Database

The darc project utilises Redis based database to provide tele-process communication.

Note

In its first implementation, the darc project used Queue to support such communication. However, as noticed when runtime, the Queue object will be much affected by the lack of memory.

There will be three databases, all following the save naming convension with queue_ prefix:

• the hostname database – queue_hostname

• the requests database – queue_requests

• the selenium database – queue_selenium

For queue_hostname, it is a Redis set data type; and for queue_requests and queue_selenium, they are both Redis sorted set data type.

darc.db.drop_hostname(link)

Remove link from the hostname database.

The function updates the queue_hostname database.

Parameters

link (darc.link.Link) – Link to be removed.

darc.db.drop_requests(link)

Remove link from the requests database.

The function updates the queue_requests database.

Parameters

link (darc.link.Link) – Link to be removed.

darc.db.drop_selenium(link)

Remove link from the selenium database.

The function updates the queue_selenium database.

Parameters

link (darc.link.Link) – Link to be removed.

darc.db.get_lock(name, timeout=None, sleep=0.1, blocking_timeout=None, lock_class=None, thread_local=True)

Get a lock for Redis operations.

Parameters

• name (str) – Lock name.

• timeout (Optional[float]) – Maximum life for the lock.

• sleep (float) – Amount of time to sleep per loop iteration when the lock is in blocking mode and another client is currently holding the lock.

• blocking_timeout (Optional[float]) – Maximum amount of time in seconds to spend trying to acquire the lock.

• lock_class (Optional[redis.lock.Lock]) – Lock implementation.

• thread_local (bool) – Whether the lock token is placed in thread-local storage.

Returns

Return a new redis.lock.Lock object using key name that mimics the behavior of threading.Lock.

Return type

Union[redis.lock.Lock, contextlib.nullcontext]

Seel Also:

If REDIS_LOCK is False, returns a contextlib.nullcontext instead.

darc.db.have_hostname(link)

Check if current link is a new host.

The function checks the queue_hostname database.

Parameters

link (darc.link.Link) – Link to check against.

Returns

If such link is a new host.

Return type

bool

darc.db.load_requests(check=False)

Load link from the requests database.

The function reads the queue_requests database.

Parameters

check (bool) – If perform checks on loaded links, default to CHECK.

Returns

List of loaded links from the requests database.

Return type

List[darc.link.Link]

Note

At runtime, the function will load links with maximum number at MAX_POOL to limit the memory usage.

darc.db.load_selenium(check=False)

Load link from the selenium database.

The function reads the queue_selenium database.

Parameters

check (bool) – If perform checks on loaded links, default to CHECK.

Returns

List of loaded links from the selenium database.

Return type

List[darc.link.Link]

Note

At runtime, the function will load links with maximum number at MAX_POOL to limit the memory usage.

darc.db.redis_command(command, *args, **kwargs)

Wrapper function for Redis command.

Parameters

• command (str) – Command name.

• *args – Arbitrary arguments for the Redis command.

Keyword Arguments

**kwargs – Arbitrary keyword arguments for the Redis command.

Returns

Values returned from the Redis command.

Warns

RedisCommandFailed – Warns at each round when the command failed.

Return type

Any

See also

Between each retry, the function sleeps for REDIS_RETRY second(s) if such value is NOT None.

darc.db.save_requests(entries, single=False, score=None, nx=False, xx=False)

Save link to the requests database.

The function updates the queue_requests database.

Parameters

• entries (List[darc.link.Link]) – Links to be added to the requests database. It can be either a list of links, or a single link string (if single set as True).

• single (bool) – Indicate if entries is a list of links or a single link string.

• score – Score to for the Redis sorted set.

• nx – Forces ZADD to only create new elements and not to update scores for elements that already exist.

• xx – Forces ZADD to only update scores of elements that already exist. New elements will not be added.

When entries is a list of Link instances, we tries to perform bulk update to easy the memory consumption. The bulk size is defined by BULK_SIZE.

Notes

The entries will be dumped through pickle so that darc do not need to parse them again.

darc.db.save_selenium(entries, single=False, score=None, nx=False, xx=False)

Save link to the selenium database.

The function updates the queue_selenium database.

Parameters

• entries (List[darc.link.Link]) – Links to be added to the selenium database. It can be either an iterable of links, or a single link string (if single set as True).

• single (bool) – Indicate if entries is an iterable of links or a single link string.

• score – Score to for the Redis sorted set.

• nx – Forces ZADD to only create new elements and not to update scores for elements that already exist.

• xx – Forces ZADD to only update scores of elements that already exist. New elements will not be added.

When entries is a list of Link instances, we tries to perform bulk update to easy the memory consumption. The bulk size is defined by BULK_SIZE.

Notes

The entries will be dumped through pickle so that darc do not need to parse them again.

darc.db.BULK_SIZE: int

Default

100

Environ

DARC_BULK_SIZE

Bulk size for updating Redis databases.

See also

• darc.db.save_requests()

• darc.db.save_selenium()

darc.db.LOCK_TIMEOUT: Optional[float]

Default

10

Environ

DARC_LOCK_TIMEOUT

Lock blocking timeout.

Note

If is an infinit inf, no timeout will be applied.

See also

Get a lock from darc.db.get_lock().

darc.db.MAX_POOL: int

Default

1_000

Environ

DARC_MAX_POOL

Maximum number of links loading from the database.

Note

If is an infinit inf, no limit will be applied.

darc.db.REDIS_LOCK: bool

Default

False

Environ

DARC_REDIS_LOCK

If use Redis (Lua) lock to ensure process/thread-safely operations.

See also

Toggles the behaviour of darc.db.get_lock().

darc.db.REDIS_RETRY: int

Default

10

Environ

DARC_REDIS_RETRY

Retry interval between each Redis command failure.

Note

If is an infinit inf, no interval will be applied.

See also

Toggles the behaviour of darc.db.redis_command().